2.3.3 Phosphorus and Productivity

Summary

As the Lake Source Cooling (LSC) system draws cold water from deep in Cayuga Lake and returns the warmed water to the shallow southern lake basin, it transfers materials within the lake's ecosystem. The mass of phosphorus (P) input to the upper waters during the lake's stratified period will increase with the implementation of LSC. In addition, the concentration of soluble reactive phosphorus (SRP), which is readily available for algal growth, will increase in the vicinity of the outfall. This section of the Draft Environmental Impact Statement (DEIS) addresses the potential impacts of this transfer of P on primary productivity and related water quality indicators. Several interrelated analyses have been performed:

• A monthly phosphorus budget of the southern lake basin during the stratified period has been calculated to estimate the relative contribution of LSC. In an effort to compare P loads of equivalent biological availability, we compared total soluble phosphorus (TSP) from the tributaries to total phosphorus (TP) from the wastewater treatment plants, and to TP from the lake's hypolimnion. Based on conservative assumptions, LSC would contribute 2.4 percent of the P loading to the southern lake basin during spring high stream flows. The monthly maximum LSC contribution to the P loading would be 6.4 percent during August.
  
  
• The impact of this additional P on chlorophyll a (a pigment used to indicate the amount of algal growth) has been estimated. The predicted impact is very small. There will be no perceptible impact on water clarity.
  
  
• The potential for secondary effects on dissolved oxygen depletion has been calculated based on the relationships between P, algal biomass (as indicated by organic carbon), and oxygen demand by decomposing organisms. According to the results of these calculations, the additional algal growth caused by P cycling through LSC will not cause measurable oxygen depletion in the lake.
  
  
• Impacts of SRP in the outfall region have been examined using CORMIX2, a mixing and dilution model. The largest SRP plume is predicted to occur in August and September, when the LSC system demand is high and the difference in SRP concentrations between the upper and lower waters is at an annual maximum. Even during this period, SRP concentrations are rapidly diluted to within 0.5 micrograms per liter (µg/l) of background at a distance of several hundred meters of the outfall diffuser. These trace concentrations of SRP will not cause measurable increases in algal growth.
  
  
• Potential impacts on the algal community have been examined based on thermal preference and nitrogen: phosphorus (N:P) ratios. The additional P from LSC would slightly decrease the N:P ratio in the outfall region, but the ratio would remain well above critical values for favoring the growth of nitrogen-fixing blue-green algae. Consequently, implementation of LSC will not cause or contribute to growth of these nuisance organisms.

From these analyses, we conclude that recirculation of P through the LSC system will have only a minor impact on primary productivity in Cayuga Lake's southern basin. No perceptible effect on the lake's visual or ecological character is projected.

2.3.3.1 Introduction.
In most northeastern lakes, the level of primary productivity is ultimately controlled by the amount and availability of phosphorus (Schindler 1974). Phosphorus is the element that limits the amount of plant and algal growth in these lake ecosystems; given favorable light and temperature conditions, plant and algal growth continues until the supply of phosphorus is depleted. The external supply of phosphorus is governed by terrestrial and watershed processes.

Phosphorus is the limiting nutrient for primary productivity in Cayuga Lake (Peterson, Barlow, and Savage 1974). In southern Cayuga Lake, the region of interest for the LSC project, phosphorus (P) inputs are approximately evenly divided between inflows from the major tributaries (Fall Creek and Cayuga Inlet) and the two wastewater treatment plants (Ithaca Area and Cayuga Heights Wastewater Treatment Plants). In contrast to other moderately productive lakes, regeneration of phosphorus from the bottom sediments is not an additional (internal) source to Cayuga Lake. The well-oxygenated hypolimnion and the iron-rich sediments prevent the diffusive flux of P from sediments.

On an annual basis, Cayuga lake is a sink of phosphorus; more P is added to the lake than is lost through the outlet. P inputs are utilized and sedimented by organisms within the lake (Likens 1974).

LSC does not represent a new source of phosphorus to Cayuga Lake. However, by drawing water near the lake bottom and returning it to the shallow waters, LSC represents a transfer of Pwithin the Cayuga Lake ecosystem that would not otherwise occur during the period of thermal stratification (June through November), when natural conditions prevent mixing of shallow and deep waters.

Phosphorus measurements in surface waters are operationally defined by the manner in which they are analyzed in the laboratory. Total phosphorus (TP) is all the P in an unfiltered sample that reacts with molybdate, after digestion. TP includes dissolved inorganic P, P in organic material, and P associated with particulate material. Total soluble phosphorus (TSP) is all the P in a filtered (or centrifuged) sample that reacts with molybdate, after digestion. TSP includes dissolved inorganic P and most of the P associated with organic matter, but excludes the fraction of P associated with particulate material. Soluble reactive phosphorus (SRP) is all the P in a filtered (or centrifuged) sample that reacts with molybdate, without digestion. SRP includes dissolved inorganic P, some P associated with small particles, and some organic P, which reacts with the acidic molybdate solution. SRP is considered by most investigators to be 100 percent biologically available.

In this chapter of the Draft Environmental Impact Statement (DEIS), we analyze the impact of the LSC phosphorus load on the southern Cayuga Lake P budget during the stratified period. Since our concern is with the potential increase in primary productivity associated with the LSC phosphorus transfer, we have made conservative assumptions that enable us to compare P loads with approximately equivalent biological impact. We calculate the P budget using TP from the wastewater treatment plants, TSP from the tributaries, and TP from LSC. Selecting these fractions as the basis of the budget calculation de-emphasizes the particulate P which is far less biologically available in Cayuga Lake. As discussed later, P in the effluent of the two wastewater treatment plants and in the LSC recirculated water tends to be predominantly soluble, and readily available for algal uptake. The TSP fraction in the tributaries excludes the significant fraction of P associated with inorganic soil particles, the majority of which is not ultimately available for algal uptake.

We also use the mixing and dilution model (CORMIX2) to examine the near-field impacts of P in the LSC return flow. We selected the SRP fraction for use in the near-field analysis for two reasons: its biological availability, and the measured gradient in concentration between the shallow and deep waters during the period of biological growth. SRP in the shallow epilimnion tends to decrease to low concentrations during the growing season as P is incorporated into algal biomass. In contrast, SRP concentrations in the lake's hypolimnion increase as algal cells and other organic materials settle and are decomposed. The areal extent of the SRP plume in the vicinity of the LSC outfall diffuser is a result of the volume of water circulated through the heat exchangers and the gradient in SRP concentration between the shallow and deep water during the stratified period.

We evaluate potential impacts of the additional LSC phosphorus load on algal biomass, the N:P ratio, and secondary water quality characteristics in southern Cayuga Lake (for example, water clarity and dissolved oxygen depletion of the lower waters). Finally, we discuss some potential impacts on primary productivity that are not related to phosphorus. These impacts include the short-term effect of construction on turbidity and macrophytes, and the long-term effect of altering the thermal regime in the vicinity of the LSC outfall.

2.3.3.2 Existing Conditions.
Since phosphorus is the limiting nutrient for plant and algal growth in Cayuga Lake (Peterson, Barlow, and Savage 1974), the field investigations in support of LSC included measuring P concentration and chlorophyll a in the region of the proposed intake and outfall. The recent data collected for the LSC impact analysis have also been compared to the historical data, measured at irregular intervals since the late 1960s, to assess the degree to which trophic status of the lake has changed. We find that the results of recent investigations are consistent with historical data. Additional discussion is included in Section 2.3.1, Environmental Setting.

