The trophic status of a lake is a function of productivity, as well as how that productivity manifests itself re lating to transparency, hypolimnetic dissolved oxygen levels, and the floral and faunal communities present. Productivity is dependent upon nutrient levels. Due to its high nitrogen:phosphorus ratio, phosphorus is most critical in dictating productivity in Otsego Lake. In-lake phosphorus levels are, in turn, largely dependent upon tributary water quality, which is a function of land forms and land uses.
The morphology of Otsego Lake is typical of oligotrophic bodies of water. However, highly calcareous bedrock and cultural effects provide ample nutrients, resulting in high standing crops, specific conductivity, and alkalinity values typical of more eutrophic situations. Recently observed changes include increasing total phosphorus and chlorophyll a concentrations, decreasing hypolimnetic oxygen levels prior to fall overturn, and decreasing Secchi transparencies. These are indicative of a shift toward eutrophy.
From 1935 to 1974, changing seston concentrations indicated in creasing standing crops. Similarly, chlorophyll a values suggest increasing biomass through the 1970s. Between then and 1988 algal productivity seemed to have dropped sharply (Iannuzzi, 1991a; b), coinciding with increased water clarity and a decreased hypolimnetic oxygen deficit (see below). Determinations conducted in 1993 and 1994 (Ramsey, Unpbl.) demonstrate a reversal of these improvements. A synopsis of the algal community is given in the "Phytoplankton" section. Recent trends in tra nsparency have been highly correlated with indicators of algal biomass, an exception being in 1993 when chlorophyll a levels were higher and transparency lower than observed in 1994. This likely was due to dominance by the blue-green Oscillatoria rubescens throughout 1993, which typically favors low light conditions (Marsden, 1989) and, during stratified conditions, would likely have minimal effects upon surface clarity.
Introduced macrophytes known to become dominant in enri ched waters became established in Otsego and most native species moved into shallower waters than those that they occupied in 1935 (Harman et al., 1980). Extensive beds of Chara sp. have disappeared in many locations resulting in uncolonized littoral substrates. Macrophyte densities appear to have increased in recent years, though this may be an artifact due to the robust nature of the introduced, newly dominant taxa (see above).
Late summer hypolimnetic dissolved oxygen levels hav e decreased sharply in recent years (see Figures 30a-d; "Dissolved Oxygen" section). From 1993 to present, concentrations in bottom waters have fallen below 1 mg/l; additionally, significant strata in the upper hypolimnion have been reduced to concentrations below 5 mg/l (Figures 30b-d). Continued decreases will pose a threat to the cold water fishes of Otsego Lake. Areal hypolimnetic oxygen deficits (12 m+ depth) were computed for several years since 1969 (Table 55). Except for the relatively fav orable conditions observed in 1988, values exceeded the lower limit of eutrophy suggested by Hutchinson and Mortimer (Hutchinson, 1957). In fact, the deficit of 1995 was over twice Hutchinson's limit.
Total phosphorus levels in the lake have gradually increased since data collection began, with the exception again being observed in 1988 (Table 22) (see "Plant Nutrient" section). It logically follows that increases in phosphorus loading, either internal or external, have occurred. Total ph osphorus budgets derived via a two year, precipitation-based study conducted throughout 1991 and between May 1, 1992 and April 30, 1993 indicated areal total phosphorus loadings of 0.29 and 0.63 gm/m2 lake surface/year, respectively (Albright, 1996; see "Stream Flow and Quality" section). These differences were presumably due to the variability of meteorological conditions during the period of study. The measured range falls very close to previous estimates based upon empirical models.
Theoretical loading, based upon population distribution and land use throughout the watershed, and calculated according to Vollenweider's criteria (1968), equaled 0.56 gm/m2 lake surface/year in the late 1970s. Similar computations, relative to the same time period, by P. J. Godfrey (Unpubl.) using formulas developed by Oglesby (Oglesby and Schaffner, 1978), give values of 0.33 gm/m2 lake surface/year. Of the fluvial inputs, those streams draining agricultural areas de livered the greatest volume of phosphorus to the lake. However, Willow Brook, draining a largely urbanized basin, delivered the most phosphorus per unit area. Undisturbed areas were drained by streams having good water quality.
While estimated to account for only 3.5% - 7.2% of the phosphorus influx, impacts by septic fields may be substantial. Much of the phosphorus entering the lake via fluvial inputs is in forms that are not readily available to algae. Also, as the bulk of the deliver y of these inputs occur in late winter and early spring, a significant portion of this phosphorus is likely incorporated into the sediments and/or flushed from the Lake prior to the onset of the first algal blooms. In contrast, that delivered by septic fields is highly soluble and easily utilized. And, since most lakeside residences are seasonal, much of this nutrient is delivered during the height of the algal growing season.
