A simple approximation for larval retention around reefs

Estimating larval retention at individual reefs by local scale three-dimensional flows is a significant problem for understanding, and predicting, larval dispersal. Determining larval dispersal commonly involves the use of computationally demanding and expensively calibrated/validated hydrodynamic models that resolve reef wake eddies. This study models variation in larval retention times for a range of reef shapes and circulation regimes, using a reef-scale three-dimensional hydrodynamic model. It also explores how well larval retention time can be estimated based on the “Island Wake Parameter”, a measure of the degree of flow turbulence in the wake of reefs that is a simple function of flow speed, reef dimension, and vertical diffusion. The mean residence times found in the present study (0.48–5.64 days) indicate substantial potential for self-recruitment of species whose larvae are passive, or weak swimmers, for the first several days after release. Results also reveal strong and significant relationships between the Island Wake Parameter and mean residence time, explaining 81–92% of the variability in retention among reefs across a range of unidirectional flow speeds and tidal regimes. These findings suggest that good estimates of larval retention may be obtained from relatively coarse-scale characteristics of the flow, and basic features of reef geomorphology. Such approximations may be a valuable tool for modeling connectivity and meta-population dynamics over large spatial scales, where explicitly characterizing fine-scale flows around reef requires a prohibitive amount of computation and extensive model calibration.     

A quick and cost-effective method for modelling water renewal in shallow coral reef lagoons

Water renewal exerts a preponderant role in eutrophication, yet few hydrodynamic indicators exist for performing quick and cost-effective assessments of ecosystem vulnerability. Using field data, we closed the water budget of a shallow coral reef lagoon recently exposed to high levels of nutrient loading that triggered green tides. Then, we tested the relevance of modelling flushing-time, a proxy of water renewal, from oceanic and atmospheric open access data. Water inflows in the lagoon were mainly driven by waves breaking on exposed reefs, but tide and wind also influenced water renewal during low-wave periods. Modelling flushing-time as a function of the wave features (significant wave height, direction, and period), tide, and wind direction provided the most convincing model, with greater contribution of wave height, and adequately reproduced observations for an independent dataset. Using this model to hindcast flushing-time over the period 2000–2019 highlighted that green tides that recently struck the area in January 2018 and June 2019 followed periods of slow water renewal, which therefore contributed to amplify the blooms. The analysis also demonstrated that renewal events even slower than those recorded in 2018–2019 are frequent in this lagoon, which highlights the high vulnerability of this UNESCO World Heritage Site to other pulses of nutrient loading. Since the methodology developed in this study can be easily applied to many other coastal barrier and fringing reefs, it offers a promising perspective for quick and cost-effective assessments of coral reef vulnerability to eutrophication and other ecosystem crises magnified by slow water renewal.

     

The Benthic Impacts of the Loch Linnhe Artificial Reef

Artificial reefs are man-made structures that are placed on the seabed to mimic natural reefs. The Loch Linnhe Artificial Reef will consist of up to 40 reef modules, when completed, each constructed using concrete blocks. As part of this complex, reef modules of approximately 200 tonnes were deployed in June 2001 and January 2002 and form the basis of this study. Sediment samples were taken adjacent to- and 5 m from the reef perimeters and characterised in terms of sediment oxygenation (redox), particle size, organic carbon (loss on ignition) and coarseness (material >1 mm and <5 mm). Changes in molluscs and ophiuroids (>1 mm) present in the sediment were linked to chemico-physical changes occurring as a consequence of the reef. Both reefs trapped macroalgal phytodetritus at the perimeters, which subsequently decomposed. This accumulated material isolated the seabed from the overlying water column and was associated with a decrease in sediment oxygenation that was linked to changes in community structure. LOI was significantly higher around one reef compared with 5 m from that reef. The reefs did not cause significant changes in the sediment texture (median particle size or coarseness) and neither LOI nor sedimentary texture changes were associated with community change.     

