Across-shelf variability of phytoplankton composition, photosynthetic parameters and primary production in the NW Iberian upwelling system

Abstract

Hydrographic conditions, phytoplankton composition and biomass, photosynthetic parameters and primary production were determined in the Ría de Vigo and adjacent shelf waters during April–May 1997 and September 1998. The sampling was designed to find the seasonal downwelling–upwelling and upwelling–downwelling transition periods characteristic of spring and autumn phytoplankton blooms. There was upwelling relaxation event followed by downwelling during both spring and autumn cruises. Temperature and salinity distributions showed that ría and shelf waters formed two distinct domains, which were separated by a thermohaline front at the mouth of the ría. The phytoplankton composition was completely different in the two environments. Cyanobacteria dominated on the shelf and constituted 46–66% of total phytoplankton biomass, while large phytoplankton (diatoms and dinoflagellates) were more abundant in the ría, especially during upwelling relaxation. However, the high shelf–ría exchange induced by a strong downwelling event on 7 September 1998 removed large phytoplankton (mainly diatoms) from the water column in the ría. Chlorophyll-specific maximum photosynthetic rates ( P m B) were significantly higher in the ría domain during upwelling relaxation, when autotrophic microplankton dominated in the interior. Primary production varied from 0.63 to 2.6 g C m −2 day −1 during the spring cruise and between 0.32 and 2.09 g C m −2 day −1 during the autumn cruise, with the highest values in the ría during both cruises. Primary production was relatively constant on the shelf with no significant differences between cruises, whereas differences were significant in the ría, with higher values during upwelling relaxation periods and lower values during downwelling. Analysis of light saturation parameters and light absorbed by phytoplanton in the water column suggests that photosynthesis was not light-limited either on the shelf or in the ría. It is concluded that upwelling–downwelling cycles were the main driving force, through changes in autotrophic microplankton biomass in the ría, that caused the variability observed in the ría–shelf system.