2.3.3.2.1 TP and SRP Concentrations.
The concentrations of TP and SRP in Cayuga Lake have been measured at irregular intervals over the past three decades. Results of research and monitoring conducted through the early 1970s have been compiled in Professor Ray Ogelsby's monograph on Cayuga Lake (Ogelsby 1978). Published literature, theses, and project files provide data from the 1970s to 1994. Since 1994, the LSC project team has measured phosphorus in the region of the proposed LSC intakeand outfall. TP data are available from 1994 to 1996; SRP data are available from 1995 to 1996.

Until recently, there has been no comprehensive, long-term phosphorus monitoring program for Cayuga Lake. In May 1996, the New York State Department of Environmental Conservation (NYSDEC) began a long-term water quality monitoring program of the eleven Finger Lakes to obtain baseline data and enable comparison with the last regional survey conducted in the 1970s. The NYSDEC monitoring program includes profile data collected monthly (May to October) at the deepest point in each lake. The 1996 data were not yet published as of the date of this document.

It is difficult to directly compare the results of the historical data with the recent data. Each investigator has measured various P fractions, at various stations, depths, and time intervals, depending on the objectives of the investigation. We discuss three summary indices of Cayuga Lake P with respect to historical and current water quality data: summer average epilimnetic TP, winter/early spring TP (measured during destratified conditions), and SRP profiles with depth.

2.3.3.2.1.1 Summer Average Epilimnetic TP.
Summer average epilimnetic TP is used as one index of a lake's trophic state and suitability for recreational use. NYSDEC, for example, has adopted a guidance value for TP in lakes of 20 micrograms per liter (µg/l) summer average (defined as June 1 to August 31). TP is measured at a 1 meter (m) depth. This guidance value was derived from opinion survey data relating to perceived water quality for recreational use. The summer average TP in Cayuga Lake's upper waters, as reported by various investigators, is compared with recent measurements obtained during the field investigations in support of LSC in Table 2.3.3-1. We conclude from these data that the recent summer TP measurements are similar to historical observations. Note that Cayuga Lake meets the NYSDEC TP guidance value of 20 µg/l.

2.3.3.2.1.2 Winter TP.
Winter or early spring TP data have been collected in Cayuga Lake as part of several investigations. The historical winter TP data are summarized in Table 2.3.3-1 and compared with the recent data collected as part of the LSC field program. We conclude from these data that the recent winter TP measurements are similar to historical observations.

Dillon and Rigler (1974) demonstrated that average water column TP concentration during winter or spring unstratified conditions is correlated with algal growth the following summer in a large number of phosphorus-limited lakes. The correlation between these variables is strong in the Finger Lakes; the relationship between winter TP and summer epilimnetic chlorophyll a for the large Finger Lakes, as published in Oglesby (1978), is plotted in Figure 2.3.3-1. Recent data collected as part of the LSC field investigations are also plotted. The potential role of the zebra mussel in altering the relationship between TP and chlorophyll a is discussed in a later section.

2.3.3.2.1.3 Profiles of Phosphorus Concentration with Depth.
Concentrations of TP and SRP in Cayuga Lake were measured at discrete depths through the water column (profile data) in 1968 to 1969 as part of the environmental analysis of the proposed Bell Station. They were also measured in the mid-1970s as part of graduate student research projects, and an evaluation of the impact of the 1973 legislated reduction in the amount of phosphorus in laundry detergents, and in 1995 and 1996 as part of the LSC investigations. The concentration of SRP, and to some extent TP, at specific depths and times is less predictable than mean values averaged over seasons. This is because the concentration at any time is a dynamic balance between many biological and physical processes. Consequently, comparisons between data sets must be carefully drawn.

The recent LSC profile data collected at biweekly intervals illustrate the degree to which the date of sampling influences the data. Summary profiles of TP and SRP collected at Station S11 are included to illustrate the depth distribution of the P data (Figures 2.3.3-2 and 2.3.3-3). Station S11 is located in the region of the proposed LSC intake, water depth 76 m (250 feet [ft]). Note that the concentration of SRP decreased in the upper waters and gradually increased in the lower waters over the stratified period. The ratio of SRP:TP increased from the beginning to the end of the stratified period in the lower waters (Table 2.3.3-2). The complete data set is included in Appendix C-1.

Because the SRP profiles change over the stratified period, comparisons with historical data must be referenced with respect to sampling date and depth. Table 2.3.3-3 is reproduced from Oglesby (1978) and summarizes the SRP concentrations measured from July 1968 to February 1969 as a function of date and depth. Each SRP concentration reported in this table is the average of measurements at several individual lake stations. The pattern of accumulation in the lower waters is masked to some extent by the lack of deep water data during the early sampling events. However, the SRP depletion in the upper waters and the higher concentrations in the lower waters during summer are notable. The TP concentrations throughout the water column measured during this investigation were relatively uniform at 20 µg/l. The winter data demonstrate SRP concentrations during destratified conditions (water column average in February 1969 was 12 µg/l), and may be compared with the LSC data (water column average in April 1996 was 15 µg/l).

SRP profiles measured at Myers Point (water depth 100 m [328 ft]) during summer 1974 are summarized in Table 2.3.3-4. The accumulation of SRP in the lower waters over the stratified period is evident in this data set, collected by Ray Oglesby and Jim McKenna of Cornell's Department of Natural Resources.

Based on comparison of the three indices (summer average epilimnetic TP, winter TP, and SRP profiles during stratification), we conclude that phosphorus concentrations in the lake that were measured as part of the LSC field investigations are similar to the historical data. Section 2.3.1, Environmental Setting, provides additional discussion of Cayuga Lake's trophic state and phosphorus concentrations.

2.3.3.2.2 Phytoplankton.
The LSC field investigations included identification and enumeration of phytoplankton in southern Cayuga Lake. Again, we were able to compare the recent data with historical data to assess the extent to which the phytoplankton community has changed since the 1970s.

Chlorophyll a concentrations, phytoplankton species composition (numbers and taxa), and phytoplankton biovolumes were monitored during portions of 1994, 1995, and 1996 as part of the LSC environmental investigations. Dr. William Schaffner, a phycologist with many years experience studying the Cayuga Lake phytoplankton, performed the phytoplankton identifications, counts, and measurements for biovolume conversions. Chlorophyll a was analyzed by Life Sciences Laboratory, which is certified in New York's Environmental Laboratory Approval Program.

The phytoplankton community of Cayuga Lake has been examined by several researchers since the first biological survey published by Birge and Juday (1912). Oglesby's (1978) monograph provides a comprehensive review of species composition, biomass, and annual succession of phytoplankton through the early 1970s. Direct comparisons of the phytoplankton data (including counts and identification of taxa) between years is difficult, due to differences in methods used to collect, preserve, and count the samples, and in the taxonomic skill of the investigators. However, qualitative and quantitative changes between 1912 and the 1970s suggest increased levels of productivity over that period. Increased productivity during the 50-year period was indicated by relatively higher numbers of blue-green algae (cyanobacteria) in the late summer, and overall increases in algal abundance.

2.3.3.2.2.1 Annual Succession of the Phytoplankton Community.
A detailed evaluation of spatial and temporal variation of the Cayuga Lake phytoplankton conducted during the 1970s concluded that annual succession dominated the observed variation in community dynamics (Godfrey 1977). On an annual basis, the phytoplankton were dominated by cryptophytes. Four distinct periods were identified:

· A spring period (early June through mid-July) where the phytoplankton community was dominated by diatoms and cryptophytes, both in numbers and biovolume. Chlorophyll a was typically at its annual peak during this period.

· A brief period during the latter half of July, where cyanobacteria comprised the highest number of plankton cells (although not the greatest biovolume).

