Phytoplankton dynamics affected by the coastal upwelling events in the Gulf of Finland in July-August 2006

Abstract

The response of phytoplankton to the observed coastal upwelling events in the central Gulf of Finland in July-August 2006 is described on the basis of chlorophyll a and phytoplankton community/biomass data. Due to the nutrient supply, advection, replacement/mixing of water masses and changes in water temperature, not only single phytoplankton species were affected but the entire phytoplankton community changed several times during a 2-month period. A faster response of nanoplanktonic species to the upwelled nutrients and high biomass of photosynthetic ciliate Myrionecta rubra were observed in the upwelling waters. The highest phytoplankton biomass was observed in the area of domed isopycnals in the context of developing downwelling 3 weeks after a relatively weak upwelling near the northern coast of the Gulf of Finland. An increase of biomass was observed also after the relaxation of a major upwelling event near the southern coast. Multi-dimensional scaling analysis matches this relatively high biomass community with the communities observed outside the area of upwelling influence 2 weeks earlier. We suggest that the observed biomass increase could be connected to the relaxation of upwelling and subsequent reversal of surface water mass movement together with the utilization of mostly regenerated but also stored nutrients.