Nitrogen Sources and Rates of Phytoplankton Uptake in Different Regions of Hong Kong Waters in Summer

Phytoplankton uptake rates of ammonium (NH4 (+)), nitrate (NO3 (-)), and urea were measured at various depths (light levels) in Hong Kong waters during the summer of 2008 using N-15 tracer techniques in order to determine which form of nitrogen (N) supported algal growth. Four regions were sampled, two differentially impacted by Pearl River discharge, one impacted by Hong Kong sewage discharge, and a site beyond these influences. Spatial differences in nutrient concentrations, ratios, and phytoplankton biomass were large. Dissolved nutrient ratios suggested phosphorus (P) limitation throughout the region, largely driven by high N loading from the Pearl River in summer. NH4 (+) and urea made up generally a parts per thousand yen50% of the total N taken up and the f ratio averaged 0.26. Even at the river-impacted site where concentrations of NO3 (-) were > 20 mu M N, NH4 (+) comprised > 60% of the total N uptake. Inhibition experiments demonstrated that NO3 (-) uptake rates were reduced by 40% when NH4 (+) was > 5 mu M N. The relationship between the total specific uptake rates of N (sum of all measured substrates, V, per hour) and the chlorophyll a-specific rates (micromolars of N per microgram of Chl a per hour) varied spatially with phytoplankton biomass. Highest uptake rates and biomass were observed in southern waters, suggesting that P limitation and other factors (i.e., flushing rate) controlled production inshore and that the unincorporated N (mainly NO3 (-)) was transported offshore. These results suggest that, at the beginning of summer, inshore algal blooms are fueled primarily by NH4 (+) and urea, rather than NO3 (-), from the Pearl River discharge. When NH4 (+) and urea are depleted, then NO3 (-) is taken up and can increase the magnitude of the bloom.