Macrozooplankton and the persistence of the deep chlorophyll maximum in a stratified lake

International audience1. Deep chlorophyll maxima (DCM) are common in deep, oligotrophic stratified lakes. The DCM refer to the maximal chlorophyll a concentration found at depth, and not at the lake surface. While control of the DCM is thought to be via physicochemical factors in many lakes, a role for zooplankton grazing in epilimnetic waters remains a possibility. The occurrence and dynamics of DCM are poorly documented in smaller lakes, where zooplankton grazing is likely to have a stronger structuring effect. In small, shallow stratified lakes, biological control by grazing may be magnified by the short vertical gradient and overall higher water temperature. 2. The respective contributions of several physical, chemical and biological parameters to the vertical distribution of phytoplankton biomass in a small stratified lake were examined. Associations between phytoplankton depth distribution and vertical gradients in temperature, light and nutrients and the density of herbivorous zooplankton were established through regressions and generalised linear models.3. Colimitation of the DCM by light from above and nutrients from below was detected. A threshold was detected at 3% incident light (100 lmol photon m 2 s 1), below which the DCM disappeared. Epilimnetic biomass was related to nutrient availability, with a threshold concentration at 4 lg P L 1, below which the DCM dominated.4. Greater stability of the water mass and more zooplankton were associated with higher phytoplankton biomass in the DCM. Stability is likely to have controlled vertical nutrient fluxes, which were intercepted by the metalimnetic phytoplankton. Zooplankton grazing of epilimnetic biomass could have increased incident light reaching the top of the metalimnion, thereby favouring proliferation of photosynthetic biomass in the DCM.5. Wind mixing events, as detected by a reduction in Lake number (LN, a measure of the influence of wind forcing on vertical structure), induced vertical intrusions of metalimnetic water, rich in nutrients and phytoplankton, into the epilimnion. We can infer that dominance of phytoplankton in the epilimnion would have occurred earlier during the summer if grazing by zooplankton had not removed epilimnetic phytoplankton. Our results suggest that, while stable stratification is necessary for initial DCM formation, zooplankton grazing may promote the persistence of a DCM