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 planktonic food web of the southern part of the Gulf of Riga, the Baltic Sea. The model was based on data from three field campaigns (3–12 May 1995; 28 June–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 in 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 efficiencies (6%) in spring and autumn. During these periods large decreases in DOC were anticipated to occur due to bacterial respiration. In summer the model suggested that bacterial respiration rate was low and that the bacterial growth efficiency was high (60%). During this period a net accumulation 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 sedimentation rates of particulate organic carbon (POC) were consistently low (<3% d−1), removing 13–29% of GPP daily. The sedimented material consisted mainly of amorphous detritus. This indicates that in spite of the relatively high 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.