MEPS 269:1-15 (2004)  -  doi:10.3354/meps269001

ABSTRACT: Microbial distribution and activities were examined in relation to the hydrodynamic conditions in the Almeria-Oran frontal area (AOF), SW Mediterranean Sea, during winter. The main objectives were to explore factors limiting bacterial growth and activities (production, ectoenzymatic activity, utilization and respiration of amino acids) in regard to dissolved organic carbon (DOC) stocks and lability in different parts of the Almeria-Oran front-jet system. The vertical distribution of total chl a, bacterial abundance and production reflected the density profiles, with a deepening of the top of the pycnocline from 25 (front) to 120 m (gyre). At the frontal site, most bacterial production was concentrated within the first 20 m (up to 19.8 nmol C l^-1 h^-1), at the boundary between the jet and Mediterranean waters, whereas bacterial abundance peaked in a wider layer (40 m) in the jet core (up to 15 × 10⁵ bacteria ml^-1). Integrated bacterial production ranged from 5.7 to 17.9 mmol C m^-2 d^-1 in the 0 to 200 m layer in the meander of the Atlantic jet and in its associated anticyclonic gyre, compared to 4.3 to 5.8 mmol C m^-2 d^-1 in typical Mediterranean waters. Heterotrophic bacteria were limited by phosphorus in the surface layers of the front and Mediterranean waters. Shipboard experiments using <0.8 µm filtered seawater incubated in the dark showed a high growth potential of the heterotrophic bacteria within the jet and the jet-gyre boundaries. Increased DOC accumulation (from 12 to 21 µM excess DOC = surface minus deep refractory DOC) occurred at the jet-gyre boundaries. From the jet towards the gyre, the decrease in potential ectoaminopeptidase activity was less than that of bacterial production, indicating that there was a greater dependence on polymeric DOC. Only the typical Mediterranean site, outside of the jet and gyre influences, was clearly net heterotrophic.

KEY WORDS: Winter · Western Mediterranean · Almeria-Oran front · Bacterial production · Ectoaminopeptidase · Respiration · Bacterial limitation · Dissolved organic carbon