Low Isles Reefs


Generalized geomorphology of a typical reef of the inner shelf, Great Barrier Reef, Australia. Figure is modified from Steers (1930) and Fairbridge (1930). Aerial photograph of Low Isles Reef, taken in 2001 by the Australian Beach Protection Authority. Compare this image with the Low Isles Reef surface map (slide 29). Aerial photomosaic of Low Isles Reef, taken in 1928 by the Allied Forces. Photo was taken to coincide with the work of the Great Barrier Reef Expedition of 1928-1929. Map based on ground surveys and aerial photograph interpretation showing the distribution of geomorphic elements of the Low Isles Reef top in 2001. Evolution of the Low Isles sand cay and positions of beachrock exposures from 1928-2001, based on analysis of aerial photographs and surface mapping. Radiocarbon dating places the age of the sand cay at c. 2550 YBP. Southward-looking view of the Low Isles sand cay at falling tide. Note initial exposure of the reef crest. Northward-looking view of the Low Isles sand cay at low spring tide. Image was taken from the northwestern reef flat. Beachrock exposure along the northern side of the sand cay at low tide. The mangrove swamp that covers the eastern side of the reef flat is visible in the distance. Multiple generations of beach rock are visible along the southern margin of the cay at low tide. Differences in strike recording changes in the orientation of the island. Ed Frankel, John Jell, and Kim Firkins attempt to drill a core. Another image of beach rock on the southern margin of the sand cay. The mainland is visible in the distance. Evolution of shingle rampart systems along the margin of the Low Isles Reef top from 1928 to 2001, based on analysis of aerial photographs and surface mapping. Ramparts, consisting of storm-deposited coral (mainly Acropora) debris, aka shingle, are a common feature of low wooded island-reefs, commonly occupying 5–10% of reef top area. They are most prevalent along the windward peripheries of reefs. Shingle rampart on the NE Reef Flat near the edge of the Mangrove Swamp. These storm-deposited accumulations occur as elongate ridges generally 0.5 to 2 m in height and are commonly asymmetric in cross section, with short, steep inward-facing sides that slope more than 45 degrees and gentle seaward-facing sides that slope less than 10 degrees. Seaward edge of a shingle rampart atop a cemented rocky platform on the eastern margin of the reef. The rocky platform is a flat, wave-washed surface consisting of cemented coral rubble (former ramparts). Radiocarbon dates for the rock platform range from 380 to 800 YBP. Area of the NE Reef Flat covered by shingle. Here, ramparts have formed a barrier that protects a subtidal pond surrounded by mangroves. Inner (north) edge of shingle rampart and adjacent subtidal pool on the southern reef margin. Cemented toes of former shingle ramparts on eastern reef margin, forming a series of features termed ‘bassett edges’ by previous workers. Close-up of a shingle rampart showing unconsolidated shingle (pencil for scale). Close-up of lithified shingle showing standard lens cap for scale. Images showing stages of coral abrasion from materials found in shingle ramparts. Increasing abrasion from left to right. Note presence of encrusing algae on most pristine example. Superimposed maps showing extent of mangrove cover on Low Isles Reef in 1928, 1945, 1982, and 2001, based on analysis of aerial photos and surface mapping. View of the western (leeward) edge of the mangrove swamp at low tide. View of the eastern (windward) edge of the mangrove samp. Note expanse of cemented coral rubble in the foreground and more recent shingle rampart in the background. Inner edge of a shingle rampart on the southeastern (windward) edge of the mangrove swamp. Shingle rampart on the southeastern (windward) edge of the mangrove swamp. Note growth of white mangroves atop the bank. Pictured is Professor John Jell. Entrance of a tidal creek entering into the northern side of the mangrove swamp. Locals have reported crocodile sightings in such settings. Map based on ground surveys and aerial photograph interpretation showing the distribution of geomorphic elements of the Low Isles Reef top in 2001. Soft corals in the Anchorage on the northern side of the reef. Mainland is visible in the background. Leeward reef crest along the Anchorage at low spring tide. Note prominence of soft corals in the foreground. Leeward reef crest along the Anchorage at low spring tide. Note prominence of soft corals in the foreground. Soft corals in the Anchorage at low tide. The murkiness of the water is normal, resulting from the presence of terrigenous mud. The most probable source of terrigenous mud is the Daintree River, the flood plumes of which occasionally reach Low Isles. Typical benthos of shallow water regions of the Low Isles Reef Anchorage (top) in 1928 (Yonge, 1930) and (bottom) in 2002. GPS in foreground is 14.5 cm long. Note change to dominance of soft coral. Windward reef margin at low spring tide. Mangrove swamp is visible in the background. Windward reef margin at low spring tide. Mainland is visible in the background. Closeup of the windward reef margin at low spring tide. Note prominence of soft coral. Northeastern reef flat in 2002 showing now dead microatolls that in 1928 flourished in the moated region north of mangrove swamp. Mangroves in the upper right are growing atop a low shingle rampart that extends southward from the NE tip of the reef. GPS in foreground is 14.5 cm long. Sandy highs and seagrass covered lows of the NW reef flat at low tide. Rippled and bioturbated sand exposed on the NW Reef Flat at low tide. The northern edge of the Mangrove Swamp is visible in the background. Coral rubble-strewn section of the NW Reef Flat at low tide, looking northward toward the Sand Cay. Surface view of seagrass covered region of the NW Reef Flat at low tide. Pencil for scale. Surface view of seagrass covered region of the NW Reef Flat at low tide. Note living macro-atoll in the foreground. SE Reef Flat at low tide. Sandy bed is covered by seagrass and brown noncalcareous algae. Here, shingle ramparts prevent full drainage of the reef top. Mangrove Swamp in background. SW Reef Flat at low tide. Surface is covered by small branching corals. South–central margin of Low Isles Reef in 1928 (top), when the area was part of an extensive moat system that supported scleractinian coral colonies (Yonge, 1930) and in 2002 (bottom), with now-dead branching and digitate coral colonies eroded to level of (dead) microatolls and overlain by mud, coarse sand, and shingle.
 
