Physical-biological coupling in the Algerian Basin (SW Mediterranean): Influence of mesoscale instabilities on the biomass and production of phytoplankton and bacterioplankton

The biomass and production of phytoplankton and bacterioplankton was invest igated in relation to the mesoscale structures found in the Algerian Curren t during the ALGERS'96 cruise (October 1996). Biological determinations wer e carried out in three transects between 0 degrees and 2 degreesE aimed at crossing a so-called event, formed by a coastal anticyclonic eddy associate d with an offshore cyclonic eddy to the west. The concentration of chloroph yll a (Chl) was maximum (> 1.2 mg m(-3)) within the cyclonic eddy and at th e frontal zones between the Modified Atlantic Water (MAW) of the Algerian C urrent and the Mediterranean waters further north. Chi (total and > 2 mum) was significantly correlated with proxies of nutrient flux into the upper l ayers. Autotrophic picoplankton and heterotrophic bacterial abundance and p roduction presented clear differences between MAW and Mediterranean water, with higher values at those stations under the influence of the Algerian Cu rrent. In general, greater differences were observed in production than in biomass variables. The photosynthetic parameters (derived from P-E relation ships) and integrated primary production (range 189-645 mg m(-2) d(-1)) res ponded greatly to the different hydrological conditions. The mesoscale phen omena inducing fertilization caused a 2 to 3-fold increase in primary produ ction rates. The relatively high values found within the cyclonic eddy sugg est that, although short-lived in comparison with anticyclonic eddies, thes e eddies may produce episodic increases of biological production not accoun ted for in previous surveys in the region. (C) 2000 Elsevier Science Ltd. A ll rights reserved.