In the Northern Adriatic Sea, marine snow dry mass varied between 3.4 and 9.1 mg 1-' during summer. In situ measurements of 02-fluxes mediated by marine snow revealed a gross primary production (GPP) ranging from 3 to 23 pg 0 2 mg-' (marine snow ash-free dry wt, AFDW) h-' 0 2 -consumption rates ranged from 1.35 to 3.7 pg O2 mg-' (marine snow AFDW) h-' and were ca 5 times the mean consumption obtained under laboratory conditions. Based on these 02-flux measurements, we eshmate that more than 90 % of the pelagic GPP is bound to marine snow aggregations and ca 70 % of the mineralization activity is mediated by marine snow associated microbes during summer. Total carbohydrate (TCHO) content in marine snow was found to be enriched by a factor of 304 as compared to the surrounding water (F = 0.82 &g TCHO ml-l). Batch cultures with marine snow were designed to investigate the influence of marine snow on the free-living microbial community. In the chambers containing marine snow, growth rates (v) of the bacterial community were 3 times the rates obtained for unenriched media (containing 1 pm filtered seawater only). Based on the batch culture experiment, it is concluded that marine snow enhances not only microbial growth on/in the organic matrix of the aggregations but also favors microbial activity of the surrounding water The degree of development of the microheterotrophic food chain was quantified using the ratio (nanoflagellate biomass/bacterial biomass) X 100. In the incubation media containing marine snow, ratios o! >l00 were maintained most of the time, indicating that additional food sources other than free-living bacteria were exploited by the nanoflagellate population. It is suggested that more sensit~ve sampling methods should be used to resolve such nutrient patches which cause heterogeneity of the pelagic environment.

Decomposition and microbial utilization of leaf material was investigated in micro-and mesocosm experiments using freshly sloughed leaves of a common Mediterranean seagrass species (Cymodocea nodosa). In the initial phase of decon~position, dssolved monomeric carbohydrates (MCHO) leached out of the material provolung a rapid (within 40 h) response of the free-living bacterial community. When ablotic leaching had ceased after ca 180 h, 13 mg MCHO-C g-' (leaf dry wt) had been lost from the material, 92 % of which was taken up by microorganisms. At the advanced stage of decomposition, a nch nanoflagellate community had developed and after 14 d of incubation a freeliving flagellate standing stock of 210 pg C per g (leaf dry wt) was still Living at the expense of decomposing leaf material. In a long-term experiment uslng htter bags, the microbial colonization pattern and the breakdown of leaf tissue were followed under reduced influence of macrofaunal shredders and physical forces. The initially uniform bacterial coverage on leaf surfaces, as followed by SEM, changed within 14 d to a 7 times more abundant and heterogenous bacterial assemblage. After more than 3 wk, leaf surfaces began to break down as indicated by crevices densely surrounded by bacteria. At the same time a protozoan community, mainly consisting of monads and choanoflagellates. developed on leaf blades, reaching a maximum density of 2.4 x 105 cells cm-' after 2 mo of degradation. Specific loss rates of weight, particulate organic carbon and nitrogen from litterbags were highest during the first 2 mo and declined thereafter to rates 5 times lower. Only 50 % of the original dry wt and organic carbon remained after 7 to 8 mo of incubation. Measurements of the O2 consumption associated with decaying leaf material indicated that 40 O/ O of the calculated C-mineralization rate can b e attributed to the decreasing organic C-concentration in the plant litter during the first 2 mo. In a later stage of decomposition, only 4 % of the O2 consumption could b e matched by the organic C-loss of the decomposing material; dissolved organic material also in the surroundmg water is proposed to be utilized by the attached microbes. It is concluded that the soluble fraction leaching out of the material in the early phase of decay is rapidly used by both the free-living and the attached bacterial comn~unity, thus supporting a microbial food web up to the protozoan level. It is suggested that most of the residual fraction of organic carbon of leaf debris is released more slowly into the water after hydrolysis by attached microbes thus indicating a loose hydrolysis-uptake coupling. Under exclusion of shredders and reduction of physical forces degradation was not complete after 231 d of incubation. Leaf debris of C. nodosa is suggested to have a small but significant impact on microbial secondary production in the study area largely vla its leachates.

