Study of the Otsego Lake zooplankton community prior to walleye

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1 58 Study of the Otsego Lake zooplankton community prior to walleye (Stizostedion vitreum) stocking, summer, 2000 Gordie Morgan 1 INTRODUCTION Zooplankton playa vital role in the Otsego Lake ecosystem. In years previous to the introduction of alewives (Alosa pseudoharengus) in 1986 (Foster, 1989), Otsego Lake waters contained relatively low algal biomass, high transparency, and a controlled phytoplankton size structure (Harman, et al. in prep). These characteristics were largely the result of the historic high abundance of large zooplankton (cladocerans and copepods) (Harman, et al. 1980). A zooplankton survey conducted by Tressler and Bere in 1935 revealed copepods as being the dominant taxa within the lake waters (Tressler and Bere, 1935). However, in 1970 (Harman, 1972) and1975 (Seaman, 1976) cladocerans, primarily Daphnia, were the dominant taxa. This high abundance of large crustacean zooplankton in Otsego Lake is no longer seen. Harman and Toner's (1993) study in 1992 showed a radical change in the zooplankton population structure. The smaller bodied, less efficient rotifers were starting to dominate the community. In the late 1990's, rotifers and small-bodied cladocera (Bosmina longirostris) were dominant (Warner, 1999). The introduction of alewives, an efficient planktivore, in 1986 has reduced the cladoceran and copepod populations to the point where they are no longer able to efficiently filter algae from the water (Warner, 1999). This caused an increase in phytoplankton biomass, leading to a decrease in transparency, and a decrease of oxygen in the hypolimnion (Haresign and Warner, 1997), which jeopardizes the cold water fishery. Concurrent studies assess chlorophyll a as an indicator of algal biomass (Durie, 2001), a physical and chemicallimnological evaluation of the lake (Albright, 2001) and evaluations of the fish communities of Otsego Lake (Meehan200 1; Gray200 1). This project is a component of a study designed to evaluate any trophic change following the recent (July 2000) stocking of 80,000 fingerling walleye (Stizostedion vitreum). Here, baseline information is provided on the zooplankton community prior to the initial stocking. It is anticipated that walleye will prey upon alewives, potentially invoking "trophic cascade" effects (Cooke et al. 1993), which may ultimately affect water quality. METHODS Zooplankton were collected on a bi-weekly basis, from 25 May to 03 August Sampling took place at TR4-C, the deepest point in Otsego Lake (Fig. 1). A fourliter Van Dorn bottle was used to collect water samples every meter, from the surface to 12 meters (the top of the hypolimnion). The individual samples were transferred to a two 1 Rufus 1. Thayer Research Assistantship, summer Present affiliation: State University ofnew York, College at Oneonta.

2 59 Shadow Brook Blackbird Bay Otsego lake Susquehanna River Figure 1. Bathymetric map of Otsego Lake showing sampling site (TR4-C). Bathymetry in feet.

3 60 liter bottle. The samples were combined to form a composite sample representative of the epilirnnion. The samples were concentrated using a #20 (63 urn) plankton cup. Distilled water was used to rinse the plankton cup. The sample was transferred into a jar with appropriate labeling (i.e. volume, date, site of sampling, contents). Lugols iodine was added (1 % of sample volume) to stain and preserve the specimens. Organisms were counted, measured (mm), and identified in the lab using a compound microscope with a Sedgwick-Rafter cell and an eye piece micrometer. The micrometer was calibrated accordingly before measurement. A Henson-Stiple pipette was used to transfer a 1 ml sub-sample onto the Sedgwick-Rafter cell. Zooplankton were identified according to Pennak (1989) and Needham (1962). Of the total volume, three 1 ml subsamples were surveyed to provide an estimate of zooplankton densities, which were converted to individuals/liter by accounting for the concentration factor. Dry weight was estimated (Peters and Downing, 1984). Filtering rates of zooplankton were recorded using an equation developed by Knoebel and Holtby (1986) involving the average length of zooplankton. The grazing index (percent epilirnnion filtered) was also calculated (Warner, 1999), as was phosphorus regeneration (Esjmont-Karabin, 1983). Temperature (needed to compute P regeneration) was calculated by averaging readings from the surface to 12 meters on each sampling date. All data were recorded on a zooplankton survey sheet for later use. Equations used are as follows: dry wt: W(ug) = 9.86 (mm)2.l (Peters and Downing, 1984) filtering rate: F(ml ind- I day-i) = length (mm)2.48 (Knoeche1 and Holtby, 1986) P regeneration (where E p = ug P. mg dry wt. -I. ind. hr., W = dry weight (ug), and T = temperature (OC»: (Esjmont-Karabin, 1984) rotifers: EN =.0879 W(ugyl.OI eo.088t c1adocerans: EN = 1.80 W(ugr l91 e O.039T copepods: EN = 1.33 W(ugr 536 e O.039T The results of this study were compared to those of 1970 (Harman, 1972) for which the preceding equations had been applied by Warner (1999), and data collected by Warner (1999). RESULTS AND DISCUSSION Table 1 provides a summary of the taxa of zooplankton collected over the summer. Table 2 provides a summary of the summer's research. Epilirnnetic temperature, zooplankton density and mean length per taxa (columns B-D) were directly observed. Mean individual dry weight (ug), dry weight (ug r l ), mean individual phosphorus