Recruitment constraints in Singapore's fluted giant clam (Tridacna squamosa) population—A dispersal model approach

Recruitment constraints on Singapore''s dwindling fluted giant clam, Tridacna squamosa, population were studied by modelling fertilisation, larval transport, and settlement using real-time hydrodynamic forcing combined with knowledge of spawning characteristics, larval development, behaviour, and settlement cues. Larval transport was simulated using a finite-volume advection-diffusion model coupled to a three-dimensional hydrodynamic model. Three recruitment constraint hypotheses were tested: 1) there is limited connectivity between Singapore''s reefs and other reefs in the region, 2) there is limited exchange within Singapore''s Southern Islands, and 3) there exist low-density constraints to fertilisation efficacy (component Allee effects). Results showed that connectivity among giant clam populations was primarily determined by residual hydrodynamic flows and spawning time, with greatest chances of successful settlement occurring when spawning and subsequent larval dispersal coincided with the period of lowest residual flow. Simulations suggested poor larval transport from reefs located along the Peninsular Malaysia to Singapore, probably due to strong surface currents between the Andaman Sea and South China Sea combined with a major land barrier disrupting larval movement among reefs. The model, however, predicted offshore coral reefs to the southeast of Singapore (Bintan and Batam) may represent a significant source of larvae. Larval exchange within Singapore''s Southern Islands varied substantially depending on the locations of source and sink reefs as well as spawning time; but all simulations resulted in low settler densities (2.1–68.6 settled individuals per 10,000 m²). Poor fertilisation rates predicted by the model indicate that the low density and scattered distribution of the remaining T. squamosa in Singapore are likely to significantly inhibit any natural recovery of local stocks.     

Experiments testing the use of waste material in estuaries as habitat for subtidal organisms

In addition to producing extensive coastal infrastructure, urbanized estuaries are also often littered with large amounts of waste material. This can be used as habitat by fish and benthic plants and animals. Apart from studies of colonization of artificial reefs, which are usually deployed specifically to enhance populations of fish or to replace degraded habitat, there have been few studies that have specifically examined the use of waste material as habitat. Neither have previous studies compared different types of waste material that one characteristically finds in urbanized estuaries, nor the use of small patches of habitat typically created by waste. Spatial comparisons of species found on previously dumped artificial material may be confounded by the fact that different types of waste may be dumped in different places, or may be of different age. This study built small patch reefs of three different types of widespread waste material (tyres, wood or metal) and compared colonization of these over 19 months to colonization of patch reefs of similar age and size made from natural sandstone. Algal assemblages were similar among the different types of reefs, with all showing more cover and diversity on horizontal surfaces. Invertebrates similarly showed few differences among reefs, although there was greater diversity, primarily due to bryozoans, on the vertical surfaces of wooden reefs. Fish rapidly colonized and used all reefs, with cryptic species showing no differences among types of reefs. Schooling species were, however, more common on all of the waste reefs than on the natural sandstone reefs. Small patches of waste material dumped in estuaries can therefore provide useable habitat for a wide range of estuarine organisms and may form a valuable resource if natural habitats continue to be degraded or lost. Although we are not advocating that rubbish simply be discarded into estuaries with the excuse that it provides habitat, removal of existing rubbish should be considered in terms of multiple changes and disturbances to the environment.     

Spatial and temporal variations in turbidity on two inshore turbid reefs on the Great Barrier Reef, Australia

This study describes the natural turbidity regimes at two inshore turbid reefs on the central Great Barrier Reef where wind-driven waves are the main agent of sediment resuspension. Many corals on inshore turbid reefs have adapted to high and fluctuating turbidity, however, anthropogenic activities such as dredging are speculated to produce larger and more prolonged turbidity events that may exceed the environmental tolerance and adaptive capacity of corals on these reefs. Natural turbidity regimes must be described and understood to determine whether and when coral communities on inshore turbid reefs are at risk from anthropogenically elevated turbidity, but at present few baseline studies exist. Here, we present turbidity data from (a) Middle Reef, a semi-protected reef located between Magnetic Island and Townsville and (b) Paluma Shoals, a reef exposed to higher energy wind and waves located in Halifax Bay. Instruments were deployed on both reefs for 16 days to measure spatial and temporal variations in turbidity and its driving forces (waves, currents, tides). Locally driven wind waves were the key driver of turbidity, but the strength of the relationship was dependent on wave exposure. Turbidity regimes thus vary markedly over individual reefs and this is reflected in community assemblage distributions, with a high abundance of heterotrophic corals (e.g. Goniopora) in reef habitats subjected to large fluctuations in turbidity (>100 NTU). A turbidity model developed using local wind speed data explained up to 75 % and up to 46 % of the variance in turbidity at Paluma Shoals and Middle Reef, respectively. Although the model was based on a brief two-week observational period, it reliably predicted variations in 24-h averaged turbidity and identified periods when turbidity rose above ambient baseline levels, offering reef managers insights into turbidity responses to modified climate and coastal sediment delivery regimes.     