Bacterial transformation of leucine into high molecular weight dissolved organic matter (HMW-DOM) material was studied in seawater cultures. Three different types of cultures were used: sodium azide poisoned cultures served as control, eukaryotic inhibitors to prevent bacterivory, and untreated cultures were used to determine the effect of grazing on bactena on the release of HMW-DOM. All these cultures were labeled wlth I4C-leucine and the distribution of incorporated label into particulate organic matter (POM) and I4C recovered in dissolved organic matter (DOM) was measured over a period of 10 d as well as the transformation into HMW-DOM. Fractionation of DOM was performed using dialysis membranes (molecular weight cutoff 50 000 daltons) and by gel filtration for substances with a molecular weight below l500 daltons. There was a shift to HMW-DOM discernible in grazed and ungrazed cultures and this DOM fraction accumulated to ca 6 % of the initially added leucine in the grazed culture within 8 d. Flagellates appeared to have only a minor influence on the formation of HMW-DOM but might be important in consuming bacterially derived HMW-DOM, representing a novel, nonpredatory way of carbon and energy transfer from bacteria to flagellates.

Die1 variations in bacterial density, frequency of bvidlng cells (FDC) and bssolved organic carbon (DOC) in the stratified water column of the Gulf of Trieste were investigated at various depths. In the surface layers morning and afternoon DOC peaks (up to 10 mg I-') were observed in 3 out of 4 diel cycles. Bacterial abundance remained fairly constant over the diel cycles; however, bacterial activity as measured by FDC showed pronounced peaks in late afternoon and dawn coinciding with the DOC maximum concentrations. The pycnocline layer exhibited DOC concentrations similar to those of the overlying waters. The frequency of dividing cells (FDC), however, remained high throughout the entire diel cycle. The observed pronounced diel variations in bacterial production were therefore largely restricted to the layers well above the pycnocline; bacterial production contributed less than 20 % to the overall diel production (500 to 1000 pg C 1-' d-') in the uppermost 5 m water body. Highest diel production was found in the pycnocline layer at the end of the phytoplankton bloom (June and July) probably reflecting the formation of nutrient-enriched microzones around decaying phytoplankton cells due to reduced sinking velocities in the pycnocline layer. Additionally, FDC-based bacterial secondary production estimates were compared with in situ incubations of dlalysis bags filled with 2 pm filtered seawater for 24 h. Bacterial biomass product~on estimates based on FDC for the 5 and 10 m layer were 3 and 14 times higher, respectively. The observed variations of microbial parameters within different layers point to the importance of small-scale investigations in both time and space.

Benthic 02-fluxes of sublittoral sediments were measured in situ using continuous recording by polarographlc 02-sensors. Three stations (7, 15, and 22 m deep) were investigated at ca 2 mo intervals in the Northern Adriatic Sea over a 19 mo period High chl a contents of the sediments were found in spring (March. April) and fall (September, October) while sediment bacterial numbers fluctuated in a n inconsistent pattern. Porewater dissolved organic carbon (DOC) exhibited strong seasonal variation with highest concentrations in September (20 mg C I-'); subsequently porewater D O C declined to ca 5 m g C I-' (March-April). In general, porewater DOC increased slightly with depth down to the 10-15 cm horizon. Benthic respiration was found to b e temperature dependent; below 10°C about 6 m g C m-' h-' were respired while at 20°C the rate was l? mg C m-' h-' Microphytobenthos gross primary production (GPP) was detected even at the deepest station and remained fairly constant from March to September (ca 100 mg C m-2 d-'1. Total community metabolism calculated over 24 h revealed net community production in spring for the shallowest station only. Highest net system consumption rates were obtained in September for all 3 stations ranging from 220 to 520 mg C m-' (24 h)-' caused by increased heterotrophic activity a s indicated by high night-time respiration rates. Calculated 02-consumption of the sediment and the water column below the thermocline indicates that subthermocline water column resp~ration was the principal cause for near-bottom hypoxia rather than sediment oxygen demand if the thermocline was at least 2 m above bottom. At a thermocline depth of 1.5 m above bottom, sediment 02-uptake and subthermocline water column respiration were equally important.