Of the macronutrients required for the growth of aquatic vegetation, phosphorus is believed to limit productiv ity in Otsego Lake. This is largely based upon the premise that lakes having total nitrogen: total phosphorus mass ratios > 14 should be limited by phosphorus [based upon the TN:TP ratio in algal biomass of 7-10 (Vallentyne, 1974; Golterman, 1975)]. This seems to be particularly true when total phosphorus concentrations are less than 30 ug/l (Downing and McCauley, 1992). Therefore, this nutrient has been monitored with the greatest intensity since 1968. Additionally, data have routinely been collected on nitrogen, as nitrate (using the chromotropic acid technique) prior to 1992 and as nitrite+nitrate (employing cadmium reduction) to present. Nitrite levels are considered to be negligible (Table 24b, "Trace Substances" section). Chlorophyll a concentrations have also been regularly determined using pigment extraction following passage through glass fiber filters; dominant algal taxa have been described intermittently (see "Phytoplankton" section for a complete overview).
Soluble reactive phosphorus (SRP-P) levels were investigated in 1973 (Oglesby, 1974), 1975 (Doremus, 1976), 1978 (Harman et al. 1980), and 1988 (Iannuzzi, 1991a; b). With few localized exceptions, concentrations have been low (< 3 ug/l). Weekly sampling in 1988 (Iannuzzi, 1991a; b) showed the lakewide average SRP-P concentration to be 0.8 ug/l. Seasonally, SRP-P concentrations decline to levels below that of detection while nitrate concentrations remain high, further indicatin g phosphorus limitation.
Throughout the summer of 1975, Doremus (1976) monitored the distribution of nitrate nitrogen. The data showed significant spatial heterogeneity (Figures 34 and 35). Nitrate appeared to be depleted in the surface waters and accumulate below 20 m from mid-June to mid-July. Spatial trends in surface NO3-N were rather striking (Figure 35). In early June, the northern end of the lake and Hyde Bay exhibited the highest levels. As the summer waned, surface nitrat e fell and then rose at the end of June. The levels then fell until the middle of August, when the lake exhibited NO3-N levels uniformly less than 0.35 mg/l.
During the summer of 1976, Godfrey (1978) monitored various physical, chemical, and biological components of the lake at two sites. Total phosphorus (TP-P) averaged 9.3 ug/l in the epilimnion and 14.4 ug/l in the hypolimnion. There was an observed reduction of TP-P in both strata over the summer; between June 28 and August 22, the mean net losses by the epilimnion and hypolimnion were 11.8 and 20.3 ug/l, respectively. This was presumed to be the result of algal sedimentation. Nitrate nitrogen levels during this period exhibited slight declines in the epilimnion (0.47 mg/l NO3-N on 6/28; 0.37 mg/l NO3-N on 8/22). Hypolimnetic levels were less variable, with the mean summer concentration being 0.52 mg/l NO3-N.
Otsego Lake has historically exhibited about 10 mg/l of inorganic carbon in the water column. Data from the 1935 Biological Survey (NYS Conservation Department, 1936) showed alkalinity levels in Otsego Lake during June and July indicative of similar conditions (8-10 mg/l bicarbonate carbon). Additional data were presented for free carbon dioxide, indicating about 0.5 mg/l of CO2-C at the surface and about 1.2 mg/l of CO2-C at the lake bottom (51 m). Inorganic forms of carbon have been discussed in greater detail in the previous s ection.
Some additional plant-nutrient data can be found in Table 24b ("Trace Substances" section), which is a summary of the chemistry of the Village of Cooperstown water supply collected between February 28, 1972 and August 25, 1994. The information must be interpreted with care since the water tested was not raw lake water but was initially drawn from the lake at a 15 m depth and was sampled at the water treatment plant. From these samples, the following plant nutrients were assayed: diss olved silica, 2.1 mg/l; average bicarbonate, 120 mg CaCO3/l (11 mg C/l), and nitrate nitrogen (NO3-N), 0.50 mg/l.
Weekly site-by-site comparisons for 1988 (Iannuzzi, 1991a; b) SRP, NO3-N, and chlorophyll a are presented in Figures 36a-d, 37a-d and 38a-d, respectively (see Figure 20a for sampling site locations). Monthly comparisons of total phosphorus (TP-P) are presented in Figures 39a-d. Figures represent epilimnion averages (based on surface concentr ations at TR4-C, and surface and mid-depth averages at TR1-C and 6-C) for the mid-lake stations, mid-lake hypolimnion/profundal concentrations at TR4-C (15 m, 30 m, bottom depths), TR1-C and TR6-C, water column averages (surface and bottom concentrations) for the shoreline stations (TR4-W, 2-W, 2-E, 3-E, 6-W, and 7-C), and center-lake (TR4-C) profiles. Table 21 presents entire lake water column averages, integrated epilimnion and shoreline averages, and hypolimnion/profundal averages for nutrients an d chlorophyll a, for the winter, spring-summer-fall (S,S,F), and full year data sets.
