Primary production and phytoplankton in three Galician Rias Altas (NW Spain): seasonal and spatial variability

Abstract

Phytoplankton species composition, chlorophyll-a concentrations and primary production were studied in the Rias of Ares, Ferrol and the Bay of La Coruña (Galicia, NW Spain) in three oceanographic situations during 1993. The observations were representative of maxima and minima phytoplankton biomass and production values in this area during the seasonal cycle. A phytoplankton bloom studied in March, was characterized by a patchy distribution of chlorophyll and high primary production rates, especially near La Coruña and in the Ria de Ferrol. High phytoplankton biomass and production was also recorded in August, when an upwelling pulse allowed for enhanced growth. Diatoms were very abundant during these blooms but while Chaetoceros socialis was the dominant species in spring other species dominated in summer, when dinoflagellates and flagellates increased. In contrast, the phytoplankton observed in December had low abundances, and correspondingly low chlorophyll concentrations and primary production rates through the study area. Photosynthetic measurements at selected stations using P-I curves indicated that phytoplankton populations were well adapted to the existing light regime. The main source of variation in photosynthetic parameters was seasonal, and no significant differences resulted between rias. Using these parameters and the measured vertical profiles of chlorophyll and irradiance, areal estimations of primary production rates were made for the entire area studied. The resulting values were scaled to the rates measured using parallel simulated in situ incubations of phytoplankton. Maximum primary production values during blooms exceeded 2400 mg C m^-2 d^-1, whereas values estimated for the winter cruise were lower than 20 mg C m^-2 d^-1. These values are equivalent to those available for the nearby Rias Baixas, but because of the larger size of the latter the annual values of the studied Rias Altas were less than 10% of the reported primary production in the most productive Galician ria (Ria de Arousa). This study provides the first estimates of up-to-date primary production values for the Rias Altas, and points out the differential role of upwelling in the nearby shelf compared to its effects on the Rias Baixas region. Upwelling events occur in localized areas outside the Rias Altas, and their associated water dynamics may cause a large export of the produced organic matter to outer shelf areas. In contast, upwelling has been detected well inside most Rias Baixas, inducing high accumulations of recently produced organic matter, that are exported to nearby coastal areas.