Temporal trends of phytoplankton and zooplankton stable isotope composition in tropical Lake Malawi

Stable isotope ratios of three seston size classes (20-100 μm, 2-20 μm, and 0.2-2 μm) and zooplankton species were analyzed to determine the plankton food web structure of Lake Malawi. Over an annual cycle, seston δ13C varied between −20.41‰ and −27.43‰ with a mean value of −24.27‰ ± 1.2 while δ13C values for zooplankton fluctuated between −22‰ and −25‰ with a mean of −23.84‰ ± 0.77. Seston δ13C fluctuations appeared to be related to changes in physical and meteorological conditions in the lake that ultimately control nutrient availability. The highest seston δ13C values observed during the rainy and mixed seasons likely result from high phytolankton growth rates. δ15N of plankton was temporally variable, suggesting short term changes in N cycling dynamics that control the supply of N to phytoplankton. Very low seston δ15N values recorded during the mixing season suggest excess NO3− availability resulting from upwelling and vertical mixing. In contrast to expectations the calanoid Tropodiaptomus cunningtoni appeared to feed at a trophic level higher than that of all other zooplankton species, including the cyclopoid, Mesocyclops aequatorialis aequatorialis. δ15N values indicate that zooplankton were nearly 2 trophic levels above seston in the early stratified season. This implies that adult zooplankton could be utilizing forms of food other than phytoplankton during this period, such as nauplii or protozoans. This extra step in the food web, and the trophic positions of large zooplankton species, may alter estimates of food web efficiency and potential fish production for Lake Malawi.