Dissolved organic carbon from cultured kelp Saccharina japonica: production, bioavailability, and bacterial degradation rates

Seaweed farming is widely perceived as one of the most environmentally benign types of aquaculture activity. In the past 10 yr, global seaweed production has doubled and reached 31.8 million t. Farmed seaweed also has important functions in local ecosystems. We focus on the production, bioavailability, and bacterial degradation rates of dissolved organic carbon (DOC) from cultured kelp Saccharina japonica. Semi in situ incubations in 2 growing seasons were conducted to estimate DOC production, and laboratory incubations were used to determine bioavailability and decay rates of DOC from cultured kelp. Results showed that DOC production was 6.2-7.0 mg C (g dry wt)-1 d-1 in the growing seasons, and the proportion of DOC in net primary production was 23.7-39.1%. The decomposition rate of DOC was 4 ± 1% d-1 and 9 ± 1% d-1 in January and April, respectively. About 37.8% remained as refractory DOC after 150 d incubation. It was calculated that the total DOC from kelp in Sanggou Bay was approximately 11.3 times of that from phytoplankton for the whole bay (144 km2). Our results suggest that more than half of the bioavailable DOC will be exported out of the bay to potentially support the wider food chain through bacterial uptake. Cultured kelp is therefore an important source of DOC in the embayment, contributes to the coastal DOC pool and provides a potential pathway for carbon dioxide sequestration.