Through 1994, regular monitoring of in-lake phosphorus concent rations had not demonstrated any incorporation of this nutrient into hypolimnetic waters from the sediments during late summer stratification. However, the most recent observations (fall, 1995) show that phosphorus levels in bottom waters gradually increased from a baseline of approximately 10 ug/l through August to 23 ug/l on October 10. Over this time interval, dissolved oxygen decreased from 3.5 mg/l to 1.1 mg/l. Such internal phosphorus release from lake sediments has been docume nted for many other water bodies. This phenomenon is generally associated with deoxygenation of waters overlying the sediments, resulting in a reducing environment and a subsequent release of phosphorus from manganese and iron complexes (Bostrom et. al., 1982; Hardt et. al., 1983; Kortmann, 1980; Marsden, 1989; Mawson et. al., 1983; Mortimer, 1971). Additionally, significant phosphorus release rates have been reported as oxygen levels drop below 2-3 mg/l (Mawson et. al. 1 989; Mortimer, 1971). While anoxic conditions have never been encountered in Otsego Lake, dissolved oxygen concentrations near or below 1 mg/l are routinely observed in deep waters prior to fall overturn. It is not currently known whether phosphorus liberated from the sediments persists in the water collumn following overturn and is ultimately available for algal uptake. The absence of internal hypolimnetic enrichment prior to 1995 may have been due to Otsego's oligotrophic history, resulting in a r elatively high phosphorus retention capacity of the lake sediments. However, since retention capacities are limited (Marsden, 1989), any trend toward cultural eutrophication may be compounded by a decrease in the phosphorus retention capacity of the sediments.
In addition to hypolimnetic internal phosphorus loading, sediment-bound phosphorus in littoral areas is constantly suspended, deposited, and resuspended as a result of wave and wake perturbations (Harman and Lindberg, 1991; France and Albright, 1996a; b) (see "Water Movement" section). High correlations have been observed between boat traffic and near-shore turbidity, suspended inorganics, and total phosphorus (France and Albright, 1996b). Recent literature suggests that phosphorus sorbed to particulate material is potentially available to some taxa of algae (Bostrom et al., 1982; Yousef et al., 1980; Premozzi and Provini, 1985). Otsego Lake seems exceptionally susceptible to boat wakes due to its unstable eulittora l substrates. This situation is due to protection from prevailing winds by surrounding hills, resulting in less natural wave action, and to artificially high lake levels as a result of a dam renovated in the 1950s. Particulates resettle in littoral areas lakewide at the average rate of 1,807 g/m2/day. In areas more protected from the prevailing winds, such as Rat Cove, (southwestern shoreline), rates of 3,033 g/m2/day prevail. Practically all the surface wave action in the latte r area is due to powerboat traffic (see "Phytobenthos" section, Figure 74).
Various trophic state indices were developed by Carlson (1977) in an attempt to quantify the degree of eutrophy in an objective manner (Cooke et al., 1993). These indices, utilizing Secchi transparency, total phosphorus, and chlorophyll a as trophic indicators, have been applied to Otsego Lake for various years since 1972 (Table 56). (It should be noted that increments of 10 represent a doubling o f phosphorus and a halving of transparency.) The trends indicate increasing eutrophy.
When empirical management models developed by Oglesby and Schaffner (1975) and Dillon and Rigler (1974) are applied to Otsego Lake, the rather sensitive position of this body of water regarding water clarity becomes immediately apparent (Godfrey, 1977b). By expanding on the work of Oglesby and Schaffner (1975), it can be shown that the relationship between Secchi disk transparency and specific phosphorus lo ading in upstate New York lakes is hyperbolic. In the early 1970s, Otsego Lake fell on the rising limb of that function; therefore, even slight changes in phosphorus loading were expected to produce relatively large changes in transparency (Figure 120). Data regarding phosphorus loading and transparency collected in the early 1990s suggests that the current position on this curve has, indeed, shifted downward. The sensitivity of Otsego to moderate changes in nutrient loading are apparent when consid ering year-to-year variations in transparency and indices of algal biomass which are presumably due to typical meteorological fluctuations. Further illustrating this situation was a decrease in transparency and increase in algal populations following the installation of a sand filter, sanitary-waste disposal system at Glimmerglass State Park in the late 1960s, which discharged into Shadow Brook and ultimately reached the lake. Mitigation of this situation, involving the conversion to subterranean di sposal in 1977, was followed by lakewide improvements. It is believed that this action, in conjunction with the state-wide, high-phosphate detergent ban in 1973, led to improved lake conditions through the 1980s.
McBride and Sanford (1996) have asserted that the trends documented herein can be attributed to normal annual variation. Their interpretation is based primarily upon an analysis of manipulated BFS Secchi transparency information collected between 1972-95. Trophic State Indices based on chlorophyll a and total phosphorus, areal hypolimnetic oxygen deficits, and extensive documentation of biotic changes which are indicative of long-term degraded environmental quality were not considered.
