Windward Reef Research Articles

Estimation of the abundance of the tropical lobster Panulirus ornatus in Torres Strait, using visual transect-survey methods

The Panulirus ornatus stock in a 25 000 km2 area of Torres Strait was estimated by making visual counts of the number of lobsters in strip transects. Pilot studies in 1988 to assess the feasibility of a full-scale survey and optimize the sampling design showed that: 4×500 m transects were the most cost-effective of the different sizes trialled; two transects per location comprised the most optimal allocation of replication; and ∼300 locations were necessary to achieve a 95% confidence interval of ±10% of the mean density found in the pilot study. Satellite imagery was used to map habitats in Torres Strait, and areas likely to be inhabited by lobsters were classified broadly into three strata: windward reef slope, submerged reef, and deep areas. The 300 locations were allocated to each stratum in proportion to its area and the estimated variance of lobster abundance within it; once allocated, the locations were positioned at random within each stratum. The main survey was undertaken over a period of 7 wk in May–June 1989, and the resulting estimate of lobster abundance was ∼14 million with a 95% confidence interval of ±21%. The surveyed population was sampled concurrently to determine its size structure: the pre-fishery year-class comprised 43% of the population; lobsters greater than legal-size comprised 57% and their average tail weight was 346 g. Thus, the estimate of stock size for the study area was 2200 to 3350 t tail weight, which is roughly ten-fold greater than the annual catch of about 250 t. The current catch is approaching the lower estimates of potential yield, calculated using simple maximal sustainable yield estimators, which suggests that the fishery is unlikely to be under threat at present and may support greater effort.

A novel in situ respirometer for investigating photosynthesis and calcification in crustose coralline algae

This communication describes a novel respirometer for measuring the metabolism in situ of encrusting coralline algae and it provides an example of the data which can be gathered. The respirometer includes a pair of UV-transparent electrode chambers: one accommodates the specimen, the other contains a light sensor. Oxygen, pH and temperature probes monitor changes in the seawater surrounding the specimen while light is recorded concurrently in the second chamber mounted adjacent. A centrifugal pump flushes the sample chamber at regular intervals during deployment. Analog signals from the probes are transmitted via underwater cables to a data-logging device contained within a submersible pressure-resistant housing. The signals are digitized, stored in RAM and later off-loaded to a shore-based microcomputer. Oxygen and pH data are converted into rates of primary production and calcification using equations associated with the alkalinity anomaly technique and experimentally determined values for the photosynthetic and respiratory quotients. On a clear day in May 1986, on the windward reef crest at Lizard Island, Great Barrier Reef, Australia, rates of primary production and calcification were determined for a specimen of Porolithon onkodes (Heydrich) Foslie. The maximum rate of net photosynthesis was 18.9 mmol O 2 · m −2 crust surface · h −1. The average rate of nocturnal respiration was 4.4 mmol O 2· m −2· h −1. The ratio of maximum gross photosynthesis to respiration was 5.25. Using a photosynthetic quotient of 1.21, the data convert to a production rate of 1.5 g C · m −2 · day −1. The maximum rate of calcification during the day was 8.2 mmol CaCO 3· m −2· h −1. At night the specimen dissolved at an average rate of 0.5 mmol CaCO 3· m −2· h −1. Net calcification was estimated to be 8.5 g CaCO 3· m −2· day −1.

Deposition and diagenesis of the windward reef of Enewetak Atoll

Six cores from the northeast reef of Enewetak Atoll reveal that 25 to 50 ft (7.5 to 15 m) of Holocene deposits unconformably overlie at least 200 ft (60 m) of Pleistocene limestone. The Holocene-Pleistocene boundary and four other unconformities within the Pleistocene section are distinguished by paleosols. The unconformities record repeated periods of subaerial emergence and death of the Pleistocene reef associated with low sea-level stands. During emergent periods, meteoric-water diagenesis altered the Pleistocene section. The resulting mineralogy and diagenetic textures are characteristic of the meteoric-vadose diagenetic environment. Aragonite generally composes from 30 to 70% by weight of the Pleistocene limestone and is absent only locally; magnesium calcite is not present. Cementation by calcite is variable but generally increases with depth. Much of the section is poorly cemented and retains high primary and secondary porosity. Micrite and mixed micrite-microspar are the most abundant cements throughout the section. They occur as meniscus, uneven, and irregularly distributed (“patchy”) pore-fill cement. In the upper parts of depositional units, the following paleosol-associated cements are common: caliche crusts, rhizocretions, needle-fiber cement, and lath-crystal cement. Phreatic-type cements are present, but generally are unimportant volumetrically. A reconstruction of the Pleistocene paleohydrology of the atoll indicates that the section has occupied the meteoric-vadose environment during emergent periods. The similarity of Pleistocene limestones beneath Enewetak and other Pacific atolls studied by drilling suggests that for many atolls, most alteration of shallow subsurface Pleistocene limestones has occurred in the meteoric-vadose diagenetic environment.