4 A B C D E F G H I J Avg. Temp. #/L Avg length mean Drywt Phos. Regen. Rate Phos. Regen. Filtering Rates % Epilimnion 25-May (deg. C) (mm) DryWt(ug) (ugll) ugp*mgdrywt-l*ind*h- 1 Rate (ugll/day) mi/ind/day filtered/day Copepods Cladocera Rotifers total Jun Copepods Cladocera Rotifers total Jun Copepods C1adocera Rotifers total Jul Copepods Cladocera Rotifers total Jul Copepods Cladocera Rotifers total Aug Copepods Cladocera Rotifers total Exce1 fonnula fonnat for: Copepods 9.86*IY'2.1 C*E (0.229*E"-0.23)*(EXP(0.039*B» O*E*C*24/ *IY'2,48 C*I/IO Cladocern.... (0.519*E"-0.23)*(EXP(0.039*B» Rotifers.... (0.0154*E"-1.27)*(EXP(0.096*B» Table 2. Summary ofmean epilimnetic temperature, zooplankton densities and mean length per taxa, as well as derived values for mean weight per individual and per 1, phosphorus regeneration per individual and per 1, filtering rates per individual and the percent ofthe epilirnnion filtered per day. The formulas used to derive these values in Exce1 are also given in a format compatable with that software. The letters in the forulas refer to the column headers across the top of the sheet. 0\ --

5 62 regeneration (ug P'mg dry weight r l 'indivual' hr- I ), phosphorus regeneration (ug rl day) filtering rate (mhndividuarl'day) and the percent ofthe epilmnion filtered daily were derived using the given formulas (formatted for an Excel speadsheet). Figure 2 shows the density ofcopepods, cladacera and rotifers over the study and Figure 3 gives their mean lengths. The historically abundant, large bodied Cladocerans, such as Daphnia pulex, are now subordinate to copepods as being the larger taxa of zooplankton in Otsego Lake. These, in tum, are dominated by smaller bodied, less efficient rotifers. Keratella cochlearis seem to be the most abundant rotifer along with Kellicotia longispina. The average length of copepods (25 May-3August) was 0.32 mm, compared to 0.25 mm for c1adocerans, which were composed almost entirely ofbosmina longirostris. In the summers of 1996 and 1997, mean c1adacera lengths were 0.03 and 0.36mm, respectively. In the summer of 1970 (prior to the introduction of alewives), they averaged 0.74 mm. Both phosphorus regeneration and the percent ofthe epilirnnion filtered per day (Figures 4 and 5) were fairly low through June, increased markedly in July, and returned to low levels by 2 August. Phosphorus regeneration averaged 4.49 ug rl day, substantially higher than that seen in the summers of 1970 (2.78 ug rl day) and (averaging 2.32 ug rl day). The mean grazing index was 12.0% ofthe epilirnnion per day in this study, similar to that in the summers of 1996 and 1997 (10.0 and 9.8 % per day, respectively). In 1970,27.8% ofthe epilimnion was filtered daily. Mean plankton biomass in 2000 was 174 ug r l, midway between that observed in (149 and 121 ug r l ) and in 1970 (235 ug r l ). It is evident that the zooplankton community has changed considerably since the introduction of alewives. Though their dry weight is only slightly lower, the shift in size has led to substantially higher rates ofphosphorus regeneration and decreased filtering rates. Both of these changes mimic, and accelerate, trophic changes induced by increased nutrient loading. Local activities are underway which are designed to address fluvial nutrient loads, which originate primarily from agricultural activities (Miner, in prep.). Managing for a zooplankton community similar to that observed in 1970 by reducing the alewife population through increased piscivory should help realize the effects ofreduced nutrient loading. CIadocerans Copepods Rotifers Bosmina longirostris Cyclops varicans Asplanchna priodontus Daphnia pulex Diacyclops bicuspidatus Brachionus spp. Senecella calanoides Cephalodella spp. Nauplius larvae Filinia longiseta Gastropus stylifer Kellicotia longispina Kerratella cochlearis Kerratella quadrata Trichocera multicrinis Table 2. Summary of zooplankton captured in Otsego Lake, summer 2000.