During 1988 (Iannuzzi, 1991a; b), winter concentrations of SRP-P, NO3-N, and chlorophyll a were more variable than those observed throughout the remainder of the year at all sample stations. No trends or interactive correlations were apparent. Winter averages for these parameters were comparably higher than those for the spring, summer and fall data sets. Initial One-Way Analyses o f Variance (ANOVA) performed on winter epilimnion and shoreline water column means revealed that significant spatial differences occurred for SRP-P and chlorophyll a at the 95% (p = 0.05) significance level. Subsequent Student Newman-Keuls test (SNK) analyses indicated no heterogeneity of winter SRP-P or chlorophyll a values between the seven sample stations, even at the 99% (p = 0.01) significance level. In addition, both tests confirmed the homogeneity of NO3-N throughout th e seven stations. Since the former test is considerably more sensitive than the latter, it is assumed that some significant differences did occur, but are likely attributed to outliers due to the dynamic nature of these parameters. Furthermore, the SNK results are based on comparisons of seasonal means between stations and are, therefore, considered more appropriate for our purposes.
Spring, summer, and fall epilimnion and shoreline water column averages for SRP-P, NO3-N, and chlor ophyll a in 1988 showed little seasonal variability. Concentrations closely approximated the epilimnion and shoreline water column averages for the year, as well as the yearly water column averages for the entire lake. Initial one-way ANOVAs for each parameter revealed homogeneous distributions throughout the nine sample stations at a >99% significance level (p < 0.01). These results were confirmed by SNK analyses of the parameter means (p = 0.01) (Iannuzzi, 1991a; b).
Results o f the one-way ANOVA performed on total phosphorus values observed for nine sample weeks during 1988 indicated that spatial heterogeneity occurred throughout the nine sample stations. However, SNK analysis at the 99% confidence interval (p = 0.01) revealed no such discontinuities. This discrepancy is attributed to winter concentrations on February 5-6 and March 4-5 which were highly variable between stations, with epilimnion and shoreline water column averages ranging from 4.6 ug/l to 11.0 u< /i>g/l. However, the small number of observations accumulated for this parameter precluded the analysis of variance by season. The high values recorded for the winter dates were adequately accounted for in the station means for the entire year, as demonstrated by the SNK results.
SRP-P water column concentrations averaged 0.8 ug/l in 1988 (Iannuzzi, 1991a; b), with an observed range of 0.0 to 10.4 ug/l. Variability within the data set is attributed to the winter concentrations a t the shoreline stations and hypolimnion/profundal averages at the mid-lake stations (Figures 36a and c, respectively). With few exceptions, weekly mid-lake epilimnion averages did not exceed the yearly averages during either of the seasonal data sets (Figure 36b).
NO3-N water column concentrations averaged 0.72 mg/l in 1988 (Iannuzzi, 1991a; b), with an observed range of 0.31 mg/l to 1.62 mg/l. Weekly variations throughout the year were minimal, with the exception of winter conc entrations at the shoreline stations which showed some variability (Figures 37a-c). Winter averages for the epilimnion and shoreline sites (0.73 mg/l) were slightly higher than those for the spring, summer, and fall data set (0.69 mg/l). Hypolimnion averages for the year (0.77 mg/l) were slightly higher than those for the epilimnion and shoreline averages (0.72 mg/l). This was attributed to average concentrations at depths to 15 m (TR4-C 15 m, 30 m, and bottom depths), excluding the bottom concentrati on averages at TR4-C which closely approximated the epilimnion averages (Figure 37c).
Chlorophyll a epilimnion and shoreline (trophogenic zone) water column concentrations averaged 0.8 ug/l (mg/m3) in 1988, with an observed range of 0.0 ug/l to 6.0 ug/l (Figures 38a, b). Weekly winter concentrations were highly variable, with an epilimnion and shoreline water column average of 1.5 ug/l. In contrast, weekly spring, summer and fall concentrations sh owed little variation with the exception of a slight increase in concentrations at the mid-lake stations following fall turnover. The epilimnion and shoreline average for the spring, summer and fall data set was 0.7 ug/l, comparably lower than that of the winter data set. Weekly hypolimnion/profundal chlorophyll a concentrations showed the same pattern of seasonal variability as the epilimnion and shoreline sites (Figure 38c). However, winter and spring-summer-fall averages were 0.8 u g/l and 0.5 ug/l, respectively, demonstrating comparably less seasonal variation.
Total phosphorus (as P) water column concentrations for the nine sample sites in 1988 averaged 5.3 ug/l, with an observed range of 2.3 to 14.0 ug/l. Marked seasonal variability was observed (Figures 39a-c). Winter concentrations averaged 7.4 ug/l under the ice, and represented the highest seasonal levels encountered.
