Physics–Biology Links in the Great Barrier Reef Eric Wolanski CONTENTS Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Land Use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Mangroves and Seagrass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Fringing Corals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Ecosystem Connectivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Climate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 INTRODUCTION The Great Barrier Reef (GBR; Figure 1) extends approximately 2600 km along the eastern coast of Australia, from just north of Fraser Island in the south (25°S) to the coast of Papua New Guinea in the north (9.2°S). It is not a continuous barrier. Instead it is a matrix of more than 2800 individual reefs (Animation 1) ranging in size from 100 to 0.01 km 2 . Inter-reefal waters form channels which surround the reefs, and these chan- nels can be several hundreds of metres to tens of kilometres wide. The assemblage of reefs and inter-reefal waters is called the GBR matrix. This matrix is located on the con- tinental shelf. The shelf is impacted by runoff from Queensland rivers. The coast is rugged with numerous bays protected by headlands. Mangrove swamps are common in these bays as well as along tidal estuaries. To the east, the shelf faces the Coral Sea with depths at points exceeding 4000 m. The shelf is generally shallow with depth at the shelf break seldom exceeding 100 m. Much of the GBR lagoon, i.e., the channel between the mainland and the mid-shelf reefs, is less than 50 m in depth. The reefs in all regions vary in shape from kidney-shaped with a lagoon (e.g., Bowden Reef shown in Figure 2) to flat platforms without a lagoon. The windward reef slope is usually very steep. At the lee side of reefs and in their lagoons, there are commonly numerous coral outcrops reaching all the way to the surface. In some areas, the reefs form ribbons; they are elongated, several kilometers long and typi- cally 1 km wide, separated by narrow passages typically 40 m deep. On the shelf next to these reefs extensive meadows of the alga Halimeda are found, forming banks 2 7 © 2001 by CRC Press LLC rising 10 to 20 m above the surrounding seafloor. Nonemergent coral reefs are also common throughout the shelf as well as seagrass beds. The surface of reefs is very rugged, with a rugosity that can vary from a few centimetres on a heavily cemented reef flat, to several metres in areas of prolific reef growth. LAND USE Humankind increasingly impacts this ecosystem. Following the findings by Talbot (Chapter 20, this book), the most severe threats may be land use, fishing, and climate change. It is unclear to what level human activities have exacerbated the infestation of crown-of-thorns starfish, which can also greatly damage the coral. Because the natural system exhibits a high degree of variability, it is difficult to quantify human impact. The precautionary principle advocated by Baker (Chapter 1, this book) would dictate that human activities should be controlled so as to minimise their impact on the environment. In practice “business as usual” apparently prevails, usually with some money thrown at science mostly for “monitoring.” There is little use of science in such a working environment, and very little communication between scientists in various disciplines, principally geomorphology, oceanography, botany, and biology. This book has been written to demonstrate the relevance of science and the need for science in planning the future of the GBR. The book emphasises multi-disciplinary processes, i.e., physics–biology links, as these emerge as the dominant forces shap- ing and controlling the ecosystem. Land-based activities clearly threaten the GBR. River water draining pristine rainforest is usually clear even during floods (Animation 2), while that from farmed areas is turbid (Animation 3). Soil erosion is intense in grazed areas (Figure 3). Sugarcane farms commonly extend to the banks of watercourses, often without a pro- tective strip of vegetation to combat soil erosion (Figure 4). Most of the freshwater wetlands have been drained for farming (Figure 5); their filtering effect has been lost. Acid sulfate soils and acid leachate often result in the coastal plains (Figure 6). Human activities at sea also threaten the GBR ecosystem. Key indicator species such as the dugong (Figure 7) are collapsing in the southern half of the GBR, down 50 to 80% in 10 years. Trawling for prawns may devastate the “inter reef” area (Animation 4) and ultimately turn it into a human-generated pasture for target species, as appar- ently has happened in the North Sea (see Chapter 20, this book, by Talbot). As