CARBON DYNAMICS IN THE GRAZING FOOD-CHAIN OF A SUBTROPICAL LAKE

Studies were conducted over a 13 month period at four pelagic sites in eutrophic Lake Okeechobee, Florida (USA), in order to quantify carbon (C) uptake rates by size-fractionated phytoplankton, and subsequent t ransfers of C to zooplankton. This was accomplished using laboratory C -14 tracer methods and natural plankton assemblages. The annual biomas s of picoplankton (<2 mu m), nanoplankton (2-20 mu m) and microplankto n (>20 mu m) averaged 60, 389 and 100 mu g C l(-1), respectively, whil e corresponding rates of C uptake averaged 7, 51 and 13 mu g C l(-1) h (-1). The biomass of microzooplankton (40-200 mu m) and macrozooplankt on (>200 mu m) averaged 18 and 60 mu g C l(-1), respectively, while C uptake rates by these herbivore groups averaged 2 and 3 mu g C l(-1) h (-1). There were no strong seasonal patterns in any of the plankton me trics. The ratio of zooplankton to phytoplankton C uptake averaged 7% over the course of the study. This low value is typical of that observ ed in eutrophic temperate lakes with small zooplankton and large inedi ble phytoplankton, and indicates ineffective C transfer in the grazing food chain. On a single occasion, there was a high density (>40 l(-1) ) of Daphnia lumholtzii, a large-bodied exotic cladoceran. At that tim e, zooplankton community C uptake was >20 mu g C l(-1) h(-1), and the ratio of zooplankton to phytoplankton C uptake was near 30%. If D.lumh oltzii proliferates in Lake Okeechobee and the other Florida lakes whe re it has recently been observed, it may substantially alter planktoni c C dynamics.