Effects of whole-lake manipulations of nutrient loading and food web structure on planktonic respiration

We assessed planktonic respiration in whole-lake manipulations of nutrient loading and food web structure in three manipulated and one unmanipulated l ake over 7 years. The manipulations created strong contrasts in zooplankton body size across a series of nutrient loads. Large-bodied zooplankton were suppressed by planktivorous fish in one lake, while in the other two manip ulated lakes, large-bodied zooplankton dominated community biomass. Nutrien ts were added as inorganic N and P. Nutrient loads ranged from background t o conditions resembling eutrophic lakes. Planktonic respiration was measure d weekly in each lake by dark bottle oxygen consumption. Respiration was lo w when lakes were not fertilized (average 8.5 mu mol O-2.L-1.day(-1)) and w as correlated with differences in dissolved organic carbon among the lakes. Respiration increased with nutrient addition to a mean range of 12-25 mu m ol O-2.L-1.day(-1); however, respiration differed among lakes at the same n utrient loading. Further, respiration was independent of dissolved organic carbon in the fertilized lakes. Differences in the intensity of zooplankton grazing as determined by food web structure strongly regulated primary and bacterial production across the range of nutrient loads. Consequently, res piration was positively related to primary production, phytoplankton biomas s, and bacterial production and inversely related to the average size of cr ustacean zooplankton.