Estuary Processes and Managment - River Dynamics and Integrated River Management - page 507

Fig. 8.46 Computed variation of DO, Chlorophyll- a , and DIN in Three Fathoms Cove, July 1987 (predicted daily
range and field data shown)
To study seasonal and long term trends of eutrophication, a multi-segment dynamic eutrophication
model (with the segments coinciding with the locations of routine biweekly water quality monitoring)
has been formulated for Tolo Harbor (Lee and Arega, 1999). The modeling yields interesting insights
about eutrophication dynamics, and provides useful information on the critical controlling factors: the
relation between nutrient removal and water quality, and the effect of global climate change such as the
greenhouse effect. Figure 8.47 shows the predicted variation of Chlorophyll- a levels in the surface and
bottom layers at an inner and outer station in Tolo Harbor over an entire year (Arega and Lee, 2000); the
median and range of monthly field measurements are also shown. It is seen that both the predictions and
measurements indicate multiple blooms over the year. In the outer harbor, where flushing rates (more
information can be found in Section 8.3) are much higher, much lower Chlorophyll- a levels are seen.
Contrary to initial expectations, the modeling results indicate nutrients often are not the controlling factor;
instead blooms are triggered by a combination of hydrologic and meteorological conditions. The sensitivity
of bloom events to water temperature helps to explain why very low dissolved oxygen levels were observed
in the harbor in 1998, even after substantial nutrient loads have been diverted into another catchment.
Environmental management of mariculture —The water quality model can be used for environmental
management of marine fish culture (Lee et al., 2003). Figure 8.48(a) shows a typical fish culture zone
(FCZ) in Hong Kong. The sustainable management of mariculture requires proper sites of the fish farms
and stocking density control. Both of these are related to the carrying capacity (CC) of the water body