The nutrients responsible for eutrophication in the Peel-Harvey System clearly derive from catchments cleared for agriculture, especially on the sandy coastal plain. One possible management option is, therefore to control fertiliser usage on the catchments.
A survey of some 120 possible management options 36,37 confirmed that fertiliser control should be a key component of a general management strategy. Attention is being directed particularly at control in the Harvey Catchment. where several approaches have been adopted. There was an immediate ban on the further clearing of perennial native bushland in the catchment. Improved predictions were made of the levels of fertiliser needed to obtain high plant yield, based on experimentation in the catchment. 1,38 Detailed, paddock-by-paddock testing of soil phosphorus levels were undertaken. and it was found that many soils were already phosphate saturated. Recommendations were made about the timing of fertiliser application, traditionally at the end of summer before the break of season. And a new formulation of phosphatic fertiliser was marketed, with a smaller proportion of readily water-soluble phosphate. 38,39 With the strong support of farm advisers, these modifications were widely accepted by many' farmers. and it is estimated that a 30% reduction in phosphorus application had taken place by 1991. 9 Longer-term options are also being promoted, such as altered land use to the growing of tree crops, which require less fertiliser and, probably more important, increase water use and so reduce the volume of water entering drainage.
Already there is some evidence that phosphorus loading from Harvey River is decreasing. 9 Nevertheless, while we can expect a fall in the amount of phosphorus reaching drainage, even if all phosphorus application ceased there remains a phosphorus store in the catchment soils which would continue to be lost to drainage over a long period, and for this reason Nodularia might not be satisfactorily controlled in the short term by the reduction in phosphorus loading in the catchment.1 In addition there are point sources of nutrient addition, including some large piggeries, sheep-holding yards, and increasing urbanisation; these are also potential sources of nutrients to the system.
For these reasons, in addition the suggestion was made that a new channel be cut to the ocean at Dawesville (Figure 1), in order to increase water exchange. It was calculated that this increased loss, coupled with a reduction in fertiliser usage, has the potential to control Nodularia blooms. After a considerable period of modelling the effects of such a channel on water exchange, tidal amplitude and salinity,1 the decision has been made to proceed with construction; this has begun, and should be complete by mid-1994. The channel would have two major effects. One is to increase the loss of nutrient-enriched water from the system (the volume exchange through the channel will, according to modelling studies, be three times the present exchange through the Mandurah channel),1 the other is to increase the salinity in early spring to levels which are not favourable to Nodularia growth. There will be a number of other effects, including a reduction of salinity in late summer, an increase in the magnitude of the daily tidal signal, and changes in invertebrate and fish populations in the direction of more marine populations. Some of the secondary effects are difficult to predict with accuracy (for example, effects of tidal regimes on fringing marshes), but the possibility of relatively minor changes in these components of the ecosystem does not outweigh the benefits of reducing eutrophication.
The control measures suggested should lead to a reduction in the populations of Nodularia in Harvey Estuary, but the effects on macroalgae are more difficult to predict. Because of the relationship between macroalgal biomass and light attenuation, which is attributed to material from Nodularia blooms entering Peel inlet, it is reasonable to predict a rise in macroalgal populations as water clarity increases. However, providing nutrient loading can he reduced, algal biomass should only remain high until surface sediment stocks have been depleted, after which there should be a fall in biomass. These changes are more difficult to model in quantitative terms and remain somewhat more speculative than the conclusions for Nodularia. It will clearly be a valuable exercise to document the changes which take place in the system after construction of the channel, as they will be important not only to the management of this estuary but to similar systems in other regions.
The Peel-Harvey estuarine system remains a key area for recreation in the expanding urban area near to the capital. It is also an important area for conservation and has been listed under the RAMSAR Convention as an important habitat for waterbirds, including migratory waders. It supports a key recreational and commercial fishery. Its informed management will remain a major challenge for research workers and for those authorities responsible for management implementation.