Development and persistence of deep chlorophyll maxima in oligotrophic lakes over the summer season

摘要

Deep chlorophyll maxima (DCM) are common features in temperate and tropical oligo-mesotrophic stratified lakes, where they can account for an important part of the primary production (Fee 1976, Cullen 1982, Pick et al. 1984, Pedros-Alio et al. 1987, Padisak et al. 2003). However, the mechanisms that lead to the development and persistence of DCMs remain to be fully understood (Perez et al. 2002, Burnett et al. 2006, Sawatzky et al. 2006). A number of hypotheses have been proposed to explain the development and persistence of DCMs in lakes: (1) the limiting resource hypothesis, stating that DCM is due to in situ algal productivity in the metalimnion, where nutrient supply comes from mineralisation of sinking epilimnetic particles and zooplankton excretion (Carmacho et al. 2000); (2) the behavioural hypothesis, stating that only mobile or mixotrophic algae species adapted to low light develop in DCM layers (Fahnenstiel & Glime 1983, Pedros-Alio et al. 1987); (3) the sedimentation hypothesis, stating that sinking of epilimnetic algae over depth along the density gradient generates DCM in the metalimnion (Cullen 1982); and (4) the depth-specific zooplankton grazing hypothesis, stating that lower grazing rate in the metalimnion lead to the maintenance of DCM (Pilati & Wurtsbaugh 2003). In fact, several of these processes may interact to control the development and persistence of DCMs in lakes (Richerson et al. 1978, Padisak et al. 2003, Burnett et al. 2006). We describe the development and persistence of DCMs using high resolution in situ fluorometry in 3 Laurentian lakes (Quebec, Canada) during summer 2002. The objective of this study was to evaluate how DCM patterns were related to environmental factors such as photosynthetic active radiation, nutrients, and crustacean zooplankton community.