· A longer period from August through mid-October with numbers of plankton and biovolume dominated by chlorophytes, gradually shifting to an increase in numbers of cyanobacteria.

· A winter period dominated (in numbers and biovolume) by cryptophytes.

This investigation (Godfrey 1977) concluded that the Cayuga Lake phytoplankton community consistently increased in number of species from minimum values in the winter (11 to 25 species) to maximum values in late summer and autumn (60 species).

2.3.3.2.2.2 Chlorophyll a Measurements.
The recent chlorophyll a data are similar to the historical Cayuga Lake data (Table 2.3.3-5), although the 1996 value is the lowest reported. The 1996 data may reflect the recent proliferation of exotic mussels (the zebra mussel, Dreissena polymorpha, and quagga mussel, Dreissena bugensis). These nonindigenous species have the potential to fundamentally alter the food web of a lake system, shifting the flow of nutrients and energy from the water column to the lake sediments. Reductions in phytoplankton biomass following mussel invasions have been reported in shallow regions of the Great Lakes (Lake Erie: Nicholls and Hopkins 1993; Saginaw Bay of Lake Huron: Fahnenstiel et al. 1995a), in the Seneca River (Effler and Siegfried 1994), and in Oneida Lake (Mellina, Rasmussen and Mills 1995).
However, the high interannual variability demonstrated in the Cayuga Lake chlorophyll data makes it difficult to identify trends with statistical assurance; more years of data would be necessary to determine that chlorophyll a had decreased. Additional investigation would be required to demonstrate a cause and effect relationship between dreissenid mussels and decreases in chlorophyll a. The issue of the duration of monitoring required to identify a statistically significant decrease in Cayuga Lake chlorophyll a was evaluated in the context of projected changes in water quality in response to the June 1973 ban on detergent phosphorus (Trautmann, McColloch, and Oglesby 1982). With three years of pre-detergent ban data, the investigators concluded that ten years of post-detergent ban monitoring would be required to detect a change in chlorophyll a of 2.1 standard deviations from the mean.

Using the calculations presented by Trautmann, McColloch, and Oglesby (1982), six years of post-zebra mussel monitoring would be required to detect a change of 1.8 times the standard deviation of the pre- mussel data set, assuming a power level of 0.90 (using six years of pre-mussel data as a baseline, with a mean summer chlorophyll a of 7.57 µg/l, standard deviation 2.25). The magnitude of the detectable change is consequently on the order of 4 µg/l mean summer chlorophyll a, which is comparable to the magnitude of the decrease measured in the 1996 data set. Fewer years of monitoring would be required to detect a regional change in chlorophyll a, since the data set would be expanded to include other lakes.

Chlorophyll a concentrations measured during the LSC field program in 1994, 1995, and 1996 are plotted in Figure 2.3.3-4. The 1995 results of chlorophyll a measurements in the region of the proposed LSC intake (Stations P4 and S11) are consistent with the seasonal pattern of chlorophyll a reported in the historical Cayuga Lake record (Oglesby 1978). Maximum concentrations of chlorophyll a were evident in the spring and early summer samples, followed by a steep decline and a gradual recovery. The 1994 LSC field program began too late to capture the spring phytoplankton and chlorophyll a. The 1996 data are anomalous in the annual pattern of chlorophyll a, as well as in the low average concentrations. Highest concentrations were measured in the summer; the spring bloom was very much reduced. The spring of 1996 was cool and wet, with a great deal of inorganic turbidity from tributary runoff.

2.3.3.2.2.3 Phytoplankton Species Composition and Biovolume.
The phytoplankton taxa identified during the 1994 to 1996 LSC field investigations are listed in Table 2.3.3-6. The complete data set, which reports the number and size of individual plankton species present in each replicate sample for each sampling date, is included in Appendix C-1. According to Dr. William Schaffner, species composition of the phytoplankton measured in the LSC field investigations is generally consistent with the historical Cayuga Lake data (W. Schaffner, personal communication, October 1996). The phytoplankton assemblage in the 1994 to 1996 samples was similar in both species composition and community structure to the assemblage examined in the 1970s.

The Cayuga Lake phytoplankton community includes many small species. Because of the differences in size of the individual species, results of the 1994 to 1996 field investigations are presented with respect to number of species, number of organisms of each species, and biovolume (numbers of organisms multiplied by their average size in the sample). Biovolume conversion factors were calculated based on the 1996 samples, and applied to the complete data set (1994 to 1996). A list of the conversion factors is included in Appendix C-1.

The 1996 plankton results monitored in the region of the proposed LSC outfall demonstrate the impact of organism size on the pattern of dominance in the community (Figure 2.3.3-5). For example, large numbers of cyanobacteria were reported present in the August sampling, but their relative contribution to the total biovolume was small. One blue-green alga, Merismopedia sp., was dominant in the August samples, but the relative contribution of this tiny organism (biovolume 1 cubic micrometer [µm3 ]) to total biovolume was small. This genus does not fix atmospheric nitrogen. Overall, diatoms comprised the largest proportion of the biovolume early in 1996 (April-late July), with cryptophytes increasing in the later samples (August and September). Data for the partial monitoring years 1994 and 1995 appears to be similar to the 1996 data (Figure 2.3.3-6).

There are inconsistent reports in the literature regarding impact of mussels on phytoplankton species composition. In-lake enclosure experiments summarized in Nalepa and Fahnenstiel (1995) indicate that zebra mussel grazing decreased the standing crop of phytoplankton groups with the exception of the blue-greens, consequently increasing their relative abundance in the phytoplankton. In contrast, Nicholls and Hopkins (1993) reported that all algal species in Lake Erie, including the blue-greens, were equally reduced by the zebra mussels; there was no shift towards blue-greens in the community.

2.3.3.2.3 Macrophytes (Rooted Aquatic Plants and Algae).
The macrophyte community in the southern basin of Cayuga Lake has been the focus of a long-term monitoring and research program, funded by the New York State Aquatic Vegetation Control Program (AVCP). The amount of macrophyte growth in Cayuga Lake is of concern due to its potential impact on recreational uses of the lake. Since the AVCP program began in 1987, annual surveys of macrophyte biomass and species composition have been conducted by Robert L. Johnson of Cornell University's Department of Ecology and Systematics. The findings are summarized in reports available through the Tompkins County Planning Department, which manages the AVCP and integrates the lake work with broader watershed and nonpoint source pollution control initiatives. This research has identified a total of 16 species of macrophytes in southern Cayuga Lake (Table 2.3.3-7).

Both the biomass and the relative dominance of eurasian water milfoil (Myriophyllum spicatum) have decreased compared with results of macrophyte surveys conducted in the 1970s. Consistent locations and sampling and analytical methods have been used. In the past five years, the population of M. spicatum in southern Cayuga Lake has declined dramatically (R.L. Johnson, personal communication, 1996). The AVCP research team and other investigators at Cornell have identified the larval form of an aquatic moth (Acentria ephemerella) selectively grazing on the meristem of M. spicatum. Both field and laboratory investigations have confirmed that the moth decreases growth and competitive success of the macrophyte.

The long-term monitoring data compiled as part of the AVCP document the high interannual variability of both biomass and species composition that is characteristic of the macrophyte community in southern Cayuga Lake. The high interannual variability is compounded by the extreme patchiness of the rooted aquatic plants; dominance can shift significantly between sampling sites. With the exception of the marked decline in M. spicatum, AVCP findings show no consistent trends in species composition or biomass of macrophytes in the southern lake basin during the ten years of record (1987 to 1996).

