Oceanography of the Gulf of Artabro (northwestern Iberian Peninsula) in early spring: General patterns

The COPLA 393 cruise was carried out from 22-29 March 1993 on the continental shelf located between the Sisargas Islands and Cape Prior (northwest Spain). Forty-one stations distributed into five sections perpendicular to the coast were sampled, with the aim of investigating spring conditions in the Gulf of Artabro. Salinity and temperature profiles were obtained, and nutrients, chlorophyll, primary production, light, specific composition and abundance of plankton (including bacteria and phytoflagellates) were determined. The present study provides the first survey of conditions prevailing at the beginning of the spring in the entire Gulf of Artabro. The hydrographic situation is relatively complex, with a quite well defined thermic-saline front perpendicular to the coastline in the central part of the Gulf of Artabro. The water west of the frontal area was more saline (13.2 °C; 35.85 psu), whereas east of the front colder and less saline waters prevailed (12.8 °C; 35.75 psu). When the cruise was carried out, the primary production (35-155 mg C m‐2 h‐1) and chlorophyll a concentration (< 2 mg m‐3) were relatively low and a dispersion of the phytoplankton communities was observed. Mean biomass of the microzooplankton and the mesozooplankton were typical of winter (8.8 and 23 mg dw, respectively). Bacteria (1.5 � 105 cel ml‐1) and microflagellates (phytoflagellate autotrophs: 790 cel ml‐1 and heterotrophs: 960 ml‐1) also had low abundances. The atmospheric conditions, which favoured an early spring bloom in February, had changed. There were areas with phytoplankton typical of winter, with predominance of microflagellates, cryptophyceae and small dinoflagellates, and other areas with low abundance of phytoplankton but with a high specific diversity. The latter were the remains of the spring bloom, which had occurred some days before the beginning of the cruise. Therefore, we suggest that the bloom-dispersion regime may be a common feature, existing normally in this gulf, as a consequence of the occurrence of early blooms in winter and the varying environmental conditions in the February-March period.