Factors Controlling Accumulation of Labile Dissolved Organic Carbon in the Gulf of Riga

Abstract

The bioavailability of dissolved organic carbon (DOC) was investigated in the Gulf of Riga during spring and summer 1996. The Gulf of Riga is a sub-basin of the southern Baltic proper, characterized by low salinity (2–7) and a riverine discharge amounting to 7% of the total volume per year. Samples were collected in the northern and southeast part of the Gulf of Riga in transects extending 5–13km offshore. During summer, DOC concentrations increased by 70–600μmC indicating an imbalance between input and degradation of organic carbon in the system. The pool size of labile DOC (L-DOC) was determined in predator-free bacterial seawater cultures enriched with nitrogen and phosphorus by following the decline in DOC during bacterial growth. L-DOC made up 30–100% of the DOC that accumulated during summer and the pool increased 2–4-fold compared with spring. Bacterial growth rate limitations were studied using incubations of unfiltered seawater enriched with nutrients or subjected to increased temperature. Nutrient additions had little or no effect on the community turnover time even when glucose, ammonium and phosphate were added together, while a 10°C shift-up in temperature increased the turnover time 10-fold. This indicates that the bacteria were growing at maximum growth rate at thein situtemperature. The results suggest that the underlying mechanism for accumulation of L-DOC was growth rate limitation of the bacterial community caused by lowin situtemperature in combination with control by predators.