Spatial overlap in lake phytoplankton: Relations with environmental factors and consequences for diversity

Opposing gradients of light and nutrients can create a variety of niche opportunities for lake phytoplankton. Theory predicts that phytoplankton vertical distribution should be associated with these gradients as different taxa maximize resource access, while minimizing competitive interactions using niche partitioning. We examined the relationships between spatial overlap (SO) of four major phytoplankton spectral groups with key biogeochemical and morphometric environmental parameters across 52 north temperate lakes. SO decreased primarily with greater thermal stratification, and where more light was available and nutrient levels lower. When greater SO did occur in highly stratified lakes, taxonomic diversity was favored through increased species evenness, but not richness. Taxonomic richness, on the other hand, increased in lakes with greater light availability, coincident with low SO. Similarly, niche partitioning associated with greater functional diversity in the range of traits present in the communities was detected when SO was low in clear lakes. Overall, our results indicate that the ability of phytoplankton to spatially separate and utilize large portions of the water column is important for augmenting species and functional trait richness related to motility and resource acquisition. However, our study also suggests that community evenness is favored when phytoplankton distributions overlap in deep chlorophyll maxima associated with stratified lakes.