Hydrography and phytoplankton in an isolated and non-pristine ria area: the A Coruña Harbour (NW Spain)

Abstract

Abstract The hydrography and phytoplankton cycle were examined from April 1994 to April 1995 in the harbour of the Ria of A Coruna, an area subjected to anthropogenic influence. The seasonal changes in the hydrography found elsewhere in Galicia (spring bloom, upwelling, summer stratification, autumn bloom and winter mixing) also occurred in the harbour. However, the phytoplankton cycle presented some local peculiarities. Phytoplankton biomass was very high in all periods (mean chlorophyll ranged from 4.7 μg l–1 in summer to 16.7 μg l–1 in spring, and phytoplankton carbon ranged from 337 μg l–1 during upwelling to 547 μg l–1 in spring), except winter. No significant differences were found among these periods. Sewage from the city of A Coruna and benthic remineralisation in summer (essentially for silicates) are the main source of nutrients in the harbour. Unlike the Rias Bajas, upwelling is a very poor source of nutrients due to the isolated situation of the harbour. The N/P ratio is lower than the Redfield ratio, with nitrogen being the limiting nutrient. As a result of these special hydrological and biogeochemical conditions, the phytoplankton community exhibits a high biomass and is dominated by large diatoms, with a negligible contribution from flagellates except during winter mixing. The anthropogenic origin of the nutrients, together with high phytoplankton biomass and water oxygen super-saturation from high primary production are all indicative of an eutrophication. Nevertheless, anoxic episodes were not observed. In the Galician Rias, eutrophication is a natural consequence of upwelling. However, the A Coruna Harbour is now described as the most important eutrophic area on the Galician coast owing to anthropogenic activities.