Inshore–offshore differences in seasonal variations of phytoplankton assemblages: the case of a Galician Ria Alta (Ria de A Coruña) and its adjacent shelf (NW of Spain)

Differences in phytoplankton assemblages were studied along a transect inshore–offshore in the Ria de A Coruña (NW Spain) and the adjacent continental shelf from April 1994 to April 1995. Samples were obtained monthly at three stations, two in the ria (one in the centre and the other in the inner harbour area), and one in the open shelf waters outside the ria. Observations were grouped into five main oceanographic stages, according to hydrography, dissolved nutrients and biological characteristics. Inshore differences between both stations of the ria were only quantitative. Phytoplankton biomass was much higher in the harbour area compared to biomass in the centre of the ria. These differences were due to accumulation processes, since nutrient concentrations, hydrography and phytoplankton composition were similar. Characteristics of oceanographic periods revealed some inshore–offshore differences. Spring bloom was very important inshore, while it was weak in the adjacent shelf. However, upwelling processes were persistent offshore, and seemed to be of less importance inshore. Nevertheless, if we take into account all information available for the shelf (since 1989 up to present), we can conclude that the spring bloom is a general characteristic of the shelf (except for 1994 when the spring bloom was very weak), with values of phytoplankton biomass in the range of that observed in the centre of the ria. However, the development of phytoplankton spring blooms in the shelf is related to the increase in surface irradiance, whilst in the ria it was mainly due to saline stratification. According to our results, the Ria de A Coruña can be considered as a bay with a large oceanic influence, except for the enclosed inner harbour zone where water turnover is very low, resulting in a retention of phytoplankton in the area. Finally, the study of microflagellate biomass can be a useful tool to discriminate between the shelf and the ria, being the contribution to phytoplankton of this component which is significantly higher in the former.