Bi-weekly Changes in Phytoplankton Abundance in 25 Tributaries of Lake St. Francis, Canada: Evaluating the Occurrence of Nuisance and Harmful Algae

Eutrophication and undesirable algae growth remain listed as beneficial use impairments in 14 Great Lakes areas of concern, including the Cornwall/St. Lawrence region. However, there is currently a lack of information regarding the distribution and frequency of undesirable algae occurrences, particularly in lotic systems influencing this region. Here, we examine phytoplankton assemblages in 25 tributaries of Lake St. Francis (St. Lawrence River) between the months of May and September (2013 and 2014) in order to evaluate abundance and taxonomic composition. Phytoplankton in the tributaries were dominated by diatoms, followed by chlorophytes, and on average only ~ 1% of overall composition were cyanobacteria. The low proportion of chlorophytes and cyanobacteria was surprising, given that these tributaries are located in watersheds dominated by agricultural activities. Stream order was not related to total phytoplankton abundance, potentially due to the highly disturbed nature of the catchments. Multiple linear regression models were developed to better understand possible chemical and physical factors contributing to phytoplankton abundance and cyanobacteria occurrence across all sites. Water temperature, total phosphorus and day of year were identified as the best predictors of phytoplankton abundance, and water temperature and depth were identified as predictors for cyanobacteria occurrence. While these models were significant, they only accounted for a low amount of the variability (between 8 and 12%) of total phytoplankton and cyanobacteria abundance. Year-to-year variation between mean phytoplankton abundances was also high, which was reflected in the low correlation between the rank order of sites between 2013 and 2014 (ρ = 0.16).