Organic matter production during late summer–winter period in a temperate sea

The quantity and quality of fresh organic matter (OM) formed by primary production in relation to phytoplankton community structure was calculated for the late summer–winter 2009/2010 in the northern Adriatic Sea (Mediterranean). Phytoplankton species, as a direct measure of fresh OM, chlorophyll a, the OM pool (DOC and POC) and lipids, including classes, were analyzed. Data for temperature, salinity and nutrients enabled deeper insight into the conditions that promoted fresh OM production and associated processes at two stations of different trophic status. Phytoplankton growth was controlled by bottom regenerated nutrients and to lesser extent by riverine nutrients. The phytoplankton community was mainly dominated by nanoplankton. Species of moderate carbon content Chaetoceros compressus, Asterionellopsis glacialis, Leptocylindrus danicus and Bacteriastrum jadranum dominated the microplankton fraction. Availability of orthophosphates was the key factor influencing fresh OM production. POC varied from 37–522 μg l−1. Freshphyto POC, i.e. carbon fixed in phytoplankton cells, contributed 7–79% to the POC pool. The DOC (890–1560 μg l−1) level decreased during the investigated period. Calculation of fresh DOC, i.e. carbon fixed during primary production and released as dissolved OM, revealed it as a minor part (0–2%) of the DOC pool. Lipid concentrations varied from 9.9–55.0 μg l−1 and 20.0–40.2 μg l−1 in the particulate and dissolved fractions, respectively. Nutrient limitation caused increased synthesis of lipids, among which energy reserve lipids triacylglycerols, which are further immobilized for the construction of glycolipids with increasing depletion of orthophosphates.