Estuary Processes and Managment - River Dynamics and Integrated River Management - page 505

Fig. 8.44 Observed change of chlorophyll and DO during a dinoflagellate bloom at Kat O, Hong Kong
incorporated in the biomass of the settled algae. The benthic segment is assumed to be anaerobic, and
organic, ammonia, and nitrate nitrogen, and biochemical oxygen demand are modeled in the sediment. In
the reducing sediment settled non-algal particulate organic nitrogen is hydrolyzed to ammonia by bacterial
action, and ammonia is also generated by the anaerobic decomposition of algal detritus. The ammonia
nitrogen may then be exchanged with the overlying water column via diffusion. No nitrification occurs in
the anaerobic sediment, while denitrification is modeled. Nitrate is present in the benthos due to diffusive
exchange with the overlying water column. In the sediment, deoxygenation due to the anaerobic
decomposition of carbonaceous and nitrogenous organic matter (CBOD and NBOD respectively) is used
to determine the (negative) DO concentration. The flux of oxygen (sediment oxygen demand, SOD) and
ammonium nitrogen across the sediment-water interface can then be computed from the respective water
column and benthic layer concentrations using a Fickian diffusion approach. Further details and various
versions of the model can be found elsewhere (Di Toro and Connolly, 1980; Ambrose et al., 1993; Lung,
1993; Arega and Lee 2000).
The water quality model can be gainfully employed to interpret the observations of phytoplankton
dynamics, and help unravel the causes and the environmental pre-conditions that would lead to severe
DO depletion—and hence a potential fish kill. Figure 8.46 shows a hindcast of water quality variation
over a two-week period in Three Fathoms Cove in July 1987. The daily variation of solar radiation, tidal
range, water temperature, and Dissolved Inorganic Nitrogen (DIN) loading are also shown. The initial
values at 00 hr, 21/7/87, are provided by measurements made on a hot sunny day, with Chlorophyll- a of