The zooplankton community in Otsego Lake has historically been dominated by the following crustacean zooplankters and the brachionid rotifers Kellicottia and Keratella (Harman et al., 1980).
Cladocera Daphnidae Daphnia pulex (de Geer) Simocephalus serrulatus (Koch) Sididae Sida crystallina (O.F.M.) Diaphanosoma brachyorum (Lievin) Bosminidae Bosmina = (Eubosmina) coregoni Baird Chydoridae Camptocercus rectirostris Schodler Leptodoridae Leptodora kindti (Focke) Copepoda Cyclopidae Cyclops varicans Sars Cyclops bicuspidatus Claus Ectocyclops phaleratus (Koch) Senecellidae Senecella calanoides Juday
In 1935, when Tressler and Bere (1936) first surveyed the zooplankton of Otsego Lake, the community was dominated by Copepoda (82%) (Figure 97). Rotifera made up 14% and Cladocera 4% of the zooplankton community. In the 1960-70s, the same species of crustaceans made up the greatest amount of biomass in the summer, whereas during the winter and spring periods the rotifers contributed comparatively greater am ounts.
Changes in population characteristics of cladocerans reflected considerable trophic changes between 1935 and 1970. On July 8, 1935, Tressler and Bere (1936) collected total Cladocera at TR4-C averaging 4,300 individuals per m3 throughout the water column. On July 8, 1970, Harman (1971a) sampled in the same manner. The average numbers of Bosmina coregoni were 30,000 organisms per m3. The addition of Daphnia pulex brings the total number up to about 3 1,000 cladocerans/m3.
Several other genera of cladocerans have been recorded from the lake in small numbers, as have the copepods Cyclops tenuis, Diaptomus oregonensis, D. minutus and the genus Epischura (Tressler and Bere, 1936). Cyclops varicans and Ectocyclops phaleratus normally occur throughout the entire water column. According to Tressler and Bere, Cyclops (Mesocyclops) tenuis is found in the upper 25 m of water whereas C. bicus pidatus occurs only below the thermocline. Diaptomus oregonensis was found only in the surface waters, D. minutus from 5-35 m, and Senecella calanoides only below 12 m. Tressler and Bere (1936) considered Anoraea and Floscularia the most abundant rotifers of the plankton. This information appears incongruous in that Floscularia is a predominantly sessile genus. The present authors are not familiar with Anoraea.
Populations of Daphnia pulex and Bosmina coregoni, the most abundant cladocerans, were studied during the summer of 1970 (Harman, 1971a). To determine their horizontal distributions within the lake, a No. 20 plankton net was used biweekly for vertical tows from near the bottom to the surface at 24 stations (Figure 98, TR- ). Units indicate relative numbers of organisms per 259 cm2 surface area at 24 stations collected over the entire water column.
The vertical distributions of these organism s were determined by samples taken at station TR4-C (Figure 98), the deepest station in the lake, at 3 m intervals from near the bottom to the surface. Collections were taken with a Kemmerer water sampler every four hours over three 24-hour periods so that diurnal migration (if any) of the organisms could be determined (Figure 99). Biweekly samples taken at noon show trends in vertical distribution throughout the summer (Figure 100). From these data, and examination of the specimens, the life cycl es and distributions of D. pulex and B. coregoni in Otsego Lake were determined.
In May, 1970, most of the population of D. pulex was composed of mature individuals. By June 15, there were many gravid females, some which exhibited faint cyclomorphosis, and a few early larval stages were present. By July 17, there were many more immatures in the population, and most mature specimens were gravid females showing definite cyclomorphosis. On August 10, most populations cons isted of large immatures and gravid females exhibiting acute cyclomorphosis. Definite cohorts were not discernable, and this condition persisted through the last sampling period (August 25). Throughout the summer, immatures tended to remain in less than 15 m of water with the majority of the adults. However, a few adults, fewer than 10 per 2-liter sample, were usually present the entire length of the water column. Bosmina coregoni exhibited a similar life cycle. No cyclomorphosis was evident. I t is apparent that in the uncrowded, comparatively homeostatic waters of Otsego Lake, little sexual reproduction takes place. The populations of these species are maintained by small populations of parthenogenic females during the winter periods.
Diel migration exhibited by Bosmina and Daphnia is minimal for both genera (Figure 99). Extreme patchiness in the horizontal distribution of both species is evident (Figures 101 and 102). Bosmina and Daphnia attained maxi mum population densities in late June. Foci for both populations were in the center of the lake at TR4-C. However, they were spatially isolated by the depths at which they occurred (Figure 100).
