August 2001
David McB Williams
CRC Reef Research Centre and Australian Institute of Marine Science
Introduction
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 (d.williams@aims.gov.au).
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.
Acknowledgements
A precautionary note
Supply of pollutants
Flood plumes
Sediments
Nutrients
Potential impacts
Sediments (coral reefs, seagrasses)
Nutrients (coral reefs, seagrasses, water column)
Status of Great Barrier Reef World Heritage Area
(observed impacts)
Difficulties of detecting
impacts
Status of coral reefs
Status of seagrasses
Status of water column
Impacts of pollutants other than sediments
and nutrients
Acknowledgements
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.
A precautionary note
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.
Supply of Pollutants
Flood Plumes
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.
Sediments
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.
Nutrients
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.
Potential impacts
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.
Sediments (coral reefs, seagrasses)
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.
Nutrients (coral reefs, seagrasses)
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.
Status of Great Barrier Reef World Heritage
Area habitats (including observed impacts)
Difficulties of detecting
impacts
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.
Status of coral reefs
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.
Status of seagrasses
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.
Status of Great Barrier Reef shelf waters
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.
Impacts of pollutants other than
sediments and nutrients
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.
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