Developing carbonate platforms: Southern Gulf of Suez, northern Red Sea — Reply

Abstract The Ashrafi reef complex represents two small (∼8–10 × ∼2 km) carbonate platforms and associated shoals located along the western side of the Jubal Strait at the Gulf of Suez's southern end. These features are developing in an active marine environment that is operating within the geologic constraints of a tectonic trough characterized by mountainous fault-controlled margins. Origin of the platforms is uncertain, but the present morphology appears to be a product of the dynamic marine setting in which they are developing. Tidal flow between the Red Sea and the Gulf results in strong rectilinear currents (commonly >50 cm s −1 ). Of the current velocities, 5–10 cm s −1 can be attributed to surface currents driven by a regional, unidirectional wind that blows from north to south down the axis of the Gulf throughout the year. Side-scan sonar data, coupled with echo-sounder profiles, direct observations via Scuba, and bottom sampling indicate that the Ashrafi platforms are actively building northward by windward reef accretion and extending to the south by sediment transport and accumulation. The drumstick shape is a product of wave-induced reef development along the northern margin and sediment distribution forced by strong tidal currents, which, coupled with unidirectional wind drift, favors net transport to the south. Small reefs and hard-grounds along the downdrift platform flanks assume a linearity consistent with the long axis of the platform. The intensely mounded sediment of the platform flanks suggests that bioejection of particles in the mound-building process, together with a strong flow field, is a downdrift sediment transport agent. Although the ambient salinity of surrounding Gulf water is ∼41‰ , platform-top water masses were measured at >47‰ . A lack of significant coral growth on the shallow platform top and the almost total exclusion of sediment-producing calcareous green algae may be related to these hypersaline conditions. Sediments for the platform flanks are largely provided by the breakdown of platform margin reef communities and in-situ organisms (largely foraminifera).

The control of nitrate and ammonium concentrations in a coral reef lagoon

One Tree Reef lagoon is surrounded by an emergent rim which restricts exchange between lagoonal waters and the surrounding ocean. For this reason, the loss rate of dissolved inorganic nitrogen (DIN) through mixing processes is slow in the central lagoon compared to rates of advective input, uptake, regeneration and loss to the atmosphere. We present some hypotheses concerning the importance of these fluxes to the observed patterns of concentration of nitrate+nitrite and ammonium. A scaling analysis of these fluxes indicates that the relative influence of advection across the windward reef crest on lagoonal concentrations changes with season and differs for the two forms of DIN. Advective flux, dominated by DIN derived from production on the algal pavements of the reef crest, is significant in controlling DIN concentration in the peripheral regions of the lagoon. Loss to the atmosphere is a more important flux from the nitrate+nitrite pool in the centre of the lagoon, particularly in summer. Regeneration is a significant input to the ammonium pool of the central lagoon in winter. The relative magnitudes of all fluxes are more similar to each other in the summer than the winter, indicating the potential for shifts in the dominance hierarchy at small time and space scales. One form of DIN in One Tree Reef lagoonal waters (nitrate+nitrite) is controlled by input and another (ammonium) by recycling as well as input. The relative importance of these fluxes changes as a result of temperature pertubations at the physiological level as well as the rate of water turnover at the “system” level. It is proposed that the degree of consistency of the seasonal concentration patterns is a function of the period, rather than the amplitude of the temporal oscillations in the fluxes controlling these concentrations. This has important implications for sampling strategies. This paper provides a conceptual framework for hypothesis testing at a manageable scale, in the context of ecosystem function.

Holocene reef sequences and geochemistry, Britomart Reef, central Great Barrier Reef, Australia

