Manipulation of zooplankton communities in waste stabilization lagoons, with a view to optimizing production for potential harvest

This project was developed as part of a program evaluating the potential coupling of biological waste treatment processes at the Werribee Treatment Complex (WTC) with the harvest of zooplankton material for commercial use. This work specifically aimed to manipulate the distribution of particular zooplankton species according to influent nutrient levels. The project was conducted on a scale that is rare for such ecological manipulations, with the flow pattern of an operational twelve pond, 70.5 hectare sewage treatment lagoon system being drastically altered to provide two comparable halves composed of paired ponds of similar size, orientation, influent and capacity. The system was subsequently subjected to a target sewage inflow level of 12 million litres a day, with the level of flow experimentally divided equally and unequally between the halves. Despite low and variable influent rates (due to unpredictable extrinsic factors), different and remarkably consistent patterns developed in the zooplankton communities for each flow regime. Zooplankton distribution and community structure changed markedly across the system following the alteration of flow from the original pattern. Under conditions of equal flow, zooplankton and various environmental concentrations - ammonia, nitrate, nitrite, phosphate and chlorophyll-a - synchronized well between corresponding ponds. Under unequal divisions of flow, both zooplankton and ammonia (representing a gross measure of overall nutrient loading) showed distinct differences between halves that directly corresponded with the new flow regimes, while patterns for nitrate, nitrite, phosphorus and chlorophyll-a reflected these changes by becoming increasingly disparate and chaotic. It is suggested that these changes in distribution represent a strong nutrient-mediated successional pattern that is overlaid on normal zooplankton seasonal succession and short-term population cycles in such highly eutrophic environments. The unique layout of lagoon systems at WTC promotes the spatial expression of this pattern in contrast to similar temporal (and smaller-scale spatial) changes documented in previous studies. Manipulations of nutrient-mediated succession as conducted in this thesis appear to promote distinct and predictable changes in the character and composition of traditionally variable zooplankton assemblages. Combined with appropriate and adjustable harvesting regimes, these could provide the first major step towards an optimized and sustainable product.