In this essay, I will describe the history of the environmental problems of the Peel-Harvey estuary in Western Australia - specifically eutrophication of this area and hope to provide some insight into its causes. I will cover a number of the management strategies which have been developed and attempted over recent years and highlight a number of studies which have taken place in the area. I hope to avoid glossing over many of the issues, however a full exposition of many of these involves much more detail than the scope of this paper allows.
My references are largely studies of the area (and other similarly affected areas) by the Department of Conservation and Environment and Environmental Protection Authority.
Extent
The Peel Inlet and Harvey Estuary (hereafter referred to collectively as the Peel-Harvey Estuary) are a conjoined inland water body with an area of 13300 ha and an average depth of 1m situated approximately 70km south of Perth, Western Australia at Mandurah. It is fed by the Serpentine and Murray Rivers to the North and by the Harvey River and Mayfields Agricultural Drain to the south with a total catchment area of 11300km2. Up until recently there was a single outflow to the ocean at the northern end of the Peel Inlet via a channel at Mandurah.
Since the early 1970's, large areas of ‘weed’ algae, primarily of the genera Cladophora, Chaetomorpha, Enteromorpha and Ulva affected the areas use as a recreational and professional fishing area and threatened the stability of the local ecosystem. By the end of the 1970's and early 1980's Nodularia spumigena bloomed in large populations initially in the Harvey Estuary and later in the Peel Inlet displacing local fish and in certain areas causing mass mortality of fish and crabs.
Hodgkin et al. (1985: 4) described the consequences of the algal blooms to the human population generally as no more than aesthetic due to the algae’s appearance as a surface scum and a black sludge after decomposition as well as various odours associated with this. However, at times concentrations of hydrogen sulphide gas produced by decaying algae could reach toxic levels (more than 1ppm) causing serious illness. Algae accumulating along the edges of the inlet also caused damage to marginal vegetation such as rushes. Tractors were used to remove these accumulations, but unfortunately these caused nearly as much trauma as the damage from the algae itself.
Causes
The causes for eutrophication of this type usually relate to the nutrients nitrogen and phosphorous. The Murray River discharges a large load of nitrogen into the Peel Inlet, and it is generally phosphorous which is in smaller supply and hence the limiting element in determining the size of the weed crop. In the Harvey Estuary, phosphorous also governs the size of the crop even though waters from the Harvey River contain little nitrogen. The reason for this is that Nodularia makes use of atmospheric nitrogen and so does not need the water to supply its requirements of this element. For this reason, there is no easy way to reduce the level of nitrogen available to the algae in the Peel-Harvey Estuary, and hence phosphorous must be the main target for preventative measures.
Phosphorous found in the estuary comes primarily from the use of phosphorous-based fertilisers in farming land within the catchment of the estuary. In addition to the use of such fertilisers, the catchment area has been extensively cleared of natural vegetation to plant seasonal, shallow-rooted crops. The construction of an elaborate drainage system to reduce flooding of these farmlands caused a huge influx of phosphorous into the estuary during the rainy season, averaging about 130 tonnes.
During a study in the late 1970’s, Hodgkin and Birch (1981) showed that the size of the Nodularia blooms was directly related to this input flow of phosphorous. Since that time various studies undertaken during the early 1980’s found that the blooms in the estuary were steadily increasing in size regardless of the phosphorous load (Hodgkin et al. (1985:4)) and suggested that this was indicative of an ecosystem “seriously out of balance”. It would appear that a large quantity of phosphorous (estimated at approx. 83 tonnes) stays in the system each year, contained in plants, sediments and decomposing material. These sediments release their phosphorous during times of low oxygen in the bottom water (which happens during calm days) and it is then available again to the algae. Generally speaking, however, only about half (approx 60 tonnes) of the phosphorous load which enters the estuary ever finds its way out to the ocean.
Management
Weed Harvesting:
Since 1974, tractors and front-end loaders have been used to remove decaying algae washed ashore. This was mainly in response to complaints from residents and had little effect on the causes of the problem. An offshore harvester was purchased in 1983 which collected floating weed in water deeper than 1.5m to prevent it from drifting ashore. This latter method had greater success in actually combating the problem as it tended to remove more of the phosphorous bearing weed from the water before it decayed and sank to the bottom of the water as sediment. These methods had little overall effect on their own, however, as the amount of weed regenerating made it impossible to keep on top of the growth. It is still used as an effective as a method of controlling the aesthetic problems presented by the algal blooms.
Managing the Phosphorous Load From Farming:
Given that the major cause of the phosphorous load is from farming it seems natural that this should be tackled first. To this end, various soil surveys have been undertaken to ascertain whether it is still appropriate for phosphorous based fertilisers to be applied to farmland within the catchment.
Three major soil types were isolated by Hodgkin et al. (1985:16) and these were identified as “loams and clays”, “sands over clays” and “deep grey sands”. The amount of phosphorous which subsequently found its way into the drainage systems was significantly different for each soil type, ranging from 5% for the “loams and clays” to 40% for the “deep grey sands”. It was also discovered that 70% of the paddocks with “sandy soils” could support crop production with no application of further phosphorous as there was sufficient supplies already in the soil. Importantly, the major fertiliser type used, “SuperPhosphate” is 84% water soluble and is subsequently readily leeched into drainage. An alternative fertiliser now in use, “Coastal SuperPhosphate” is only 20% soluble, therefore much less is lost to drainage.
