Bacterial processes in the intermediate and deep layers of the Ionian Sea in winter 1999: Vertical profiles and their relationship to the different water masses

Abstract

Dissolved and particulate organic carbon, bacterial biomass, microbial enzymatic activities (EEA: leucine aminopeptidase, β‐glucosidase, and alkaline phosphatase), bacterial production, respiration rates, and bacterial growth efficiency were determined in 10 stations of the Ionian Sea (winter 1998–1999) with the aim of characterizing the recycling of biogenic carbon and phosphorus in the different water masses, previously identified on the basis of their hydrographical properties. All microbial activities decreased markedly with depth, with a sharp increase in the benthic boundary layer, where potential remineralization rates of phosphorus up to 1.03 μg P·dm−3d−1 and bacterial carbon production of 0.078 μg C·dm−3 d−1 were recorded. Those rates were close to the surface ones; the bacterial growth efficiency was also around 20%, similar to the surface value, sustaining the microbial food chain at the bottom. The daily hydrolysis of the organic carbon pool estimated by EEA varied from 0.67% (Ionian Surface Water) to 0.02% (Deep Water). Alkaline phosphatase activity was generally low in the intermediate and deep layers, in relation to the higher inorganic P content. The last facts support the hypothesis that deep waters of Ionian Sea, and in general of the entire Mediterranean basin, because of their young age, carry a larger amount of labile dissolved organic carbon, which reduces the need for a high recycling activity by bacterial community. As a matter of fact, a relatively higher activity per cell in carbon production rates was found in the deep layer where a large volume of the very recently formed Cretan Sea Outflow Water was present.