Water quality and phytoplankton along a 500 kilometer transect of the St. Lawrence River from its outflow at Lake Ontario

Observation of phytoplankton and water chemistry along the main channel of the St. Lawrence River was made at a high spatial resolution (every 12 km) in order to infer the factors that influence development of this phytoplankton community. The phytoplankton community in the main channel was collected over a 10-d period (mid July 2018) from the headwaters to near the beginning of the St. Lawrence River estuary. Data for georeferenced sample locations include: size fractionated chlorophyll-a concentration (>20 µm, 20-2 µm, 2-0.2 µm), total phosphorus, nitrate, chloride, sulfate, and phytoplankton community composition in four groups (Chlorophyta & Euglenophyta, Heterokontophyta and Pyrrophyta; phycocyanin-rich Cyanobacteria, phycoerythrin-rich Cyanobacteria & Cryptophyta) measured using a FluoroProbe. Total phosphorus concentration in river water increased with distance downstream (154 to 2,750 nM) and phytoplankon biomass (1.4 to 10.5 µg chl-a/L) was strongly correlated to the concentration of total phosphorus. Diatoms, chrysophytes and dinoflagellates dominated the phytoplankton community at the outlet of Lake Ontario and total chlorophyll-a concentrations increased three-fold with 500 km transit downstream from Lake Ontario; phycocyanin-rich Cyanobacteria showed the greatest proportional increase (227%). Observations support the hypothesis that the phytoplankton community composition in this large river is strongly influenced by the headwater characteristics (Lake Ontario) and gradually influenced by entrainment of nutrient-rich tributary waters.