shown by Johnson et al. (Chapter 3, this book), the ecological condition of remaining riparian vegetation in most developed catchments is “poor” to “very poor” and the condition of freshwater wetlands “moderate” to “poor.” Johnson et al. describe broad-scale changes in landcover in GBR catchments. They demonstrate that since European settlement, there has been a substantial reduction in the area of Melaleuca, rainforest, and Eucalyptus-dominated landcover patterns. Their results do not support the view that grazing lands are important sources of sediment on a unit- area basis. Rather, they indicate that land under sugarcane and, by implication, changes in land use, which involve clearing of trees have a detrimental impact on water quality. However, on a catchment basis, it is clear that grazing is likely to be the 8 Oceanographic Processes of Coral Reefs © 2001 by CRC Press LLC principal contributor of sediment and possibly nutrients to the GBR. Johnson et al. conclude that reform is required at policy, planning, and enterprise levels if the impacts of terrestrial activities on the ecological, economic, and social values of the GBR are to be minimised in the future. Furnas and Mitchell (Chapter 4, this book) estimate the riverine nutrient inputs, mostly N and P, to the GBR. They illustrate that this can have a significant effect on both nearshore and shelf-scale nutrient budgets. This input has increased severalfold since the advent of European agricultural practises, but the data are insufficient to accurately quantify the increase. MANGROVES AND SEAGRASS Sediment and nutrients are carried downstream toward the sea and mix with seawa- ter in the mangrove-fringed estuaries. Wolanski et al. (Chapter 5, this book) describe the filtering effect of mangroves. They show that the water circulation in mangrove swamps operates at many scales, from the large-scale tidal dynamics to the small- scale flows around individual roots and pneumatophores. They quantify the residence time of water in mangrove creek, which typically varies between 7 and 50 days. Freshwater inflow, via surface and groundwater flow, and evapotranspiration affect water quality by generating stagnation and aeration zones. In the dry season all the freshwater from riverine inflow evaporates; a salinity maximum zone exists iso- lating the upstream mangroves from the ocean. Bioturbation, principally by crabs, facilitates groundwater flow which ventilates the soils and helps flush out the excess salts in the soils. The recruitment of floating mangrove seeds in the swamp is enhanced only in the dry season. Mangrove creeks are naturally self-scouring even without freshwater runoff. Land reclamation of mangrove swamps for human devel- opment reduces the tidal asymmetry and the natural self-scouring effect resulting in siltation of mangrove channels. The vegetation generates small-scale turbulence maintaining the sediment in suspension in the tidally inundated forest except for a few minutes near slack high tide. As a result the fine sediment from the rivers and the muddy coast spreads all through the mangroves without forming natural levees along the banks. The deposition zones of riverine and oceanic sediment are distinct. The response of mangroves to a sea-level rise depends largely on the availability of fine sediment to infill the swamp. In shallow coastal waters the detritus from mangrove vegetation and the plankton mucus facilitate the flocculation of fine sediment in sus- pension. This generates huge, muddy, micro-aggregates that settle rapidly. This process, together with hydrodynamic trapping effects, enables mangroves, seagrass, and coral reefs to exist in close proximity. These micro-aggregates are very sticky and if they happen to touch live copepods they can glue onto them and entrain them down- ward to their death. The water circulation over the mud shoals fringing the mangroves encourages the recruitment of prawn larvae spawned offshore. The mangroves appear vital to the maintenance of prawn fisheries. Wolanski et al. conclude that the water circulation in mangroves and their coastal water profoundly influences the biology of the system. Physics–Biology Links in the Great Barrier Reef 9 © 2001 by CRC Press LLC 10 Oceanographic Processes of Coral Reefs Duke and Wolanski (Chapter 6, this book) describe how, notwithstanding their beneficial filtering role, mangroves have been, and still are, destroyed for develop- ments. Relying on the filtering effect of mangroves, they argue that rehabilitation of mangroves is necessary to restore downstream marine ecosystems, in re-establishing the vegetation necessary to trap and bind sediments washed from the land, and to reduce the