The high interannual variability is not statistically correlated to measured environmental factors (including streamflow, incident light, water temperature, nutrient and sediment loading, nutrient status of the water, and lake sediment). Biological factors, rather than nutrients or sediment relationships, may be significantly affecting changes in species.

The presence of zebra mussels can increase the growth and areal extent of macrophytes in shallow aquatic systems by decreasing turbidity and increasing light penetration through the water column (Skubinna and Batterson 1995). Macrophytes may therefore increase in the southern lake basin in response to the mussels. The AVCP provides a mechanism for identifying and quantifying these changes.

2.3.3.3 Impacts of the Proposed Action.

2.3.3.3.1 Projected Changes in Phosphorus Load and Concentration.

2.3.3.3.1.1 Total Phosphorus Budget.
The potential phosphorus-related LSC impact on southern Cayuga Lake involves the circulation of phosphorus within the water that is drawn through the intake and returned at the outfall. The concept of circulated phosphorus as a "new" source is only meaningful during the stratified period, when natural conditions prevent the mixing of upper and lower Cayuga Lake waters. In this section, the estimated TP budget for the southern Cayuga Lake basin (from all sources) is discussed, and calculations for the additional TP circulated from the hypolimnion to the epilimnion as a result of LSC are presented.

The calculation of the percent of the annual southern lake budget represented by LSC depends on a series of assumptions regarding flows and TP concentrations of the point and nonpoint phosphorus sources. A complicating factor lies in the different forms of phosphorus in each of the sources. The ratio of soluble to particulate phosphorus is not the same in each of the three sources (treatment plant effluent, tributaries, and water recycled from the hypolimnion). SRP is more readily available to phytoplankton than is particulate P. Since our concern is with the potential increase in primary productivity associated with the additional LSC phosphorus load, it is important to compare loads with equivalent biological impact.

The recent and historical profile data indicate that the proportion of hypolimnetic TP that is SRP increases over the stratified period. Treatment plant effluent is measured and reported as TP, but research indicates that it contains predominantly soluble phosphorus and organic phosphorus (Young et al. 1985). In contrast, tributary TP includes phosphorus associated with suspended particles. Only a small fraction of this particulate phosphorus is available for supporting plant and algal growth.

In an effort to accurately represent the potential contribution of LSC to the phosphorus budget of the southern lake basin, we have made conservative assumptions regarding the tributary fraction of the TP load. We calculate the phosphorus budget using TSP from the tributaries, TP from the treatment plants, and TP from LSC. The phosphorus budget, including a sensitivity analysis of the impact of different assumptions on the conclusions, is summarized in Appendix C-13.

2.3.3.3.1.1.1 Wastewater Phosphorus.
Point sources include the Ithaca Area and the Cayuga Heights Wastewater Treatment Plants, and total phosphorus loading from these point sources is relatively well-characterized. The NYSDEC issues permit limits of flows, concentration, and mass loading of TP, and municipal wastewater treatment plants are required to file self-monitoring reports documenting their performance. The Ithaca Area Wastewater Treatment Plant (IAWWTP) is currently permitted for a flow of 0.44 cubic meters per second (m3/sec) or 10 million gallons per day (mgd), with a TP limit of 1,000 µg/l. The effluent TP limit of 1,000 µg/l applies to all wastewater treatment facilities greater than 0.044 m3/sec (1 mgd) that ultimately discharge to the Great Lakes.

The Special Joint Subcommittee overseeing the IAWWTP (comprised of representatives of the City and Town of Ithaca and the Town of Dryden) have received conditional approval from NYSDEC for a revised discharge permit reflecting an expanded discharge of 0.57 m3/sec (13 mgd). The approval is conditional upon action to eliminate sewer overflows and performance testing.

Under New York State's policy for permitting expansion of existing wastewater discharges to lakes, the mass load of phosphorus cannot increase if flow increases. Therefore, the TP budget was developed with the assumption that the permitted TP load from this treatment plant will remain the same, at 37.8 kilograms (kg) or 83.4 pounds (lbs) per day. The Cayuga Heights Wastewater Treatment Plant is permitted for a flow of 0.09 m3/sec (2 mgd), with a TP limit of 1,000 µg/l. The daily permitted TP load from this facility is 7.6 kg (16.7 lb).

2.3.3.3.1.1.2 Tributary Phosphorus.
Two major tributaries, Fall Creek and Cayuga Inlet, flow into southern Cayuga Lake. As discussed in Section 2.3.1.3, these two tributaries account for approximately 40 percent of the direct drainage into Cayuga Lake. Flows in both tributaries are gauged by the United States Geological Survey (USGS), and reliable flow records exist.

The phosphorus concentration data in these two southern tributaries are less certain than the flow records. A comprehensive evaluation of the Fall Creek watershed was conducted in the early 1970s as part of an investigation of nutrient flux in agricultural watersheds (Bouldin 1975). This research effort was designed to quantify loading of four phosphorus fractions (MRP [equivalent to SRP], TSP, total particulate phosphorus, and labile particulate phosphorus) and one form of nitrogen (nitrate N). MRP is essentially equivalent to SRP, but samples were centrifuged (not filtered) to separate particulate materials.

Additional sampling of Fall Creek and the other southern tributaries (Cayuga Inlet, Cascadilla Creek and Six Mile Creek) was conducted between March 1987 and February 1989 as part of the AVCP. The objective of the monitoring program was to collect storm and dry weather samples, analyze for the same suite of analytical parameters measured from 1972 to 1974, and determine whether the average concentrations were comparable. Dr. Bouldin again directed the effort. These data indicate that the flow-weighted TSP concentration in Fall Creek had not significantly changed since the field monitoring program conducted in the 1970s. The 1987 to 1989 flow-weighted average concentration was 24 µg/l TSP, within the range reported for 1972 to 1974 (mean 26 µg/l, standard error of the mean 5 µg/l). The flow-weighed average TSP concentration in Cayuga Inlet measured in 1987 to 1989 was 23 µg/l. Based on these results, the Fall Creek and Cayuga Inlet TSP concentrations were assigned values of 26 µg/l. The sensitivity analysis presented in Appendix C-13 is based on the range associated with the standard error of the mean (21 to 31 µg/l).

Bouldin (1975) defined the four phosphorus fractions he measured in the Fall Creek samples based on operational procedures. First, samples were centrifuged at 35,000 times the force of gravity for 30 minutes to separate dissolved and particulate phosphorus. Bouldin reported that centrifugation removed mineral particles greater than 0.02 x 10-3 millimeters (mm) in diameter, but that larger organic particles would remain suspended. As defined by Bouldin, the four phosphorus fractions included:

· MRP (Equivalent to SRP). MRP was defined as the fraction reacting colorimetrically with molybdate without persulfate digestion. Phosphorus forms measured as MRP are predominantly dissolved inorganic phosphorus, but also include phosphorus associated with small particles dissolved by the acid molybdate reagent, and some amount of organic phosphorus hydrolyzed during the analytical procedure.

· TSP. The second fraction, TSP, was measured by the colorimetric molybdate reaction after the sample aliquot underwent persulfate digestion. TSP presumably includes all of the soluble and low-density particulate organic phosphorus that is oxidized by persulfate.

· Total Particulate Phosphorus. Total phosphorus was measured on the particulate material as the third fraction.

· Labile Particulate Phosphorus. The amount of labile or sorbed phosphorus on the particulate material was measured by a sorption isotherm method to determine the fourth operationally defined phosphorus fraction in the Fall Creek samples. This labile phosphorus is considered by soil scientists to be available for plant growth.

