Nutrient limitation in Hong Kong waters inferred from comparison of nutrient ratios, bioassays and 33P turnover times

Abstract

There is a need to determine the spatial and temporal dynamics of nutrient limitation to decide which nutrients should be removed during sewage treatment in Hong Kong. We compared 3 methods to assess potential or actual nutrient limitation. Ambient nutrient ratios were calculated, and nutrient enrichment bioassays were conducted, along with 33 P turnover times. Comparison of nutrient ratios and bioassays demonstrated that the ambient inorganic nutrient ratios, based on the Redfield Si:N:P ratio of 16:16:1, were a rapid and effective method that could be used to predict the potentially limiting nutrient of phytoplankton biomass, except in eastern waters in summer, since the DIN:PO 4 uptake ratio was occasionally below the Redfield ratio. The agreement between nutrient limitation indices of growth rate and biomass yield suggested that phytoplankton biomass and growth rate were P-limited in southern waters, with more stable conditions during summer. In contrast, a lack of agreement between these indicators showed that phytoplankton growth in potentially P-limited cases in western waters and Victoria Harbour was controlled by physical processes (e.g. strong hydrodynamic mixing and dilution). The limiting factor for phytoplankton growth varied spatially and temporally. In summer, there was a change from physical processes (e.g. the rapid dilution and possible light limitation due to strong turbulent mixing) in hydrodynamically active western waters and Victoria Harbour to P limitation, or N + P co-limitation, in southern and eastern waters with more stable conditions. In winter, phytoplankton growth was regulated by strong wind-induced vertical mixing. Hence, different seasonal sewage treatment strategies should be considered for nutrient removal.KEY WORDS: Nutrient ratios · Nutrient enrichment bioassays · 33 P turnover times · Pearl River discharge · Sewage effluent · Hong Kong · Nutrient limitation · Light availability Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 388: [81][82][83][84][85][86][87][88][89][90][91][92][93][94][95][96][97] 2009 nutrient loading from the Mississippi River watershed (Malakoff 1998).The concept of nutrient limitation has been the keystone to understanding eutrophication impacts (Smith et al. 1999), and determining the limiting nutrient for phytoplankton growth was the central theme of eutrophication research in the 1970s (Jong 2006). Nutrient enrichment causes a change in phytoplankton biomass yield, as well as growth rate. Hence, the concept of nutrient limitation has 2 meanings: the limitation of biomass and/or of growth rate (Paasche & Erga 1988).Nutrient availability in coastal waters can be strongly influenced by both freshwater inputs and oceanic and tidal exchange, with the latter typically diluting nutrient concentrations (Gobler et al. 2005). It has been debated which nutrient, N or P, limits primary production in coastal waters. Nitrogen has traditionally been viewed as the nutrient limiting productivity in coastal ...

Long-Term and Seasonal Changes in Nutrients, Phytoplankton Biomass, and Dissolved Oxygen in Deep Bay, Hong Kong

“…In the outer bay, the high flushing rate was likely responsible for low phytoplankton biomass (Lee and Qian 2003). The influence of physical processes on the regulation of phytoplankton growth was also observed in the nearby western waters of Hong Kong (Xu et al 2008(Xu et al , 2009). …”

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

“…The contrast between high chl a-specific photosynthetic rates observed in the incubation experiments and the low in situ chl a concentrations in spring suggested that the processes (e.g., dilution, mesozooplankton grazing, and vertical mixing) that were excluded in the incubation experiments were the primary factors that were responsible for the relatively low in situ chl a concentrations. This suggestion was supported by a series of simultaneous nutrient enrichment bioassays, which showed that the ambient nutrients could support algal growth for a few days, and the algal biomass increased several times when dilution and vertical mixing were reduced in the bottle incubations (Xu et al 2009). …”

“…In addition, the Pearl River discharge might transport phytoplankton into southern waters and partially contribute to the high algal biomass in southern waters. An unusually large summer bloom (up to 73 μg chl L −1 ) was observed in this region (Stn S12), accompanied by a drawdown of PO 4 to limiting concentrations (Xu et al 2009). The frontal region of the plume has also been found to have high chl a concentrations relative to inshore and offshore waters in other coastal waters such as Chesapeake Bay (Breitburg 1990;Harding 1994) and the Mississippi River plume (Grimes and Finucane 1991).…”

“…4 and 5). Fast-growing chain-forming diatoms such as Skeletonema costatum, Chaetoceros curvisetus, Thalassiosira sp., Pseudo-nitzschia delicatissima and Pseudo-nitzschia pungens, and dinoflagellates such as Scrippsiella trochoidea are the most common phytoplankton species in Hong Kong waters (Ho 2007;Xu et al 2009). Therefore, diatoms are dominant in Hong Kong waters and in agreement with previous studies in other eutrophic and turbulent coastal waters (Trembly and Legendre 1994).…”

Temporal stability of marine phytoplankton in a subtropical coastal environment

“…Port Shelter is a sheltered water body relatively far away from anthropogenic pollutions and the influence from a large river-the Pearl River west of Hong Kong. Its hydrology and chemistry background has been extensively studied (Yung et al 2001;Xu et al 2009). Owing to the influence of China coast current, the nutrient concentrations are usually elevated during the wintertime in Port Shelter (Yin 2003;Chen et al 2009).…”