The first known reference to the phytoplankton of Otsego Lake was in the biological survey of the Delaware and Sus quehanna watersheds (Tressler and Bere, 1936). The data consisted of total cell counts within selected taxonomic groups of algae and measures of total suspended solids. At that time (summer 1935), Mougeotia was the most abundant green alga in the lake. The blue-greens reached a maximum in late July when Chroococcus and Aphanocapsa were the dominant members of the algal community. That summer Otsego did not exhibit an algal bloom over the entire lake, yet there were local conce ntrations of Anabaena and Botryococcus that were considered to be "...slight shore blooms." Secchi readings (2) averaged just over 5 m throughout that study period. Asterionella and Synedra were the most abundant diatoms (Godfrey, 1977b).
In 1968, when research at the Field Station began, intense blooms of blue-greens dominated by Anabaena occurred from mid-to-late summer. Secchi transparencies throughout the lake were reduced to 2 m or less for periods of 1 to 3 weeks, although average annual readings were 5.3 m. Large populations of the diatoms Fragilaria and Synedra were present throughout the year, a situation typical of alkaline lakes in this climate. The annual summer blooms of Anabaena indicated nutrient enrichment, but severe stress by organic pollutants (usually correlated with dense populations of euglenoids) appeared to be absent (Harman et al., 1980).
In 1973, phytoplankton (cell counts and chlorophyll < i>a) were sampled on two occasions as part of a general survey conducted by Oglesby, Mills, and Chamberlain (Harman and Sohacki, 1976). In 1975, an extensive survey of the seasonal and spatial distribution of chlorophyll a was conducted by Doremus (1976). Summer phytoplankton populations distributed in Otsego (via chlorophyll a determination) were highest in the center of the lake off Five Mile Point. Secchi data averaged 3.7 m over the year. Low transparencies in the northern p ortions of the lake were observed despite low chlorophyll a concentrations. This would indicate silt in suspension, which is reflective of soil characteristics and runoff in the northern portion of the watershed, as well as the resuspension of fine particulate sediments due to wave action in this shallow region (Harman et al., 1980). An additional source of early information concerning increases in algal standing crop is data on seston organic weight collected on July 23, 1935 (Tressler and Bere, 1936), July 23, 1971 (Sohacki, 1971), and August 15, 1975 (Doremus, 1976). Sohacki noted a 1.5 fold increase since 1935, whereas Doremus demonstrated a threefold increase. However, the time of sampling by Doremus differs by nearly a month from that of the others. Since much of the seston organic weight is phytoplankton, and phytoplankton biomass exhibited a 1.7-fold increase between July and August in 1976, it is difficult to draw any conclusions concerning the change in seston organic wei ght between 1971 and 1975.
In the late 1970s, definitive studies characterizing the phytoplankton community in Otsego Lake were conducted by Godfrey (1977a; 1977b; Harman et al., 1980). Cell counts were made during July, 1975 (Seaman, 1976), 1976, and 1977 (Harman et al., 1980) in an attempt to duplicate the methods of Tressler and Bere (1936). Although somewhat different methods of collection and enumeration were used for 1976, comparable data can be reconstructed. The metho ds used in 1935 and 1975 have been shown to sample adequately only the net plankton and large nannoplankton (Godfrey, 1977b); smaller taxa may thus be under represented. Cell counts were grouped into four categories: diatoms, green algae, blue-green algae, and protozoans in earlier studies. Except for the latter group, the same categories have been used here. "Protozoans" encompass the chrysophyceans and pyrrophytes. Cell counts from the 1976 data of algal species > 20 um in size were compiled into these groups. Figure 83 illustrates the broad comparisons noted between 1935 and studies to date. Differences between the latter years were greater than those between 1935 and 1973. The species obtained between 1976 and 1977, during which the entire lake was surveyed (Godfrey, 1977a; Harman et al., 1980) are presented in Table 36.
