Land-based dissolved organic nitrogen dynamics and bioavailability in Jiaozhou Bay, China

Dissolved organic matter (DOM) samples from forestry, livestock breeding, urban domestic and agricultural sources around Jiaozhou Bay were incubated in seawater to examine DOM dynamics and bioavailability, which is the availability of DOM to degradation by microbes. The fluorescence signals of protein- and humic-like compounds were used as a proxy for bioavailable and refractory materials, respectively, whereas the evolution of SUVA(254), dissolved organic nitrogen (DON) and dissolved organic carbon (DOC) concentrations were used to discuss bioavailability. The high bioavailability DON sources were from livestock breeding and urban domestic sources, with no significant difference, followed by agricultural and forestry sources, with respective degradation rate constants of 0.36 +/- 0.01 (mean +/- SD) d(-1), 0.32 +/- 0.03 d(-1), 0.22 +/- 0.04 d(-1), and 0.18 +/- 0.01 d(-1), and with bioavailable DON (BDON%) representing 70% +/- 10%, 67% +/- 7%, 62% +/- 8% and 37% +/- 12% of the total DON, respectively. The turnover times of BDON were 3-6 days, far less than the flushing time of Jiaozhou Bay. Approximately 66% of land-based DON may contribute to DIN by microbial remineralization in Jiaozhou Bay, with the main sources being urban domestic, agricultural and livestock breeding. DON seemed to exhibit higher bioavailability than DOC. The temporal evolution of fluorophores and optical indices revealed that bioavailable DOM from livestock breeding and urban domestic sources was high. Spectroscopic properties indicated that the degree of humification of the organic molecules changed significantly for DON from livestock breeding and urban domestic sources, followed by agricultural sources, and forestry sources were the lowest. SUVA(254) values and HIX were negatively correlated with DON bioavailability, which can be used as a refractory index and for predicting bioavailable DON.