current muddiness of coastal waters. Without this recovery, the longer- term prognosis is not good for maintaining existing seagrass meadows or inshore coral reefs in the GBR. Quantifying the sediment-induced degradation of seagrass and inshore coral reefs is also the focus of Fortes (Chapter 7, this book). He describes the effects of siltation on seagrass, coral reefs, and mangroves with several examples coming from sediment-degraded sites in the Philippines and Thailand. For seagrass, the decrease of light availability modifies the distribution and species composition of seagrass beds. Seagrass responses to increased sedimentation also include adjustments in ver- tical stem elongation or horizontal rhizome expansion, or by re-colonization from seeds. For corals, sediment deposition and suspended sediments affect coral commu- nity structure differently. Sedimentation is among the important factors that deter- mine coral abundance, growth, and distribution. In the Philippines, Acropora completely buried with littoral sediment (16% silt, 38% fine sand, and 38% coarse sand) experienced high mortality. Less sensitive taxa (e.g., Porites), however, were found to recover within a month of exposure. The comparatively fewer number of white and dark bands observed in Porites at a more silted site indicated slower growth rate when compared to colonies at a less silted site. At the population level, density of silt-induced lesions varied among reefs, with smaller colonies and more lesions observed in more exploited and silted areas above a sedimentation threshold rate of about 25 mg/cm 2 /day. Sedimentation affects coral metabolism by decreasing photo- synthetic production, increasing relative respiration, and increasing carbon loss through greater mucus output. For mangroves, high silt loads appear beneficial in the Philippines and Thailand. Mangroves next to rivers draining watersheds larger than 10 km 2 are the most profitable target areas in the efforts promoting natural and artifi- cial colonization of Rhizophora apiculata. FRINGING CORALS The relative cover of corals as opposed to algae may parameterise the health of coral reefs. Since (natural) river floods as well as tropical cyclones thus commonly impact coastal reefs, this relative abundance parameter fluctuates spatially and temporally. Nevertheless it appears possible to quantify the impact of human-induced increases in sediment and nutrients on these reefs. To do that, McCook et al. (Chapter 8, this book) use a mathematical ecological model to formalise and explore potential syner- gistic effects of natural disturbances and eutrophication (increased nutrients) in caus- ing coral reef degradation. The model demonstrates that terrestrial runoff may have serious indirect and long-term impacts when acting in combination with storms, coral bleaching, or crown-of-thorns starfish outbreaks. They demonstrate that the © 2001 by CRC Press LLC Physics–Biology Links in the Great Barrier Reef 11 combined impact of natural, acute disturbances and long-term, chronic, human- induced eutrophication may result in reef degradation even when the system is able to recover from either impact alone. Reinforcing the findings of McCook et al., Fabricius and De’ath (Chapter 9, this book) quantify the influence of turbidity in determining spatial patterns of soft coral biodiversity on the GBR. They find that turbidity and sedimentation affect the generic richness of soft corals, suggesting that a loss of biodiversity could result if turbidity increases due to land use practises which generate soil loss. Also, taxonomic inventories are found to be better indicators of environmental conditions and human impacts than are assessments of total cover. Finally, Fabricius and De’ath found that richness and cover change more within a single site between 0 and 18 m depth, than between reefs hundreds of kilometres apart along the shelf at the same depth. This finding implies that valuable additional information could be gained in a cost-efficient way if monitoring and survey programs covered several depth zones rather than a single depth. ECOSYSTEM CONNECTIVITY Most rivers draining into the GBR are highly seasonal, with often a negligible flow for most of the year and the bulk of the discharge occurring during river floods which usually last only about 1 or 2 weeks. During floods the water discharge is enormous and this generates unsteady river plumes. King et al. (Chapter 10, this book) study these plumes in order to determine the intensity, duration, and frequency of plume impacts on reefs within the GBR. Plume trajectories are complex and event-driven — the wind and the complex bathymetry with headlands and islands