Eutrophication in three Canadian Areas of Concern: Phytoplankton and major nutrient interactions

Abstract

The Bay of Quinte, Hamilton Harbour and Toronto Harbour are all coastal regions of Lake Ontario that have experienced eutrophication and all have been designated as ‘Areas of Concern’ under the terms of the Great Lakes Water Quality Agreement. An assessment of the phytoplankton communities in relation to nutrient (P,N,Si) regimes was undertaken during 2015 (Bay of Quinte) and 2016 (Hamilton Harbour and Toronto Harbour) in order to compare and contrast the dynamics of eutrophication in the three ecosystems. Bay of Quinte was found to be phosphorus and silica enriched, but nitrogen limited which resulted in a phytoplankton community dominated by both filamentous diatoms and diazotrophic (N–fixing) cyanobacteria. Hamiton Harbour was phosphorus and nitrogen enriched, but silica depleted with a community dominated by small and large phytoflagellates in addition to experiencing cyanobacteria blooms. Toronto Harbour, by contrast, showed only moderate phosphorus enrichment and no nitrogen limitation, but some silica depletion; phytoplankton was dominated by smaller flagellates and pennate diatoms. Our findings suggest that while phosphorus was a key factor causing cultural eutrophication, other nutrients including nitrogen and silica also had important roles in determining the biomass and composition of the algal standing crop. Future management activities need to consider how the interactions of phosphorus with other nutrients (nitrogen, silica) affect the dynamics of the phytoplankton community in order to promote the recovery of eutrophic ecosystems.