Determining which of the measured fractions would be "biologically available" once delivered to Cayuga Lake is not a simple task. Bouldin (1975) presented the following hypotheses regarding this issue: all MRP is available, most TSP is available, labile or sorbed phosphorus is of limited importance (and present in very small amounts), and total particulate phosphorus is of no consequence to Cayuga Lake ecology, considering that the Cayuga Lake sediments are not an internal source of P. However, sediments deposited in the littoral zone may provide nutrients for macrophyte growth. Bouldin (1975) relied on the TSP fraction for loading calculations and comparisons between lakes. We have therefore chosen the Fall Creek TSP data set as the basis for estimating tributary phosphorus input in this southern Cayuga Lake phosphorus budget.

Selecting the TSP fraction reduces the seasonal variation in TP loading through the lake tributaries. Particulate phosphorus is delivered to the lake on an event basis; the majority of suspended solids are delivered to the lake within a short period. Flow-weighted average TSP data have been used as the basis of the calculation. Since TSP concentrations were not correlated to flow, there was no systematic bias introduced by using the flow-weighted concentration to characterize both high and low flows.

In summary, the tributary phosphorus data used in this analysis were based on the Fall Creek data set collected between 1972 and 1974, and were verified by additional sampling and analysis conducted from 1987 to 1989 by David Bouldin, Robert Johnson, and Elizabeth Moran as part of the AVCP (Tompkins County Planning Department 1990). The TSP fraction, as operationally defined (centrifuged samples, digested supernatant), is used as a conservative estimator of TP load through the tributaries. TSP is assumed to be comparable to wastewater treatment plant TP and hypolimnetic TP in terms of its biological effect. The tributary phosphorus load is summarized in Table 2.3.3-8.

2.3.3.3.1.1.3 LSC Phosphorus.
The volume of water to be recirculated by the LSC system has been estimated by the project engineers based on an iterative process intended to optimize system design in light of projected campus demand and efficiencies of scale. The TP budget analysis includes monthly values for the LSC maximum permitted flows. Actual flows through the system averaged over periods of days, weeks, or months will always be less than the flows included in the permit application. The concentration of TP in the hypolimnion at the depth of the LSC intake has been estimated at 20 µg/l each month, which represents the upper 95 percent confidence interval around the mean concentration measured at the depth of the proposed intake. The permitted monthly LSC flows, and the corresponding TP loads, are summarized in Table 2.3.3-9.

2.3.3.3.1.1.4 Summary Budget.
The southern Cayuga Lake phosphorus budget with LSC has been estimated on both a monthly and annual basis (Table 2.3.3-10). Two scenarios are presented: (1) average tributary flows, considering that future tributary flows will be consistent with the long-term hydrology of the two streams; and (2) dry year tributary flows, corresponding to a return frequency of about one year in ten. Both scenarios assume existing permitted TP loads from the two wastewater treatment plants, and permitted LSC flows with a hypolimnetic TP of 20 µg/l. As calculated on this basis, LSC would contribute an additional 3 to 7 percent each month during the stratified period to the phosphorus budget of southern Cayuga Lake. Figure 2.3.3-7 shows the relative percent contribution of the tributary and wastewater phosphorus sources in the southern lake basin. Note that the summer period of full LSC system use and diminished tributary flows represents the largest percentage contribution of LSC.

Conservative assumptions regarding the tributary inflows and phosphorus content, hypolimnetic TP concentration, and the LSC flows have been made throughout the analysis. For example, only inflows from the two large tributaries and the two wastewater treatment plants are included in the budget. Inflows from smaller tributaries, groundwater, and atmospheric deposition are not included. The relative impact of these smaller sources would be greater during the summer period, when the LSC impacts are at their largest. Another conservative assumption is assigning 20 µg/l TP to the hypolimnetic waters for the entire stratified period, and coupling that concentration with the LSC permitted maximum monthly flows each day. The LSC system will not circulate that volume, nor transfer that mass load of phosphorus, more than a few days each month. Finally, considering TSP from the tributaries instead of TP is a conservative assumption that was selected to estimate fractions of phosphorus load with comparable biological availability.

The effect of each of these individual assumptions is to overstate the LSC impact on the southern lake basin; the net effect of all of the cumulative assumptions is therefore to provide a worst case estimate of the LSC impact. Even so, the analysis demonstrates minimal impact. The estimated LSC contribution ranges from a low of 2.4 percent during the spring high flow period to a high of 6.4 percent during August low flows, during an average hydrologic year. During a dry year ( return frequency once in ten years), the relative contribution of LSC increases only about 0.5 percent.

2.3.3.3.1.2 SRP Concentrations in the Outfall Region.
The preceding section documented the small potential LSC impact on the southern basin's TP budget. In this section, we examine the potential for localized enrichment of the upper waters with SRP. As discussed earlier, there are higher concentrations of SRP in Cayuga Lake's hypolimnion than epilimnion as the summer growing season progresses. Because the LSC system would recirculate hypolimnetic water into the lake's photic zone (the zone of light penetration in the upper waters where photosynthesis can occur) during the period of phosphorus-limited growth, this natural difference in SRP concentration would create a region of higher SRP concentration at the outfall.

The mixing of the LSC return flow with the lake's upper waters was analyzed using the model CORMIX2. The model enabled researchers to predict mixing and dilution of the LSC return flow with the lake water through a multiport, submerged diffuser. CORMIX2 was developed by Professor Gerhard Jirka and is supported by the United States Environmental Protection Agency (USEPA). The design of the LSC outfall and diffuser was interactive with the CORMIX projections. The projections of mixing and dilution of the LSC outfall plume focused on temperature (which was discussed in Section 2.3.2.2.4) and SRP.
The CORMIX2 projections require specification of ambient water quality conditions and system design parameters. The CORMIX2 input parameters used in the projections are summarized in Table 2.3.3-11. The diffuser length and orientation, and the number, alignment, and size of ports were designed in an iterative manner to meet specific environmental and engineering objectives. These objectives included discharging at a high velocity, ensuring rapid initial dilution, and minimizing interaction with the benthic zone and the shoreline. The minimum 2.7 m (9 ft) depth of the overlying lake water at the diffuser was specified based on navigational safety considerations.

Projections of the SRP plumes associated with the LSC return flow are plotted in Figures 2.3.3-8A to 2.3.3-8E. The projections are presented on a monthly basis during the stratified period. The excess SRP in the discharges (the difference between concentrations at the intake and outfall) and the distance from the outfall at which the excess SRP is reduced to 1 µg/l, 0.5 µg/l, and 0.1 µg/l are presented in Table 2.3.3-12. The following conclusions may be drawn from this analysis:

· Excess SRP occurs in model predictions for June through October. The largest difference in SRP concentration between the LSC intake and outfall (10 µg/l) is predicted to occur in September. When the lake circulates naturally (late November through late May), there is no gradient in concentration of SRP or other chemicals between the intake and outfall.

· The excess SRP increases during the stratified period. Maximum values are projected to occur in August, September, and October.

· The largest SRP plume is projected to occur in September, when both the excess SRP and the demand for campus cooling are high.

· Even in September, the SRP plume associated with the LSC return flow is projected to be small. The areal extent of elevated SRP is predicted to be very limited. The SRP concentration is projected to return to within 0.5 µg/l of background at a distance of less than 300 m (1,000 ft) from the outfall diffuser. As discussed in a following section, this limited region of slightly elevated SRP concentration is not projected to cause detectable changes in algal biomass or water clarity.

