Catchment run-off and land-based sources of pollution

2011

 

Coastal habitats are susceptible to many impacts that arise from the adjacent lands, and from rivers that discharge into the gulfs, coastal lakes and lagoons and directly to inshore waters. The species and habitats that occupy these marine areas are often well adapted to the dynamics of variable levels of salinity and contaminants such as suspended sediments and nutrients, although their capacity to withstand these pressures is limited. Extensive and frequent extreme weather events, or persistent low-level pollution from rivers, may exceed the capacity of many species to resist such pressures. If these impacts occur broadly across a region, or persist locally for a long time, they will lead to irreversible change in habitats and species distributions. Examples from New South Wales and Western Australia illustrate these problems.

More than half the estuaries in New South Wales are subject to double the natural levels of sediment and nutrient inputs, and around one-third of catchments are more than 50% cleared of natural vegetation.18 These and other pressures are directly linked to the poor water quality found in a high proportion of New South Wales estuaries—only 11% of the estuaries were found to comply more than 90% of the time with guidance levels for chlorophyll-a—and to losses of coastal vegetation, including seagrasses, which are estimated to have been reduced by more than 30% from their natural (pre-European colonisation) extent.18

The Northern Rivers region of New South Wales has 46 estuaries (25% of the total in the state) that cover 350 square kilometres (20% of the total state estuarine area) and drain an estuary catchment area of 49 600 square kilometres (39% of the total in the state). The 46 estuaries comprise 20 barrier rivers and lakes that are generally open, 23 creeks and lagoons with intermittently open entrances, and 3 brackish water bodies. Measured against benchmarks in recent history and using comparisons with the existing conditions in other New South Wales estuaries (which may also be degraded), a number of indicators are rated as very poor, including seagrasses, saltmarsh and chlorophyll in the water column. Many of the estuaries are under pressure from excessive inputs of sediments and nutrients, and altered freshwater inputs and hydrological regimes.18,73

Tuggerah Lakes in New South Wales is a barrier estuary with a long history of urbanisation of the catchment, including reclamation of foreshore wetlands and structural realignment of water passages between the individual lakes and the opening to the ocean. About half of the wetlands (the upstream ‘biological filter’ system) are already lost, including 85% of the saltmarsh, and urban development is directing surges of stormwater into the lakes. These changes contribute to problems such as ‘black ooze’ (monosulfidic black ooze causes rapid oxygen depletion of lake and drainage waters when the ooze is mixed with oxygenated waters during disturbance) and serious degradation of water quality in the lakes.74 The lakes system has been subjected to a long series of structural solutions (such as dredging of the lake bed) over many years, and is currently funded for major ongoing restoration and environmental management works under the Australian Government’s Caring for our Country program.

As with most such estuaries, coastal lakes and lagoon systems, many issues and many authorities are involved in management attempts to reduce environmental impacts and restore more desirable natural conditions. The New South Wales Government ‘owns’ the Tuggerah Lakes, while Wyong Shire Council is the main manager of the catchment that flows into and affects the environmental health of the Tuggerah Lakes estuary. A number of state and federal authorities have a role in management. Private and community sector organisations also have a direct interest in the management of the lakes, including community groups, the real estate industry, various recreational groups and commercial fishers.
The most recent assessment of Tuggerah Lakes indicates that, although turbidity is ranked as fair, important ecological aspects are in good or very good condition, including fish, seagrasses and saltmarshes, suggesting that restoration efforts have been at least partially successful.73,75

In Mandurah (Western Australia), major nutrient and algal bloom problems have a long history in the Peel–Harvey Estuary, caused principally by nutrient pollution from upstream agricultural lands.76 The $57 million Dawesville Channel was opened in 1994 to create an artificial opening from the Peel–Harvey Estuary to the ocean, to increase flushing in the estuary and reduce the frequent and extensive algal blooms and nutrient pollution problems. Subsequently, local residents observed a temporary improvement in conditions, but deteriorating water quality and adverse biological conditions returned within five years of the channel opening. These included further major algal blooms and deterioration of some indicator species to levels equivalent to those documented before the channel.77 Thus, despite a large investment of public funds, restoration efforts may not have been able to persistently improve environmental conditions in this estuary.

