Hong Kong is surrounded by estuarine, coastal and oceanic waters. In this study, monthly averages over a 10 year time series of salinity, temperature, chlorophyll
a (chl
a), dissolved oxygen (DO), dissolved inorganic nitrogen (DIN), silicate (SiO
4) and orthophosphate (PO
4) at three representative stations around Hong Kong were used to examine if excess nitrogen in estuarine influenced waters is due to P limitation.
The monthly distribution clearly shows
the dominant influence of
the seasonal change in
river discharge in
the Pearl River
estuary and adjacent coastal waters. In winter,
the river discharge is small and more oceanic waters are dominant and as a result, salinity is high, and chlorophyll and nutrients are low. In summer, when
the river discharge is high, salinity decreases and nutrients increase. DIN is very high, reaching 100 μM in
the estuary. This indicates over enrichment of nitrogen relative to P and consequently
there is an excess of N in coastal waters of Hong Kong. P remains low (
1 μM) and can potentially limit both phytoplankton biomass and N utilization which was demonstrated in field incubation experiments. P limitation would result in excess N being left in
the estuarine influenced waters south of Hong Kong. Phosphate concentration is lower in
the Pearl River
estuary than in many o
ther eutrophied estuaries. Therefore, this relatively low PO
4 concentration should be a significant factor limiting a fur
ther increase in
the magnitude of algal biomass and in
the degree of eutrophication in
the Pearl River
estuary. The export of
the excess N offshore into
the nor
thern South China Sea may result in an increase in
the size of
the region that is P limited in summer.