Dynamics of dissolved organic carbon in a coastal ecosystem

Abstract

In the Bothnian Sea, there was a marked seasonal variation of dissolved organic C (DOC) in 1990–1992, with a large increase in DOC concentrations in summer at two stations. The accumulation of DOC at the coastal station persisted for 5 months, reaching peak values 24–31% above the mean winter value (288 µ M). At the offshore station DOC concentrations were elevated throughout the water column in July, reaching 14% above the mean winter value (291 µ M). The DOC concentration at the coastal station was significantly correlated to water flow in an adjacent river, suggesting that the source of the summer DOC increase was largely explained by riverine input. Bioassays indicated that a large portion (22–99%) of the introduced DOC was degradable by bacteria after inorganic nutrients were added. A negative correlation between DOC and phosphate concentration was also found, suggesting that the system was P deficient in summer. The accumulation of DOC in summer was thus possibly caused by slow bacterial degradation due to phosphate deficiency and transient accumulation of refractory DOC. An annual C balance at the coastal station indicated an insufficient supply of C from phytoplankton production to support the C demand of the system; at the offshore station the budget was close to balanced. The results suggest that riverine DOC had a major impact on coastal DOC dynamics and that it was partly used in the microbial food web in the bay.