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LAND-OCEAN INTERACTIONS IN THE COASTAL ZONE March 2000 No. 14 LOICZ NEWSLETTER to the sea is considered by some to The threat mud poses to be of the most threatening impacts on coastal reefs of the Great Barrier the Great Barrier Reef Reef (Bell and Elmetri 1995). of Australia. HISTORICAL CHANGES Eric Wolanski1 and Norman Duke2 1Australian Institute of Marine Science, The impact of man on this sector of [email protected] the coastline of the Great Barrier 2 University of Queensland Reef has been dramatic. Old pho- [email protected] tographs of the Cairns waterfront suggests that the Cairns beach was The Cairns coast (Figure 1) of the sandy at the end of the 19th century, Great Barrier Reef of Australia has it is now muddy and the bed is 1.5 been severely impacted by man- to 2 m higher. This is affirmed by induced erosion. Over the last 100 oral accounts gathered from people years in this area much of the forest living in Cairns through 1930 to the and natural coastal vegetation and 1950s. The consensus is clear and wetlands have been modified to uniform, namely there was a sandy permit urban, port, industrial and beach before, the Esplanade was agricultural development (Wolanski This is the fourteenth newsletter used as a swimming and picnicking 1994, Larcombe et al. 1996, of the Land Ocean Interactions recreational area and there was Wachenfeld et al. 1997). The in the Coastal Zone (LOICZ) even an open saltwater swimming coastal rivers have become drains International Project of the enclosure at the mouth of Trinity bringing eroded mud to settle in the IGBP. It is produced quarterly Inlet - now a mud bank. estuaries, in the coastal shallows to provide news and information and on the inshore reefs. Additional regarding LOICZ activities. Further evidence for this very rapid mud from dredging Cairns port is accumulation of mud comes from a dumped in coastal waters. The re- comparison of the navigation charts sulting addition of nutrient-rich mud from 1878 and 1972 (Figure 2). Figure 1: Map of the coast of the Great Barrier Reef near Cairns Figure 2: Chart of Trinity Bay showing the 0 and 2 depth contours (at low spring tides) in 1878 and 1972. page 2 LOICZ NEWSLETTER This suggests that on the wave- Before European settlement, the tion. This pioneer study was mostly sheltered eastern side of Trinity Bay holocene deposition rate of mud in carried out near Low Isles offshore the intertidal flat has prograded was about 6 m in 6000 years, ie 1 Port Douglas. The scientists mea- seaward by up to 1 km (see the metre/1000 year (P. Larcombe sured visibility weekly and found changes in the 0 m depth contours). pers. com.). In the 20th century, clear water (mean visibility ~ 11 m) Also the 1878 chart refers to the man apparently increased this de- during the South East trade wind tidal flat as a sandy mud bank that position rate to 1 to 2 metres/100 season (the windy, dry season dries at low water spring tides. In years, resulting in: the 1897 sandy when river runoff is negligible). In 1999 this tidal flat dries at 1.5 m beach now buried by 1-2 m of mud; 1997, for the same dry season and above low spring tides and is soft the establishment of new man- distance offshore, the visibility off- mud. The 1878 chart refers to the groves on the western coast of Trin- shore from Double Island was only Cairns beach as a white sand ity Bay since 1952; a wider mud flat 50% of what it was 70 years ago beach barely covered at high water. over the western side of Trinity Bay; near Low Isles (Wolanski and In 1999 the only sand there is a and shallower water by 1-2 m on Spagnol submitted). Assuming the strip of sand a few meters wide and the eastern side. 1927 data are an indicator of natu- dumped there by man to beautify ral conditions, these data suggest the area, everything else is mud. THREATS TO THE GREAT BAR- mud is increasingly being exported Thus a 1.5 m thick layer of mud has RIER REEF from Trinity Bay toward the Great apparently accumulated in the last Mud from Trinity Bay is readily re- Barrier Reef. 100 years and covered the original suspended by wind and is exported beach. This increased siltation has northward alongshore in a turbid Long-term effects of low-level sedi- also apparently resulted in the re- coastal band during southeast trade mentation on coral reefs include cent (since 1952) colonisation of winds. This season lasts six tissue lesions and diseases, de- mangroves along the coast be- months of the year. Wolanski and creased calcification, net productiv- tween the airport and the city Spagnol (submitted) collected ity and growth, and shifts in abun- (Figure 3). oceanographic data to assess the dances and species composition offshore extent of the dispersion of (Dodge