ABSTRACTHolocene reef development was investigated by coring on Britomart Reef, a mid‐shelf reef, 23 km long and 8 km wide situated 120 km north of Townsville in the central Great Barrier Reef (GBR). Two holes were drilled, Britomart 1 on a lagoon patch reef, and Britomart 2 on the windward reef crest. The Holocene reef (25·5 m) is the thickest yet recorded in the GBR and overlies an uneven substrate of weathered Pleistocene limestone.Mineralogical and geochemical analyses show that magnesian calcite and aragonite were converted to low Mg‐calcite below the Holocene‐Pleistocene disconformity. Corals above the interface have 7500–8500 ppm Sr, but 1650–1500 ppm just below it, decreasing to 400–800 ppm downwards. The intermediate Sr values could be due to partial replacement of aragonite by calcite or higher original Sr content in the corals.Three units are recognized in the Holocene: (1) coral boundstone unit, (2) coral framestone unit, and (3) coral rudstone unit. The coral boundstone unit forms the top 5 m of both cores and is algal‐bound coral rubble similar to the present reef top. The coral framestone unit is composed of massive head corals Diploastrea heliopora and Porites sp., and is currently forming in patch reefs situated in the lagoon and along the reef front. The coral rudstone unit comprises coral rudstone and floatstone with unabraded, and unbound, coral clasts in muddy matrix. This matrix may be up to 30% sponge chips.Radiocarbon dating indicates the reef grew more rapidly under the lagoon than under the reef front from 7000 to 5000 yr BP. The rate of reef growth matched existing estimates of sea‐level rise, but lagged approximately 1000 years (5–10 m) behind it. Most of the reef mass accumulated between 8500 and 5000 yr BP as a mound of debris, perhaps stabilized by seagrasses or algae. Accretion of the reef top in a windward direction between 5000 and 3000 yr BP created the present, steep reef‐front profile.

Developing carbonate platforms: Southern Gulf of Suez, northern Red Sea

The Ashrafi reef complex represents two small (∼8–10 × ∼2 km) carbonate platforms and associated shoals located along the western side of the Jubal Strait at the Gulf of Suez's southern end. These features are developing in an active marine environment that is operating within the geologic constraints of a tectonic trough characterized by mountainous fault-controlled margins. Origin of the platforms is uncertain, but the present morphology appears to be a product of the dynamic marine setting in which they are developing. Tidal flow between the Red Sea and the Gulf results in strong rectilinear currents (commonly >50 cm s −1). Of the current velocities, 5–10 cm s −1 can be attributed to surface currents driven by a regional, unidirectional wind that blows from north to south down the axis of the Gulf throughout the year. Side-scan sonar data, coupled with echo-sounder profiles, direct observations via Scuba, and bottom sampling indicate that the Ashrafi platforms are actively building northward by windward reef accretion and extending to the south by sediment transport and accumulation. The drumstick shape is a product of wave-induced reef development along the northern margin and sediment distribution forced by strong tidal currents, which, coupled with unidirectional wind drift, favors net transport to the south. Small reefs and hard-grounds along the downdrift platform flanks assume a linearity consistent with the long axis of the platform. The intensely mounded sediment of the platform flanks suggests that bioejection of particles in the mound-building process, together with a strong flow field, is a downdrift sediment transport agent. Although the ambient salinity of surrounding Gulf water is ∼41‰, platform-top water masses were measured at >47‰. A lack of significant coral growth on the shallow platform top and the almost total exclusion of sediment-producing calcareous green algae may be related to these hypersaline conditions. Sediments for the platform flanks are largely provided by the breakdown of platform margin reef communities and in-situ organisms (largely foraminifera).

Stratification in a small lagoon in the Great Barrier Reef

Temperatures were measured in a small lagoon in the windward reef flat of Davies Reef in the central Great Barrier Reef and examined on three time scales to gain three perspectives on thermal stratification and the trapping of bottom water. Profiling for stratification and dye revealed layering where bottom water was trapped and released by the successive capping and uncapping of the lagoon by a diurnal thermocline. A 1-month monitoring array revealed a solar synchronization, with the temperature of reef-flat water exceeding temperatures of lagoon water by up to 1 5�C within 1 h of midday, and lagoon stratification lagging this by 1 h. There was also a lunar synchronization with mixing proceeding during nocturnal rising tides. Lagoon surface and bottom temperatures were also monitored for 11 months. The amplitude of the diurnal stratification showed no coherence either with the amplitude of the tide (marked spring-neap tides) or with scalar wind stress. The low frequency amplitude of the diurnal oscillation was coherent with the longshore wind vector at periods near 3 6 days and in a band approximately from 10 to 40 days. Daily stratification increased when winds were poleward and decreased when winds were equatonvard. Events of flushing were separated on average by 9 h, but the most frequently observed separation was 5 h and only 10% of separations exceeded 18 h during the 11 months.

Circulation in Enewetak Atoll lagoon1

Currents at Enewetak Atoll, Marshall Islands, were measured on the reef margins, in the channels, and in the lagoon. Lagoon circulation is dominated by wind‐driven downwind surface flow and an upwind middepth return flow. This wind‐driven flow has the characteristics of an Ekman spiral in an enclosed sea. Lagoon flushing is accomplished primarily by surf‐driven water input over the windward (eastern) reefs and southerly drift out the South Channel. Mean water residence time is 1 month, while water entering the northern portion of the atoll takes about 4 months to exit.

