Planktonic carbon and nitrogen cycling off northwest Spain: variations in production of particulate and dissolved organic pools

Abstract

The coupling of carbon and nitrogen fluxes between dissolved and particulate organic pools was examined at a coastal location off Galicia (northwest Spain). Monthly measurements of dis- solved and particulate carbon and nitrogen stocks, along with inorganic carbon and nitrogen uptake and release of dissolved organic substances were obtained between October 1998 and September 1999. Carbon fluxes were determined using incubations of plankton with inorganic 14 C and nitrogen fluxes were determined using incubations of plankton with 15 N-ammonium. Dissolved organic car- bon (DOC) was often released near the surface, while maximum release of dissolved organic nitro- gen (DON) generally occurred near the base of the euphotic zone. Both rates were linearly related to the corresponding uptake rates; however, the percentages of extracellular release relative to total uptake (PER) were highest during low-productivity periods, when both carbon and nitrogen PER exceeded 50% in deep layers. PER values were generally below 20% near the surface. Despite max- imum values of both DOC (0.63 µmol C l -1 h -1 ) and DON release (0.06 µmol N l -1 h -1 ) being measured during spring and summer, volumetric values of both rates were not significantly correlated through the year. Such a differential behaviour caused a mismatch between DOC:DON release ratios and the C:N ratios of seston at discrete depths. In contrast, euphotic-zone integrated release rates showed average DOC:DON ratios reflecting the C:N ratios of seston. These results suggest that most of the organic matter available for microbial plankton near the coast derives from phytoplankton, which in turn is sustained by a rapid recycling of organic matter when external nutrient inputs are reduced.