Phytoplankton Uptake of Nitrate and Ammonium in Different Regions of Southern East China Sea

Marine nitrogen cycle is composed of multiple transformations of nitrogenous compounds, catalyzed primarily by microbes. With anthropogenic activities altering total nutrient loads to the ocean, it has changing the community structures of microorganisms in the ocean. Therefore, it is important to understand the adaptations of different size-fractionated microorganisms to varying nitrogen forms. In this study, the kinetics of nitrate and ammonium uptakes by size-fractionated natural assemblages were measured using ¹⁵N-tracer technique during two cruises in the southern East China Sea. Three regions were sampled, one impacted by coastal water, one impacted by oligotrophic Taiwan Warm Current, and one impacted by upwelling water. In oligotrophic stations, pico- and nanoplankton dominated the phytoplankton assemblages. Significant differences of nitrogen uptake were also found. The potential maximum uptake rates of ammonium were around 50-fold higher than those of nitrate. This result indicated that ammonium should be the major nitrogen source for pico/nanoplankton in oligotrophic region. In contrast, pico/nanoplankton and microplankton were near equal dominated in phytoplankton community in eutrophic regions. Interestingly, there were no significant differences in ammonium maximum uptake rates between pico/nanoplankton and microplankton, but higher maximum uptake rates of nitrate were detected in microplankton than those in pico/nanoplankton. These results suggest that microplankton are not only able to uptake ammonium, but also possess the ability to take up nitrate.