Each of the “Sky Lakes” on the Shawangunk Ridge in Ulster County, New York have zooplankton communities that vary in size and density. These lakes also vary in pH and food web complexity; Lake Awosting (pH~4.5) is fishless while Lakes Minnewaska (pH~6.0) and Mohonk (pH~7.0) both support fish communities. We tested the effects of acidity on zooplankton/phytoplankton food web interactions in filtered (0.7 µm) Lake Mohonk water adjusted with H2SO4 to pH ~4.5, 5.5, and 6.8 to mimic the conditions of our three lakes. We added cultures of four green algae species (Ankistrodesmus, Chlorella, Scenedesmus, and Selenastrum species) to 250 ml bottles of each water acidity treatment. Half of the twelve replicates contained three Daphnia pulex and half remained as zooplankton-free controls. We suspended all bottles in the dark on a rotating wheel in an incubator at 15°C for approximately 72 hours. We quantified the clearance of algae by D. pulex in each acidity treatment by calculating differences in initial and final chlorophyll a concentrations while accounting for algal growth in the zooplankton-free treatments. We also recorded pH change, D. pulex mortality, and phytoplankton cell densities.
Results/Conclusions
The pH approached neutral in all samples over time due to photosynthetic consumption of CO2; however, zooplankton presence had no effect on changes in acidity (F=0.51, p=0.480). Algal concentrations differed in each acidity treatment with the presence of zooplankton (F = 25.84, p<0.001), but the three acidity treatments did not differ from one another (F = 1.45, p = 0.251). Furthermore, the differences in acidity had no effect on realized algae consumption by zooplankton (F = 0.19, p = 0.831). The initial concentration of chlorophyll a (10.6 𝝻g/L) was over five times higher than what we typically measure in Lake Awosting (~2 µg/L). Under these experimental conditions, zooplankton and phytoplankton processes were not modified by the initial acidity; however, at lower initial concentrations of algae with sustained pH levels over the course of the experiment, zooplankton could become stressed and exhibit an optimal pH for feeding rates. In the future, we plan on testing the effects that different initial algal concentrations have on pH change and zooplankton at different acidities. This will enable us to effectively test the sensitivity of aquatic ecosystems to changes in acidity due to anthropogenic driven acidification and subsequent recovery from acid rain.