David McB Williams
The impact of changes in water quality, as a result of changing land use run-off from the land, is arguably the single greatest environmental threat to the Great Barrier Reef World Heritage Area (Zann 1995). Debate surrounding the issue has often been ill-informed because of the complexity of the issues, the scattered state of relevant studies and the steady stream of new studies. The first aim of this paper is to draw together and integrate the best available scientific information on the impacts of terrestrial run-off on the Great Barrier Reef World Heritage Area (outside of the river mouths). The second is to be a point of entry for those who wish to review and explore the relevant primary literature. To achieve both these goals, it is intended that this be a 'living document' to be based on the CRC Reef website and regularly updated. Suggestions of significant references omitted or key new findings are encouraged (firstname.lastname@example.org).
The format of this document is aimed at making the key scientific results as accessible and transparent as possible. It is divided into sections on: Supply of Pollutants; Potential Impacts; Status of the GBRWHA (Observed Impacts). The Status sections are not intended to be complete overviews but to focus on water quality related issues and the GBR. The first authors cited in the right hand column are the primary sources for each section and in many cases are quoted verbatim. The intention is to present the published views of the authors. The Executive Summary draws together the key results of each section.
This paper started as an overview to the CRC Reef Board and was further developed as a result of two workshops organised by the CRC Sugar and CRC Reef in March and May of 2000 and a number of iterations with attendees of the last workshop.
This review has benefited greatly from the assistance of numerous people including those involved in the May 2000 workshop. In particular, my thanks to David Haynes, Jon Brodie, Britte Schaffelke, Lawrence McCook, Michelle Devlin, Miles Furnas, Jochen Muller, Helene Marsh, Alan Mitchell and Katharina Fabricius.
There is currently significant cause for concern for the impacts of terrestrial run-off on nearshore coral reefs, seagrasses (see below) and estuaries and rivers (not covered here) in the Great Barrier Reef World Heritage Area. These impacts are a result of both past and present land use practices. This review focuses on what's happening outside the river mouths rather than trends in land use. It is important to understand, however, that while some significant improvements have been made in sustainable land use (eg the introduction of trash blanketing in some canelands and some increased attention to reducing sediment run-off in grazing lands), other factors negatively impacting on the World Heritage Area such as continued expansion of farming into marginal areas, continued increases in fertiliser application and loss of riparian vegetation and wetlands, continue to threaten to make the situation worse. If stronger action is not taken to further reduce run-off of sediment, nutrients and biocides, the present threat to these habitats will worsen. A precautionary approach to the management of nearshore areas is recommended given the relative lack of studies and the history of impacts of terrestrial run-off on coral reefs elsewhere.
Most pollutants from the land are delivered to the GBRWHA during major flood events. Episodic high inputs of particulate and dissolved matter during flood events are an important and natural part of the ecology of the Great Barrier Reef and the associated continental shelf and estuarine environments. Discharge from both wet (eg Wet Tropics) and dry (Fitzroy and Burdekin) river catchments is dominated by large flood events associated with tropical cyclones and monsoonal rainfall. These flood events significantly raise nutrient and sediment loads in the rivers - particularly during the first major flood following the dry season.Flood plumes are generally constrained close to the coast by prevailing south-east winds, the buoyancy of the plumes and Coriolis effects (effects of the earth's rotation). While some inshore reefs regularly experience floodwaters, most mid-shelf reefs see plumes less frequently than every ten years. Under unusually calm conditions, plumes can travel as far as some outer shelf reefs but their duration offshore is short.Concentrations of contaminants in rivers during floods give some idea of relative concentrations in a plume close to the river mouth and immediately adjacent habitats, such as seagrasses, but these concentrations rapidly change in time and as the plume moves. This is a result of rapid consumption of dissolved nutrients by phytoplankton and subsequent changes in the plankton communities and the dynamics of contaminants and sediment particles.