Recruitment and larval connectivity of a remnant Acropora community in the Arabian Gulf,United Arab Emirates

Coral cover and community structure in the Arabian Gulf have changed considerably in recent decades. Recurrent bleaching events have dramatically reduced the abundance of previously dominant Acropora corals and have given space to other more thermally resistant coral taxa. The loss of Acropora spp. has reduced reef structural complexity and associated biodiversity. Sir Bu Nair Island (SBN) is a nature reserve in the United Arab Emirates that sustains some of the last dense and extensive Acropora stands in the southern Gulf. This study investigated coral recruitment at a southern coral reef on SBN and examined larval dispersal and reef connectivity between SBN and other local and regional reefs through an agent-based model coupled with a 3D hydrodynamic model. Recruitment was surveyed with settlement tiles deployed from April to September 2019. Contrary to other reefs in the Gulf, we found that Acropora is indeed the major coral recruiter settling at SBN reefs, followed by Porites. The models indicate that SBN reefs are mostly self-seeding but also connected to other reefs in the Gulf. SBN can supply coral larvae to the neighbouring islands Siri and Abu Musa, and nearby reefs along with the north-eastern Emirates, Iranian coast and Strait of Hormuz. Findings highlight the importance of SBN to protect remnant populations of the locally almost extinct Acropora in a region where natural coral recovery is increasingly sparse.

     

The large-scale influence of the Great Barrier Reef matrix on wave attenuation

Offshore reef systems consist of individual reefs, with spaces in between, which together constitute the reef matrix. This is the first comprehensive, large-scale study, of the influence of an offshore reef system on wave climate and wave transmission. The focus was on the Great Barrier Reef (GBR), Australia, utilizing a 16-yr record of wave height from seven satellite altimeters. Within the GBR matrix, the wave climate is not strongly dependent on reef matrix submergence. This suggests that after initial wave breaking at the seaward edge of the reef matrix, wave energy that penetrates the matrix has little depth modulation. There is no clear evidence to suggest that as reef matrix porosity (ratio of spaces between individual reefs to reef area) decreases, wave attenuation increases. This is because individual reefs cast a wave shadow much larger than the reef itself; thus, a matrix of isolated reefs is remarkably effective at attenuating wave energy. This weak dependence of transmitted wave energy on depth of reef submergence, and reef matrix porosity, is also evident in the lee of the GBR matrix. Here, wave conditions appear to be dependent largely on local wind speed, rather than wave conditions either seaward, or within the reef matrix. This is because the GBR matrix is a very effective wave absorber, irrespective of water depth and reef matrix porosity.     

An experimental study of the habitat preferences and movement patterns of copper,quillback, and brown rockfishes (Sebastes spp.)

Synopsis A study was designed to test whether habitat association affects the movement patterns and habitat preferences of copper, quillback, and brown rockfishes. Resighting of tagged rockfishes (512 tagged rockfishes and 726 resightings) from July 1986 through June 1988 indicated that home ranges, movements from reefs, and tendency to return from experimental displacement (up to 8.0 km) were significantly different on the three habitat types compared. On high relief rocky reefs, rockfishes maintained small home ranges (most within 30 m²) and displayed strong reef fidelity that was not affected by season; no off-reef movement was detected and rockfishes generally returned from displacements. On artificial reefs, rockfishes also maintained small home ranges (most within 30 m²), however, there were pronounced seasonal habitat preferences. In the summer, artificial reefs become less suitable; considerable off-reef movement occurred and rockfish did not return from displacements. In contrast, during fall and winter, rockfishes remained on artificial reefs and returned from displacements. On low relief rocky reefs, rockfishes maintained considerably larger home ranges (most within 400 m² and some up to 1500 m²) and habitat use was strongly affected by season; rockfishes only inhabited low relief reefs in the summer and only returned from displacements in the summer coincident when peak algal cover. In addition, there was substantial movement between artificial reefs and adjacent low relief reefs; many rockfishes leave artificial reefs in the summer, move to low relief reefs, and return to artificial reefs in the fall when kelp disappears on low relief reefs. Through habitat assessment, movement in response to habitat quality, and ability to home, these rockfish maintain a flexible behavior for optimal habitat use.     

