Estuary Processes and Managment - River Dynamics and Integrated River Management - page 506

Fig. 8.45 Schematic of water quality model for dissolved oxygen and algal dynamics
around 12 mg/m 3 . A prolonged period of overcast skies, thunderstorms, and heavy rainfall then ensued.
Water temperature was generally high in the summer, above 25 ć . The computed DO stays fairly high for
the first few days; the predicted range of DO is generally greater for sunny days. The model predicts a
steady decline in DO down to a sustained period of critical low level, below 2 mg/L, around July 29. This
coincided with a massive fish kill that occurred on July 29 during an unusual protracted (8 hours) period
of slack tide. Observations of cell counts suggest that the causative species for this incident was the
non-toxic dinoflagellate ( Peredinium faeroense ). The bloom developed from mid-July to a peak on July 28;
peak cell counts went up to 0.5 million per mL; however, cell counts quickly decreased down to about 6300
per mL on July 29. The bloom subsided by July 31, with DO restored to high values on August 3 when
the water was dominated by high densities of diatoms. This simulation demonstrates clearly the importance
of hydro-meteorological conditions and the sediment oxygen demand on DO (due to decomposition of
settled algae) in the fish culture zone.
In general, the DO predictions are often quite satisfactory; the same, however, cannot be said for
predictions of algal biomass. Although general trends can be simulated, the model framework is limited
in its description of the complicated growth and death patterns during an algal bloom. The prediction of
bloom collapse under nutrient limited and oxygen depletion conditions remains a challenging task. For
example, a more realistic sinking hypothesis (than an assumed constant algal sinking rate) may have to be
adopted in order to correctly reproduce the timing of the bloom collapse; as the nutrients get depleted,
laboratory experiments on algal cultures have revealed the sinking velocity of the algal cells (with changed
structure) can increase by as much as six times.