OCEANOGRAPHIC PROCESSES OF CORAL REEFS: Physical and Biological Links in the Great Barrier Reef - Chapter 6

6 Muddy Coastal Waters and Depleted Mangrove Coastlines — Depleted Seagrass and Coral Reefs Norman C. Duke and Eric Wolanski CONTENTS Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 The Importance of Water Clarity in GBR Waters . . . . . . . . . . . . . . . . . . . . . . . . . . 78 Loss of Catchment Vegetation — More Mud in Estuarine Waters . . . . . . . . . . . . . 79 Mud Accumulation and New Mangroves in Downstream Parts of Estuaries and Nearshore Areas. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 The Cost of Ignoring the Role of Mangroves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 A Consequence of Catchment and Mangrove Degradation — The Loss of Seagrass. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 Conclusion — A Holistic Strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 INTRODUCTION Along the tropical northeastern coast of Queensland is one of the outstanding biotic ecosystems in the world, the Great Barrier Reef (GBR), attested to be the only biotic structure in the world visible from space. This complex series of reef communities is based on tiny coral polyps and deep accumulations of their carbonate skeletons over eons. The resulting barrier to ocean waves has created a vast and relatively sheltered coastal lagoon in which other complex biotic tropical ecosystems have flourished in association with coral reefs. Two types of ecosystems dominate these sheltered waters, namely, the mostly sub-tidal seagrass meadows in the extensive coastal lagoon, and mangrove and salt marsh growing along the upper intertidal zone and within all estuaries. These ecosystems are highly dependent not only on each other, but also on prevailing environmental conditions in a dynamic equilibrium. It is also of fundamental importance that each of these biotic communities is based on plants for the provision of both their physical living structure as well as via complex trophic food webs which support a myriad of associated organisms. In this 77 © 2001 by CRC Press LLC 78 Oceanographic Processes of Coral Reefs way, mangroves, salt marsh, seagrass, and reef-building corals provide primary pro-duction by photosynthesis and fixation of atmospheric carbon — a function and role which extend well beyond their mere presence and benefit as habitat. Accumulation of carbon is particularly important for the structure created not only by reef building corals of the GBR, but also within the extensive mangrove forests (Duke, 1997; Alongi, 1998; Alongi et al., 1998). Like coral reefs, mangrove forests provide essential shelter and protection for coastal shorelines in northeastern Queensland. Where corals provide the first line of protection in offshore clearer waters, mangroves provide a second level of protection along nearshore areas where water clarity is often muddy and sediments and substrate are typically soft. Such con-ditions are typically unsuitable for corals. These ecosystems are therefore seen as mutual and symbiotic since each ecosystem cannot prevail or dominate where the conditions might be reversed. The advantage in the relationship for mangroves is based primarily on shelter from strong wave action provided by the coral reef barrier, allowing mangroves to have colonised and stabilised the estuarine soft sediments deposited in river mouths. By contrast, the advantage for corals is based on consis-tently high levels of water clarity primarily, and secondarily on the regular supply of nutrients from terrestrial runoff. Mangroves essentially support these conditions by acting as filters to trap fine sediments and improve water clarity, by binding and hold-ing sediments with their specialised root structures. In this way, nutrients from the land may disperse offshore in relatively clearer waters suitable for coral reef devel-opment (see Fabricius and De’ath, Chapter 9, this book). THE IMPORTANCE OF WATER CLARITY IN GBR WATERS The fundamental role of photosynthesis in each of the key ecosystems demonstrates their dependence on available light. Above the water, these conditions are in common with terrestrial plants, but there are marked differences for those ecosystems restricted to sub-tidal environments, particularly the reef-building corals and seagrass meadows. For these ecosystems, relatively clear water is