Minimal population densities occurred during late May. At that time, the populations were highest at the south end of the lake (Figures 101 and 102). The data illustrated in Figure 101 exhibit a second population maximum of Daphnia in September. However, when this is compared with Figure 102, it is apparent that it is an artifact resulting from the patchiness in distribution (TR4 exhibited highest populations present at that date).
Leptodora kindti Focke was found in low densities during the entire summer. Although the numbers of individuals collected were small, they do show evidence of horizontal movement of population centers. Before May 29, none were collected. On that date, 34 specimens were collected at TR3-E. On June 30, individuals were only collected north of TR3. O n July 17, some were collected south of TR4. By July 29, they were scattered throughout the lake. On August 10, they were again collected south of TR4. By August 25, only five specimens were found in the entire lake (at TR3 and TR5).
In 1975, Seaman (1976) surveyed the zooplankton utilizing methods similar to Tressler and Bere (1936). Her results are illustrated in Figure 103. Populations of Rotifera, Cladocera, and Copepoda had all increased significantly between the years 1935 and 1975 (Fig ures 104-106). Copepods continued to dominate the community, comprising 61% of the individuals collected, cladocerans 23% and rotifers 16%.
In 1990, Wigen (1991a) collected samples using a 10 liter Van Dorn Sampler at TR4-C at 3 m intervals. Samples were passed through a #20 (63 um) plankton cup prior to processing. Results are illustrated in Figure 107 and Figures 104-106. Drastic changes had occurred since 1976. The Rotifera accounted for 95% of the plankton populations, with both C ladocera and Copepoda together composing only 5% of the individuals collected.
In 1992, a survey was conducted by Harman and Toner (1993) using methodologies described by Wigen (1991a). The results are illustrated in Figure 108. Rotifer abundance had declined to 75%. In 1993 (Figure 109) (France and Taylor, 1994), the relative abundance of Rotifera was effectively the same at 76%.
In 1988, alewives (Alosa pseudoharengus) were first found in the lake (Foster, 1990). Alewives ar e visually oriented planktivores and, as such, are selectively predaceous on large zooplankters. Thus, zooplankton species composition can be significantly altered by their presence (Cooke, et al., 1993; Pennak, 1989). The zooplankton exhibiting the largest sizes of individuals in the lake, which seem to be those most severely impacted, are the crustaceans. The taxa possessing the largest individuals in this class are Daphnia, Leptodora, and the Suborder Calanoidia. They seem to be tho se most severely impacted. Leptodora kindti, a very large species of cladoceran, was not found in any of the quantitative 1993 samples. Individuals of this species have been observed in small quantities since the 1970s (Harman, 1993b). Only two individuals were seen in plankton collections during the summer of 1993.
In 1993, France and Taylor collected zooplankton once a month from January through March and September through December, biweekly in April and May, and weekly during the months of June, July, and August. Samples were collected at 0, 4, 8, 12, 16, 20, 30, 40, 44, and 48 meters from the surface at station TR4-C, the deepest location on Otsego Lake (on 3/10/93, samples were collected from 0 to 30 meters; on 4/28/93 samples were taken from 0 to 40 meters; and on 2/8/93 samples were taken between 4 and 20 meters). Methods were similar to those employed by Wigen (1991a); detailed methodology is presented in the original manuscript (France and Taylor, 1994). Nauplii were included with copepod data.
Table 40 (France and Taylor, 1994) shows the total number of individual zooplankton counted for each sampling date during 1993. These totals are the sum of the organisms found at all sampled depths. Table 40 also contains the mean number of individuals per liter in the water column on that date. Rotifers, cladocerans, and copepods are each recorded as a single taxonomic unit. Cladoceran and copepod totals are further combined into total crustacea counts in o rder to compare them to total rotifer counts. Table 40 also reduces the data to rotifer/total crustacea ratios to clearly illustrate differences in populations.
Below is a list of the organisms that were observed and identified in 1993. Note that there are some taxonomic changes from previous studies due to revision of the nomenclature as presented by Pennak (1989). Entries marked with an asterisk were found only occasionally.