Salt marsh shoreline geomorphology influences the success of restored oyster reefs and use by associated fauna

Restoration is increasingly implemented as a strategy to mitigate global declines in biogenic habitats, such as salt marshes and oyster reefs. Restoration efforts could be improved if we knew how site characteristics at landscape scales affect the ecological success of these foundation species. In this study, we determined how salt marsh shoreline geomorphologies (e.g. with variable hydrodynamic energy, fetch, erosion rates, and slopes) affect the success of restored intertidal oyster reefs, as well as how fauna utilize restored reefs and forage along marsh habitats. We constructed oyster reefs along three marsh shoreline geomorphologies in May 2012: 1) “creek” (small‐fetch, gradual‐sloped shoreline), “ramp” (large‐fetch, gradual‐sloped shoreline), and “scarp” (large‐fetch, steep‐sloped shoreline). Following recruitment, oyster spat density was greatest on ramp reefs; however, 2 years later, the highest adult oyster densities were found on creek reefs. Total nekton and blue crab catch rates in trawl nets were highest in the creek, while piscivore catch rates in gill nets were highest along the scarp shoreline. We found no difference in predation on snails in the salt marsh behind constructed reef and nonconstructed reference sites, but there were more snails consumed in the creek shoreline, which corresponded with the distribution of their major predator—blue crabs. We conclude that oyster reef construction was most successful for oysters in small‐fetch, gradual‐sloped, creek environments. However, nekton abundance did not always follow the same trends as oyster density, which could suggest constructed reefs may offer similar habitat‐related functions (prey availability and refuge) already present along existing salt marsh borders.     

Native Predators Do Not Influence Invasion Success of Pacific Lionfish on Caribbean Reefs

Biotic resistance, the process by which new colonists are excluded from a community by predation from and/or competition with resident species, can prevent or limit species invasions. We examined whether biotic resistance by native predators on Caribbean coral reefs has influenced the invasion success of red lionfishes (Pterois volitans and Pterois miles), piscivores from the Indo-Pacific. Specifically, we surveyed the abundance (density and biomass) of lionfish and native predatory fishes that could interact with lionfish (either through predation or competition) on 71 reefs in three biogeographic regions of the Caribbean. We recorded protection status of the reefs, and abiotic variables including depth, habitat type, and wind/wave exposure at each site. We found no relationship between the density or biomass of lionfish and that of native predators. However, lionfish densities were significantly lower on windward sites, potentially because of habitat preferences, and in marine protected areas, most likely because of ongoing removal efforts by reserve managers. Our results suggest that interactions with native predators do not influence the colonization or post-establishment population density of invasive lionfish on Caribbean reefs.     

Hydrodynamic diameters of RNA tumor viruses. Studies by laser beat frequency light scattering spectroscopy of avian myeloblastosis and Rauscher murine leukemia viruses.

The diffusion constants of avian myeloblastosis virus (AMV) and murine leukemia virus (MuLV) (Rauscher) suspensions in buffer and in 30% sucrose were determined by laser beat frequency light scattering spectroscopy at a series of temperatures ranging rom 5 to 25 degrees. By the use of the Stokes-Einstein equation, the following hydrodynamic diameters are calculated at 20 degrees: MuLV, 154 plus or minus 3 nm in sucrose and 145 plus or minus 7 nm in buffer; AMV, 144 plus or minus 3 nm in sucrose and 138 plus or minus 4 nm in buffer. While the diameters measured in buffer were temperature independent, the diameters measured in sucrose decreased by about 20% as the temperature was raised from 5 to 25 degrees. The concentration of virus particles in the suspensions ranged from 10 7 to 10 9 particles/ml. The absolute particle concentrations are estimated within plus or minus 30% by determining the dilution needed to reach a concentration sufficiently low that the particle number fluctuation contribution was comparable to that of the interference scattering. Particle weights of 3.9 x 10 8 daltons for MuLV and 4.0 x 10 8 daltons for AMV were calculated from the diffusion constants and from our own experimentally determined sedimentation coefficients. From these particle weights and the hydrodynamic diameters of the viruses, we calculated the per cent of the hydrodynamic volume of the viruses which could be freely penetrated by water, viz., 57% for AMV and 69% for MuLV.     