Estimates of increases in sediment yield into the Great Barrier Reef Lagoon since European settlement (based on models of catchment erosion) are highly variable from catchment to catchment but range from about 1.6 to 4.1 based on models of sediment erosion. The most recent estimates (July 2001) of change in run-off of sediments from the land since 1800 is a 3 to 4-fold increase. Many biologically active and toxic trace elements associated with agrochemical products are attached to sediments. Most of this sediment is deposited close to the coast, particularly in the northward facing bays such as Bowling Green Bay, Cleveland Bay and Princess Charlotte Bay. It is believed that increased sediment supply to the Great Barrier Reef will not increase sediment accumulation or turbidity at most coral reefs, because these factors are not currently limited by sediment supply. Turbidity in nearshore areas is primarily caused by wind-driven re-suspension of bottom sediment. Most of this sediment is not recent but has accumulated over the last five or six thousand years as the sea has inundated the continental shelf and risen to its current level.
Most of the nutrients (nitrogen, N and phosphorus, P) required by the pelagic and reef communities of the Central GBR are derived from recycled biological material. External sources of nutrients come from rivers, rainfall, upwelling from the Coral Sea and from nitrogen fixation of atmospheric N by the blue-green alga, Trichodesmium. Terrestrial run-off, estimated at 70km3 of water per year, is the largest external source of nutrients to the GBRWHA. The most recent estimate of increases in run-off of phosphorous and nitrogen from the land to the GBRWHA compared to pre-1800 levels is a 6 to 10-fold increase in phosphorous and a 2-fold increase in nitrogen. The extent to which this nutrient run-off has increased the total amount of nutrients to the marine environment, and the nearshore zone in particular, is uncertain.The growth of phytoplankton and seagrasses in GBR waters appears to generally be constrained by the availability of nitrogen, rather than by phosphorus or silicate. The nitrogen most immediately available to plants and animals is dissolved inorganic nitrogen (DIN), primarily ammonia and nitrate. Land use increases the available DIN to the GBR. In nearshore waters <20m deep, the area most impacted by terrestrial run-off, wind-generated wave action re-suspends bottom sediment, releasing nutrients. The cycling of nutrients between water column and the benthos and from particulate to dissolved forms in general is not well understood in GBR waters and is critical to understanding the impact of run-off from the land on the GBR.
Because of the behaviour of flood plumes and the maximum depth of sediment re-suspension by non-cyclonic waves, any adverse effects of land-based inputs on the GBRWHA are likely to be restricted to nearshore areas - broadly within 20km of the coast and in waters less than 20m deep.
Thriving coral reefs with high coral cover, and in some cases high diversity, do occur in episodically turbid nearshore waters of the GBR. Deposition of sediments near river mouths may, however, threaten seagrasses and there are anecdotal but unconfirmed accounts of coastal coral reefs in the Wet Tropics being buried by sediment. It's not clear whether this happened to thriving reefs or reefs where corals had already died as a result of other causes. Significant increases in sedimentation on nearshore coral reefs would be likely to cause changes in community structure and create less favourable habitats for hard corals, zooxanthellate soft corals and calcareous coralline algae. The latter are critical in reef building. Loss of reef structure filled in by sediment may lead to a reduction in numbers of herbivorous fish and a subsequent increase in macroalgae.
For many years the greatest concern regarding increased levels of nutrients on coral reefs has been that at a certain threshold level of nutrients, increased growth of macroalgae will occur and corals will be overgrown. More recently, studies have challenged both the concept of a simple threshold level of nutrients and the concept that increasing nutrient levels will inevitably lead to increased growth of macroalgae. Many studies demonstrate that herbivores readily consume increases in algae and that the primary cause for coral reefs shifting to algal-dominated reefs is likely to be declines in herbivore numbers through disease (eg sea urchins in the Caribbean) or chronic over- fishing. The major herbivorous fishes on the Great Barrier Reef are not targeted by fishers and are rarely caught. Experimental studies exposing corals to artificially high levels of nutrients have demonstrated direct effects on corals including changes in coral growth and calcification, disruption of reproduction (embryo development, fertilisation rates) and changes in settlement success of planulae. It has been suggested that a primary cause of crown-of-thorns outbreaks may be heavy terrestrial run-off increasing nutrient inputs into reef waters - the 'terrestrial run-off hypothesis'. This could be a result of natural run-off or it could be exacerbated by changes in land use. Evidence for a linkage between extreme rainfall events and the last three outbreaks of crown-of-thorns starfish on the Great Barrier Reef has recently increased. A relationship between outbreaks and changes in land use has neither been demonstrated nor disproven.
