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

Differences in phytoplankton assemblages were studied along a transect insh ore-offshore in the Ria de A Coruna (NW Spain) and the adjacent continental shelf from April 1994 to April 1995. Samples were obtained monthly at thre e stations, two in the ria (one in the centre and the other in the inner ha rbour 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 b etween both stations of the ria were only quantitative. Phytoplankton bioma ss was much higher in the harbour area compared to biomass in the centre of the ria. These differences were due to accumulation processes, since nutri ent concentrations, hydrography and phytoplankton composition were similar. Characteristics of oceanographic periods revealed some inshore-offshore di fferences. Spring bloom was very important inshore, while it was weak in th e adjacent shelf. However, upwelling processes were persistent offshore, an d seemed to be of less importance inshore. Nevertheless, if we take into ac count all information available for the shelf (since 1989 up to present), w e can conclude that the spring bloom is a general characteristic of the she lf (except for 1994 when the spring bloom was very weak), with values of ph ytoplankton biomass in the range of that observed in the centre of the ria. However, the development of phytoplankton spring blooms in the shelf is re lated to the increase in surface irradiance, whilst in the ria it was mainl y due to saline stratification. According to our results, the Ria de A Coru na can be considered as a bay with a large oceanic influence, except for th e enclosed inner harbour zone where water turnover is very low, resulting i n a retention of phytoplankton in the area. Finally, the study of microflag ellate biomass can be a useful tool to discriminate between the shelf and t he ria, being the contribution to phytoplankton of this component which is significantly higher in the former. (C) 2001 Elsevier Science Ltd. All righ ts reserved.