Seasonal Dynamics and Potential Fate of Size-fractionated Phytoplankton in a Temperate Nearshore Environment (Western Gulf of St Lawrence, Canada)

Abstract

Environmental conditions and seasonal dynamics of the <5 and >5μm phytoplankton were monitored in a temperate nearshore environment (Baie des Chaleurs, western Gulf of St Lawrence, Canada). Following the spring bloom, there was a sharp decrease in the contribution of >5μm phytoplankton to the chlorophylla biomass and primary production, so that the <5μm fraction accounted for up to 80–90% of both biomass and production throughout the summer. Strong summer stratification prevented wind-driven mixing and nutrient replenishment by transient upwelling events. The compensation depth for phytoplankton growth was close to the ≤25m bottom, so that only cells growing near the bottom could have been limited by light. In spite of the high production of <5μm phytoplankton (>0·5g C m−2d−1), their biomass remained relatively constant, which is indicative of a tight balance between production and export, involving active microbial grazing. Production per unit biomass of >5μm phytoplankton [8–126mg C (mg Chlorophylla d)−1] was generally higher than that of the small size fraction [13–56mg C (mg Chlorophyllad)−1], suggesting active photosynthesis by the former. Seasonal changes in size-fractionated production and biomass indicate that >5μm phytoplankton experienced intense export. This is consistent with the accumulation of large cells with low photosynthetic efficiency near the bottom during the period of strong stratification, probably resulting from the sinking of intact phytoplankton. In addition, there is indirect evidence of mesozooplankton grazing on large phytoplankton during summer. The coexistence of microbial and herbivorous feeding modes during summer corresponds to a multivorous food web.