Submarine Lithification on Windward Reef Slopes: Capricorn-Bunker Group, Southern Great Barrier Reef

ABSTRACT Windward reef slopes of the Capricorn-Bunker Group, southern Great Barrier reef, are the sites of extensive submarine lithification. Both crystalline (aragonite and bladed Mg-calcite spar) and microcrystalline (micrite and peloids) cements occur within skeletal pores and chambers, whereas interskeletal cements are mainly microcrystalline. These cements are considered to be contemporaneous with reef growth and have developed in an environment characterized by slow accretion rates. Continuous precipitation of aragonite cement has, in some places, been inhibited by the formation of Mg-calcite micrite. Apparent dissolution of some of the Mg-calcite cements was observed, but the processes involved are unknown.

Deposition of Enewetak Atoll Reef, Middle Pleistocene to Holocene: ABSTRACT

Cyclic deposition and diagenesis of a large atoll reef, associated with major eustatic sea-level fluctuation, is documented in six cores from the northeast (windward) reef of Enewetak Atoll. To 250 ft (76 m) depth, the reef section comprises six primary depositional packages that represent periods of reef growth during Pleistocene and Holocene high sea-level stands. These depositional packages are separated by unconformities, former subaerial exposure surfaces, which are demarcated by paleosols. During emergent periods at low sea-level stands, the reef was subjected to meteoric-water diagenesis. However, the depositional textures generally are well preserved. Diagenetic textures, most indicative of meteoric-vadose diagenesis, usually have only partly replaced original tex ures. An oceanward shift of reef environments through time is apparent in a large-scale view of the cores. Reef crests in succeedingly younger depositional packages apparently are offset oceanward. Younger marginal lagoonal deposits unconformably overlie older backreef deposits that unconformably overlie older reef-crest deposits. Within depositional packages, influence of rising sea level on facies development is evident. Subaerial surfaces flooded by rising sea level were rapidly colonized by both solitary corals and patch reefs of diverse composition. Rapid upgrowth of oceanward reef crests led to moderation of wave energy in backreef areas and great increase in deposition of sediment which locally buried and killed patch reefs. Shoaling of backreef areas by sediment aggradation to a tabilized sea level resulted in renewed development of patch reefs and deposition of coarser sediment. End_of_Article - Last_Page 713------------

Copepod swarms: Attributes and role in coral reef ecosystems

Monospecific aggregations of copepods were observed and sampled from eleven coral reefs, two sea grass habitats, and several marine lakes in the Central and Northern Great Barrier Reef Provinces of Australia and in Palau, Western Caroline Islands. Acartia australis, Acartia bispinosa, Oithona oculata, and Centropages orsinii regularly occurred at swarm densities of from 500,000·m−3 (A. australis) to 1,500,000·m−3(O. oculata). During the day copepods swarmed over white sand on the lagoon side of windward reefs, adjacent to or downstream from isolated coral outcrops, with size of swarm dependent on size and shape of the nearby coral head. Swarms 1–2 m in diameter were common; they occasionally exceeded 30 m3. Seasonally these large swarms coalesced and carpeted the lagoon floor with copepods. In Palau O. oculata also swarmed near the surface in a continuous linear band in the solution notch that undercuts the limestone islands. Acartia bispinosa swarmed at the surface in open water in the marine lakes of Palau and over sea grass beds in the lagoon at low water. Swarming probably serves several adaptive functions, but protection from predators appears of major importance. Visual behavior and orientation to surrounding objects maintained swarm integrity, as did mechanical winnowing by currents. At night, swarms dispersed throughout the water column and swarming species sometimes dominated the nocturnal plankton within the reef lagoon. Previous investigations of coral reef zooplankton have not sampled swarming copepods properly and prior estimates of food available to the reef community must be re‐evaluated.

Coral Reef Morphogenesis: A Multidimensional Model

Windward reef morphogenesis is a dynamic process directly controlled by the growth potential of dominant corals and coralline algae relative to wave energy and sea level rise. Moderate wave energy favors vertically rapid growth that is porous and uncemented; high wave energy favors slow but compact growth. Growth potentials of Caribbean and Indo-Pacific reefs are probably equivalent. Major differences are probably not due to biological or direct climatic factors but are in part related to differing patterns of sea level rise resulting from Holocene crustal adjustment. The nature and position of antecedent foundations developed by high interglacial and interstadial sea levels is especially critical and is largely controlled by regional tectonic factors, especially long-term subsidence.

