Zooplankton Abundance, Biomass and Trophic State in Some Venezuelan Reservoirs

Abstract

Zooplankton is a key community in trophic webs of lake and reservoir ecosystems, due to its role in the energy flow from phytoplankton to higher trophic levels. However, zooplankton density, expressed as number of individuals per area or volume units, does not necessarily provide accurate information about community biomass, because zooplankton consists of a great variety of groups or animal species of a large size range. Moreover, the biomass of the zooplankton species is an important and necessary parameter to calculate the secondary production of this community. Consequently, estimation of zooplankton dry weight constitutes an important contribution for the study of trophic-web structure in aquatic ecosystems, considering its relationship with the trophic status of the water bodies. In Venezuela, there is a scarcity of data concerning zooplankton dry weight and/or biomass. From more than 100 reservoirs in the country, there is some information about zooplankton ecological aspects from around 20 of them. Because of this, the goal of this study was to gather data collected from 13 reservoirs with different trophic status, in order to establish relationships among zooplankton abundance and biomass with phytoplankton biomass (as chlorophyll a) and the trophic status of reservoirs. We studied the following reservoirs, distributed from the North–Eastern to North–Central regions of Venezuela: Agua Fría, Taguaza, Lagartijo, El Cigarrón, Tierra Blanca, El Cují, Clavellinos, El Andino, El Pueblito, La Mariposa, La Pereza, Quebrada Seca and Suata. Analysis of data was based on results from 6-12 monthly sampling periods at each reservoir. Samples for estimating phytoplankton biomass (as chlorophyll a) were collected using an opaque van Dorn bottle (3 l) in the euphotic layer of reservoirs, preserved in cold and dark conditions until its analysis in the laboratory, and its concentration was estimated by extraction of photosynthetic pigments with ethanol after filtering through glass-fiber filters (Nusch & Palme method). Zooplankton samples were obtained in the limnetic region of the water bodies by vertical tows in the oxygenated strata using plankton net (77 µm of mesh size). Samples were preserved in 4% formaldehyde (final concentration). Abundance was determined by counting animals in Sedgwick-Rafter chambers (1 ml), according to the Wetzel & Likens method, whereas biomass was estimated as dry weight (d.w.) after desiccation at 60°C for about 20-24 h, according to the Edmondson & Winberg method. Reservoir trophic status ranged from ultra–oligotrophic (Agua Fría) to hypertrophic (Suata), considering the phytoplankton biomass (values ranging between 2.16 and 92.89 µg/l) and total phosphorus concentrations (from 4 to more than 1500 µg/l). In relation to zooplankton, copepods were dominant in almost all reservoirs; rotifers and ostracods were present in high numbers in some of them. Generally, higher values of abundance and biomass were registered in the more eutrophic systems (1130 individuals/l and 1127.26 µg d.w./l in Quebrada Seca, and 753 individuals/l and 2026.14 µg d.w./l in Suata), whereas the lower values were registered in the more oligotrophic ones (24 individuals/l and 48.51 µg d.w./l in Agua Fría, and 86 individuals/l and 28.71 µg d.w./l in Taguaza). Considering the mean values of each reservoir, zooplankton abundance showed a positive and significant correlation with zooplankton biomass (r=0.718) and chlorophyll a (r=0.778), while zooplankton biomass showed a positive and significant correlation with chlorophyll a (r=0.751). These results support a strong correlation between zooplankton abundance and biomass, and among these variables and the trophic status of the studied Venezuelan reservoirs.