Primary producers must respond to the diel changes in light availability. Therefore, detection of diel cycles in bacterial activity would imply tight coupling between the production of photosynthetic dissolved organic carbon (DOC) and its consumption by bacteria. Absence of diel cycles, on the contrary, would indicate that bacteria depend largely upon allochthonous organic carbon and that bacteria are not tightly dependent on photosynthetically produced autochthonous carbon. In 1993 and 1994 we sampled 3 sites in the NW Mediterranean Sea several times a day, and measured several microbial parameters as well as the vertical profiles of DOC along the diel cycle. The sites were selected so that one was on the continental shelf and, thus, was more Influenced by coastal runoff; a second one was over the shelf slope and a third, oceanlc one was located further offshore over a depth of 2000 m. LVe found clear die1 cycles in bacterial total and specific activity always in the oceanic stations and sometimes in the shelf slope stations. Diel changes were detected as changes in both DNA and protein synthesis rates. These diel cycles were accompanied by diel changes in the distribution of total DOC, and by diel changes in the proportion of bacteria containing visible nucleoids. Noon estimates of bactenal activity were more than twice the daily average in the oceanic site, but they were less different in the other 2 sites. DOC chanyed daily by 15 pM (5 to 15% of the total stock) For bacterial activity to explain the diel changes In DOC concentration, bacteria should have growth efficiencies lower than 10 O/o in general, and lower than 2 "h in the oceanic station.
From September 1994 to September 1995 a time-series station in the Ria d e Vigo (NW Spain) was monitored fortnightly. Dissolved organic carbon (DOC) was analysed by high temperature catalytic oxidation. Dissolved organic nitrogen (DON) was determined by the Kjeldahl method, after removal of inorganic nitrogen from the sample. The time courses of DOC and DON changes were parallel. The average C/N molar ratio of dissolved organic matter (DOM) was -15. DOM was strongly influenced by physical and biological processes. During the upwelling season, the entry of DOM-poor Eastern North Atlantic Central Water (ENACW) controlled DOM levels in subsurface waters. Biologically produced DOM excess in surface waters was uncoupled with chlorophyll a on a daily time-scale. A tentative partitioning of DOM during the upwelling season has been inferred from mixing of oceanic and freshwater endmembers. The refractory pool, -70% of total DOC in surface water, was carried by upwelled ENACW (6Oor>, 10%) of which was semi-refractory) and continental water (10%). Net production of semi-labile DOC occurred in the bottom layer (-10 pM C) The average DOC excess in surface waters coinparcd to bottom waters was 21 pM C ,with a C/N molar ratio of 12 The excess was a mixture of labile and semi-labile material wlth a recycling tlme > S d, which represented -23 and -13 %) of the net primary production for C and N respectively. The average DOM excess/[POM (particulate organic matter) + DOM excess] ratio in surface waters was -0.4 and -0.3 for C and N respectively, indicating that POM was the most important pool of organic matter net produced in the inner ria. During the downwelllng season DOM was balanced by the external inputs and the DOM excess in surface waters was due to the freshwater contribut~on.
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