Dynamics of the pelagic food web in St. Georges Bay, southern Gulf of St. Lawrence

Abstract

Phytoplankton standing stock and production increased between June and October in St. Georges Bay, Nova Scotia, Canada. Inorganic nitrogen is believed to be the limiting nutrient in the Bay and it is deduced that in situ regeneration must have satisfied most of the summer demand by primary producers. The proportion of organic matter sedimented, relative to that produced by phytoplankton during this time, depended upon the degree of water stratification. Sedimentation of organic carbon and nitrogen amounted to between 9 and 14 % of that assimilated by phytoplankton during maximum stratification in August, implying that -90 % of the organic matter produced was recycled above the thermocline. Although copepod biomass decreased, production increased in late June and remained high throughout the summer because of rapid development at high summer temperatures. The production of mackerel and lobster larvae, estimated from studies in other years, was greatest from late July to mid-August. Thus, maximum consumption of the products of phytoplankton production in the water column above the thermocline and minimum transfer of particulate organic matter to the benthos coincided with maximum growth and production rates of copepods and their predators. Calculations of potential food consumption by larval and adult fish, estimated from biomass, showed that standing stocks of planktonic prey organisms could not support the biomass of fish present. The predictable occurrence of warmer waters with rapid production of prey organisms and the restricted exchange with offshore waters makes St. Georges Bay a successful nursery ground for pelagic spawners.