et al. 1974, Cortés and Risk the mud along the coast north of 1985, Peters and Pilson 1985, Cairns. For six weeks in August Rogers 1990). Mud in suspension and September 1997, five oceano- as muddy marine snow also physi- graphic moorings were maintained cally smothers coral reef organisms in a cross-shelf transect just north (Fabricius and Wolanski, 2000). of Double Island from the coast to Along this coast, muddy marine Pixie Reef. The moorings logged snow is probably enhanced by data every 10 min on currents and runoff from farms and pastures, turbidity. and appears to be the norm rather than the exception. In coastal waters the suspended solid concentration fluctuated THE PROTECTIVE ROLE OF widely, peaking at 1,000 mg l-1 (1 kg MANGROVES m-3) on windy days, at which time The mangrove forests in the Cairns the visibility was zero (the divers area are located mainly along the could not see their hand against the Barron River and Trinity Inlet, and face mask). Terrigenous mud was they have been measurably re- also found to be exported to off- duced since 1952 (Figure 3); de- shore in a near-bottom muddy layer spite some local increases there a few metres thick (a nepheloid has been a net loss of 15% for the layer). In these offshore waters Barron River mouth, and a net loss events of high suspended solid of 24% for Trinity Inlet. Mangrove concentration (>50 mg l-1 peaking in losses are due to the expansion of one event at 200 mg l-1) occurred the airport, urban areas, industry, occasionally. This suggests that the port facilities and land reclaiming mid-shelf reefs of the Great Barrier for sugar cane farms. In particular Reef in the Cairns area are also a bund wall has destroyed a large ultimately threatened by mud. area of mangroves along the east- ern shore of Trinity Inlet. These Mud in suspension decreases the losses have occurred mainly in visibility, an indicator of the ability of downstream areas, where man- light to be transmitted through the groves best trap mud they capture water. Without light there can be no from suspension in coastal waters photosynthesis and plants cannot (Furukawa & Wolanski 1986). survive, neither can they sustain Based on studies of sedimentation other species which depend on the in mangroves near Cairns airport plants for their food. Knowledge of (Furukawa et al. 1997), the 8.22 Fig 3 Changes to mangroves of Trinity Inlet and 2 the mouth of the Barron River (1952-1998). the past visibility is derived from the km of mangroves that have been 1928-1929 British Museum Expedi- lost would have trapped about 12- page 3 LOICZ NEWSLETTER 25,000 tonnes of sediment per grove plants compared to upstream Inlet, beginning with the removal of year, or 0.5-1 million tonnes over stands, but they are the sites of the bund wall in Trinity Inlet. In this the last 40 years since the man- dominant biomass and widest spa- latter case, however, tis would have groves were destroyed. This mud tial extent. The reason for this to be done in a controlled manner to is now polluting coastal waters of occurrence is related to the supply avoid large-scale contamination by the Great Barrier Reef. This esti- of nutrients with seasonally reduced acid sulfates formed since the bund mate is approximate. However salinities, but the main reason is the walls were constructed. Finally, the even if it was too high by a factor of supply of sediment. Mangroves, mud dredged from Cairns port 2 or 3, it still shows the vital role of particularly Rhizophora species, should be readily used to help man- mangroves in protecting Great Bar- readily colonise mud banks and by groves colonise muddy intertidal ar- rier Reef waters from excessive tur- doing so, they can trap and bind-up eas, instead of dumped at sea bidity (see also Duke 1997). sediments for thousands of years. where it pollutes coastal waters. In These areas may remain stable this way mangroves can be used to Historically these mangrove-fringed provided the processes of recruit- protect this and other areas along estuaries have undergone dramatic ment and tree replacement are not the Great Barrier Reef from excess changes also prior to European set- interrupted. In this way, established mud. tlement (Duke 1997). Trinity Inlet mangrove forests act to regulate was once the mouth of the Mul- and improve water quality. Without CONCLUSIONS grave River before volcanic activity, mangroves, these sediments would The Great Barrier Reef region is many thousands of years ago, redi- be readily flushed further offshore justly inscribed on the World Her- rected this large river to join the during both seasonal and episodic itage list because it is one of the Russell River which enters the sea incidents of increased land catch- mega-biodiverse prime natural as- much further south.