essential for their existence. Beyond this, additional factors influence coral reef development, particularly where climate and geomorphology might also affect water clarity. At a greater scale, the GBR offshore barrier edge becomes increasingly closer to the Australian coastline toward the northern part (Figure 1). This occurrence gener-ally corresponds with decreasing size of coastal catchment areas. Drainage from coastal areas into GBR waters is dominated by three chief catchment systems: Normanby (24,319 km2 in area), Burdekin (129,860 km2), and Fitzroy (152,640 km2) (also see Johnson et al., Chapter 3, this book). Reef development offshore, ~50, ~100, and ~200 km from the respective estuarine outflow points of these systems, correlates with catchment area. The relationship is indicative of the influence of catchment runoff on nearshore reef development where larger river catchments with higher annual freshwater outflows appear less favourable to coral reef development. A close relationship has evolved between the largely mangrove-fringed shoreline of the northeast Australian coastline and the coral reefs of the GBR. This relationship © 2001 by CRC Press LLC Muddy Coastal Waters and Depleted Mangrove Coastlines 79 involves a delicate, but dynamic, balance between sediment discharge from catch-ment runoff (determined largely by the area of exposed lands, cleared of natural veg-etation — in concert with rainfall) and the amount of riparian and estuarine fringing vegetation (determined partly by the area of mangrove vegetation). Where sufficient amounts of sediment were trapped and held within the estuarine mangrove forests, this had resulted in coastal waters being relatively free of suspended material. Over many thousands of years, these relatively stable and sustainable conditions had allowed for the development of the extensive reef system of the GBR we find today. However, this balance has been severely upset within the last 200 years, and there are important indications of a steady and dramatic decline in coastal ecosystems of the GBR region (e.g., Capelin et al., 1998; Larcombe et al., 1996; Wachenfeld et al., 1997; Wolanski, 1994; Wolanski & Duke, 2000; Zeller, 1998). Much of the dete-rioration appears related to increased levels of water turbidity, seen as muddier coastal waters and shoreline margins (Fabricius & Wolanski, 2000). Furthermore, as discussed in the chapters by Lough (Chapter 17, this book) and Skirving and Guinotte (Chapter 18, this book), there is a corresponding increase in severity and frequency of associated events like coral bleaching and dieback of seagrass. In other instances (Onuf, 1994; Schoellhamer, 1996), a dieback in seagrass meadows was attributed to both low light availability within unusually turbid waters and burial from deposition of suspended sediments from runoff. This process is accelerated by the steady decline in mangrove and salt marsh habitat resulting from human development in coastal areas. These factors are related also to large-scale and on-going clearing of most catchment vegetation, including riparian areas, freshwater wetlands, and tidal mangrove wetlands. The effect has been compounded further by the development of extensive built-up (converted) areas surrounding remaining areas where runoff waters have been channelled directly into coastal waters instead of soaking into soil and being taken up into vegetation and sub-surface aquifers. Furthermore, as discussed in the chapter by Johnson et al. (Chapter 3, this book), the extensive land clearing has lead to higher peak runoff flow rates which equate to significantly greater erosion and removal of sediment into downstream areas, partic-ularly the estuaries. LOSS OF CATCHMENT VEGETATION — MORE MUD IN ESTUARINE WATERS Over the last 100 to 200 years, the catchment areas of all coastal river systems in the GBR region have been impacted by land use change involving the conversion of nat-ural habitat into grazing lands, agricultural cultivation, and mining, as well as urban and industrial development areas. These often dramatic alterations in land use have resulted in the severe decline of natural vegetation, and a rapid increase in erosion of catchment sediments. This erosion has also been increased unnecessarily by the ill-advised depletion of riparian vegetation throughout most catchment areas. For instance, many coastal rivers and