Phytoplankton Biomass and Production in Subtropical Hong Kong Waters: Influence of the Pearl River Outflow

Abstract

The size-fractionated phytoplankton biomass and primary production were investigated in four contrasting areas of Hong Kong waters in 2006. Phytoplankton biomass and production varied seasonally in response to the influence of the Pearl River discharge. In the dry season, the phytoplankton biomass and production were low (<42 mg chl m −2 and <1.8 g C m −2 day −1 ) in all four areas, due to low temperatures and dilution and reduced light availability due to strong vertical mixing. In contrast, in the wet season, in the river-impacted western areas, the phytoplankton biomass and production increased greater than five-fold compared to the dry season, especially in summer. In summer, algal biomass was 15-fold higher than in winter, and the mean integrated primary productivity (IPP) was 9 g C m −2 day −1 in southern waters due to strong stratification, high temperatures, light availability, and nutrient input from the Pearl River estuary. However, in the highly flushed western waters, chl a and IPP were lower (<30 mg m −2 and 4 g C m −2 day −1, respectively) due to dilution. The maximal algal biomass and primary production occurred in southern waters with strong stratification and less flushing. Spring blooms (>10 μg chlaL −1 ) rarely occurred despite the high chl-specific photosynthetic rate (mostly >10 μg C μg chla) as the accumulation of algal biomass was restricted by active physical processes (e.g., strong vertical mixing and freshwater dilution). Phytoplankton biomass and production were mostly dominated by the >5-μm size fraction all year except in eastern waters during spring and mostly composed of fast-growing chain-forming diatoms. In the stratified southern waters in summer, the largest algal blooms occurred in part due to high nutrient inputs from the Pearl River estuary.

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

“…5c), which was much higher than the highest values (∼20%) reported in the mid-Atlantic Bight off southern New Jersey and Chesapeake Bay (Bradley et al 2010a, b), but these values must be interpreted cautiously due to the potential for overestimation. N Uptake by Size-Fractionated Phytoplankton and Effects of a Rain Event Primary production was 2 to 40 μM C h −1 (or ∼0.5 to 2 g C m −3 day −1 ) at the surface in this study, which was within the range of ∼0.5 to 2.7 g C m −3 day −1 in the wet season during 2005-2006(Ho et al 2010). The chl a biomass and primary productivity in surface waters are mostly dominated by the 5-20-μm size fraction, mainly fastgrowing chain-forming diatoms in the wet season in Hong Kong waters (Ho et al 2010;Xu et al 2008).…”

Influence of seasonal monsoons on net community production and CO&lt;sub&gt;2&lt;/sub&gt; in subtropical Hong Kong coastal waters

“…Seawater samples for the primary production measurements were pre-screened through a 200 µm mesh net, and then a 50 ml sample was transferred to acid-washed glass tubes to which 0.4-2 µCi (14.8-74 kBq) of 14 C-labelled sodium bicarbonate (NaH 14 CO 3 ) was added (see details in Ho et al, 2010). Duplicate tubes were wrapped with different layers of screening to provide light fields corresponding to approximately 100, 55, 30, 10, and 1% of surface irradiance.…”

“…Dissolved organic carbon (DOC) also increased to 200 µM in Victoria Harbor in both the dry and wet seasons (Yuan et al, unpublished data, 2010). Hence, the increase in inorganic nutrients and organic carbon not only increased primary production in summer (Ho et al, 2010), but also produced an increase in bacterial production (BP) and BR (Yuan et al, 2010a).…”

“…6), cold offshore water moves shoreward with high DIC and DO in the winter dry season. Light availability was reduced due to strong vertical mixing (Ho et al, 2010), which resulted in a negative NCP (−84 ± 50 mmol C m −2 d −1 ) in all three areas (Fig. 6).…”

River export of nutrients to the coastal waters of China: the MARINA model to assess sources, effects and solutions

“…Some studies analyzed specific locations, such as the Pearl River estuaries (Ho et al 2010;Xu et al 2010a), while focusing on the current management of water resources (Cui et al 2007;Lu et al 2007;Weng 2007); others involved the Yangtze River ). In addition, eutrophication in selected lakes in China has been studied (Qu et al 2005).…”