Decreased production of surface-active organic substances as a consequence of the oligotrophication in the northern Adriatic Sea

Abstract

The dynamics of surface-active substances (SAS) in the northern Adriatic Sea were studied based on monthly sampling from 1998 to 2010. The data from two stations on the eastern and the western part of the northern Adriatic basin, differing significantly in terms of their trophic status, are presented and discussed. As a consequence of oligotrophication the concentration of SAS, primarily produced by phytoplankton, decreased lately (period 2005–2010) in the upper water column. A higher decrease was observed at a site further away from the influence of freshwater, implying that oligotrophic regions are much more sensible to the environmental changes. Besides the change in SAS concentration, the changes in its distribution between the dissolved and the particulate fraction and their acidity characteristics along the water column have been also observed in the recent period. We suggest that production of SAS shifted to deeper layers. This probably due to the phytoplankton depth increasing in response to the changed vertical distribution of nutrients. This implies increased importance of regenerated nutrients near the bottom for the phytoplankton blooming lately. In the period 2005–2010 pico- and nano-phytoplankton might significantly contribute to new SAS production near the bottom during summer. The observed changes of SAS may result in the changes of biogeochemical cycling of macro- and microconstituents of sea.