Inorganic and Organic Nitrogen Uptake by Phytoplankton and Bacteria in Hong Kong Waters |
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| Authors: | Xiangcheng Yuan Patricia M Glibert Jie Xu Hao Liu Mianrun Chen Hongbin Liu Kedong Yin and Paul J Harrison |
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| Institution: | (1) State Key Laboratory of Tropical Oceanography, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou, China;(2) Division of Environment, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China;(3) Horn Point Laboratory, University of Maryland Center for Environmental Science, PO Box 775, Cambridge, MD 21613, USA;(4) School of Marine Sciences, Zhong Shan (Sun Yat-Sen) University, Guangzhou, China |
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| Abstract: | 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. |
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