Some estuaries typically have re-occurring blooms in particular areas. For example, the Berowra estuary near Sydney has a continually high biomass of algae in the middle reaches near Calabash Point. Harmful algal blooms also occur intermittently, which result in closure of the Sydney rock oyster aquaculture facilities situated in the downstream reach of the estuary. Closure of the estuary following algal blooms has a significant impact on the local community, due to the importance of the area for boating and swimming. Berowra Estuary is a drowned river valley estuary (tidally dominated), which joins the Hawkesbury River estuary about 24 km from the Pacific Ocean. The estuary has a waterway area of about 13 km2 and drains a catchment of approximately 310 km2. A study was instigated to determine when and why the blooms occur in the mid-reaches of the estuary (Rissik et al. 2006).
The flushing time in Berowra Estuary was influenced most by the volume of water in each section of the estuary. Flushing time is the time taken for water in a specified region of the estuary to be moved from this region due to replacement (dilution) by incoming fresh water or by tidal dynamics. The volume of water at each reach was determined by the depth and width of the estuary. Upstream the estuary is narrow and fairly shallow; mid-stream the estuary is wide and deep; and downstream the estuary is wide and shallow. These factors translate to flushing times of 1.5 days for the upstream site, 7 days for the midstream site and 1 day for the downstream site (Figure 3.1). Flushing times in the mid-stream reach were sufficiently long for both primary and secondary production to take place in warm summer temperatures.
Primary production was greatest in the mid-reaches (bloom area), indicating that conditions supported rapid growth. Zooplankton was more abundant in the areas with the highest phytoplankton biomass. Small zooplankton was found to respond most rapidly to changes in the phytoplankton. This increase in concentrations of small-sized zooplankton, which were dominated by copepod nauplii, suggested that when more food was available, zooplankton production took place. The high levels of phytoplankton concentrations in the mid-reaches of the estuary indicated that their production was at a rate at which biomass could not be controlled by zooplankton grazing. Only when other factors that reduced primary production rates, such as reduced light intensity, occurred, could the zooplankton assimilate the bloom.
From a manager's perspective, flushing times in various reaches were an important determinant of phytoplankton biomass. To reduce blooms
Croplands_'< _Hawkesbury River
Croplands_'< _Hawkesbury River
in the mid-reach, flushing times in the deeper sections would have to be reduced to periods of 1-2 days which would involve undertaking works such as filling the deep holes to reduce the depth of the estuary. Such highly engineered solutions would be prohibitively expensive and would have major impacts on the estuary's ecology. Unfortunately, zooplankton grazing was unable to consistently control phytoplankton biomass during warm temperatures and light intensities of summer, as such grazing would only be likely to reduce the biomass effectively if phytoplankton production rates declined.
The most effective options to reduce blooms are those which result in less nutrients being discharged to the estuary from run-off, sewage discharge, directly from homes and some boats. The estuary receives tertiary treated discharge from two sewage treatment plants and also receives stormwater from a number of drains. Solutions were delivered by working with sewage-treatment managers, undertaking educational campaigns, building nutrient-reduction devices, such as constructed wetlands and gross pollution traps, and repairing broken sewerage infrastructure in the catchment. To assist management, an algal bloom monitoring buoy was moored near Calabash Point, which automatically sends an e-mail to the local council when the chlorophyll-a level exceeds 20 ^g.L"1.
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