2.3.3.3.1.3 Sensitivity Analysis.
Additional CORMIX runs were made to evaluate how changes in the assumed velocity of the lake currents affect the size of the SRP plumes. The projections discussed above were made assuming ambient current velocity of 2 centimeters per second (cm/sec) or 6.6 x 10-2 feet per second (ft/sec). Sensitivity analyses were performed with weak (0.5 cm/sec [1.6 x 10-2 ft/sec]) and strong (10 cm/sec [0.3 ft/sec]) currents. The projected distances at which SRP in the outfall plume returns to within 0.1 µg/l of background are summarized in Table 2.3.3-13. Note that weaker ambient lake currents are projected to contribute to a somewhat larger plume, as the return flow from LSC pools in the region of the outfall diffuser.

2.3.3.3.2 Projected Secondary Effects on Water Quality.
Phosphorus added to the upper waters by the LSC system during the stratified period has the potential to stimulate the growth of phytoplankton. Excessive growth of phytoplankton is associated with degradation of the aquatic environment; water clarity decreases with increasing algal growth and the public perceives a less desirable recreational and aesthetic resource. Dissolved oxygen depletion of the lower waters is another secondary impact of excessive phytoplankton growth. In this section, the potential secondary effects of LSC phosphorus on Cayuga Lake are evaluated.

2.3.3.3.2.1 Potential Effect on Water Clarity.
In recognition of the central role of phosphorus in lake management, several state regulatory agencies (including the NYSDEC) have adopted in-lake guidance values to protect primary and secondary recreational uses of the water bodies. The derivation of the guidance value is based on user surveys that correlate perception of a lake's suitability for recreational and aesthetic uses with measures of Secchi disk transparency, TP, and chlorophyll a.

The New York State (NYS) guidance value of 20 µg/l TP was derived based on surveys conducted through the state's Citizen Statewide Lake Assessment Program (CSLAP). The survey results indicated that the uses of the waters should not be impaired if the biweekly summer average (June 1 to September 30) TP concentration measured at a depth of 1 m at a mid-lake station did not exceed the 20 µg/l guidance value. The fact sheet issued in support of the NYS guidance value for phosphorus notes that similar surveys in Minnesota and Vermont detected significant regional variation in public perception based on differences in baseline water quality (NYSDEC Bureau of Technical Services and Research, October 22, 1993, New York State Fact Sheet: Ambient Water Quality Value for Protection of Recreational Uses).

Projected compliance with the NYS phosphorus guidance value provides one scale by which the potential LSC impacts can be assessed. Data from Station P4 or S11, in the mid-southern lake basin, were used to evaluate how Cayuga Lake water compares to the state TP guidance value under current conditions (Table 2.3.3-14). These data indicate that southern Cayuga Lake currently meets the ambient water quality guidance value for TP in ponded waters, since the summer average concentration is consistently below 20 µg/l. Since the maximum concentration of TP in the circulated hypolimnetic water is at or below the guidance value (20 µg/l), we conclude that implementation of the LSC project would not cause exceedances of the guidance value in the southern lake basin.

2.3.3.3.2.2 Potential Impacts on Dissolved Oxygen Depletion.
Another approach to estimating the potential for secondary water quality impacts uses stoichiometric relationships between phosphorus, chlorophyll a, carbon, and dissolved oxygen (DO). This approach examines the short-term impacts of adding LSC phosphorus, by estimating the amount of algal growth (measured by chlorophyll, and converted to grams of carbon) potentially occurring as a result of the additional TP load to the system. The amount of oxygen in grams required to decompose the additional phytoplankton biomass is then calculated, and is divided by the volume of the lake's hypolimnion to estimate the potential for measurable DO depletion in units of grams per cubic meter (1,000 µg/l).

The stoichiometric approach assumes phosphorus-limited algal growth; each "unit" of LSC phosphorus is assimilated by a "unit" of phytoplankton. However, lakes and rivers with large populations of zebra mussels may have low phytoplankton standing crops and high ambient concentrations of SRP (Effler and Siegfried 1994; Fahnenstiel et al. 1995b). Consequently, the extent to which zebra mussels alter the Cayuga Lake ecosystem will affect the extent to which quantitative predictions can be made using this method. Another assumption is that the ratio of major nutrients in the algal biomass remains relatively constant. It is probably reasonable to assume that this is the case over the summer growing season, although short-term fluctuations in ratios are likely as algae can store nutrients in excess of their metabolic needs (a process called luxury consumption).

Published ratios of phosphorus: chlorophyll a: carbon: oxygen demand were used to estimate the potential dissolved oxygen depletion of the lake's lower waters in response to the LSC phosphorus load (Table 2.3.3-15). The analysis includes the following assumptions:

· The LSC loading is calculated using maximum TP in the lower waters, and maximum monthly permitted flows for May through October.

· The ratio of chlorophyll a to phosphorus in algal cells is estimated using the USEPA's compendium of published values from the literature (Bowie et al. 1985). The mean value reported in the reference (1 µg chlorophyll a per µg phosphorus) has been used in our analysis. We have used the reported range in values (0.5 to 2 µg chlorophyll a per µg phosphorus) to bracket the analysis. Bowie et al. (1985) do not specify the statistical distribution associated with this range.

· The amount of algal organic carbon associated with the projected chlorophyll a is also indexed to published values (Bowie et al. 1985). Mean (75 µg carbon per µg chlorophyll a), low (50 µg carbon per µg chlorophyll a), and high (100 µg carbon per µg chlorophyll a)values are assigned and carried through the sensitivity analysis.

· The oxygen requirement for biochemical oxidation of the organic carbon is estimated at 2.67 µg oxygen per µg carbon, based on published values (Thomann and Mueller 1987).

· The volume of the hypolimnion during the stratified period is based on data reported in Sunderam et al. (1969).

· The DO depletion associated with biochemical decomposition of the additional algal biomass is estimated by dividing the grams of oxygen demand by the volume of the entire hypolimnion, or (worst case) the volume of the hypolimnion south of Myers Point.

The projected depletion of DO is on the order of 5 µg/l-month (range 1 to 10 µg/l-month). Godfrey (1977) reported background DO depletion rates between 400 and 1,000 µg/l-month in the lowest waters. DO concentrations in the lower waters are typically between 9,000 and 11,000 µg/l. Cumulative DO depletion associated with LSC is estimated to be on the order of 20 µg/l DO over the stratified period. The amount of DO depletion created by the additional algal biomass resulting from LSC phosphorus circulation to the epilimnion is essentially undetectable, given the natural variability of the system.

If the excess algae that result from LSC phosphorus circulation settle in the southern lake basin, the localized impact of their decomposition could be slightly higher, since the size of the DO resource is smaller. Based on lake bathymetry, Cayuga Lake south of Myers Point contains approximately 10 percent of the total hypolimnetic volume. With a corresponding 10 percent of the DO resource, the LSC impact could increase an order of magnitude from the calculations presented above, to an estimated 50 µg/l per month over the stratified period, and a cumulative depletion of 200 µg/l over the stratified period, which represents a small fraction of the resource (9,000 to 11,000 µg/l). Even in this worst case projection, the DO depletion would be undetectable, given the natural variability of the system.

From this analysis, we conclude that LSC will have no ecologically significant impact on the DO concentrations of the lower waters. It is important to note that this analysis was performed using conservative assumptions throughout. These projections consequently represent upper bounds on the potential LSC impacts.

2.3.3.3.3 Projected Biological Impacts.

2.3.3.3.3.1 Short-Term (Construction Phase).
The aquatic plant habitat will be disturbed during the construction in the lake. Excavation of sediments in the shallow littoral zone will be necessary to bury the LSC intake and outfall pipe; the excavation will destroy rooted aquatic plants and algae, and create turbidity that will temporarily diminish the light available for both macrophytes and phytoplankton.

