Contribution of urea to occurrence of phytoplankton in the eutrophic coastal environment Uranouchi inlet [Japan]

Urea is considered to be one of the most biologically available form in dissolved organic nitrogen (DON) compounds in marine environments, and it is therefore important to clarify the dynamics of urea pools in order to understand the marine nitrogen cycle properly. In the present study, the distribution of urea and the uptake by phytoplankton in seawater were examined to elucidate the contribution of urea to the growth of phytoplankton in a eutrophic coastal environment, Uranouchi Inlet, Kochi Prefecture in Japan from 2002 to 2004. Urea-N concentration in surface seawater was 0.1-48.6 (average 2.87+-SD 5.58) microM, while dissolved inorganic nitrogen (DIN) concentration was 0.16-22.7 (average 2.26+-SD 2.88) microM, and DON (including urea) concentration was 2.61-185 (average 14.0+-SD 20.4) microM. Urea-N concentration was, on average, 18.0% of dissolved total nitrogen (DTN: DIN+DON), 21.9% of DON, and 114% of DIN. Urea concentrations tended to increase in summer. Urea-N uptake (UP) rates using sup(14)C-urea of phytoplankton, which were calculated by subtraction of bacterial UP under DCMU treatment from total UP, were 2.57-55.1 nmol-N lE-1 hE-1. The contribution of UP by phytoplankton to the total UP in the seawater, was about 96%. Chl a-specific urea uptake rates were 0.58-8.70 nmol-Nmicrog-chl aE-1 hE-1. Photosynthetic rates calculated using sup(14)C-NaHCO3 were 243-5,345 nmol-C lE-1 hE-1. Assuming that the C/N ratio of phytoplankton was 7.8, assimilation rates of total nitrogen calculated from the photosynthetic rates were 31.2-685 nmol-N lE-1 hE-1. The contribution of UP to total nitrogen assimilation by phytoplankton was 2.3-29 (average 10)%. The present results show that urea is distributed at a relatively high concentration level throughout the year in eutrophic seawater and the urea is mainly assimilated and used as a nitrogen source by the phytoplankton communities. Therefore, this suggests that urea significantly contributes to the occurrence of phytoplankton in eutrophic coastal environments.