Subsidy quantity and recipient community structure mediate plankton responses to autumn leaf drop

Although ecologists appreciate the important contributions of cross-ecosystem subsidies to a variety of ecological processes, relatively little is known about the ability of detrital subsidies from temperate forests (e.g., leaves and leaf leachates) to stimulate plankton in freshwater lakes and ponds, especially immediately after leaves fall. We hypothesized that subsidy quantity interacts with zooplankton community structure to determine how phytoplankton and zooplankton respond to leaf additions, predicting that phytoplankton would peak at intermediate levels of leaf addition due to a tradeoff between increasing nutrients and decreasing light, while Daphnia would continue to increase due to their ability to exploit increased microbes. To test this hypothesis, we manipulated the amount of freshly-abscised sugar maple leaves added to artificial pond mesocosms with contrasting zooplankton community structures (with vs. without added Daphnia) in a factorial experiment and then monitored the responses of water chemistry, phytoplankton, and zooplankton to treatments over the next four weeks. Leaf additions increased total phosphorus (TP) concentrations but decreased pH, water clarity, and total nitrogen (TN) to TP ratios; the increases in TP and decreases in TN: TP were smaller in ponds with added Daphnia. Consistent with expectations, chlorophyll a increased following modest leaf additions in the ponds without Daphnia and decreased in response to the largest leaf addition in both zooplankton treatments, probably due to a lack of light. Daphnia increased with increasing leaf quantity in the+Daphnia ponds, as predicted. Surprisingly, the dominant copepods (mainly diaptomids) also increased with leaf quantity in the ponds without Daphnia. We conclude that modest leaf additions can stimulate phytoplankton and zooplankton in autumn, especially in small lakes and ponds surrounded by forest, and suggest that this hot moment'' (sensu McClain et al. 2003) for cross-ecosystem studies be further evaluated in natural systems.