6 63 800, , Copepods ;;: = _ Cladocerans :I u --.- Rotifers ;: "'., ;;--- "'..-:---_1 u ======L"----=----- z o L!=:::::,=;:;I;;::::::::=====::::::::::::;:L J 14-May 24-May 3-Jun 13-Jun 23-Jun 3-Jul 13-Jul 23-Jul 2-Aug 12-Aug Sampling Dates Figure 2. Abundance of the major zooplankton taxa, summer I 0.30 T <Il..., b ' Copepods _ Cladacerans Rotifers I May 24-May 3-Jun 13-Jun 23-Jun 3-Jul 13-Jul 23-Jul 2-Aug 12-Aug Sampling Dates Figure 3. Average zooplankton lengths per sampling date, summer ::::. bb t:: u t:: 2.0 u OJ) ụ May 24-May 3-Jun 13-Jun 23-Jun 3-Jul 13-Jul 23-Jul 2-Aug 12-Aug Sampling Dates --+- Copepods _ Cladocerans Rotifers Figure 4. Phosphorus regeneration rates for crustacean zooplankton, summer 2000.

7 64 'E , , I-+- Copepods... i.i: JE j F j I---C1adocerans c:: I L '--- I.. Rotifers : ' =-==""""-::----= =""""-= L-::=S:;::==:====!-J 14-May 24-May 3-JUll 13-JUll 23-JUll 3-Jul 13-Jul 23-Jul 2-Aug 12-Aug Sampling Dates Figure 5. Zooplankton grazing index (percent ofthe epilimnion filtered per day), summer REFERENCES Albright, M. F. In prep. Otsego Lake limnological monitoring. Summer SUNY Oneonta Bio. Fld. Sta., SUNY Oneonta. Cooke, D. G, Welch, E. B., Peterson, S. A. and Newroth, P. R Restoration and management oflakes and reservoirs. (2 nd Ed.). pp Lewis Publishers; Boca Raton. Durie, B Chlorophyll a analysis of Otsego Lake, Summer SUNY Oneonta Bio. Fld. Sta., SUNY Oneonta. Esjmont-Karabin, J Phosphorus and nitrogen excretion by lake zooplankton (rotifers and crustaceans) in relation to the individual body weights of the animals, ambient temperature, and presence of food. Ekologia Polska 32: Foster, J. R Introduction of the alewife (Alosa pseudoharengus) into Otsego Lake. In 22 nd Ann. Rept., pp SUNY Oneonta Bio. Fld. Sta., SUNY Oneonta. Gray, M. S Comparative age and growth of Centrarchid fishes in Otsego lake following the introduction of alewives. SUNY Oneonta Bio. Fld. Sta., SUNY Oneonta. Haresign, S. and Warner, D. M Filtering rates of Otsego Lake zooplankton. In 30 th Ann. Rept. SUNY Oneonta Bio. Fld. Sta., SUNY Oneonta. Harman, W. N Aquatic ecological studies. In 5 th Ann. Rept. SUNY Oneonta Bio. Fld. Sta., SUNY Oneonta.

8 65 Hannan, W. N., Albright, M. F. and Warner, D. M Trophic Changes of Otsego Lake, N.Y. following successive fish introductions In preparation. SUNY Oneonta Bio. Fld. Sta., SUNY Oneonta. Hannan, W. N., Sohacki, L. P. and Godfrey, P The limnology of Otsego Lake. In Lakes ofnew York State, Vol. II: ecology ofthe lakes of East-Central New York, ed. J. A. Bloomfield, pp New York, NY: Academic Press, Inc. Knoechel, R. and Holtbly, B Construction of a body length model for the prediction of cladoceran community filtering rates. Limnology and Oceanography. pp Miner, M.M Water quality monitoring of five major tributaries in the Otsego Lake watershed, summer SUNY Oneonta Bio. Fld. Sta., SUNY Oneonta. Needham, P.R. and Needham, 1. G A guide to the study of fresh-water biology. Holden-Day, Inc.; San Francisco. Pennak, P. W Fresh-water invertebrates ofthe United States: protozoa to mullusca (3 rd Ed.). Wiley and Sons; New York, NY. pp Peters, R. H. and Downing, 1. A Emperical analysis of zooplankton filtering and feeding rates. Limnology and Oceanography, 29 (4). pp Seamen, B A comparison of the plankton distribution of Otsego Lake between the years 1935 and In 8 th Ann. Rept. SUNY Oneonta Bio. Fld. Sta., SUNY Oneonta. Tressler, W. L. and Bere, R A limnological study of some lakes in the Delaware and Susquehanna watersheds. In A biological survey ofthe Delaware and Susquehanna watersheds. pp Albany, NY: 1. B. Lyon Company Printers. Warner, D. M Alewives in Otsego Lake, NY: A comparison oftheir direct and Indirect mechanisms ofimpact on transparency and chlorophyll a. Occasional Paper No. 32. SUNY Oneonta Bio. Fld. Sta., SUNY Oneonta.