Reef Accretion Rates Research Articles

Holocene coral reef accretion in Hawaii: a function of wave exposure and sea level history

In the high Hawaiian Islands, significant accretion due to coral reef growth is limited by wave exposure and sea level. Holocene coral growth and reef accretion was measured at four stations off Oahu, Hawaii, chosen along a gradient in wave energy from minimum to maximum exposures. The results show that coral growth of living colonies (linear extension) at optimal depths is comparable at all stations (7.7–10.1 mm/y), but significant reef accretion occurs only at wave sheltered stations. At wave sheltered stations in Hanauma Bay and Kaneohe Bay, rates of long term reef accretion are about 2.0 mm/y. At wave exposed stations, off Mamala Bay and Sunset Beach, reef accretion rates are virtually zero in both shallow (1 m) and deeper (optimal) depths (12 m). At wave sheltered stations, such as Kaneohe Bay and Hanauma Bay, Holocene reef accretion is on the order of 10–15 m thick. At wave exposed stations, Holocene accretion is represented by only a thin veneer of living corals resting on antecedent Pleistocene limestone foundations. Modern coral communities in wave exposed environments undergo constant turnover associated with mortality and recruitment or re-growth of fragmented colonies and are rarely thicker than a single living colony. Breakage, scour, and abrasion of living corals during high wave events appears to be the major source of mortality and ultimately limits accretion to wave sheltered environments. Depth is particularly important as a modulator of wave energy. The lack of coral reef accretion along shallow open ocean coastlines may explain the absence of mature barrier reefs in the high Hawaiian Islands.

Growth of the staghorn coral Acropora formosa at Houtman Abrolhos, Western Australia

The growth (extension rate, number of radial branches, skeletal mass, branch diameter) of the␣staghorn coral Acropora formosa (Dana, 1846) was examined at four sites on the Beacon Island platform at Houtman Abrolhos, in subtropical Western Australia (28°S). Sites were at depths of 7 to 11 m, with variable exposure to weather and swell conditions. Two sites on the western reef slope were partly exposed to the oceanic swell, and two sites in the lagoon were largely protected from wave action. Linear extension rate between 1994 and 1995 varied significantly between sites, with greater linear extension at the more protected lagoonal sites. However, accumulation of skeletal mass per branch and number of newly initiated radial branches did not vary significantly between the sites. Carbonate was deposited in similar amounts, but either as porous, rapidly extending branches, or as denser branches which extended more slowly. Branch extension rate over 11.5 mo ranged from a mean of 50.3 mm (range=13 to 93 mm) at a reef slope site to a mean of 76.0 mm (range=31 to 115 mm) at a sheltered lagoonal site. Mean extension rates were almost twice that previously reported for this species in Houtman Abrolhos (37 to 43 mm yr−1) from a shallower site where environmental conditions were apparently sub-optimal. Growth was within the range reported for A. formosa from tropical sites, which is consistent with the relatively high calcification and reef-accretion rates recorded for Houtman Abrolhos in geological and metabolic studies. The role of reduced coral growth-rate in limiting coral reef formation at high latitudes remains equivocal.

Distribution and Abundance of Macroborers in an Upper Miocene Reef System, Mallorca, Spain: Implications for Reef Development and Framework Destruction

The importance of boring organisms within reef environments, as agents of framework destruction, and thus as controls to rates of reef accretion, has been widely recognised. Relatively little, however, is known about variations in the distribution of macroborers (principally sponges, bivalves and worms), within individual reef systems. The Upper Miocene reefs of Mallorca present a unique opportunity in which to assess such distributional variations. Borings are preserved as casts within the molds of former corals, and these trace fossils typically exhibit highly characteristic morphologies. Based on the distribution of different bore morphologies, it has been possible to assess variations in both the distribution and abundance of individual borers, and the intensity of boring activity across the reef system. Different parts of the reef are characterised by different associations of boring organisms, with the highest diversity of borers, and the most intense boring activity, observed within corals from the lagoonal facies. The principal control on distributional patterns appears to have been the morphology and structure of the reef framebuilders. Massive corals are always more extensively bored, and typically contain a higher diversity of boring organisms than branched or platy corals. On a more local scale, variations in sediment accumulation rates, and degree of substrate encrustation appear to have exerted some degree of control. Rates of reef accretion would have been influenced not only by the abundance of different boring organisms in different parts of the reef, but on the response of different framebuilders to boring activity. Platy and branched corals, whilst typically less intensely bored, would have been more vulnerable to the effects of boring, while massive corals, which were often extensively bored, were more capable of surviving such damage. This preservational bias has important implications for interpreting styles and rates of reef accretion.