珊瑚礁区碳循环研究进展

珊瑚礁是海洋中生产力水平最高的生态系统之一,其碳循环受到有机碳代谢(光合作用/呼吸作用)和无机碳代谢(钙化/溶解)两大代谢过程的共同作用,过程十分复杂.珊瑚礁植物的光合作用保证了有机碳的有效补充,动物摄食及微生物降解等生物过程驱动了珊瑚礁区有机碳高效循环,只有不超过7％的有机碳进入沉积物,而向大洋区水平输出的有机碳通量变化幅度较大,主要受到水动力条件的影响.珊瑚礁区碳酸盐沉积(无机碳代谢)是全球碳酸盐库的重要组成部分,年累积量达到全球CaCO3年累积量的23%～26%,是影响大气CO2浓度的重要组成;珊瑚礁是大气CO2源或汇则取决于净有机生产力与净无机生产力的比值(ROI),当ROI <0.6时,珊瑚礁区是大气CO2的源,反之,则是大气CO2的汇.     

River-reef connectivity in the Meso-American Region

The accumulation and seasonal impact of riverine discharge on coral reefs of the Meso-American Region (MAR) were estimated using a numerical simulation of river runoff dispersion. River-reef connectivity, or source-sink dynamics of terrestrial runoff was further assessed from more than 400 watersheds of the region onto discrete coral reef areas. Using land use for 2003 and 2004 in the MAR, this work builds upon a Regional Ocean Modeling System simulation of the MAR validated by ocean color satellite data, and on the monthly river nutrient and sediment load and discharge provided by the World Resources Institute using the N-SPECT model. Analysis of the variability of simulated runoff transport to the reefs showed that reefs of the Mesoamerican Barrier Reef System (MBRS) were mostly impacted from June to September, following the peak time of river discharge. At that time, the coastal and oceanic circulations contribute quickly to expel the runoff from the MBRS. High runoff concentration waters leaving the eastern coast of Honduras during the months of October to December return to the southwestern MAR in March as they are entrained in a cyclonic gyre. Coral reefs of the MAR are thus impacted twice, first from the coastal side with runoff of local origin and later from the oceanic side with runoff from mixed origin. High probability of connectivity between rivers and remote reefs is established as this study revealed that river runoff from the north shore of Honduras has a wide-spread impact on most of the coral reefs of southern Belize, while watersheds on the Gulf of Honduras are mostly connected to coral reefs in the northern shore of Honduras. Although the level of remote influence (or runoff concentration reaching the reef) is lower than the local, the cumulative effect of numerous remote river-reef connections remains significant. Communicated by Biology Editor Michael Lesser     

Halimeda bioherms of the northern Great Barrier Reef

The reefless tract directly behind the ribbon reefs on the outer shelf off Cooktown supports a luxuriant growth of Halimeda that, during the Holocene, has developed into bioherms. These mounded biodies of unconsolidated sediment have formed banks that vary in height between 2 and 20 m. Combined shallow, high-resolution seismic reflection profiles and side-scan sonar have diferentiated three areas of biohermal complexes behind the ribbon reefs of Cooktown. Observations by SCUBA and submersible plus the sedimentology of the bioherms indicate that they are in situ accumulations. Evidence from dating of cores suggests that the Halimeda bioherms began to grow about 10 000 years B.P. and their growth has continued to the present time, even though their tops are presently restricted to a depth of -20 m. It is suggested that the origin and morphology of the bioherms are related to a specific hydrodynamic phenomenon, involving jets of nutrient-rich, upwelled oceanic water intruding onto the outer shelf via the narrow passes between the ribbon reefs, and forming eddies behind the ribbons.     