Difficulties in making definitive statements of impacts of terrestrial run-off on GBRWHA habitats include: acute and relatively frequent natural disturbance of these habitats; the relatively short duration of monitoring programs (20 years or less); the lack of unambiguous pristine controls for comparison because many of the major changes in land use occurred before monitoring of coastal and reef ecosystems was initiated; and a poor understanding of the capacity of the waters of the continental shelf to buffer and absorb cumulative changes - in particular the fate and fluxes of nutrients are poorly understood.
Based on limited research to date, clear impacts of enhanced run-off of sediments, nutrients and contaminants (as a result of land use) on coral reefs of the Great Barrier Reef ecosystem have proven difficult to detect. Impacts are unlikely for the majority of reefs that are located well offshore. There is, however, cause for concern of potential impacts on the coastal and island fringing reefs and nearshore patch reefs within 20 km of the shore in the Wet Tropics from Port Douglas south to Hinchinbrook and from the Whitsundays (Gloucester Island) south to Mackay. This area includes 209 reefs (approximately 28% of the total number of nearshore reefs of the GBR) covering 135 square kilometres (approximately 3% of total area of nearshore reefs in the GBR). They are a unique part of the ecosystem and include all the inshore reefs in the Cairns and Whitsunday regions. This concern is primarily based on estimated increases in nutrient and sediment run-off from the land compared to pre-European times (varies among catchments), anecdotal community observations of changes in the nearshore reefs of the Wet Tropics (Hinchinbrook to Port Douglas) and observations of a mismatch between substantial past reef building capacity and non-existent or limited present reef-building capacity for two sites in the Whitsundays close to the mouths of the Proserpine and O'Connell Rivers.On occasions nearshore reefs have been found to experience concentrations of nutrients and sediments that could give cause for concern (as indicated by field and laboratory experiments) and on other occasions flood plumes that are usually relatively ephemeral - lasting hours or days - have also persisted for up to three weeks after initial flooding at large distances away from the river mouth. There is very little information, however, on the doses - how much for how long - the reefs actually see during a flood plume. This is a critical area for further research on the impacts of terrestrial run-off on coral reefs of the GBRWHA.Significant impacts of land use on nearshore reefs could potentially go undetected for a considerable time. Healthy coral reefs have evolved to recover from acute natural disturbances such as cyclones, floods, and crown-of-thorns outbreaks. The consequences and recovery from these events can be readily observed by the amount and diversity of corals and fishes, for example. It is these aspects of reefs that are most readily monitored. Effects of chronic impacts such as human-induced increases in nutrients and sediments on nearshore reefs are going to be much more difficult to observe. They would occur gradually over time and would not be as dramatic as the effects of cyclones or crown-of-thorns outbreaks. A gradual shift in the community structure of affected reefs may occur due to differences in the ability of species to cope with a changing environment. Such changes may have occurred prior to the start of existing monitoring programs. Experiments are also suggesting that any effects on adult corals are at first likely to be sub-lethal, such as decreased reproductive success or survival of recruits, and these are not readily observed. One of the greatest concerns is that these gradual, unseen impacts may only be detected when coral reefs fail to recover from acute natural disturbances.