In order to recognize the strengths of trends in areal hypolimnetic oxygen demand (AHOD), total phosphorus (TP-P), Secchi disk transparency, and annual precipitation with time, data from 1988 to 1995 were analyzed and plotted (Figure 121). Recognizing the limitations of correlation s based on low numbers of sample points, trends are nevertheless clear. There is great variability in annual precipitation and no meaningful trend over time. Water quality indicators do show strong trends.
Correlations between precipitation and these variables (Table 57) show the weakest trends with Secchi transparency, the strongest with total phosphorus. Clear is the high correlation between AHOD and total phosphorus, illustrating the importance of the management of phosphorus to reduce AHO D, which could well become limiting to cold water fish populations in Otsego Lake. Each point plotted in Figure 121 and used to compile Table 57 represents numerous samples taken annually. Relationships between dissolved oxygen, chlorophyll a, and total phosphorus-P are further illustrated in Figures 29a-f (see "Dissolved Oxygen" section).
While appropriate interpretation of these data are necessary to ascertain the development of trends in changing water quality, in this case they po se few short-term management concerns. Phosphorus loading, from both external and internal sources, must be minimized to maintain late-summer oxygen concentrations in hypolimnetic waters adequate to support the cold water fishery. Release of phosphorus from the profundal sediments prior to fall overturn in 1995 attests to the urgency of the current situation.
Trophic indicators in Otsego Lake are intimately linked with its food web structure. Despite epilimnetic productivity bordering on t hat of mesotrophic waters in 1976, Otsego Lake exhibited a relatively low algal standing crop (see "Phytoplankton" section). As shown, the loss rates due to grazing, sedimentation, and decay were very high (Godfrey, 1977b). Epilimnetic populations in late summer, although relatively low, were dominated by large phytoplankton species much more so than most of the nearby Finger Lakes (Mills, 1975; Godfrey, 1977a). Presumably, the production of small algae was selectively reduced by zooplankton, which c annot easily forage on the existing larger algae (Porter, 1977). Comparisons of zooplankton counts for Otsego (Harman and Sohacki, 1976) in 1975 showed substantially greater cladoceran abundances in Otsego at that time than in any of the Finger Lakes (Table 35). Otsego and Skaneateles were most similar in trophic state (the other lakes were mesotrophic to eutrophic). Both cladocerans and copepods were more abundant in the 1970s in Otsego than in Skaneateles (Harman et al., 1980). At that time, Godfrey (1977b) stated that... "Otsego Lake would thus seem to have the potential for a higher algal standing crop and less transparency than currently (1973) observed, without any increase in nutrient loading". Factors that reduce the zooplankton grazing rate, such as the introduction and population irruption of the alewife, would be expected to result in greater algal standing crops.
The introduction of the alewife in 1986, and the concomitant decrease in populations of large zooplankters, has resulted in a drastic reduction in cisco populations and apparently has negatively affected whitefish, smelt, and most other planktivores (Foster, 1993; Frost, 1993). This size-selective predation (Brooks and Dobson, 1965) has had similar repercussions on other water bodies. Cooke et al. (1993) present several case studies describing the effects of top-down interactions between predators and zooplankton and phytoplankton communities. In addition to direct competition for food resources, va rious fishes may be suffering declines as a result of alewives foraging upon their eggs and fry. Populations of both coregonids showed signs of stress in the early 1990s (Keenen and Ketola, 1993). In 1995, several cisco and whitefish have been observed that appear healthy, possibly indicating some stabilization of their populations at reduced levels. Lake trout and Atlantic salmon have exhibited excellent growth since alewife populations increased (Sanford, 1994).
Because of a lack of curren t data on alewife populations, their impact on nutrient cycling and availability cannot be ascertained. Alewife introduction and irruption has been concurrent with increasing symptoms of eutrophy thus complicating the relationships between internal and external loading. Summer algal standing crops have increased. It is assumed that reduced grazing by zooplankton leads to an increased rate of sedimentation of algal cells to profundal waters where respiration and decomposition depress dissolved oxygen concentrations. In theory, restoration of historic populations and species composition of crustacean zooplankton through increased piscivory should decrease algal standing crop, increase clarity, and help minimize hypolimnetic oxygen deficits. While such protocols have been successfully implemented in other lake management plans (Cooke et al., 1993), DEC Finger Lake fisheries managers with on-going salmonid stocking programs similar to one proposed for Otsego reported no noticeable decline in a lewife abundance (Mcbride and Sanford 1996). Lacking the alternative of alewife control, reducing phosphorus loading seems to be the only viable option to maintain Otsego's current conditions.
The delicacy of the Otsego Lake ecosystem requires special emphasis. The above mentioned changes which have resulted from nutrient loading, both internal and external, and introductions of exotic organisms, have impacted those ecosystem characteristics most valuable to society in a way that seems to hav e been disproportionally large.