Phylum Rotifera unknown #4 unknown #5 Class Monogonota Order Flosculariacea Family Conochilidae Conochilus uniconis Family Filiniidae Filinia longiseta Order Ploima unknown ploimate Family Notommatinae Cephalodella sp.* Family Synchaetidae Ploesoma truncatum Polyartha vulgaris Synchaeta stylata Family Gastropodidae Gastropus stylifer Ascomorpha chromogster Chromogaster sp.* Family Trichocercidae Trichocerca multicrinis Family Asplanchidae Asplanchna priodonta Family Brachionidae Kellicottia longispina Keratella cochlearis Keratella quadrata Family Colurellidae unknown #12 Phylum Arthropoda Class Crustacea Order Cladocera Family Chydoridae Alona sp.* Family Daphnidae Daphnia pulex (Leydig) Family Bosminidae Bosmina longirostria (O.F.M.) Order Eucopepoda Suborder Cyclopoida Family Cyclopodidae Microcyclops varicans (Sars) Diacyclops bicuspidatus thomasi (Forbes) Ectocyclops phaleratus (Koch) Suborder Calanoida Family Senicellidae Senecella calanoides (Juday)
Four organisms that were not positively identified were included in the rotifer totals: unknown rotifers #4, 5, 12, and an unknown (originally called unknown #1) in the Order Ploima. Unknown #4 (Figure 110) was seen in small numbers on 6/10, 8/5, and 10/11. Number 4 is roughly the size of Polyartha. Unknown #12 (Figure 111) was observed on 8/26, 11/9, and 12/7. Seventy-five percent of #12 carried eggs, and individuals appeared to be encased in a gelatinous material. Unknown # 12 was tentatively assigned to the family Colurellidae. Unknown ploimates (Figure 112) were found in every sample and were often as numerous as Keratella cochlearis and Polyartha vulgaris, which were the most abundant rotifer taxa found throughout the year. Unknown #5 is a small cylindrical shaped rotifer (Figure 113). Ploesoma truncatum did not appear until 8 /19/93, when it was the most abundant rotifer taxa. Gastropus and the colurellids had not been previously identified in Otsego Lake.
The Crustacea were dominated by cyclopoid Copepods with the Cladocera Bosmina longirostris and Daphnia pulex being the second and third most numerous, respectively. Calanoid copepods were found in nearly every sample, but in modest numbers as compared to cyclopoids.
Noontime collections of zooplankton throughout 1993 found them con centrated in the upper 20 meters (Figures 114a-w) of the water column. The most notable exceptions to this trend occurred a few times in the winter, once in April, and twice in mid-June.
Figures 97, 103, 107, 108, and 109 illustrate changes in rotifer and crustacean populations since 1935. It is believed that the planktivorous cisco (Coregonus artedii) was introduced between 1925 and 1942 during regular whitefish stockings (Newell, 1976). While ciscos consume zooplankton, they are n ot as efficient zooplankton predators as alewives. Populations have exhibited symptoms of severe stress (Keenan and Ketola, 1993) since the alewives were introduced.
Phytoplankton distribution and abundance in Otsego is directly related to the amount of nutrients available in the lake, particularly phosphorous. Zooplankton population sizes are largely correlated with changes in the phytoplankton standing crop and predation pressures. All else being equal, the overall increases in crustacean zooplankton populations between 1935 and 1975 were assumed to be indicative of increasing eutrophication (Harman et al., 1980).
The domination of rotifers first observed by Wigen in 1990 has continued through 1994, though the differences are not as extreme as were exhibited in 1990 (compare Figure 107 to Figures 108 and 109). The slight increase in Crustacea in the summer of 1992 correlates with a massive alewife die-off in the winter and spring of 1991 (Foster, 1993). The number of ro tifers in most plankton communities range between 40 to 500 individuals per liter (Pennak, 1989). Data from Otsego Lake, collected in 1993, are comparable (Table 40). The maintenance of extremely high standing crops of crustacean zooplankters that pertained through the 1970-80s was apparently unique.
The recent decline in populations of crustacean zooplankton and the dramatic increases in the rotifer populations appear to be a direct result of the introduction of the alewives. Lack of a lgal grazing by these larger zooplankton, as well as excessive phosphorus loading (see "Stream Flow and Quality" and "Plant Nutrient" sections), have led to a marked increase in the standing crops of phytoplankton species. High phytoplankton standing crops have, in turn, had negative effects on water clarity (see section on "Transparency") and on hypolimnetic oxygen concentrations (see section on "Dissolved Oxygen").