An analysis of the textural variability displayed by inter-reef sediments of the Impure Carbonate Facies in the vicinity of the Howick Group

Recent surface sediments from the continental shelf in the vicinity of the Howick Group have been analysed to determine their textural, chemical and skeletal composition. There is a decrease in terrigenous particle size from quartz sand in the nearshore area to mud offshore. Carbonate particle size decreases from gravel to mud moving away from the reef. Multivariate statistical analysis of grain size data has determined the influence of various sources. Q-mode cluster and factor analysis distinguished four sediment types and indicated that three factors are responsible for 94 % of the textural variation. R-mode cluster analysis showed three distinct size populations. They result from intermixing of material from different sources and do not necessarily reflect distinct modes of transportation. The distribution of the textural sediment types displays concentric elliptical patterns around each platform reef in a zone approximately 2 km wide, within which there is a size gradient from coarse sand and gravel near the windward reef edge to fine sand near the leeward margin.

Calcium Carbonate Production of the Mare Incognitum, the Upper Windward Reef Slope, at Enewetak Atoll

Corals and algal pavement produce calcium carbonate more slowly on the windward reef slope of Enewetak Atoll than on the reef flat despite the high standing crop of reef-building organisms on the slope. The capacity of reefs to remain at or near sea level is therefore not determined primarily by growth on the seaward slope.

Calcium Carbonate Production of the Mare Incognitum , the Upper Windward Reef Slope, at Enewetak Atoll

Corals and algal pavement produce calcium carbonate more slowly on the windward reef slope of Enewetak Atoll than on the reef flat despite the high standing crop of reef-building organisms on the slope. The capacity of reefs to remain at or near sea level is therefore not determined primarily by growth on the seaward slope.

ORGANIC CARBON PRODUCTION ON THE WINDWARD REEF FLAT OF ENIWETOK ATOLL1

Oxygen and carbon dioxide were used to make two separate but simultaneous estimates of organic carbon production on the windward reef flat of Eniwetok Atoll. The methods give correlated results, showing a metabolic ratio (ΔCO2:ΔO2) of −1, uncorrected for gas exchange.A predominantly algal community had both a higher gross production and a higher ratio of gross production to respiration than one visually dominated upstream by algae and downstream by corals. The two communities exhibited similar respiration rates, perhaps as a result of limitations imposed by O2 diffusion rates.The mixed coral‐algal community had an overall gross production to respiration ratio near 1; the coral‐dominated part of that community is apparently heterotrophic. The autotrophic (algal‐dominated) portion of the community may limit the metabolic rate of the heterotrophic (coral‐dominated) portion.

Coral reef and associated invertebrate communities (mainly molluscan) around Mahé, Seychelles

Coral reef and associated invertebrate communities (mainly molluscan) around Mahé, Seychelles

The Morphology of the Fringing Coral Reefs along the East Coast of Mahé, Seychelles

Two types of fringing coral reefs are found along the coast of Mahe: windward reefs exposed to the South East Monsoon, on whose slopes buttresses and grooves are well developed, and sheltered reefs protected from this monsoon, on which buttresses and grooves are not found. The sheltered reefs are dissected by a network of deep channels and depressions which originate in one of three ways: the influx of fresh water from streams, the mechanism by which these reefs advance seaward, and as gaps between fringing reefs around adjacent granite islands. Active linear reefs occur seaward of the sheltered reefs. Two submarine terraces can be recognized: one at about -10 m. and the other at around -20 m. These may result from Pleistocene fluctuations in sea level. The different levels and linear reefs in the forereef environment of the sheltered reefs lead to a complex bottom topography which controls the nature and distribution of the associated unconsolidated sediments.

TAXONOMY AND DISTRIBUTION OF THE MELOBESIOID ALGAE ON RONGELAP ATOLL, MARSHALL ISLANDS

Seven growth forms of the species Neogoniolithon frutescens (Foslie) Setchell and Mason, Porolithon onkodes (Heydrich) Foslie, P. craspedium (Foslie) Foslie, and P. gardineri (Foslie) Foslie found on Rongelap Atoll are described in detail. Two of the growth forms are new combinations: N. frutescens f. flabelliformis (Foslie) Lee and P. gardineri f. abbreviata (Foslie) Lee. Porolithon craspedium and P. gardineri seem to be especially adapted to the atoll environment. The largest individuals and greatest density of organisms were produced by P. craspedium f. craspedium at the leeward reef margin and P. gardineri f. subhemisphaerica at the windward reef margin. With the exception of P. onkodes, a somewhat ubiquitous species, the distributional pattern displayed by the different species on the reef flat at Rongelap indicates that each has a narrow tolerance range for local variations in the environment.