Microbial availability and degradation of dissolved organic carbon and nitrogen in two coastal areas

Microbial availability and degradation rates of dissolved organic carbon (DOC) and nitrogen (DON) were estimated at two coastal stations (Horsens Fjord and Darss Sill) in Denmark, by measuring the decrease in DOC and DON concentrations during long-term laboratory incubations (150 days). The experiments used two different treatments: one only receiving a microbial inoculum and another additionally to the inoculum, carbon and phosphate to ensure nitrogen limitation. The additions of carbon and phosphate led to increased DON bioavailability in all experiments. The incubations showed that bioavailable DOC (BDOC) accounted for 22 ± 13% of total DOC in Horsens Fjord and 14 ± 5% at Darss Sill. Bioavailable DON (BDON) accounted for 43 ± 10% (Horsens Fjord) and 28 ± 12% of DON (Darss Sill). The linear relations between BDOM and DOM suggested that the DOC variations in Horsens Fjord were controlled by the available fraction, while this was only partly the case for DOC at Darss Sill and DON (both stations), showing that the refractory pool to some degree controlled the seasonal variations in DOM at these coastal stations. Additionally we found that DOC and DON were cycled at approximately the same speed, probably due to a high carbon demand of the microbial community. Calculating the amounts of DON degraded within the two areas using the obtained decay rates showed that compared with the ambient inorganic nitrogen levels BDON contained a large proportion (52 ± 37%, Horsens Fjord and 74 ± 19%, Darss Sill) of the bioavailable nitrogen (BDON + DIN). These calculations further suggested that bioavailable DOM was washed out of the respective areas and could contribute to heterotrophic growth in adjacent waters.