Abstract
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. I t is suggested that more sensit~ve sampling methods should be used to resolve such nutrient patches which cause heterogeneity of the pelagic environment.
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