Carbon dynamics in the 'grazing food chain' of a subtropical lake

Abstract

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 transfers of C to zooplankton. This was accomplished using laboratory 14C tracer methods and natural plankton assemblages. The annual biomass of picoplankton (<2 μm), nanoplankton (2-20 μm) and microplankton (>20 μm) averaged 60,389 and 100 μg C l-1, respectively, while corresponding rates of C uptake averaged 7,51 and 13 μg C l-1 h-1. The biomass of microzooplankton (40-200 μm) and macrozooplankton (>200 μm) averaged 18 and 60 μg C l-1, respectively, while C uptake rates by these herbivore groups averaged 2 and 3 μg C l-1 h-1. There were no strong seasonal patterns in any of the plankton metrics. The ratio of zooplankton to phytoplankton C uptake averaged 7% over the course of the study. This low value is typical of that observed in eutrophic temperate lakes with small zooplankton and large inedible 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 time, zooplankton community C uptake was >20 μg C l-1 h-1, and the ratio of zooplankton to phytoplankton C uptake was near 30%. If D.lumholtzii proliferates in Lake Okeechobee and the other Florida lakes where it has recently been observed, it may substantially alter planktonic C dynamics.