Spatial patterns in benthic communities and the dynamics of a mosaic ecosystem on the Great Barrier Reef,Australia

The benthic communities of the Great Barrier Reef (GBR) have been characterized as a mosaic with patches at scales of tens to hundreds of kilometres formed by clusters of reefs with comparable environmental settings and histories of disturbance. We use data sets of changes in cover of abundant benthic organisms to examine the relationship between community composition and the dynamics of this mosaic. Our data were compiled from seven annual video surveys of permanent transects on the north-east flanks of up to 52 reefs at different shelf positions throughout most of the GBR. Classification analysis of these data sets identified three distinct groups of reefs, the first dominated by poritid hard corals and alcyoniid soft corals, the second by hard corals of the genus Acropora, and the third by xeniid soft corals. These groups underwent different amounts of change in cover during the period of our study. As acroporan corals are fast growing but susceptible to mortality due to predators and wave action, the group of reefs dominated by this genus displayed rapid rates of growth and loss of cover. In contrast, cover in the remaining groups changed very slowly or remained stable. Some evidence suggests that competition for space may limit growth of acroporan corals and thus rates of change in the group dominated by xeniid soft corals. These contrasting patterns imply that susceptibility to, and recovery from, disturbances such as cyclones, predators, and bleaching events will differ among these groups of reefs.     

Depth estimation using diameter-thickness ratios in larger benthic foraminifera

The diameter-thickness ratio (D/T) of larger benthic foraminifera varies indirectly with depth. Increased turbulence thickens the test, whilst decreased light intensity causes a flatter test. It is shown that in a nearshore-offshore transect the D/T gradient is similar between reefs, but stretched over a larger depth gradient. In facies analysis an increase in D/T can either be caused by decreased hydrodynamic energy, increased transparency or increased depth (or a combination of these).     

Ecohydrodynamics of Cold-Water Coral Reefs: A Case Study of the Mingulay Reef Complex (Western Scotland)

Ecohydrodynamics investigates the hydrodynamic constraints on ecosystems across different temporal and spatial scales. Ecohydrodynamics play a pivotal role in the structure and functioning of marine ecosystems, however the lack of integrated complex flow models for deep-water ecosystems beyond the coastal zone prevents further synthesis in these settings. We present a hydrodynamic model for one of Earth''s most biologically diverse deep-water ecosystems, cold-water coral reefs. The Mingulay Reef Complex (western Scotland) is an inshore seascape of cold-water coral reefs formed by the scleractinian coral Lophelia pertusa. We applied single-image edge detection and composite front maps using satellite remote sensing, to detect oceanographic fronts and peaks of chlorophyll a values that likely affect food supply to corals and other suspension-feeding fauna. We also present a high resolution 3D ocean model to incorporate salient aspects of the regional and local oceanography. Model validation using in situ current speed, direction and sea elevation data confirmed the model''s realistic representation of spatial and temporal aspects of circulation at the reef complex including a tidally driven current regime, eddies, and downwelling phenomena. This novel combination of 3D hydrodynamic modelling and remote sensing in deep-water ecosystems improves our understanding of the temporal and spatial scales of ecological processes occurring in marine systems. The modelled information has been integrated into a 3D GIS, providing a user interface for visualization and interrogation of results that allows wider ecological application of the model and that can provide valuable input for marine biodiversity and conservation applications.     

Modeled connectivity of <Emphasis Type="Italic">Acropora millepora</Emphasis> populations from reefs of the Spratly Islands and the greater South China Sea

The Spratly Island archipelago is a remote network of coral reefs and islands in the South China Sea that is a likely source of coral larvae to the greater region, but about which little is known. Using a particle-tracking model driven by oceanographic data from the Coral Triangle region, we simulated both spring and fall spawning events of Acropora millepora, a common coral species, over a 46-yr period (1960–2005). Simulated population biology of A. millepora included the acquisition and loss of competency, settlement over appropriate benthic habitat, and mortality based on experimental data. The simulations aimed to provide insights into the connectivity of reefs within the Spratly Islands, the settlement of larvae on reefs of the greater South China Sea, and the potential dispersal range of reef organisms from the Spratly Islands. Results suggest that (1) the Spratly Islands may be a significant source of A. millepora larvae for the Palawan reefs (Philippines) and some of the most isolated reefs of the South China Sea; and (2) the relatively isolated western Spratly Islands have limited source reefs supplying them with larvae and fewer of their larvae successfully settling on other reefs. Examination of particle dispersal without biology (settlement and mortality) suggests that larval connectivity is possible throughout the South China Sea and into the Coral Triangle region. Strong differences in the spring versus fall larval connectivity and dispersal highlight the need for a greater understanding of spawning dynamics of the region. This study confirms that the Spratly Islands are likely an important source of larvae for the South China Sea and Coral Triangle region.