It should be noted that cell count comparisons are fraught with difficulties. The greatest of these is the equal weight given algae of vastly differen t sizes. Clearly, the resources necessary to support one large alga could support great numbers of individuals of smaller species. As long as the relative proportions of the biomass contributed by each of the size groups remains constant over the years, comparisons of cell counts should be reasonably meaningful. However, if there has been a shift to species of a different size, cell counts would be very misleading (Mills, 1975). Also, the timing of the maxima of each plankton group limits the uti lity of using short-term studies to fully gauge the long-term changes in algal communities.
The seasonal pattern of total chlorophyll a (integrated average, 0-15 m) in Otsego Lake for 1976 was one of relative constancy at quite low concentrations, resembling an inverted plot of Secchi disk transparency (Godfrey, 1977b). Secchi readings averaged 4.4 m for the year. Maximum chlorophyll a levels occurred from August to October with a secondary peak in late June, and the lowest concentrations occurred in mid-July. Compared to the previous year (Doremus, 1976), there seemed to have been very little change in total chlorophyll a concentrations. A late June peak was not observed in 1975, and the minimum concentration occurred approximately a week earlier. The rather limited amount of data from 1973 shows values for May and June that were substantially lower than those found in later years. However, the analytical technique used to determine those early summer values (co ncentration of algal pigments by centrifugation rather than filtration) probably underestimated the actual concentration. The information obtained in August 1973 was collected by methods similar to those of later years (Oglesby, 1974, in Harman and Sohacki, 1976). Here, the correspondence is closer to comparable dates in 1975 and 1976 (Harman et al., 1980). Figure 84 illustrates the seasonal trend of chlorophyll a observed in 1976; also included are additional data collected be tween 1973 and present (see below). Graphed values represent integrated euphotic zone averages.
The vertical distribution of total chlorophyll a in 1976 was also relatively constant (Figure 85). An increase in near-bottom values occurred on May 28 and a pronounced metalimnetic peak was observed on June 28, but otherwise the total chlorophyll a in the water column was relatively homogeneous. The vertical distribution of total phytoplankton biomass (Figure 86) exhibited mor e pronounced heterogeneity with depth of increased phytoplankton biomass. Hypolimnetic maxima occurred frequently; the most extreme case occurred on June 28. The data for 1975 (Doremus, 1976) show slightly more persistence of higher concentrations near the bottom of the metalimnion or in the hypolimnion, but that trend is not prominent. Spatial variation in surface chlorophyll a is shown in Figure 87.
In 1976, diatoms predominated in late spring and early summer (Figure 88, Table 37) followed by a shift to greens and blue-greens in late summer and fall. Cryptophytes were generally persistent throughout the summer. Except for modest abundances in early spring, chrysophyceans were a relatively minor component of the phytoplankton. Pyrrophytes became a significant portion of the biota in late August. Net plankton predominated, especially in early June (Figure 89). The combined total of net plankton and large nannoplankton accounted for 75-95% of the total phytoplankton biomass.
The average composition of the water column for taxonomic groups and size groups is shown in Table 38. Overall, diatoms were the dominant taxonomic group, followed by chlorophytes and pyrrophytes. Most phytoplankton were species of relatively large size; 85% were larger than 20 um over most of the sampling dates. Smaller phytoplankton (< 20 um) tended to be somewhat more prevalent below the thermocline.
Figure 90 details the temporal changes in the depth distribution of taxonomic groups in 1976. In May, diatoms were the dominant taxa at all depths and were homogeneously distributed. Thermal stratification had developed by June 11, and the relative abundance of diatoms in the epilimnion was waning. The dominant organisms were Asterionella formosa and Synedra radians. By July, the relative abundance of diatoms in the epilimnion was at a minimum, while below the thermocline amounts were still relatively large. In August, the hypolimnetic population reach ed its lowest proportions while the epilimnetic diatoms showed signs of resurgence. Cyclotella bodanica was the dominant diatom species. By October, diatoms, primarily A. formosa and C. bodanica, were again homogeneously distributed, but not dominant.
