PARENT SESSION
Oral Session #56: Aquatic Ecology: Plankton. Presiding: N. Hairston.
Wednesday, August 8, 2001. 1:00 PM to 5:00 PM. Hall of Ideas I.
Effect of terrestrial subsidies on pond planktonic communities.
Geddes, Pamela1, Leibold, Mathew1, 1
ABSTRACT- External subsidies are fluxes of nutrients, matter, or energy that couple seemingly independent systems, affecting local community structure and function. Theory suggests these subsidies can alter the structure of food webs and stabilize their dynamics. In this study, we addressed the effect of terrestrial subsidies as humic compounds on food web dynamics and ecosystem functioning of planktonic communities. Humic substances leach out from terrestrial leaf litter, acting as a source of colored organic carbon that can influence plankton dynamics through various mechanisms. We manipulated humic substance concentrations (control=no addition, low, and high) in 1000L-cattle tanks. We inoculated mesocosms with hyperdiverse phytoplankton and zooplankton communities and sampled them for chlorophyll-a and zooplankton ash-free dry mass. We also measured productivity, respiration, and P:R ratios as indicators of ecosystem functioning. Zooplankton biomass was significantly higher in the high treatment than in controls, but there were no significant differences in chlorophyll-a among treatments. Productivity and respiration were significantly higher in controls, but the P:R ratios did not differ among treatments. The coefficient of variation of zooplankton biomass in the high treatment was significantly higher than in controls, providing evidence against stabilizing effects of the manipulated subsidies. These results suggest that external subsidies in the form of humic substances did not affect phytoplankton biomass or ecosystem functioning as P:R ratios, but they significantly increased zooplankton biomass and its variability relative to controls.
KEY WORDS: external subsidies, humic substances, food web dynamics, ecosystem functioning