Major seagrass beds occur in coastal areas particularly vulnerable to run-off. Major potential anthropogenic threats include reduction in the light available for photosynthesis, burial by sediment and a reduction in function caused by herbicide run-off in areas adjacent to catchments with intense cropping. The major subtidal seagrasses in the Great Barrier Reef region are low biomass species that are relatively tolerant to low light levels in the water column. Many intertidal and subtidal species are naturally ephemeral and regenerate from seed banks provided these are not damaged by extreme weather events. The levels of nitrogen and phosphorus in some species of intertidal seagrasses in the Central Section of the Great Barrier reef are highly variable. The significance of this variation is unknown. There is circumstantial evidence that the impacts of extreme weather events such as floods and cyclones on seagrass habitats can be influenced by land use. For example, anecdotal evidence suggests that the loss of seagrass from Hervey Bay following the 1992 floods was unprecedented in the last 100 years, even though the magnitude of the flood was not. It was concluded that the impacts of natural disturbance on seagrass bed can be exacerbated by poor catchment management and the resultant increase in sediments and nutrients entering coastal waters. This increase may affect the ability of seagrass beds to recover from damage caused by natural events. Data are not available to indicate the extent of change in seagrass habitats off the east coast of Queensland over an extended time-frame (decades). The natural variability of the Great Barrier Reef seagrasses contributes the uncertainty of their status. However, work done in Moreton Bay to the south of the Great Barrier Reef region suggests that this it is likely that changes in water quality have reduced the depth range and hence areal extent of at least some species of subtidal seagrasses in the region. Evidence for major declines in the numbers of dugong (which feed specifically on seagrass) along the Queensland coast south of Cooktown in recent decades has given rise to speculation that changes in the distribution and community composition of the seagrass habitats may be a contributing cause.
Assessment of the status of Great Barrier Reef (GBR) shelf waters is complicated by the large natural spatial and temporal variability in nutrients and the pelagic communities (unlike most reef systems studied elsewhere) and a lack of long-term records. Evidence of nutrient enrichment of the GBR lagoon has been suggested based on historical studies at Low Isles and comparisons of nutrient levels in the GBR lagoon with coral reefs waters elsewhere. However, more extensive phytoplankton studies have found biomass and composition consistent with an unimpacted system and failed to find evidence of large-scale eutrophication. A large-scale monitoring program of chlorophyll a (a surrogate for nutrient concentrations) begun in 1992 has found the highest mean average concentrations of chlorophyll a in the inner section (within 20km of the coast) between Townsville and Port Douglas, including the Wet Tropics area of greatest concern in terms of potential impacts. This probably results from the extensive river run-off in this region. It is not clear whether these concentrations have been enhanced as a result of recent changes in land use. It is also not yet clear whether or not concentrations of chlorophyll a have increased over the monitoring period.
Pollutants impinging on the GBRWHA (other than unnaturally high levels of nutrients and sediment) include insecticides and herbicides, heavy metals and polyaromatic hydrocarbons. Studies to date have generally found low concentrations of these pollutants, indicative of a relatively unpolluted environment. Exceptions are coastal sites adjacent to human activity such as ports and harbours, urban centers and areas adjacent to intensive agricultural activity. Elevated pollution concentrations are generally the consequence of effluent discharge, urban stormwater and agricultural and industrial run-off. There is concern, however, that much of the data on pollutants in the GBRWHA are dated and a call has been made for an update of information on the distribution and impact of potential pollutants. Just as attention is turning to monitoring of non-lethal effects of nutrients and sediments on reef communities, so attention should be given to more sophisticated and sensitive indicators for monitoring impacts of other pollutants.The pollutant of greatest current concern for potential impacts on the GBR, in addition to dissolved inorganic nitrogen, is the herbicide diuron. Significant levels of diuron, used extensively in cropping, have been found in the sediments adjacent to all rivers examined in the high rainfall (Wet Tropics) coast between Port Douglas and Townsville and the Fitzroy River. Diuron has also been detected in inter-tidal seagrasses between Cairns and Townsville and is a potential threat to seagrasses.