Chlorophytes exhibited two maxima over the growing season. Their relative abundance became substantial by June 11, 1976, particularly that of Ankistrodesmus falcatus, but a tremendous increase in diatoms reduced their re lative contribution to the community on June 28, even though their absolute biomass was still increasing. The demise of the epilimnetic diatom populations by July left the chlorophytes, mainly Sphaerocystis schroeterii and Oocystis lacustris, below the thermocline. In October, chlorophytes were of minor significance. Cyanophytes were an insignificant part of the community until August. By October, they averaged 36% of the biomass throughout the water column. The dominant species was Chroococcus prescotti, although Anabaena flos-aquae and Aphanizomenon flos-aquae were also common. Chrysophyceans were relatively important only on June 11. Cryptophytes were the only taxonomic group to exhibit a preference for the hypolimnion. Although noticeable throughout the season, except on May 28, they were most prominent during July and August. Four species, Cryptomonas erosa, C. ovata, C. pusilla, and Rhodomonas lacustris, were the dominants of this group. The remaining group, the pyrrophytes, displayed the opposite tendency, with a high relative abundance above the thermocline, especially in August. In June, the pyrrophytes were dominated by Peridinium cinctum, with a shift to Ceratium hirundinella in late summer.
Algal productivity, as measured using 14C incorporation rates, reached maximum integrated values of 1,740 mg C/m2/day in July and August of 1976 (Figure 91) (Godfrey, 1977b). Spring values wer e 25-69% of the summer maximum. The temporary decline in productivity on June 28 coincided with the decline of the Asterionella formosa bloom. Summer productivity levels similar to those of Otsego Lake have been reported for Lakes Ontario, Michigan, and the eastern basin of Erie (Vollenweider et al., 1974). Compared to a large variety of lakes whose productivity data have been compiled by Wetzel (1975), Otsego Lake had, at that time, values similar to lakes on the borderline between ol igotrophy and mesotrophy.
The depth distribution of productivity in Otsego Lake exhibited by 14C incorporation rates (Figure 92) in 1976 typically followed the theoretical curve of light penetration. Exceptions to the rule were the May 20 homothermal conditions of spring overturn, the June 28 diatom die-off, and the August 20 productivity maximum below the thermocline. In the latter case, Ceratium hirundinella, a large, presumably slow-growing dinoflagellate dominated the ep ilimnion, while the large nannoplankton, Cyclotella bodanica and Oocystis lacustris, dominated below the thermocline. This hypolimnetic community was relatively small but growing vigorously.
Throughout 1988, as part of an intensive limnological characterization of Otsego Lake, Iannuzzi (1991a; b) conducted weekly chlorophyll a analyses at 13 sites around the lake. Lakewide epilimnetic concentrations were low, averaging 0.8 ug/l (Figure 84). Correspondingly, transp arency was relatively high, averaging 4.7 m at TR4-C, and total phosphorus low, with a lakewide mean of 5.3 ug/l. Chlorophyll a epilimnetic and shoreline water column concentrations ranged from 0.0 to 6.0 ug/l. Weekly winter values were highly variable, with an epilimnion and shoreline average of 1.5 ug/l. In contrast, weekly spring, summer, and fall concentrations showed little variability, the exception being a slight increase at the mid-lake stations following fall o verturn. Spatial fluctuations were considerably less than were observed in 1975. Weekly hypolimnion and profundal chlorophyll a concentrations showed the same pattern of seasonal variability as the epilimnetic and shoreline sites. However, the variations were not as significant. Winter values averaged 0.8 ug/l and the spring-summer-fall mean was 0.5 ug/l. The depth distribution of chlorophyll a at TR4-C is given in Figure 93.
The most recent characterization o f the phytoplankton community was conducted throughout 1993-94 by Ramsey (Unpbl.). Cell counts and chlorophyll a determinations were performed bi-weekly, or monthly during ice cover, at TR4-C on samples collected at 4 m intervals through the euphotic zone (i.e. 20 m depth). Additionally, samples were collected throughout the hypolimnion on a monthly basis.
