Acidification Effects on the Algal–Zooplankton Interface

An in situ mesocosm experiment was done to assess the impacts of acidification on algal and zooplankton biomass and C assimilation, and on the efficiency of energy transfer in the grazing food chain. Triplicate mesocosms were left untreated (pH 8.5), or were acidified to pH 6.5, 5.5, or 4.5 with H2SO4 over 9 d. Algal biomass was reduced at pH 6.5 and lower, but showed no further decline across the gradient of pH 6.5–4.5. Algal C assimilation rates consistently declined with decreasing pH, reflecting a shift in dominance to larger, less productive cells at pH 5.5 and 4.5. Zooplankton biomass and productivity also declined with decreasing pH, as nearly all crustacean herbivores became extinct. In the low-pH treatments, only cyclopoids and rotifers persisted. Overall, there were significant reductions in the ratios of zooplankton/algal biomass and zooplankton/algal C assimilation (ecological transfer efficiency) with declining pH. The latter was a result of reduced grazer biomass, rather than reduced grazing efficiency; the mean zooplankton P/B ratio at pH 4.5 exceeded that measured in the higher pH treatments.