interacting with the prevailing currents to generate patchiness. While mid-shelf reefs can be affected under extreme conditions with a minimum dilution of one part river water to three parts seawater, coastal reefs are more frequently and severely affected. Some mid-shelf reefs and most offshore coral reefs may be far enough offshore to be spared the direct impact of increased sediment and nutrient pulses from land use; nevertheless they may still be impacted indirectly. Cappo and Kelley (Chapter 11, this book) describe examples of the landscape interconnections of biotopes to demonstrate the biological and energetic pathways essential to the integrity of the GBR as an ecosystem. Clear gradients and links are shown between biotopes, from offshore reefs to coastal reefs and mangroves, in “places, processes and protein.” The popular view of reefs as somewhat self-contained biological islands, which are linked through episodes of larval dispersal with other reef systems, seems invalid. Further, non-reef (“inter-reef”) communities are shown to be important “load-bearing” ele- ments in terms of the integrity and health of the larger system. The importance of sea- sonal migrations between biotopes appears crucial. Assuming the habitat is not changed by trawling, Gribble (Chapter 12, this book) quantifies the effect of commercial fishing using a trophic-based ecosystem model of the GBR calibrated with results from extensive surveys in the far northern GBR. The model focusses on the effect of trawling on the penaeid prawn community and © 2001 by CRC Press LLC inter-reef habitat. The model suggests only a minor negative impact on prawn popula- tions by trawling because positive effects on the prawns negated the negative effects of direct harvest. These positive effects include the removal of predators or competi- tors as bycatch and the discarded bycatch, which made up a proportion of the diet or were consumed by animals eaten by the prawns. The impact of trawling varied between different species of prawn. The model suggests that a gradual reduction in trawl effort to 50% of current levels resulted in a 59% reduction in Penaeus esculen- tus (tiger prawn) and a 4% reduction in Metapenaeus endeavouri (endeavour prawn). The model also suggests that this reduction in trawl effort would include a dramatic increase in sea-turtle numbers and an increase in small fish omnivores, but also would result in a decrease in species that feed on discards such as seabirds, groupers, sharks, and rays. Clearly, the biodiversity of the GBR appears to be already measurably affected by fishing. Carleton et al. (Chapter 13, this book) describe the effects of water flow around coral reefs on the distribution of pre-settlement fish. The physics–biology links are dominant in determining this distribution. For the observed concentrations of pre- settlement fish to be reproduced in advection-dispersion models, it is necessary for the larvae to swim to and remain near the reef. It is known, from field and laboratory experiments, that such behaviour is reasonable. In accordance with observations, the location of the “hot spots” of larvae varies with the size of the fish. To protect against the ultimate failure of coral reef fisheries, managers have advocated the introduction of marine fisheries reserves. For these reserves to function correctly, these reserves must be situated upstream from the sink reefs open to fish- ing. It is thus necessary to differentiate source reefs (these require maximum protec- tion) from sink reefs (these can be fished). Spagnol et al. (Chapter 14, this book) demonstrate that a tidal blocking effect prevails in the GBR in areas of reef density. By this process the low-frequency currents are steered away from the region during spring tides but not during neap tides. This blocking effect appears to be due to the formation of a tidal boundary layer around reefs at spring tides. At such times the whole assemblage of reefs is largely impervious to the mean longshore currents pre- vailing upstream in low reef density areas. The connectivity between reefs is thus a function of both the reef density and the tidal range. The present location of protected reefs in the GBR was chosen without consideration of this effect. The oceanography appears to be important also to the black marlin fisheries. As Speare and Steinberg demonstrate in Chapter 15 (this book), the arrival of black mar- lin off the Ribbon Reefs around September is coincident with the development of a strong east Australian current. The circulation within the Coral Sea apparently pro- vides a reliable mechanism to facilitate the arrival of mature and gravid fish. Apparently, also, the juveniles