Chlorophyll a levels in 1993 were higher than had been previously observed (Figure 84), with a annual average of 7.2 ug/l in the euphotic zone. In contrast to the distribution observed in 1988 (Iannuzzi, 1991a; b), levels were highest during the summer, averaging 8.7 ug/l (Figure 84). Concentrations through the spring, winter, and fall averaged 7.7, 6.3, and 5.1 ug/l, respectively. In late May, a metalimnetic maxima was established. As summer progressed, levels continued to increase, with peak concentrations increasing with depth. From May 25 to August 19, the highest chlorophyll a conce ntrations were encountered between 12 m and 16 m depth. The maximum value of 31 ug/l was observed at 20 m depth, presumably below the euphotic zone (i.e. below the depth at which light penetration is 1% ambient radiation) on July 8. Following this date, maximum levels gradually shifted upward, at the same time decreasing in intensity. By September, this summer maxima was abruptly terminated, with a more homogeneous situation following. Below 20 m depth, concentrations generally decreased wi th depth, with the average levels in this zone being 3.0 ug/l. The depth distribution of chlorophyll a is given in Figure 94.
Cell counts conducted throughout 1993 indicate that the phytoplankton community was dominated by the cyanophyte Oscillatoria rubescens throughout much of the year. This species comprised over 80% of the algal community, by cell count, throughout the winter months and over 95% throughout the spring. Peak populations were observed in April, when t his species accounted for 99% of the community. The dominance by this species was particularly apparent at greater depths; as summer stratification progressed, it accounted for virtually all of the community from 12 to 20 m. This tendency for Oscillatoria to favor low light conditions has been described elsewhere (Marsden, 1989).
In 1993, the balance of the algal community was composed primarily of diatoms. Synedra, Melosira, Fragillaria, Cylclotella, and Asteronella co ntributed significantly to the community throughout the winter, then all dropped markedly. Following the onset of summer stratification, Synedra rumpens resurged and was a dominant species throughout the summer. Diatoms were essentially absent during September and October, then returned to dominance following fall overturn. Diatoms generally were homogeneously distributed in the top 8 to 12 m of the lake.
The crysophyte Dinobryon was present from mid-March to mid-July, thoug h never in significant numbers. Cell counts were highest between 4 and 8 m depth. The chlorophyte Chlamydomonas globosa was present in low numbers in January, then dominated the surface waters in February and March. From April to August, Chlamydomonas was absent from the community, then C. dinobryonii and C. angulosa reappeared and increased steadily through the year's end, when together they accounted for 35% of the community. Figures 95a-v illustrates the temporal cha nges in the depth distribution of the observed taxonomic groups. Table 38 provides a comparison of the general taxonomic composition and size distribution between these data and those collected in 1976.
Chlorophyll a concentrations observed throughout 1994 (Fig. 84) were substantially lower and more homogeneously distributed than those in 1993 (Ramsey, Unpbl.), with levels averaging 3.7 ug/l throughout the euphotic zone. With the exception of a surface value of 20 ug/l noted on January 18, no value exceeded 6 ug/l. The mean hypolimnetic concentration was 1.0 ug/l. Following fall overturn, the mean concentration throughout the water column was greatest at 4.2 ug/l. The distribution of chlorophyll a for 1994 is illustrated in Figure 96.
Chlamydomonas was dominant from the onset of 1994 until mid-spring, particularly in surface waters. This group was essentially absent from the community from May to November, then resurged to regain dominance at the years end. The chlorophyte Chlorella vulgaris dominated in June and July, then disappeared in August. Diatoms appeared in March and gradually increased through May, when Synedra was the major component of the community. Diatoms were significant throughout the summer, decreased during the fall, then resurged through November and December. Oscillatoria rubescens was common during the winter months, but, unlike the previous year, was negligible after March. The blue-greens Chroococcus and Coelspharium dominated from September to November. Anabaena was present, though not significant, in September and October.