In summary, the type of fertiliser used and the amounts applied in the past have been found to be inappropriate for continued use on the mainly sandy soils of the coastal plain. Enlightenment of farmers to these facts began in the mid-1980’s and this has helped to significantly reduce the runoff of phosphorous into the estuary.
Managing the Retained Phosphorous Load:
The problems of the retained phosphorous load are not so easy to address. It is necessary to increase the export of phosphorous into the ocean from the inlet and the main hope for this is in the recently completed Dawesville Channel. This channel was cut through from the western edge of the estuary to the ocean to supplement the inadequate channel at Mandurah. Mathematical modelling predicts that exchange volume of water through the Dawesville Channel will more than double that of the original Mandurah Channel. It should also have the side effect of increasing the flow through the Mandurah Channel by about 20%. In addition, a greater level of exchange between the Harvey estuary and the Peel inlet should take place. It is hoped that the time taken to completely flush the estuary should decrease by a factor of three. The quantity of phosphorous exported should greatly increase with this extra flushing and over time the stores of phosphorous in the estuary decrease.
Studies of the estimated effects to flora and fauna from the new channel seem to suggest there will be little detriment to those which are “desirable”. However there are fears that larger tidal effects may be caused by the channel. This will disrupt present feeding patterns of “migratory waders”, birds which feed on the intertidal areas at low tide and also shallow waters on inland, non-tidal areas. The larger tides may also decrease the available area of favoured roosting sites for wading and fish eating birds such as pelicans and cormorants. It has been proposed to provide additional roosting sites using dredge spoil. These would afford the birds a safe haven, generally inaccessible by predators.
Managing the Algal Blooms:
In addition to the phosphorous, Nodularia require optimum salinity of 3-30ppt (part per thousand) and low light levels. Currently, the estuary waters provide near perfect conditions for growth.
The level of salt in the estuary ranges from 3ppt in winter to 50ppt in summer. Considerable quantities of phytoplankton also enjoy the nutrient levels in the waters. These, combined with a high level of suspended sediment in the Harvey estuary cause the water to be relatively opaque to light.
Tidal effects from the ocean through the channel should even out the salinity change to about 25-40ppt and also improve the clarity of the water by reducing suspended sediment. Therefore, removing nutrients from the estuary, levelling off the salinity and improving water clarity should adversely affect the growth of Nodularia. Blooms that do occur should certainly be lesser in magnitude and shorter in time span.
Management Overview:
The current management solution relies on the Dawesville Channel to achieve all of the above. Coupled with continued weed harvesting and lower phosphorous inflow noticeable reduction of the algae formation should be evident within five years after which time the appearance of algae should be limited to years with exceptionally high rainfall.
Conclusion
It is intriguing to study such a “cause and effect” situation such as this and note the amount of time it often takes for “real” solutions to be effected. Although signs of ecological imbalance appeared over thirty years ago, and proposals from various scientific bodies have been on the table for at least fifteen, only recently there appears to have been any real change in the attitude to this problem. Hodgkin et al. (1985) undertook a series of interviews and discussions with various local residents and bodies and this makes for humorous reading. Local residents appeared to be largely in the dark about the various proposals afoot and saw many other problems (eg mosquitoes) as a much more important issue than the algae. They also suggest that the Dawesville Channel project should be finished by 1989! It is hard to ascertain the reasons for the time it took from the initial studies to finally getting the wheels in motion. It certainly is an eye-opener how many studies appear to have been undertaken. The Department of Conservation and Environment lists no less than 29 separate documents about the problem in a four year period from 1981-85 and two large reports in the same time span.
It is certainly hoped that this management plan will succeed and that the advantages of the Dawesville Channel will outweigh the disadvantages. However it seems that predicting the outcomes of such a “drastic” measure verge upon the black arts. It concerns me that in many of the brochures for public consumption which I acquired in researching this topic, the negative results of this important undertaking were largely glossed over. In its blithe optimism, a technocentric solution such as this may yet cause damage unforeseen due to the overly positive view taken during the studies.
In the end, it all comes down to a matter of “wait and see”.
Ken Taylor, May 1994
References:
Wood, G. 1975. An Assessment of eutrophication in Australian inland waters. Australian Water Resources Council. Technical Paper no. 15. Australian Government Printer, Canberra
Hodgkin, E.P., Birch, P.B.,1981. Eutrophication of the Peel Inlet and Harvey Estuary Poster - Department of Conservation and Environment, Bulletin 117.
Humphries,R.B. and Croft C.M. 1983. Management of the Eutrophication of the Peel-Harvey Estuarine System. Department of Conservation and Environment, Bulletin no. 165
Hodgkin, E.P., Birch, P.B., Black, R.E. and Hillman K. 1985.The Peel-Harvey Estuarine System - Proposals for Management. Department of Conservation and Environment, Report no. 14
