Estuary Processes and Managment - River Dynamics and Integrated River Management - page 504

Fig. 8.43 The massive 1998 Hong Kong red tide: (a) severe fish kill at fish culture zone in Lamma Island (From Yang
and Hodgkiss, 1999); (b) reported red tide movement
There is a genuine need to study bloom dynamics to provide a scientific basis for mariculture management.
Long term field monitoring and modeling work in Hong Kong was initiated during 1987-1989; the
emphasis was to understand the often perplexing observed DO variations in many of the 26 marine fish
culture zones. It is of interest to understand the environmental factors and preconditions which lead to
fish kills, and to predict the occurrence of severe DO depletion.
Field observations —The nature of DO and algal dynamics can best be illustrated by field observations.
A total of seventeen 26-hour water quality field surveys were carried out in a weakly-flushed fish culture
zone in Tolo Harbor (Lee et al., 1991a). The 26-hour field studies subsequently gave way to the development
of a telemetry system to continuously monitor algal and dissolved oxygen dynamics (Lee and Lee, 1995).
Since 2000 real time algal dynamics research stations have been deployed at two sites in Hong Kong,
with the objective of developing a real time forecasting and warning system (Lee et al., 2000; Wong,
2004). Figure 8.44 shows the changes in the algal biomass (chlorophyll fluorescence) and DO recorded
by real time monitoring at Kat O, Hong Kong. The measurements were recorded at three depths during a
dinoflagellate ( Gonyaulax Polygramma ) bloom over the course of two weeks (Lee et al., 2000). It is seen
that the diurnal DO fluctuations mirror the algal biomass; the difference between surface and bottom DO
concentrations is also significant during the bloom period but relatively small on days without algal
growth. This suggests the possibility of using DO data to gain insights into the algal dynamics—e.g., the
vertical structure of a diatom bloom may be fundamentally different from the case when vertical migration
of dinoflagellates can influence the DO consumption pattern.
Water quality modeling —Herein a summary is given of the elements of a water quality model that
has been useful in practical application (Fig. 8.45). Phytoplankton growth and the associated nutrient and
DO dynamics are governed by a number of interacting physical, biological, and chemical processes which
vary in time and space: river inflows and tidally driven circulation, mass transport by advection and
dispersion, algal growth as a function of environmental forcing (organic loads, solar radiation, rainfall
and nutrient input), nutrient regeneration, deoxygenation, and reaeration. Phytoplankton dynamics and
nutrient kinetics are based on a generally accepted framework (Bowie et al., 1985; Thomann and Mueller,
1987; Lee et al., 1991a,b; Ambrose et al., 1993). In order to simulate nutrient and DO exchange between
the water column and the sea bed, an additional benthic layer is provided. Particulate organic nitrogen
and phosphorus settle into the benthic sediment along with the respective nitrogen and phosphorus