In addition to impacts in enclosed coastal waters, such as the examples above, land-based sources of pollution can have serious impacts in open coastal waters. On the Queensland coast adjacent to the Great Barrier Reef, there are 38 major river catchments, including some of Australia’s largest rivers (such as the Burdekin and Fitzroy rivers), and these combined sources deliver substantial amounts of sediments and nutrients into the shallow coastal waters of the nearshore lagoon system. The catchments now deliver 2–10 times more nutrients and sediments to the lagoon waters than they did before European settlement.31 They also deliver significant amounts of pesticides to the reef and lagoon waters, although the impact of these chemicals on habitats and species are as yet unclear.32 Nonetheless, the combined impacts of the sediments, nutrients and agricultural chemicals reaching the coral reef systems of the Great Barrier Reef are considered to be highly significant. Models have estimated that minimising agricultural run-off could reduce macroalgal cover, which threatens the viability of corals on reefs across the Great Barrier Reef, by 39% on average, and increase the richness of hard corals and phototrophic octocorals on average by 16% and 33%, respectively.78

Figure box 6-10

Satellite image of the Burdekin River flood plume on 22 February 2008

From 2000 to 2006, the Burdekin and Fitzroy river catchments received relatively small amounts of rainfall (around 670 millimetres annually), leading to only limited river plumes flowing into the Great Barrier Reef lagoon. From 2007 to the 2011 wet season, this changed significantly. Monsoonal or cyclonic rainfall sometimes reached the annual average for the catchment in a few weeks, causing small, medium and large river flood plumes along the entire east Queensland coast that extended well into the lagoon.

The flood plume shown by the true-colour satellite image above extended more than 40 kilometres into the Great Barrier Reef lagoon and was caused by significant rainfall from several low pressure systems.79 This flood plume merged with the wide band of flood-affected waters following the coast in a south-to-north direction, originating mainly from the Fitzroy, Pioneer and Proserpine rivers. The clear, beige colour shows water masses that are strongly dominated by freshwater suspended sediment, such as clays, whereas the water with a darker brown colour is a mix of fresh and marine water, with more dissolved and particulate organic material. The mid-shelf broad green band south of the plume is likely to be a phytoplankton bloom that resulted from the increase in nutrient availability provided by the river flood plume waters. The satellite image shows that the coarse material of suspended sediments is deposited near the coast, while the finer particulate and dissolved fractions merge into a 30–40-kilometre-wide band that gradually disperses towards the Reef. In some cases, these floodwaters may disperse through the inner, mid and outer reef into the Coral Sea, and occasionally curve back towards the outer reefs tens to hundreds of kilometres north of their source rivers. There is evidence that an increase in frequency, intensity or duration of these flood plumes causes increased primary production during the wet season through phytoplankton growth, and this may contribute to decreased resilience of the coral systems of the Reef.

The long Great Barrier Reef coastline (2000 kilometres) and the short-term duration of floods make monitoring the flood plumes difficult. However, several institutes (Commonwealth Scientific and Industrial Research Organisation, James Cook University, Australian Institute of Marine Science, Great Barrier Reef Marine Park Authority) regularly combine their field sampling efforts and expertise to monitor such events and assess the impact of floods on the water quality of this world-renowned ecosystem.

Source: Blondeau-Patissier38

The evidence indicates that, although we can point to many small-scale successes, the problems of land-based pollutant sources, coastal development and catchment run-off are likely to be much more effectively resolved by systems that deliver prevention rather than cure. Both prevention and cure can be complex and expensive, and take a long time to implement and produce results. Whereas the pathway to effective prevention is moderately clear, achieving a successful cure for impacts of coastal development once they have occurred is not only difficult and costly, but also uncertain. Unfortunately, management systems around Australia appear to have difficulty learning from past failures, and this impedes the application of more effective planning for prevention rather than applying a cure.

Ward T (2011). Marine environment: Catchment run-off and land-based sources of pollution. In: Australia state of the environment 2011, Australian Government Department of the Environment and Energy, Canberra, https://soe.environment.gov.au/science/soe/2011-report/6-marine/3-pressures/3-6-pollution, DOI 10.4226/94/58b657ea7c296