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.
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Sampling ̶ Forty-eight georeferenced stations (Fig. 1) were sampled from surface water of the main channel of the St. Lawrence River during daylight hours over the period of July 14-24, 2018 from the R/V Lampsilis. Water was collected through an in-hull sampling port (depth = 0.8 m) whilst the vessel was underway (5-10 knots; 9-19 km/h) and processed within 0.5 hours. Sample sites were selected based on the presence of the ship in the main channel with the aim to sample as many sites as possible. Sampling was conducted between Kingston, Ontario (at the head of the St. Lawrence River in eastern Lake Ontario) and Deschaillons-sur-Saint-Laurent, Quebec, a non-tidally-influenced point in the river approximately 70 km upstream from Quebec City (where the St. Lawrence River estuary begins). Consecutive Station Number in the data set refers to the sequence that stations were visited. Chemical analyses ̶ Total phosphorus was determined in duplicate on unfiltered samples (35 mL) of river water collected in borosilicate tubes conducted using colorimetric analysis following persulfate digestion. Dissolved anions (nitrate, sulfate, and chloride) were measured by ion chromatography in water filtered through 0.2-μm syringe filters (Whatman). All samples were stored and refrigerated (4 °C) until analysis (within 4 weeks of collection). Size-fractionated Chlorophyll-a ̶ Chlorophyll-a (chl-a) was determined from parallel filtration of samples collected on 0.2-μm and 2-μm pore-size polycarbonate filters (47 mm diameter; Millipore), as well as 20-μm pore-size nylon filters (47-mm diameter; Millipore); all filtrations were conducted in duplicate. Chl-a retained on the different size class filters was extracted (24 h, 4°C, dark) in 90% acetone and quantified with a Turner Designs TD-700 fluorometer using a non-acidification protocol. Phytoplankton community composition ̶ River water was filtered (< 0.2-μm) and used for background correction of dissolved fluorometric colored substances for each individual river water sample that was assessed for phytoplankton community composition using a pigment-specific fluorometer (bbe FluoroProbe, with WorkStation 25; bbe Moldaenke, GmbH). The fluorometer used factory settings for phytoplankton.