Seasonal variability of plankton blooms in the Ria de Ferrol (NW Spain): I. Nutrient concentrations and nitrogen uptake rates

The frequent plankton blooms in the Ria de Ferrol (Galicia, NW Spain) were analysed by measuring changes in dissolved nutrient concentrations and plankton uptake rates. Four cruises were made during February, May, July and September 2000 to study seasonal differences between blooms. Dissolved inorganic nutrients, oxygen, particulate and dissolved organic carbon and nitrogen, along with chlorophyll-a concentrations were measured in three selected stations representative of inner, middle and outer zones of the ria. In addition, gross uptake of nitrate and ammonium was estimated using 15N-tracers. During February only there were no accumulations of phytoplankton inside the ria, despite the high concentration of nutrients. In contrast, large accumulations of phytoplankton were found during the other three cruises (up to 27.8 mg chlorophyll-a m−3 in September), when nutrient concentrations were much lower; however, inorganic nutrients were never exhausted in surface waters. Values of the inorganic nitrogen-to-phosphate ratio <10 during blooms in the surface during spring and summer suggest that the nitrogen supply control bloom development. In this way, nitrate uptake dominated in February (mean±SE, f-ratio=0.76±0.01) and July (0.64±0.05), while ammonium uptake dominated in May (0.25±0.07) and September (0.42±0.05). Due to the low input of continental water during the study, nutrient dynamics and fluxes were largely dependent on shelf-ria exchanges and in situ regeneration, which helped to sustain a relatively large biomass of plankton in absence of direct effects of the coastal upwelling. The succession of phytoplankton blooms in the Ria de Ferrol produces a progressive transformation of nitrogen from inorganic to organic forms (both particulate and dissolved) from winter to late summer. Such accumulations of organic matter would maintain remineralisation rates matching plankton uptake, with the result of no net accumulation of inorganic nutrients inside the ria.