Geochronology and subsurface stratigraphy of Pukapuka and Rakahanga atolls, Cook Islands: Late Quaternary reef growth and sea level history

Eustatic sea-level cycles superposed on thermal subsidence of an atoll produce layers of high sea-level reefs separated by erosional unconformities. Coral samples from these reefs from cores drilled to 50 m beneath the lagoons of Pukapuka and Rakahanga atolls, northern Cook Islands give electron spin resonance (ESR) and U-series ages ranging from the Holocene to 600,000 yr B.P. Subgroups of these ages and the stratigraphic position of their bounding unconformities define at least 5 periods of reef growth and high sea-level (0–9000 yr B.P., 125,000–180,000 yr B.P., 180,000–230,000 yr B.P., 300,000–460,000 yr B.P., 460,000–650,000 yr B.P.). Only two ages fall within error of the last interglacial high sea-level stand (∼125,000–135,000 yr B.P.). This paucity of ages may result from extensive erosion of the last intergracial reef. In addition, post-depositional isotope exchange may have altered the time ages of three coral samples to apparent ages that fall within glacial stage 6. For the record to be preserved, vertical accretion during rising sea-level must compensate for surface lowering from erosion during sea-level lowstands and subsidence of the atoll; erosion rates (6–63 cm/1000 yr) can therefore be calculated from reef accretion rates (100–400 cm/1000 yr), subsidence rates (2–6 cm/1000 yr), and the duration of island submergence (8–15% of the last 600,000 yr). The stratigraphy of coral ages indicates island subsidence rates of 4.5 ± 2.8 cm/1000 yr for both islands. A model of reef growth and erosion based on the stratigraphy of the Cook Islands atolls suggests average subsidence and erosion rates of between 3–6 and 15–20 cm/1000 yr, respectively.

Responses of coral reefs and reef organisms to sedimentation

Unprecedented development along tropical shorelines is causing severe degradation of coral reefs primarily from increases in sedimentation. Sediment particles smother reef organisms and reduce light available for photosynthesis. Excessive sedmentation can adversely affect the structure and function of the coral reef ecosystem by altering both physical and biological processes. Mean sediment rates and suspended sediment concentrations for reefs not subject to stresses from human activities are < 1 to ca 10 mg cm-* d-' and < 10 mg I-', respectively. Chronic rates and concentrations above these values are 'hlgh'. Heavy sedmentation is associated with fewer coral species, less live coral, lower coral growth rates, greater abundance of branching forms, reduced coral recruitment, decreased calcification, decreased net productivity of corals, and slower rates of reef accretion. Coral species have different capabilities of clearing themselves of sediment particles or surviving lower light levels. Sedlment rejection is a function of morphology, orientation, growth habit, and behavior; and of the amount and type of se lment . Coral growth rates are not simple indicators of sediment levels. Decline of tropical fisheries is partially attributable to deterioration of coral reefs, seagrass beds, and mangroves from sedimentation. Sedimentation can alter the complex interactions between fish and their reef habitat. For example, sedimentation can lull major reef-building corals, leading to eventual collapse of the reef framework. A decline in the amount of shelter the reef provides leads to reductions in both number of individuals and number of species of fish. Currently, we are unable to rigorously predict the responses of coral reefs and reef organisms to excessive sedimentation from coastal development and other sources. Given information on the amount of sediment which will be introduced into the reef environment, the coral community composition, the depth of the reef, the percent coral cover, and the current patterns, we should be able to predict the consequences of a particular activity. Models of physical processes (e.g. sediment transport) must be complemented with better understanding of organism and ecosystem responses to sediment stress. Specifically, we need data on the threshold levels for reef orgarusms and for the reef ecosystem as a whole the levels above which sedimentation has lethal effects for particular species and above which normal functioning of the reef ceases. Additional field studies on the responses of reef organisms to both temgenous and calcium carbonate sediments are necessary. To effectively assess trends on coral reefs, e.g. changes in abundance and spatial arrangement of dominant benthic organisms, scientists must start using standardized monitoring methods. Long-term data sets are critical for tracking these complex ecosystems.

Accretionary Styles in Shelf-Edge Reefs, St. Croix, Virgin Islands: ABSTRACT

Fourteen reef cores were taken in water depths of 8-30 m (25-100 ft) along two contrasting shelf margins on St. Croix, Virgin Islands. Seven of the cores were drilled vertically into the deep reef rimming the narrow (150-300 m, 500-1,000 ft) shelf of Cane Bay. Seven horizontal cores were drilled into the steeply sloping reef walls of Salt River submarine canyon. The Cane Bay cores indicate initial Acropora colonization on a rubble slope of 5,000-6,000 years B.P. (at a water depth of 5 m or 16 ft). A subsequent 3,000-year hiatus in reef accretion was ended by recolonization of head corals (at a water depth of 8 m or 25 ft). The Salt River cores show alternating Montastrea annularis framework and open or sediment-filled voids up to 2 m (6 ft) across. This pattern is related to the complex system of channels, caverns, and overhangs common in this steep reef face. At Cane Bay, vertical accretion rates averaged 1.16 m/1,000 years (45.7 in./1,000 years), with a range of 0.17-1.69 m/1,000 years (6.7-66.5 in./1,000 years). Lateral accretion rates at Salt River averaged 1.03 m/1,000 years (40.6 in./1,000 years), with a range of 0.84-1.38 m/1,000 years (33.1-54.3 in./1,000 years). The surprisingly high lateral accretion rates at Salt River are largely the result of down-faulted reef blocks causing repetitions of section. Comparison of reef-accretion rates based on the cores with calcification potential for the reef (primarily as coralgal growth) indicates that more than 60% of the carbonate incorporated in the reef is ultimately reduced to sediment. Unlike that on Pacific reefs, this material is moved seaward and off the shelf edge, primarily during major storms. Such early degradation of the reef relates to the under representation of reef framework in the ancient record. End_of_Article - Last_Page 488------------