streams have become nothing more than straightened, featureless drains with little to stop the export of sediments (Figure 2). These drainage channels rapidly © 2001 by CRC Press LLC 80 Oceanographic Processes of Coral Reefs carry eroded mud to settle in estuaries, and to be carried to coastal shallows and inshore reefs. The impact of these disturbances on coastal catchments has no equal in recent geological time scales. In the study by Neil and Yu (1996), a relationship was shown between catchment runoff and unit sediment yield (USY) in Queensland coastal catchments. When this model was applied to geological data of deteriorating late Holocene climate over 6000 to 7000 years ago in the Brisbane River area, it showed mean flow-weighted sediment concentrations increased from about 90 to 150 mg/l (Capelin et al., 1998). By contrast, mean flow-weighted sediment concentrations increased to 525 mg/l as a consequence of land use intensification following European settlement over the last 200 years in the same area. For any particular level of runoff, based on the data from coastal river systems in the GBR region (Neil & Yu, 1996), the change from natural to disturbed systems involved an increase of 3.5 times the sediment load observed prior to catchment disturbance. As might be expected, the amount of disturbance varies from system to system, as detailed in the chapter by Johnson et al. (Chapter 3, this book). One indication of current catchment condition is provided by the amount of remaining natural vegeta-tion. In Table 1, seven catchment systems in Queensland are shown from Trinity Inlet, around Cairns, to the Moreton Region, around Brisbane. The percentage of remain-ing natural vegetation ranged from 14.9% in the Port Curtis region, to greater than 90% in the Hinchinbrook area. MUD ACCUMULATION AND NEW MANGROVES IN DOWNSTREAM PARTS OF ESTUARIES AND NEARSHORE AREAS Sediment washed down from the catchments has accumulated in estuaries and along nearshore coastlines. Direct evidence of these sediments is notable today as mud along foreshore areas, as well as the often enlarged and new areas of downstream estuarine mangroves. Examples of this can be seen in Trinity Inlet and the Pioneer River estuaries. In Trinity Inlet, the coastline has become muddier over the last 100 years (Wolanski & Duke, 2000). Evidence for the increase in mud, and a general change from a sand–mud-dominated foreshore to the present-day mud-dominated foreshore, is found in several historical records, including old marine charts, historical photo-graphs, and anecdotal accounts of long-time residents. An example of an historical photograph compared with a recent photograph of the same location is shown in Figure 3. The 1878 navigation chart of this area refers to the tidal flat as a sandy mud bank that dries at low water spring tides. In 1999 this tidal flat dried at 1.5 m above low spring tides. An assessment of such images provided evidence of mud accumu-lation up to around 1.5 m above the level of 100 years earlier. A map showing change in mangrove vegetation over 46 years in Trinity Inlet (Figure 4) indicates that accumulation of soft mud sediments was generally a com-mon feature of the region. The areas of new mangroves in Trinity Inlet are all located © 2001 by CRC Press LLC TABLE 1 Changes to Mangrove/Salt Marsh and Catchment Vegetation in Several Estuaries in Queensland until the 1990s, Noting up to 50 Years of Change Estuarine System Period of Comparison (Source) Trinity Inlet 1952–1998 (Wolanski & Duke, 2000) Johnstone River 1951–1992 (Russell & Hales, 1994) Moresby River 1951–1992 (Russell et al., 1996) Hinchinbrook Channel Islands 1943–1991 (Ebert, 1995) Pioneer River 1948–1998 (this article) Port Curtis (Calliope River) 1941–1989 (QDEH, 1994) Moreton Region (SE Queensland) 1974–1987 (Hyland & Butler, 1988; Capelin et al., 1998) Catchment Area (km2) 336 1634 142 1490 2255 21899 Remaining Natural Bushland (km2) 172 886 78 ~95 ~522 336 5694 Remaining Natural Bushland (%) 51.2 54.2 55.3 38.36 ~35 14.9 26.0 Area of Mangrove Vegetation (km2) 31.65 2.02 28.73 0.47 6.29 60.88 136.04 Net Change in Mangroves (km2) 7.67 0.26 6.40 1.2 2.75 16.36 11.32 Net Change in Mangroves (%) 19.5 14.8 28.7 0.47 30.4 21.2 8.3 New Mangroves (km2) 0.92 0.26 6.40 1.2 0.52 1.45 New Mangroves (%) 3.1 14.8 28.7 9.0 2.4 Note: 1 km2 100 ha. © 2001 by CRC Press LLC ... - tailieumienphi.vn