Environmental factors controlling the vertical distribution of phytoplankton in lakes

Observations from single lake and experimental studies predict that vertical habitat heterogeneity in lakes can influence phytoplankton community structure. We examined the nature of water column physical habitat structure (light penetration, thermocline depth and shape and relative thermal resistance to mixing), and in turn, how these structures influenced the distribution of bulk chlorophyll a and the biomass of several major phytoplankton groups across 45 lakes in eastern Canada, within two lake districts which varied in watershed geology and water chemistry. Across all lakes, more pronounced temperature gradients favoured the distribution of bulk phytoplankton into more defined layers. The depth at which peak chlorophyll a was observed was affected by temperature heterogeneity and environmental factors related to light penetration. Peak depths and vertical heterogeneity of the major phytoplankton groups were differentially related to epilimnetic water colour and phosphorus concentration across all lakes. Further insight was gained by comparing the physical structure and phytoplankton responses in the two regions. Lakes from the Laurentians Region had less wind exposure, shallower thermoclines, but greater vertical temperature variability than ill the Eastern Townships Region. As a result, total and major phytoplankton group biomass showed more heterogeneous distributions in the Laurentians. The depth of peaks in total biomass and for the major phytoplankton groups was similar in both regions; the exception being a deeper chlorophyte maximum in the ETR, suggesting that there may be important differences between regions in the taxonomic composition of thin group.