MEPS 214:25-41 (2001)  -  doi:10.3354/meps214025

ABSTRACT: An extensive survey of nitrogen forms and phytoplankton of the euphotic zone on the northern Spanish shelf was made in March 1992. Dissolved inorganic nitrogen (DIN) included nitrate, nitrite and ammonium, which were determined by colorimetric analysis. Dissolved organic nitrogen (DON) was determined by persulfate oxidation of filtered seawater. In addition, particulate nitrogen, chlorophyll a, sestonic proteins and primary production measurements were carried out at selected stations. Surface waters in the western region (Galicia) were characterized by the presence of Eastern North Atlantic Central Water of subtropical origin (ENACW_t). This water mass extended over the southern Bay of Biscay (Mar Cantábrico) where large phytoplankton accumulations occurred. Chlorophyll a concentration and primary production rates measured in the Mar Cantábrico were significantly higher than in Galicia. DON reached concentrations of up to 10 mmol m^–3 in phytoplankton rich areas, particularly in the eastern part of the southern Bay of Biscay, where primary production was also high. Positive correlations were found between DON and particulate nitrogen, and between chlorophyll and primary production, while a negative correlation was found between DON and DIN. On average, dissolved nitrogen was ca 95% of total nitrogen, and DON formed from 52% (Galicia) to 61% of total nitrogen (Mar Cantábrico). The average concentration of DON in the Mar Cantábrico (7.82 mmol N m^–3) was almost double that of Galicia. Using salinity as a conservative tracer, DON was partitioned and the excess DON (DON_excess) compared to the expected concentration resulting from mixing between continental water and ENACW_t at each depth was computed. We found lower correlation values between DON_excess and phytoplankton biomass and production than between total DON and these variables, which suggests an uncoupling between phytoplankton and DON production and the participation of microplanktonic grazers in the release of DON in surface waters. However, considering nitrogen stocks integrated in the euphotic zone in each region, DON_excess accounted on average for only 7% of total nitrogen in Galicia, while in the phytoplanktonrich Mar Cantábrico it accounted for 26%. This suggests that phytoplankton spring blooms are one of the main sources of organic matter in this pelagic ecosystem.

KEY WORDS: Nitrogen speciation · Dissolved organic matter · Phytoplankton · Primary production · Northern Spain