These impacts will be localized and temporary, and will not measurably alter the lake's productivity. A closed bucket dredge will be used to minimize turbidity caused by the excavation. Silt curtains will be used in the construction area, which will extend approximately 20 m from the centerline route of the LSC intake and outfall pipelines. In addition to the area along the pipelines, an area just north of where the pipelines exit the shoreline will be dredged to accommodate a transfer barge for dredged sediments. An estimated 6,900 square meters (74,000 square ft) of lake bottom will be directly impacted by these dredging activities. An outline of the affected region and additional discussion are presented in Section 2.3.5 (Lake Sediments). In addition to the direct impact area, we anticipate that some turbidity will escape the control measures. Best management practices to minimize turbidity impacts are described in a later section.

2.3.3.3.3.2 Long-Term (Operations Phase).

2.3.3.3.3.2.1 Phytoplankton and Chlorophyll.
Implementation of LSC could potentially affect both biomass and species composition of Cayuga Lake's phytoplankton community. The biomass effect has been discussed above in terms of the potential increase in chlorophyll a associated with circulation of phosphorus to the upper waters during the stratified period. The data supporting this calculation were presented in Table 2.3.3-15; the TP budget indicates a potential total mass increase of approximately 500 kg (1,100 lb) (range 250 to 1,000 kg) phosphorus to the upper waters between June and October (with LSC at permitted maximum flows each day). If all the additional TP is incorporated into algal biomass, chlorophyll a could potentially increase by a similar amount, given a 1:1 stoichiometric ratio between chlorophyll a and TP (the range reported in the literature is 0.5 to 2).
It appears reasonable to assume that the additional chlorophyll would accumulate in the lake's photic zone south of Myers Point. The volume of this portion of Cayuga Lake is estimated at 20 million cubic meters (700 million cubic ft) (using 10 m [33 ft] depth of the photic zone and 20 square kilometers [km2] or 7.7 square miles [mi2] surface area). We have estimated the increased concentration of chlorophyll a represented by an increase of the projected magnitude (5 x 105 g) in this volume of water (2 x 108 m3). On a daily basis, the maximum TP mass load from LSC could support 3.15 kg (6.9 lb) chlorophyll a, which would represent an undetectable increase in the southern lake basin (less than 0.02 µg/l, assuming 3.15 kg [6.9 lb] is added to the 2 x 108 m3 volume of the photic zone). The estimated potential cumulative increase in concentration of chlorophyll a is approximately 2.5 µg/l (range 1.25 to 5 µg/l) over the June to October period. Even if the potential increase in phytoplankton growth is restricted to the region of the outfall, the increase in chlorophyll a would be very small.

The potential decrease in water clarity in the southern lake basin associated with this amount of additional chlorophyll a would be minimal. The relationship between chlorophyll a and Secchi disk transparency in the southern lake basin is shown in Figure 2.3.3-9. Note the high degree of scatter in the relationship, suggesting that other factors, such as inorganic turbidity, play a significant role in moderating light penetration and water clarity in the southern lake basin.

A significant limitation to applying this empirical relationship is the uncertain role of zebra mussels in altering the phosphorus:phytoplankton relationship in the Cayuga Lake ecosystem. The relationship between chlorophyll a and TP was developed based on data collected prior to the recent mussel invasion. Researchers throughout the Great Lakes have documented significant decreases in concentrations of chlorophyll a and TP, and increases in Secchi disk transparency associated with proliferation of the mussels. The changes represent a re-partitioning of nutrients and energy flow in the system, with a shift away from water column phytoplankton production and towards benthic production (Fahnenstiel et al. 1995b). Based on data reported in other aquatic systems and the most recent Cayuga Lake data, zebra mussels have the potential to significantly alter the chlorophyll and TP relationship in Cayuga Lake.

2.3.3.3.3.2.2 N:P Ratio.
Species composition of the lake phytoplankton, as well as biomass, might change with implementation of the LSC project. The ratio of nitrogen to phosphorus, and the water temperature would both be altered in the region of the LSC outfall diffuser. There are published criteria regarding the relative competitive success of phytoplankton groups with respect to these parameters. Nitrogen-fixing blue-green algae (cyanobacteria) are rare in the Cayuga Lake algal community under existing conditions. Nitrogen-fixing cyanobacteria are often present in eutrophic lakes (Howarth et al. 1988) and may contribute to nuisance conditions of algal blooms and low water transparency. The relative proportion of nitrogen-fixing cyanobacteria in the phytoplankton community is reported to increase as the epilimnetic ratio of total nitrogen to total phosphorus decreases; nitrogen-fixing blue-greens are rare when the ratio exceeds 29 to one (Smith 1983). On a molar ratio basis, cyanobacteria generally comprised less than a few percent of the planktonic biomass when the total nitrogen:total phosphorus ratio was greater than 65:1 (Howarth, Marino, and Cole 1988).

The N:P ratio of nutrient loading to lakes has been demonstrated to be a good predictor of nitrogen fixation by cyanobacteria (Flett et al. 1980). The performance of this loading ratio as a predictor of the composition of the algal community suggests that the relative availability of nitrogen and phosphorus, not just their relative concentrations, is important (Howarth, Marino, and Cole 1988). When the N:P ratio of nutrient loading decreases below 16:1, nitrogen fixation appears to be favored. In Cayuga Lake, the N:P ratio of the nutrient load is significantly above this threshold value. For example, Likens (1974) reported an annual input-output balance for Cayuga Lake based on data collected from 1970 to 1971. Nitrogen loading (nitrate plus ammonia) from runoff and precipitation was estimated at 11,320 metric tons per year. Total P loading was estimated at 170 metric tons per year, for a calculated N:P loading ratio of 66:1.

An alternative hypothesis regarding the conditions under which cyanobacteria could be favored has been advanced by McQueen and Lean (1987) based on data from Lake St. George, Ontario. Theseinvestigators concluded that the relative contribution of blue-greens to the phytoplankton biovolume could be predicted by water temperature and nitrate-N concentrations. Based on their findings, the summer blue-green composition will stay below 3 percent of the total algal biovolume when water temperature is less than 21°C (69.8°F) and/or when the ratio of nitrate-N:TP remains above 5. At elevated temperatures, and when the ratio of nitrate-N:TP is less than five, blue-green biomass could be expected to increase. It is notable that the predominant blue-greens in Lake St. George are not nitrogen fixing species.

Cayuga Lake has high ambient concentrations of dissolved inorganic nitrogen (defined as the sum of nitrate, nitrite and ammonia). Results of 1994 monitoring are similar to historical data summarized in Oglesby (1978); nitrate concentrations average 1,000 µg/l, and ammonia N concentrations average 25 µg/l in the southern lake basin. TP concentrations are low, averaging 20 µg/l on an annual basis. The ratio of total inorganic nitrogen (TIN) to TP is therefore high in Cayuga Lake, averaging on the order of 1.025:0.01, or over 100 to one. Including the N incorporated into algal biomass and dissolved organic N would further increase the ratio. The ratio of dissolved inorganic N (DIN) to soluble reactive P (SRP) is also high in Cayuga Lake under current conditions, varying from 400:1 to over 1,000:1 during the summer period.

Based on these ratios, nitrogen-fixing blue-green algae are not favored under natural conditions. The phytoplankton monitoring results discussed in Section 2.3.3.2.2 demonstrate that cyanobacteria comprise only a small proportion of the algal community. The dominant species of cyanobacteria present in Cayuga Lake are not nitrogen fixing organisms.