may time their southerly migration to coincide with the seasonal abundance of food in coastal waters. Speare and Steinberg identify a number of environmental factors which may be responsible for the considerable inter-annual variation in catch rates. Drew (Chapter 16, this book) demonstrates that tidal currents through the narrow channels in the outer barrier of the northern GBR are strong enough at spring tides to cause Bernoulli upwelling from below the thermocline in the adjacent Coral Sea. 12 Oceanographic Processes of Coral Reefs © 2001 by CRC Press LLC Resulting pulses of cold, nutrient-rich water are carried into the nutrient-depleted waters of the GBR lagoon by discrete tidal jets. This nutrient pumping may sustain the extensive meadows of the alga Halimeda which grow atop 20-m-thick deposits of Halimeda-rich gravel just behind the outer barrier. Similar upwelling into the surface waters of the Coral Sea on the ebbing tide probably sustains the large amounts of phy- toplankton found just outside the reefs. Phytoplankton pumped by the tidal jets can provide a secondary nutrient source after re-mineralisation. CLIMATE In Chapter 17 (this book) Lough describes how the climate of the GBR is dominated by large inter-annual fluctuations that are attributed largely, but not exclusively, to the El Niño phenomenon and to tropical cyclones. There are also large spatial gradients in rainfall and sea-surface temperature, with resulting gradients in impacts on corals. The future of the GBR in a greenhouse-induced warmer world may depend on what will happen to the El Niño-Southern Oscillation (ENSO). A change toward more fre- quent and/or intense ENSO events would lead to reduced rainfall and river flow into the GBR with a likely reduction in the frequency of disturbance by tropical cyclones. More frequent and/or intense anti-ENSO conditions would significantly increase the level of disturbance to the GBR through increased rainfall, river flood events, and enhanced tropical cyclone activity in the vicinity of the reef during summer. Either of these scenarios is likely to be superimposed on warmer land and sea-surface temper- atures. It is also possible that both ENSO and anti-ENSO events become more intense. Reef-building corals appear to be living close to their upper thermal tolerance limits. Mass coral bleaching (which can cause significant coral mortality) is a stress response to higher than average sea-surface temperature during the seasonal warm season. A mass bleaching event occurred in the 1998 austral summer. Skirving and Guinotte (Chapter 18, this book) used satellite data to describe the physical conditions that formed these warm water masses and controlled their movement over the GBR. They demonstrate that a strong correlation existed between the location of these warm water masses and the incidence of bleaching. They also show that vertical mixing was enhanced in areas of high reef density. In these areas exces- sive surface heating was thus prevented and the incidence of coral bleaching was reduced. MANAGEMENT As Suzuki stated in the Introduction, all the GBR management program can focus on is human beings and the way they interact with the natural world. As Cappo and Kelley (Chapter 11, this book) suggested, human activities should be managed with reference to a model that reflects some of these basic physical and biological processes and linkages between reefs, the “inter reef,” and through the coastal fringe into the catchments. What happens if there is neither the scientific background nor Physics–Biology Links in the Great Barrier Reef 13 © 2001 by CRC Press LLC the political will to manage the human impact on reefs? This is described by Dutton et al. (Chapter 19, this book) for Indonesia, the global epicenter of marine biodiver- sity. Direct impacts from overfishing and less direct impacts of runoff from agricul- ture, forest logging, mining, and urbanisation, particularly during the past 30 years, have ravaged coral reefs and related ecosystems. Less than 30% of Indonesia’s reefs are now in good condition. The economic losses caused by overfishing and reef degradation have been estimated at (USD) $410,000/km 2 /year. Dutton et al. describe that several trial programs have been initiated for coral reef management. They show that outside these few, small protected areas there is very little “on ground” manage- ment activity, and that even within these protected areas management effort is largely ineffective and sporadic. Will the GBR be similarly degraded by human activities, notwithstanding the sound scientific knowledge marine scientists have of the ecosystem functioning? Malcolm Fraser, the former Prime Minister of