The effect of the freshwater-sea transition on short-term dissolved organic carbon bio-reactivity : the case of Baltic Sea river mouths

Abstract: Terrestrially-derived dissolved organic carbon (DOC) is transported to estuarine systems via river runoff, where it is largely degraded by microbial communities. I studied the combined influence of salinity (NaCl), microbial community (marine vs. freshwater) and inorganic nitrogen (N) and phosphorus (P) availability on DOC degradation rates in 10 major rivers in Sweden sampled during summer. The DOC degradation (inferred from dissolved O2 consumption rates) was determined on filtered river water during one week (7-days) in vitro experimental bioassays, applying the factors alone and in all possible combinations. Compared to the controls, addition of salt (10 psu) caused significant decreases in the DOC degradation in most cases, by 21-49%. In two cases, a significant P limitation was found, and a few additional rivers responded positively to P addition, although the effect was not statistically significant. Additions of N had no significant effect on the DOC degradation. Only one river – Pite älv – showed significant interactions effects between N and P, or N and S, but these effects were relatively small. Interactions between P and salt were found in two cases (Helgeån and Nyköpingsån).These interactions were strong and the results suggest that P additions significantly dampened the salt stress. Tests in which the microbial community was manipulated showed that salt stress was similar for both freshwater and marine microbes. In fact bacteria from the coast outside Malmö and ambient river mouth communities showed a similar negative response to salt additions. Finally, possible salt induced flocculation and particle formation was tested. According to results, there was no particle formation that significantly affected the DOC degradation rates. This study highlights the need for further studies of interactions between nutrient and salt concentrations on the fate of the terrestrially-derived DOC in the marine environment.