The issue of the magnitude of potential change in this ratio with implementation of LSC can be addressed using results of the CORMIX analysis of local enrichment with SRP in the region of the outfall. These results demonstrate that even in the local region closest to the LSC outfall, the ratio of TIN to SRP remains well above any threshold considered favorable to nitrogen fixing blue-green algae (Table 2.3.3-16). The projected shift in the TIN:SRP ratio over the stratified period, with and without LSC, is shown in Figure 2.3.3-10. Recall that the LSC return flow will be cooler than background conditions in the southern lake basin during the summer and early fall (as discussed in Thermal Characteristics, Section 2.3.2). Based on the data from Lake St. George, blue-greens are not likely to be favored under the temperature and nutrient conditions associated with LSC.

2.3.3.3.3.2.3 Water Temperature.
The impact of the LSC outfall on the temperature in the southern lake basin will be localized, due to the relative volume of the return flow and to the proposed diffuser, which will maximize initial mixing of the flow from the heat exchangers with ambient lake water. Projections using CORMIX (refer to Section 2.3.2) have demonstrated the limited spatial extent of the mixing zone associated with the LSC return flow. Water temperatures in the outfall region return to background conditions within several hundred meters of the diffuser, even during those months with the greatest flows and difference in temperatures between background and LSC return flow.

Phytoplankton and macrophyte communities will be exposed to only a small area of altered lake water temperature. Recall that the LSC return flow will be close to uniform temperature throughout the year. Summer water temperatures in the immediate vicinity of the outfall will be cooler than background lake conditions, and winter water temperatures adjacent to the outfall will be warmer. The phytoplankton and macrophyte communities in the outfall region are unlikely to be affected by this slightly altered thermal regime. Photosynthetic rates of all plants are stimulated by increasing temperatures within a given range; the range is determined by species and acclimation (Barko, Adams, and Clesceri 1986). However, the magnitude of the projected differences in water temperature at the outfall will be small (within 2°C [3.6°F]), and within the natural variability of ambient conditions.

The generalized thermal preferences of different phytoplankton groups are plotted in Figure 2.3.3-11. The projected temperature differences in response to LSC either in the immediate vicinity of the outfall or throughout the southern lake basin would not be sufficient to alter the phytoplankton species composition.

2.3.3.3.3.2.4 Macrophytes.
Once construction is complete, the LSC system will have only minimal impact on macrophytes in the southern lake basin. Based on datacollected in other systems, and results of various investigations completed on Cayuga Lake, distribution and abundance of macrophytes are primarily controlled by light availability, sediment texture, herbivorous insects, and sediment nutrient content (Tompkins County Planning Department, December 1994). Water temperature interacts with the light regime (light intensity and photoperiod) in affecting the growth rate and morphology of macrophytes, and the formation of reproductive structures (Holmes and Klein 1987). Temperature may be as important as light in influencing the competitive relations between coexisting species (Barko, Adams, and Clesceri 1986). However, most macrophyte species have relatively high thermal optima, in the range of 28 to 32°C (82.4 to 89.6°F), and the small and localized changes in lake water temperature due to LSC would be insufficient to affect their competitive success. The LSC return flow during the spring and fall seasons has the potential to slightly extend the growing season for macrophytes. However, since the affected area will be very limited, no overall impact on the macrophyte community in the southern lake basin is anticipated.

Sediment texture may be altered in the region of the LSC outfall, as flow through the diffuser ports during system start-up will likely resuspend surficial sediments and transport them away from the outfall. Deposition of particulate materials will not occur in the immediate vicinity of the outfall. These effects on sediment texture might adversely affect the environment for macrophyte growth in the immediate vicinity of the outfall, since essential nutrients (especially nitrogen) are supplied to macrophytes from recently deposited sediments in infertile lakes (Roger, McFarland, and Barko 1996). However, investigations conducted during the AVCP indicate that the Cayuga Lake sediments have an adequate supply of nutrients to support macrophyte growth.

The light environment for macrophytes in the vicinity of the outfall is not projected to change in response to turbidity from algae, based on the minimal increase in chlorophyll a associated with additional phosphorus loading with long-term operation of LSC. However, the LSC return flow may have less inorganic turbidity than is typical in the southern lake basin, particularly during storm events. The LSC outfall will, at times, appear as a clear area in the turbid plumes that flow into the southern lake basin through the tributaries. This occasional increase in light intensity should not persist long enough or consistently enough to increase macrophyte growth.

2.3.3.4 Mitigating Measures.
Appropriate construction practices (best management practices) will minimize the creation of turbidity during construction activities associated with the outfall and intake structures. These measures (including silt curtains, handling and disposal of dredge spoils) are discussed in detail in Section 2.3.5.3.

The outfall diffuser is proposed as an appropriate measure to mitigate the potential impacts of increased phosphorus loading to the southern lake basin. The diffuser will maximize initial dilution of the return flow with the epilimnion, thus limiting the spatial extent of the region of elevated SRP concentrations. As discussed in Section 2.3.2.3, temperature effects will be spatially limited as well.

2.3.3.5 Unavoidable Impacts.
The following impacts on phosphorus and productivity will occur with implementation of LSC. Impacts are summarized as short-term (construction and system startup) and long-term (operations phase).

2.3.3.5.1 Short-Term Impacts.

· Turbidity in the water column will increase during excavation activities as a result of sediments stirred up on the lake bottom. Best management practices (closed bucket dredging, upland disposal of removed sediments, and silt curtains) will be employed to minimize this impact.

· Macrophytes will be physically removed from a limited region of the lake bottom during installation of the LSC pipelines.

2.3.3.5.2 Long-Term Impacts.

· LSC will increase phosphorus loading to the southern lake basin during the stratified period by recirculating hypolimnetic water. The LSC phosphorus load represents a small monthly increase (between 3 and 7 percent) to the existing TP budget of the southern lake basin during the stratified period. This estimate assumes maximum LSC flows and hypolimnetic TP concentrations. The percent contribution of LSC under operating conditions will be less.

· LSC will increase SRP concentration in the immediate vicinity of the outfall during the summer period when the background concentrations are currently low. The CORMIX projections demonstrate that SRP concentrations in the return flow are rapidly diluted to within 0.5 µg/l of background concentrations. The increase in concentration is very small, and any associated increase in algal production will have no discernable impact on water clarity.

· The return flow of hypolimnetic water will cause a slight decrease in the ratio of inorganic nitrogen to inorganic phosphorus in the immediate vicinity of the LSC outfall. Based on the limnological literature, the decrease in the ratio will not affect the competitive environment for blue-green algae in Cayuga Lake.

· The potential exists for lower turbidity water (a clear plume) at the LSC outfall to be evident during periods of high turbidity associated with tributary runoff. The impact on primary productivity is not expected to be significant.

· Deposition of particulate materials in the immediate vicinity of the LSC diffuser will be diminished, which will limit the supply of nutrients available to macrophytes. This impact is considered minor, since Cayuga Lake sediments have an adequate supply of nutrients to support macrophyte growth.

· Water temperature in the immediate vicinity of the LSC outfall will be affected. As discussed in Section 2.3.2, Thermal Characteristics, the LSC return flow will be cooler than background conditions during late spring, summer, and early fall months, and warmer than background during late fall, winter, and early spring months. The rate and duration of primary productivity in the local outfall region will be altered. Because the LSC outfall will return water through a submerged diffuser, temperature changes are rapidly moderated. Temperature in the outfall region will return to within 0.5°C (0.9°F) of background conditions within approximately 300 m (1,000 ft) of the outfall, under worst case (August) conditions. The spatial extent of the potential impact of temperature on primary productivity will be very limited.

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