The Pearl River Estuary is a subtropical
estuary and
the second largest in China based on discharge volume from
the Pearl River. Processes in
the estuary vary spatially and temporally (wet vs dry season). In
the dry season at
the head of
the estuary, hypoxic and nearly anoxic conditions occur and NH
4 reaches >600 μM, NO
3 is
300 μM and nitrite is
60 μM indicating that nitrification and denitrification may be important dry season processes in
the region extending 40 km upstream of
the Humen outlet. There are very few biological studies conducted in this upper section of
the estuary in ei
ther
the dry or wet seasons and hence
there is a need for fur
ther research in this region of
the river. In
the wet season,
the salinity wedge extends to
the Hongqimen outlet and oxygen is low (35–80%saturation). Nitrate is
100 μM, silicate
140 μM; and phosphate is relatively low at
0.5 μM, yielding an N:P ratio up to
200:1 in summer. Nutrients decrease in
the lower
estuary and primary productivity may become potentially P-limited. Eutrophication is not as severe as one would expect from
the nutrient inputs from
the Pearl River and from Hong Kong's sewage discharge. This
estuary shows a remarkable capacity to cope with excessive nutrients. Physical processes such as
river discharge, tidal flushing, turbulent dispersion, wind-induced mixing, and estuarine circulation play an important role in controlling
the production and accumulation of algal blooms and
the potential occurrence of hypoxia. Superimposed on
the physical processes of
the estuary are
the chemical and biological processes involved in
the production of
the bloom. For example,
the 100N:1P ratio indicates that P potentially limits
the amount of algal biomass (and potential biological oxygen demand) in summer. While extended periods of hypoxia are rare in Hong Kong waters, episodic events have been reported to occur during late summer due to factors such as low wind, high rainfall and
river discharge which result in strong density stratification that significantly dampens vertical mixing processes. Nutrient loads are likely to change over
the next several decades and monitoring programs are essential to detect
the response of
the ecosystem due to
the future changes in nutrient loading and
the ratio of nutrients.