Inorganic and Organic Nitrogen Uptake by Phytoplankton and Bacteria in Hong Kong Waters

Measurements of uptake rates of inorganic (NO3 − and NH4 + ) and organic (urea, glycine, and glutamic acid) N, and indirect estimates of total N uptake by bacteria, were made in four contrasting environments in sub-tropical Hong Kong waters in summer of 2008. In addition, the effects of several days of rain on N uptake rates were studied in eastern waters. Although ambient NO3 − was the dominant form of N in Hong Kong waters, the dominant N form taken up by phytoplankton was usually NH4 + and organic N, including urea and amino acids, rather than NO3 − . Hence, because of the low NO3 − uptake, there was a long turnover time for NO3 − (100 days), and most of the NO3 − was apparently transported offshore into deeper shelf waters. In eastern waters where NH4 + was undetectable, NO3 − uptake rates were positively correlated with phytoplankton cell size. In contrast, potential rates of glutamic acid uptake were negatively correlated with phytoplankton size. N uptake rates in the smaller size fraction (0.7-2.8 μm) were less affected by the rain event, and smaller phytoplankton appeared to outcompete larger cells after several days of rain. The surface (PN)-specific N uptake rates in the >8-μm fraction decreased from 0.02 to 0.0001 h −1 , while the smaller fraction only exhibited a one- to threefold decrease after the rainfall. In contrast, bacterial production and N uptake were not affected by the rain event, and bacteria N uptake accounted for 10- 60% of the total N uptake by phytoplankton.