Low Isles Reef
 
Photographs and diagrams are from Tracy Frank. Photographs of modern reef surfaces were taken between 2000 and 2004, mainly during low spring tides when the reef top was accessible by foot. Historical photographs of the Low Isles Reef surface dating to 1928 were scanned from Yonge (1930).
 

Image Gallery

This gallery of images provides an opportunity to compare two disparate settings of reef development in the Great Barrier Reef (GBR) Province.  Low Isles Reef (16°23’ S, 145°34’ E) is one of 46 coral reef platforms unique to the inner shelf of the northern GBR, which support both sea grass and mangrove growth. Accommodation space on this buildup is largely filled, so carbonate sediment production is minimal. Low Isles Reef lies within reach of flood plumes that emanate from the Daintree River. Moreover, surrounding inter-reef sediments contain a significant proportion of terrigenous mud. In 1928–29, Low Isles Reef was the site of the first, and arguably most comprehensive, study ever carried out on a modern coral reef, mounted by members of the Great Barrier Reef Committee and the Royal Society of London (Yonge, 1930). This year-long ‘‘Great Barrier Reef Expedition’’ to Low Isles yielded a detailed 1:5000 map showing the principal geomorphologic elements of the reef flat. Subsequent to the expedition, investigators returned to note changes, in particular those occurring in the wake of major cyclones (Moorhouse, 1933, 1936; Fairbridge and Teichert, 1947, 1948; Stephenson et al., 1958; Stoddart et al., 1978). In addition to ground surveys, numerous aerial photographs of Low Isles Reef are available, dating from 1928 to the present. Frank and Jell (2006) compiled data from these surveys and aerial photographs into a series of diagrams that document changes in the extent of major reef top elements from 1928-2001. These diagrams are also provided in this gallery. 

Relevant References
 
 FAIRBRIDGE, R.W. and TEIchert, C., 1947. The rampart system at Low Isles 1928–1945. Reports of the Great Barrier Reef Committee, 6, 1–16.

FAIRBRIDGE, R.W. and TEIchert, C., 1948. The Low Isles of the Great Barrier Reef; a new analysis. Geographical Journal, 111, 67–88.

Frank, T.D., and Jell, 22, 474-486. , J.S., 2006. Recent Developments on a Nearshore, Terrigenous-Influenced Reef: Low Isles Reef, Australia. Journal of Coastal Research

MOORHOUSE, F.W., 1933. The cyclone of 1934 and its effects on Low Isles, with special observations on Porites. Reports of the Great Barrier Reef Committee, 4, 37–44.

MOORHOUSE, F.W., 1936. The recently formed natural breastwork on Low Isles. Reports of the Great Barrier Reef Committee, 4, 35– 36.

STEPHENSON, T.A.; ENDEAN, R., and BENNETT, I., 1958. An ecological survey of the marine fauna of Low Isles, Queensland. Australian Journal of Marine and Freshwater Research, 11, 241–267.

STODDART, D.R.; MCLEAN, R.F.; SCOFFIN, T.P., and GIBBS, P.E., 1978. Forty-five years of change on low wooded islands, Great Barrier Reef. Philosophical Transactions of the Royal Society of London, B 284, 63–80.

YONGE, C.M., 1930. A Year on the Great Barrier Reef: the Story of Corals and the Greatest of their Creations. London, UK; New York: Putnam. 

 
Tuesday, September 17, 2013
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