Summer Plankton Distributions Associated with the Physical and Nutrient Properties of the Northwestern Gulf of St. Lawrence

Abstract

Two transects, north–south and east–west, through the Anticosti gyre and the Gaspé current were sampled for physical properties, nutrient and plankton concentrations over a 5-d period during July 1976. Temperature, salinity, σt and nutrient isolines formed a dome characteristic of cyclonic circulation with the apex within the Anticosti gyre, but a strong shallow thermocline prevented nutrient enrichment of the surface mixed layer. Surface nitrate and silicate concentrations were very low, except for two anomalies, throughout the region sampled, with phosphate concentrations higher. Phytoplankton biomass was uniformly low and dominated by nannoplankton (< 15 μm) at all stations except that in the Gaspé current at which netplankton (15–202 μm) was important and the total biomass considerably higher. The higher biomass and importance of larger celled species suggest higher nutrient turnover in the Gaspé current. Nitzschia delicatissima dominated the species composition in the Gaspé current, dinoflagellates and microflagellates being of more importance within the gyre and along the north shore. Zooplankton biomass was distributed fairly uniformly over the entire region but species composition at the shallower stations was characterized by small copepod species (Pseudocalanus sp., Acartia sp.) and fish larvae, the gyre stations being characterized by larger copepod species (Calanus sp.), euphausids, and decapod larvae. Physical properties, nutrients, and plankton were distributed quite uniformly except for the marked variability in the measurements made in the strongly advective Gaspé current station. Distributions of derived parameters that were estimated at 5 m depth suggest nutrient limitation at most stations within the gyre, with the exception of the two stations at which surface nutrient anomalies were detected, while at the nearshore stations, in the Gaspé current and along the north shore, better physiological state and lack of nutrient limitation were observed. Short-term variability in these parameters complicate geographic interpretation, but trends were discernable. Estimated daily primary production was an order of magnitude less than that reported previously. Key words: Anticosti gyre, Gaspé current, primary production, nutrient limited growth