Carbon flow patterns in the planktonic food web of the Gulf of Riga, the Baltic Sea: a reconstruction by the inverse method

We used the inverse method to reconstruct a carbon flux model for the plank tonic food web of the southern part of the Gulf of Riga, the Baltic Sea. Th e model was based on data from three field campaigns (3-12 May 1995; 28 Jun e-7 July 1994; 26 August-3 September 1993). The carbon flow model indicated very different channelling of gross primary production (GPP) in the three different periods. In spring 55% of GPP was channelled to detritus, while i n summer 35% was exudated and channelled to dissolved organic carbon (DOC). In autumn, 42% of GPP was channelled to the zooplankton compartments. The model suggested high bacterial respiration rates and low bacterial growth e fficiencies (6%) in spring and autumn. During these periods large decreases in DOC were anticipated to occur due to bacterial respiration. In summer t he model suggested that bacterial respiration rate was low and that the bac terial growth efficiency was high (60%). During this period a net accumulat ion of DOC apparently occurred. Respiration was the most important loss of organic carbon in all seasons, corresponding on average to 152, 59 and 391% of GPP daily in spring, summer and autumn, respectively. Daily sedimentati on rates of particulate organic carbon (POC) were consistently low (< 3% d( -1)), removing 13-29% of GPP daily. The sedimented material consisted mainl y of amorphous detritus. This indicates that in spite of the relatively hig h level of eutrophication, the planktonic system in the Gulf of Riga has a high short-term carbon retention and recycling capacity, and little POC is lost from the upper water column through sedimentation. (C) 1999 Elsevier S cience B.V. All rights reserved.