MICROPLANKTON ASSEMBLAGES ASSOCIATED WITH SALINE FRONTS DURING A SPRING BLOOM IN THE CENTRAL CANTABRIAN SEA - DIFFERENCES IN TROPHIC STRUCTURE BETWEEN WATER BODIES

An intrusion of high salinity water which flowed into the central Cantabrian Sea in spring 1987 produced two fronts, on-shelf and at the outer-shelf. A density gradient from saline origin appeared at coastal stations as a result of the surface circulation pattern characteristic of this season. This feature gave rise to a chain-forming diatom bloom at coastal stations limited by the on-shelf front. The saline water body was dominated by microflagellates, mostly heterotrophs, and was chemically characterized by high nitrite and inorganic nutrient concentrations and low dissolved oxygen values, suggesting the dominance of regeneration over uptake processes. Significant differences between the west and east transect of this water body were detected and related to a different successional state of a post-bloom phase. Microplankton populations at oceanic stations were formed by diatoms, which were from different species to those of coastal waters, and small dinoflagellates. The convergent circulation at the outer-shelf frontal margin favoured passive accumulation of phytoplankton biomass. Our results suggest that a different trophic structure prevailed in each water body. In coastal and oceanic waters primary production would flow through the classical food chain, whereas within the saline water body the microbial loop would be involved in the process of energy transfer.