The spring turnover total phosphorus water column average was 7.1 ug/l. Immediately following the onset of spring turnover, TP-P levels began to decline, reaching a summer minimum average of 4.4 ug/l from approximately late May through September. During October, water column averages began to rise, reaching their fall peak during November. TP-P averaged 5.0 ug/l during fall overturn (Iannuzzi, 1991a; b) (Figure 39e).
Between the early 1970s and Iannuzzi's 1988 work, Otsego lake apparently experienced significant reductio ns in phosphorus concentrations. They are attributed to the 1973 New York State phosphate detergent ban and the conversion of the Glimmerglass State Park secondary waste treatment system (with Otsego Lake as the ultimate receiving water) to subterranean disposal. Godfrey (1978) used methods developed by Oglesby and Schaffner (1978) for estimating phosphorus loading from various watershed sources. Based upon that model, it was estimated that those changes resulted in a 33% reduction in total phosphoru s loading. Reduced in-lake phosphorus concentrations encountered in 1988, along with corresponding low chlorophyll a levels and increased transparency, indicate that this estimate of reduction may have been conservative (Iannuzzi, 1991a; b).
Data collected since 1992 includes the analyses of total phosphorus (as P) (persulfate digestion followed by single reagent ascorbic acid procedure) and nitrite+nitrate (cadmium reduction method) on samples collected monthly during periods of ice cover and bi-weekly throughout open water conditions. Lakewide TP-P averages for 1992, 1993, and 1994 were 10.0 ug/l, 12.9 ug/l, and 10.0 ug/l, respectively. This would indicate an approximate doubling in the concentration of this nutrient between 1988 and the present; concurrent changes in transparency, chlorophyll a, and hypolimnetic oxygen concentrations are reflective of this change. While TP-P levels varied both temporally and spatially during periods of summer strat ification throughout this period, no significant net changes in concentration were observed in either the epilimnion or the hypolimnion in any given year. Table 22 describes the annual average concentrations, summer epilimnetic and hypolimnetic averages, and concentration of TP-P during spring and fall overturn for 1972 to present. Figures 40-42 plot the distribution of total phosphorus for 1992, 1993, and 1994, respectively.
Nitrite+nitrate (NO2+NO3) levels over this per iod tended to be relatively constant. Lakewide averages ranged between 0.55 mg/l and 0.63 mg/l NO2+NO3-N. Throughout summer stratification, NO2+NO3 levels generally decreased in the epilimnion and increased in the hypolimnion. These concentrations and trends are similar to those reported by Godfrey (1978). Table 23 describes annual characteristics of this nutrient between 1973 and the present.
Investigations throughout 1993-94 by Ramsey (Unpbl .) indicate a marked shift in the algal community structure in recent years (see "Phytoplankton" section). During 1993, chlorophyll a averaged 5.86 ug/l; epilimnetic concentrations averaged 7.10 ug/l. The community was dominated by cyanophytes (e.g. Oscillatoria rubescens) throughout most of the year. During 1994, chlorophyll a averaged 2.99 ug/l on a lake wide basis and 3.01 ug/l in the epilimnion. Diatoms and chlorophytes composed most of the biomas s; cyanophytes were significant in the fall. These data are reflective of total phosphorus concentrations. Higher TP levels result in greater algal biomass and favor dominance by cyanophytes; corresponding decreases in transparency and late-season hypolimnetic dissolved oxygen concentrations generally follow (Hutchinson, 1967; Cooke et al.4, 1993).
During 1993, conditions resulted in greater productivity than had ever been observed in Otsego Lake. Excessively high spring runoff result ed in high TP-P concentrations at spring overturn (Albright, 1996). Interestingly, Oscillatoria rubescens, which is generally associated with more enriched conditions (Cole, 1994), was found to dominate prior to spring runoff (Ramsey, Unpbl.). This may have been the result of phosphorus introduction during several intense runoff events monitored prior to the 1992 fall overturn (Albright, 1996). Transparency during 1993 was somewhat greater than in 1994 despite the significantly higher chlorop hyll a levels. This is likely due to the tendency of Oscillatoria spp. to favor low light conditions (Marsden, 1989). This is supported by peak chlorophyll levels generally being observed below the thermocline at approximately 16 m from the surface. Late summer dissolved oxygen concentrations in both 1993 and 1994 were observed to decline significantly in the upper hypolimnion, apparently as the result of algal respiration and/or decomposition. Deoxygenation deep in the hypolimnion was also significant during this period, with levels falling below 0.5 mg/l in 1993 and below 1.0 mg/l in 1994. These severe oxygen trends are likely to correspond with trends in phosphorus increases. Elevated algal standing crops as the result of decreased grazing by zooplankton may exacerbate this problem. This issue is the consequence of the recent introduction of alewives (Alosa pseudoharengus) (Foster, 1990) (see "Dissolved Oxygen" section).