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Noctiluca scintillans , a heterotrophic dinoflagellate responsible for most of the red tides in Hong Kong waters was investigated to determine the influence of biotic and abiotic factors in determining its population dynamics. N. scintillans first occurred in January when temperature was ~ 17 °C, and reached a maximum of 1980 cells L − 1 one month later. N. scintillans interacted with trophic compartments from picoplankton to mesozooplankton, but its population development was largely related to the availability of phytoplankton, especially diatoms. Growth rates in microcosms (with mainly biological factors) ranged from − 0.38 to 1.11 d − 1 and were similar to the field in situ growth rates (physical and biological factors). Thus, optimum hydrographical conditions (temperature and water stability), and a rich food supply were necessary for N. scintillans to bloom. Its spatial distribution pattern was a result of biological and physical coupling, but mainly controlled by physical accumulation processes such as winds, tides and currents. [ABSTRACT FROM AUTHOR]

Phytoplankton uptake rates of ammonium (NH), nitrate (NO), and urea were measured at various depths (light levels) in Hong Kong waters during the summer of 2008 using N 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. NH and urea made up generally ≥50% of the total N taken up and the f ratio averaged 0.26. Even at the river-impacted site where concentrations of NO were >20 μM N, NH comprised >60% of the total N uptake. Inhibition experiments demonstrated that NO uptake rates were reduced by 40% when NH was >5 μM 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 NO) was transported offshore. These results suggest that, at the beginning of summer, inshore algal blooms are fueled primarily by NH and urea, rather than NO, from the Pearl River discharge. When NH and urea are depleted, then NO is taken up and can increase the magnitude of the bloom. [ABSTRACT FROM AUTHOR]

Abstract: A monthly comparative study of mesozooplankton biomass and composition between a eutrophic Pearl River estuarine site (WE) and a mesotrophic coastal-oceanic site (EO) in Hong Kong waters was conducted to examine the response of mesozooplankton to nutrient-rich riverine discharge. Although the annual average mesozooplankton biomass was higher at WE than at EO, they were not statistically significant. Variations of mesozooplankton biomass at both stations followed similar trends of Chl a concentrations, with the exception of July at WE where mesozooplankton biomass was low but total Chl a was high. This mismatch may be due to the high flushing effect of the Pearl River discharge in summer and a time lag in mesozooplankton population growth. On the other hand, the composition of mesozooplankton was significantly modified by riverine discharge and eutrophication conditions. While small copepods dominated at both sites, the eutrophic estuarine water had a high abundance of barnacle and polychaete larvae, while cladocerans, bivalve larvae, gastropod larvae and chaetognaths mainly occurred at EO. Eutrophication increased the top-down role of copepods in the grazing community, revealed by an increase in the percentage of copepods in the total metazoan mesozooplankton, especially during the period of high river discharge. Moreover, mesozooplankton diversity at the two stations was similar, and they both showed relatively higher diversity during autumn and winter and lower diversity during summer, especially at WE. These results suggest that, despite high nutrient and Chl a concentrations in estuarine waters, mesozooplankton biomass were not enhanced compared to coastal waters with no river impact, possibly due to poor food quality and increased predation in the eutrophic estuarine waters. [Copyright &y& Elsevier]