Dynamics of nutrients and phytoplankton biomass in the Pearl River estuary and adjacent waters of Hong Kong during summer:preliminary evidence for phosphorus and silicon limitation

Abstract

The Pearl River in the south of China is the second largest river in China in terms of discharge volume. Two cruises were made to investigate the dynamics of nutrients and phytoplankton biomass in June and July 1998, across the Pearl River estuary to the adjacent territorial waters of Hong Kong. On-deck incubation experiments of 5 mixtures of freshwater from the surface with seawater from below the halocline were conducted to simulate time scales of phytoplankton blooms for each freshwaterheawater mixture and to examine uptake of nutrients. In July, phytoplankton growth rates increased with salinity of the mixtures, with the lowest growth rate (0.81 d-') in freshwater and the highest (2.41 d-l) in 100% seawater ( s a h i t y = 29). PO4 was lower in freshwater (0.3 PM) than in seawater (1.2 pM), whereas concentrations of NO3+ NH4 + urea (-80 FM) and SiO, (150 FM) were higher in freshwater than those in seawater (25 pM for nitrogen and 26 pM for SO,). During the incubation PO, disappeared first, indicating that P was limiting the phytoplankton biomass. All mixtures reached the maximum in phytoplankton biomass in 3 to 4 d. There was a regional maximum of phytoplankton biomass that occurred at the seaward edge of the estuarine plume during June The region moved eastwards (away from the estuary) to the southern waters between Lantau Island and Hong Kong Island during July. Mixing dagrams of NO3 and Si04 showed conservative behaviour with sabnity in the estuary. These observations suggest that dilution by freshwater outflow was a controlling factor in determining the distribution of nutrients and phytoplankton biomass in the estuary due to high flow during June and July. The regional maximum of phytoplankton biomass was comparable to that resulting from the incubation and coincided with the exhaustion of PO, during July. On the estuanne (west) side of the regonal maximum, chl a fluorescence increased during 24 h incubations, but decreased at the station with the maximum and on the east side, suggesting the possible hutation of nutrients to the phytoplankton community. In the same eastern waters, both PO, and SiO, were very low. However, NO, and NH, remained abundant, suggesting possible CO-limitation by phosphorus and silicon. We hypothesize that considerable seasonal rainfall in June and early July might have contributed an additional source of nitrogen to the water column, which resulted in the exhaustion of PO, and SiO, before nitrogen.

Shifts in limiting nutrients in an estuary caused by mixing and biological activity

“…A decrease in DIN : SRP ratios, to a certain extent, is generally consistent with the results of bioassays that there is a shift from P to N limitation (Doering et al 1995;Yin et al 2001;Smith 2006). Because surface seawater contains low nutrients, most previous studies have focused on changes in riverine nutrient loadings or additional sources of P in estuaries to explain the nonlinear decrease of N : P ratios across the salinity gradient (Fisher et al 1992;Yin et al 2004;Harrison et al 2008).The switch from P-limited riverine water to N limitation with increasing salinity is expected, because surface seawater generally has a DIN : SRP ratio of # 3 (Fanning 1992), although in the Mediterranean Sea, DIN : SRP ratios may exceed the Redfield ratio of 16 in winter (Krom et al 1991;Kress and Herut 2001). Previous studies have implied that a sudden shift in the limiting nutrient across estuaries is a salinity-related phenomenon (Caraco 1988;Doering et al 1995), but whether this shift is mainly because of changing riverine loadings, increasing salinity stress, additional sources of phosphorus, or just simply through mixing remains unclear.…”

Relationship between nutrients and plankton biomass in the turbidity maximum zone of the Pearl River Estuary

“…Phosphorus was the potential limiting nutrient in PRE (Yin et al, 2000). The particle-bound P would release into the water when the ambient PO 4 3 − concentration decreased, which potentially alleviated P limitation in the Pearl River estuary.…”

Distributions and sea-to-air fluxes of nitrous oxide in the South China Sea and the West Philippines Sea

“…First, many rivers that are rich in NO 3 À and NO 2 À flow into the SCS. In particular, two of the largest rivers in the world (the Mekong and Pearl Rivers) enter from the north, contributing large amounts of nutrients and organic matter to the SCS, especially in the wet season (Yin et al, 2000;Chen et al, 2008). Also, the high-nutrient riverine water fuels higher productivity in the SCS relative to the WPS.…”