Australia, states in his Foreword to this book that the current generation should not take unnecessary risks to satisfy economic imperatives if there is even the smallest chance of spoiling any part of his heritage. Joe Baker, in his Introduction to this book, states that Economic Rationalism has taken over from the Precautionary Principle, and this is so contrary to the principles of Ecologically Sustainable Development. Talbot (Chapter 20, this book) reviews the state of the GBR environment based on the scientific understanding we now have of the ecosystem. He concludes that without fresh thinking and fundamental attitudinal and management changes, the GBR, just like the Indonesian reefs that Dutton et al. describe, will not “survive” as we enjoy it today. The GBR will be slowly and contin- uously degraded both biologically and aesthetically. ACKNOWLEDGMENTS Every chapter in this book has been peer-reviewed in the same way that scientific papers are reviewed before publication in scientific journals. It is a pleasure to thank the many reviewers in Australia and overseas. Thanks are also due to the Australian Institute of Marine Science (AIMS), IBM–Australia, and the IBM International Foundation that made possible our advances in visualisation in coral reef science, Simon Spagnol whose modelling and visualisation skills are invaluable, and Katie Moore who prepared the visualisation CD accompanying this book. Editing of this book was facilitated by a G. Lemaitre Fellowship at the Catholic University of Louvain and an F. Mosey Fellowship at the University of Western Australia. 14 Oceanographic Processes of Coral Reefs © 2001 by CRC Press LLC Physics–Biology Links in the Great Barrier Reef 15 a FIGURE 1 (a) Location map of the GBR of Australia. (b) Three-dimensional rendering of the GBR between about 24°S (left) and 11°S (right). Note the mountain range running more or less parallel to the coast all along the length of the ecosystem. Note the steep con- tinental slope and the coral reefs scattered on the shelf, principally the mid- and outer-shelf. b FIGURE 2 Three-dimensional rendering of the bathymetry of Bowden Reef, a typical kidney-shaped reef. This reef is about 9 km long and is surrounded by waters about 60 m deep. The windward reef slope is very steep. At the lee side of the reef and in the lagoon there are numerous small coral outcrops reaching all the way to the surface from depths of typically 20 m. The pins locate measurement sites. © 2001 by CRC Press LLC 16 Oceanographic Processes of Coral Reefs FIGURE 3 Photographs taken in the Burdekin River catchment showing (a) overgrazing and (b) the result- ing soil erosion and (c) a typical cattle-induced erosion gully. (Photographs a and b are courtesy of, respec- tively, Dr. Esala Teleni and Dr. Scott Smithers.) a FIGURE 4 Photograph of land cleared for sugarcane farming near Tully. No protective strip of vegetation is kept along water courses at this site. © 2001 by CRC Press LLC [...]...Physics–Biology Links in the Great Barrier Reef 17 FIGURE 5 Photograph of a pumping station to drain wetlands converted to sugarcane farms, near Ingham FIGURE 6 Photograph of acid sulfate leaching from poorly managed areas FIGURE 7 A dugong killed by a fishing net ANIMATION 1 A fly-by of the GBR from south to north Note the changing width of the continental shelf and the successions of areas of high and low reef. .. 20 01 by CRC Press LLC 18 Oceanographic Processes of Coral Reefs ANIMATION 2 A video clip showing clear water draining from pristine rainforest near Ingham (18.5°S), during a river flood ANIMATION 3 A video clip showing sedimentladen, turbid water draining from farms near Ingham (18.5°S) during a river flood ANIMATION 4 A video clip of a trawl net in operation over the non -reef, inter -reef area of the. .. during a river flood ANIMATION 4 A video clip of a trawl net in operation over the non -reef, inter -reef area of the GBR, illustrating the ability of the net to destroy the natural benthos (This animation is reproduced with kind permission of CSIRO and Dr Ian Poiner.) © 20 01 by CRC Press LLC . in mangroves and their coastal water profoundly influences the biology of the system. Physics–Biology Links in the Great Barrier Reef 9 © 20 01 by CRC Press LLC 10 Oceanographic Processes of Coral. other reef systems, seems invalid. Further, non -reef (“inter -reef ) communities are shown to be important “load-bearing” ele- ments in terms of the integrity and health of the larger system. The. con- tinental slope and the coral reefs scattered on the shelf, principally the mid- and outer-shelf. b FIGURE 2 Three-dimensional rendering of the bathymetry of Bowden Reef, a typical kidney-shaped reef.