A comparison of phytoplankton communities of the deep chlorophyll layers and epilimnia of the Laurentian Great Lakes

Phytoplankton biomass and primary productivity within Great Lakes deep chlorophyll layers (DCL) remain largely uninvestigated. Consequently, the taxonomic makeup of DCL phytoplankton communities, as well as the mechanisms regulating their formation and maintenance, is poorly understood. We examined 6 years of phytoplankton compositional characteristics of Great Lakes summer DCL and epilimnetic communities as well as spring communities from isothermal water columns. DCLs were regularly observed during summer stratification in all lakes with the frequent exception of Lake Erie. Relative compositions of summer chlorophyte and cryptophyte assemblages were not different between the epilimnion and DCL, but DCL phytoplankton communities from other algal groups were distinct from their epilimnetic counterparts and comprised an integration of phytoplankton from the overlying epilimnetic assemblages and relict taxa characteristic of spring. Summer epilimnetic communities were characterized by higher abundances of cyanophytes, and centric diatom communities were dominated by Cyclotella sensu lato (i.e. species within Cyclotella and closely related genera). Cyclotella species exhibited distinct patterns of vertical distribution, with small-bodied taxa being partitioned heavily into the epilimnion, while larger-bodied forms tended to occupy the DCL. Vertical size partitioning was exemplified by larger mean individual cell sizes in epilimnetic siliceous algae (diatoms and chrysophytes) in the DCL compared to the epilimnion, while the opposite pattern was exhibited by cyanophytes. These findings demonstrate the importance of stratification intensity to vertical structuring of summer phytoplankton communities and imply that changing stratification regimes (such as that due to recent climate change) may exert profound effects on Great Lakes primary producer communities.