Species Composition and Seasonal Succession of the Zooplankton Community of Eutrophic Lahontan Reservoir, Nevada

The zooplankton community of eutrophic Lahontan Reservoir, Nevada was studied during a 22-month period of 1980 and 1981. The zooplankton community consisted of seven species of cladocerans, two species of copepods and nine species of rotifers. The dominant zooplankters were the cladocerans Daphnia galeata, D. schodleri, Diaphanosoma leuchtenbergianum, and Bosmina coregoni, the copepods Cyclops vernalis, and Diaptomus sicilis, and the rotifer Keratella. Mean zooplankton density was 37.2/1 in 1980 and 64.7/1 in 1981. Overall, copepods comprised 50 percent, cladocerans 34 percent, and rotifers 16 percent of the total population. Biotic and abiotic factors are discussed in relation to their potential influence on zooplankton community structure. Lahontan Reservoir, Nevada, located in the arid Great Basin, exhibits characteristics of a culturally eutrophic waterbody (Cooper et al., 1983). Nutrient enrichment of the reservoir was traced primarily to seven waste treatment plants that discharge into the Truckee and Carson rivers (RichardHaggard, 1983). A secondary, but significant, source of nutrient loading to the reservoir is runoff from approximately 29,000 ha of irrigated agriculture. In an attempt to improve the water quality in Lahontan Reservoir, a restoration plan is underway that includes reductions in nutrient loading via a combination of land application or tertiary treatment of sewage effluent and "best management practices" for diffuse source loading. Water quality models applied to the reservoir indicate that a reduction in nutrient loading will modify phytoplankton structure and decrease biomass and production (French et al., 1983). Changes in the phytoplankton community can trigger ecological changes in the zooplankton community (Patalas, 1972; Ravera, 1980; Edmondson and Litt, 1982). Because zooplankton life histories are relatively short they are almost instantaneous indicators of changes in water quality (Brooks, 1969). In this paper we document the prerestoration species composition of the Lahontan Reservoir zooplankton community and the spatial and temporal variation in zooplankton abundance. STUDY SITE.-Lahontan Reservoir, situated on the Carson River drainage, is a storage impoundment located approximately 72 km southeast of Reno, Nevada. The dam was constructed during 1911-1915 by the U.S. Bureau of Reclamation and is part of the Newland's Irrigation Project. The reservoir receives the entire flow of the Carson River in addition to a transbasin diversion from the Truckee River. Morphologic and hydrologic characteristics of Lahontan Reservoir are presented in Table 1 and Fig. 1. Symptoms of eutrophication include an annual bloom of the cyanophyte, Aphanizomenon flosaquae that has exceeded concentrations of 150,000 cells/ml. At the peak of the bloom surface chlorophyll a concentrations have reached 350,gg/l (Cooper and Vigg, 1984). Blue-green algae dominate the phytoplankton community from May or June through October. Gross primary production for 1980 ranged from 133 to 318 g C/m2/yr in the least and most productive areas T E SOUTHWESTERN NATUR LIST 30(2):239-252 MAY 31, 1985 This content downloaded from 207.46.13.149 on Wed, 19 Oct 2016 04:09:41 UTC All use subject to http://about.jstor.org/terms The Southwestern Naturalist TABLE 1.-Morphologic and hydrologic characteristics of Lahontan Reservoir, Nevada. Characteristic Value Maximum length 27 km axi u width 4 k Maximum depth 26 m ean depth 8.1 Surface area 4,410 ha Spillway elevation 1,269 m Capacity 3.57 X 108 m3 Annual Inflow Carson River 3.26 X 108 m3 Truckee Canal 2.18 X 10I m3 Hydraulic Residence Time 0.77 yrs Watershed Area Carson River 5,050 km2 Truckee Canal 1,190 km2* Shoreline length 96 km Shoreline development ratio 4.09 Mean annual water level fluctuation 9.4 m *This value represents 27.5% of the Truckee River watershed above the Truckee Canal diversion which is the percentage of the Truckee River annual flow that enters Lahontan Reservoir. of the reservoir, respectively. Hypolimnetic oxygen depletions are common during stratification periods in the monomictic northern basin (Fig. 1). The shallower, relatively turbid, and less productive southern basin of the reservoir is polymictic. Lahontan Reservoir is nitrogen deficient and relatively phosphorus rich with growing season TN:TP ratios less than 5 to 1. Other studies show that low N:P ratios favor the dominance of nitrogen-fixing blue-green algae (Schindler, 1977; Flett et al., 1980; Smith, 1983). Assays using the acetylene-reduction technique confirmed that nitrogen-fixation by A. flos-aquae is occurring in Lahontan Reservoir (Cooper and Vigg, 1984). METHODS.-Zooplankton samples were collected monthly with a Wisconsin plankton net (153pA mesh) from four sampling locations on the reservoir (Fig. 1) from February 1980 through November 1981. Each sample consisted of a vertical tow from 10 m to the surface and was preserved in 40 percent ethyl alcohol (Pennak, 1953) in the field. Zooplankton were counted in a Sedgewick-Rafter counting chamber with a binocular microscope at 40-100X magnification. Species were identified using the taxonomic keys of Pennak (1953), Brooks (1957), Yeatman (1959), and Ward and Whipple (1959). Net phytoplankton was field filtered through a 64A mesh net and counts made in a SedgewickRafter counting chamber (Welch, 1948). Diatoms were enumerated using the membrane filter technique described by McNabb (1960). Algal species were identified with the taxonomic works of Hustedt (1930), Ward and Whipple (1959), Prescott (1970), Patrick and Reimer (1975), Vinyard (1979) and LaRivers (1978). RESULTS AND DISCUSSION.-Zooplankton Composition.-The Lahontan Reservoir zooplankton community was composed of seven cladocerans, two copepods, and nine rotifers (Table 2). Between February 1980 and November 1981 reservoir zooplankton were dominated by the copepod Cyclops vernalis, closely followed by Daphnia schodleri and D. galeta (Table 2). The cladocerans Diaphanosoma leuchtenbergianum and Bosmina coregoni, the copepod Diaptomus sicilis, and the rotifer Keratella also had significant populations (> 1.0 percent with respect to overall relative abundance). The average standing crop during the study for copepods was 24.8/1 and ranged from 8.7 to 73.6 individuals per liter; cladocerans averaged 16.7/1 240 vol. 30, no. 2 This content downloaded from 207.46.13.149 on Wed, 19 Oct 2016 04:09:41 UTC All use subject to http://about.jstor.org/terms May 1985 Cooper and Vigg-Composition and Seasonal Succession of Zooplankton