Some aspects on phytoplanktonic primary production of particulate (POC) and dissolved (DOC) organic carbon in pelagic systems and the importance of phytoplankton-derived DOC in meeting heterotrophic bacterial carbon demand were studied. Filtration is a usually needed step in phytoplankton ecology studies before subsequent treatment of the matter retained on the filter or of the filtrate. The use of different filters had notable effects on picoplanktonic (<2 micro-m) assemblages, i. e. selecting for a sample size-distribution different to the original and changing the heterotrophic bacterial activity in the filtrate, especially in oligotrophic systems. These unavoidable changes could be easily quantified by flow cytometric techniques. The filter used was a critical factor when partitioning of primary production between particulate and dissolved was aimed. Glass-fiber filters were inadequate to separate POC from DOC as they adsorbed a variable fraction of produced DOC, yielding production values closer to total organic carbon (TOC) than to POC, as shown in experiments undertaken under different hydrographic conditions; hence the use of membrane filters was recommended.

The photosynthesis-irradiance (P-E) relationships were studied in relation to well-defined SW Mediterranean mesoscale structures: an association of a cyclonic and an anticyclonic eddy in the Algerian Basin and the quasi-persistent Alboran Sea anticyclonic gyre. In the Algerian Basin only algal POC production was estimated while in the Alboran Sea TOC, POC and DOC production rates were simultaneously measured, allowing for the assessment of the relationship between the fraction of total primary production released in the dissolved form or percent extracellular release (PER) and irradiance within natural communities. PER values increased significantly at low irradiances and with the sampling depth. In both sub-basins hydrographic phenomena causing nutrient inputs in the surfacemost layers had a noticeable effect in planktonic biomass and production rates, with 2- to 3-fold increases in usual values for the Mediterranean.

In two summer cruises carried out in 1995-1996 and 1998 in the Southern Ocean time-course C-14-uptake experiments and compartmental analysis were used to estimate phyoplanktonic POC and DOC production. This method allowed for the correction of the possible bacterial uptake of DOC during the incubation. The rate of dissolved primary production was correlated to the total and particulate ones, but not to chlorophyll a concentration, suggesting that the availability of recently-fixed photosynthate determined the fraction being released extracellularly, involving some active release together with possible passive diffusion. With all data pooled a significant inverse relationship existed between PER and total primary productivity. In offshore Antarctic waters the bacteria-phytoplankton coupling through algal-released products was considerably stronger than previously considered. With bacterial growth efficiencies taken from the literature this source of DOC proved enough to meet bacterioplankton requirements of labile organic carbon.