Does zooplankton grazing affect seston size-structure and areal hypolimnetic oxygen depletion in lakes?

In four New Zealand lakes of differing trophic status, we examined the abil ity of zooplankton grazing to alter the size structure of the seston and th ereby affect the rates of areal hypolimnetic oxygen depletion (AHOD). In ge neral, the study lakes had relatively low abundances of Daphnia, reflecting a trend for many New Zealand lakes to be copepod dominated. Seston size-st ructure shifted towards larger particles as the proportion of copepods in t he zooplankton community increased but Daphnia had no discernible effects o n the seston size-structure. An empirical model, based on North American an d European lakes, that predicts areal hypolimnetic oxygen depletion from ch lorophyll-a, mean depth and euphotic depth was used as a null model to gene rate predictions of AHOD for the study lakes. Differences between observed and predicted AHOD were used to test the effects of zooplankton community s tructure and particle size-structure on AHOD. The empirical model predicted AHOD in the study lakes remarkably well and only 4 % of the variation in o bserved AHOD remained to be explained. Analysis of the residuals of the mod el's fit to the data indicated that neither seston size-structure nor effec ts of zooplankton grazing on seston size-structure explained significant re sidual variation in AHOD among the lakes. The net effects of zooplankton gr azing on seston size-structure and AHOD were confounded by apparent contras ting and scale-dependent zooplankton interactions. Among lakes, zooplankton biomass was tightly linked to trophic state and, hence, was also positivel y related to AHOD whereas, within lakes, zooplankton grazing was negatively related to AHOD, possibly as a result of the grazers affecting the sedimen tation of autochthonous organic matter.