Isolated Coral Reef Research Articles

Local coral connections within an atoll reef system underlie reef resilience and persistence

AbstractThe recovery of isolated reef systems is a complex process that is usually associated with the supply of coral larvae from distant reefs (or large‐scale connectivity). However, a frequently neglected process is the potential for supply within the reef itself (or local connectivity). In this study, we quantify and characterize the role of local connectivity over 21 yr of simulated annual coral spawning on an isolated coral reef atoll using outputs from a high‐resolution biophysical model (< 150 m horizontal resolution) along with network analysis. We find that approximatively half of the coral reef larvae dispersal remains local (within 100 s m to 10 s km of release location), while the remaining half contributes to long‐distance dispersal (> 100 s km) and is exported away from the system. Local dispersal plays a pivotal role in creating a highly‐connected network across the reef, enhancing exchanges of larvae within the same reef patches (local retention), across reef zones (e.g., lagoon, reef flat), and across the larger reef system. Finally, we show that this highly‐connected network exhibits a certain level of robustness, even when exposed to environmental stressors such as thermal‐induced mortality. Our findings highlight the previously overlooked role of local scale dispersal in driving recovery of isolated reef systems and emphasize the importance of targeted local management actions, indicating that efforts directed at enhancing and preserving local connectivity can have a substantial impact on the overall health and resilience of isolated reef ecosystems.

Neogene carbonate platform development in the southern South China Sea: Evidence from calcareous microfossils

Biogenic reefs are unique geologic bodies that are the products of biological activity in tropical and subtropical oceans and critical to global carbon cycling. The South China Sea (SCS) has developed largest carbonate platforms in the Western Pacific. The fully-cored Well NK-1, which is drilled through the reef complex on the Meiji Atoll, Nansha Islands, offers good opportunity for assessing the long-term evolutions of biogenic reefs in the SCS. The stratigraphic ages based on palaeomagnetic and Sr isotopic data are controversial in some intervals of core NK-1. To reveal the evolution history of Nansha Islands and their controlling factors, more credible age constraints on core NK-1 are needed. This research presents new records of calcareous microfossils (calcareous nannofossil and foraminifer) abundances and diversities in the biogenic reef carbonates of core NK-1. Based on the assemblages of the calcareous microfossils, seven biostratigraphic datums are recognized from the sequence of core NK-1, revealing a general continuous succession of the earliest Miocene to Pleistocene age. By combining with the magnetostratigraphic data, the development and evolution of carbonate platform in Nansha Islands are reconstructed. The bottom age of the biogenic reef sequence of NK-1 is estimated for 23.03 ± 1.06 Ma, close to the Oligocene/Miocene boundary. The new chronology highlights three short hiatuses of 3.6–2.4 Ma, 11.1–10.0 Ma, and 13.5–11.9 Ma, which are well correlated with the prominent exposed surfaces at 210.3 m, 464.6 m and 561.6 m, respectively. It reveals remarkably similar evolution history of carbonate platforms in the SCS since Neogene by comparing with records of stratigraphy and growth rate from deep drilling cores on isolated coral reefs in Nansha and Xisha Islands. The results showed that the SCS carbonate platform developments, which go through Late Oligocene / Early Miocene initiation phase, Early-Middle Miocene rapid growth phase, Middle-Late Miocene erosion phase, Miocene-Pliocene stable development phase and Quaternary growth phase, are mainly influenced by global sea level changes, environmental factors and regional tectonism such as the SCS basin spreading, Sabah Orogeny Movement and Dongsha Movement.

Spatial and temporal patterns in the coral assemblage at Clipperton Atoll: a sentinel reef in the Eastern Tropical Pacific

Isolated coral reef habitats are unique systems to study the natural dynamics of coral traits and their natural acclimatization, adaptation, and recovery from global-scale stressors such as thermally induced bleaching events. This study evaluates the spatial and temporal changes in coral community attributes (diversity, live cover, and coral assemblage structure) over 14 years (2005–2019) at Clipperton, an extremely remote Eastern Tropical Pacific (ETP) atoll. The atoll exhibited overall high coral cover (~ 50–60%) dominated by massive species (Porites spp.), yet we observed large variation (44–56%) in coral community attributes among survey years (2005, 2016, 2019) with depth explaining most of the variation. Live coral cover increased in 2019 after a severe thermal stress event (El Niño, 2015–2016) and many tropical cyclones, which also caused a shift in assemblage structure from branching Pocillopora to massive Porites in the shallower reef zones, resulting in a less well-defined depth gradient. These changes in coral assemblage structure may have long-term effects on the configuration of the physical reef framework of the well-conserved coral reef ecosystems at Clipperton and consequently may alter the ecological functionality of one of the most important biogeographic stepping stones in the central Pacific and ETP regions.

Being Isolated and Protected Is Better Than Just Being Isolated: A Case Study From the Alacranes Reef, Mexico

Isolated reefs suffered from overfishing to a lesser extent than coastal reefs systems. Nevertheless, the exploitation of coastal areas forced fishers to move offshore to compensate for their decreasing catches, helped by an overall increase in their technological capabilities. Thus, these once pristine isolated areas are under increasing threat and should be considered a protection priority. Here, we compared the fish and epibenthic community of two isolated coral reefs in the southern Gulf of Mexico: one is the Alacranes reefs a Marine Protected Area (MPA) with two No-Take Zones (NTZs) and the other is Bajos del Norte an Open Area (OA). Alacranes reef is ~135 km off the northern coast of the Yucatan Peninsula while Bajos del Norte ~160 km. Our results show that despite being isolated, the OA reefs never reach the same biomass levels of MPA or NTZ reefs which in some cases exceeded 1 ton/ha. Furthermore, 70% of the reefs within NTZ and MPA had a large predatory fish (e.g. sharks, jackfish, groupers) biomass relative contribution larger than 25%, while only two reefs in the OA had a relative piscivore contribution of more than 25%. Large predatory fish have a larger contribution to overall biomass within NTZs than MPAs reefs, even at low biomass values, suggesting that NTZ is more effective in protecting large predators. NTZ and MPA reefs also showed from 5 to 10% higher coral cover than OA which had a higher erect algae cover (~22.9%). Indicating that the disruption of the trophic pyramid caused by the fishing pressure in OA reefs is reflecting on the trophic cascades controlling the benthic communities balance. These isolated areas represent the last pillars of natural diversity and are under such an increasing anthropogenic pressure that just being isolated is not enough anymore to guarantee a safe zone from detrimental activities like overfishing. Immediate establishment of protective NTZ is needed to maintain the role they play as a natural wilderness capital.

Functional convergence in macroalgal assemblages of isolated coral reefs in the Mozambique Channel

The understanding of macroalgae functions and processes requires a good understanding of the spatial distribution of the functional diversity of macroalgae. In coral reef environments, this information remains fragmentary. Here, based on 314 species sorted according to a set of 10 functional traits, the functional niches of macroalgae at three remote coral reefs of the Iles Eparses in the Indian Ocean (Europa, Glorioso, and Juan de Nova) are described. For the comparison of intra- and inter-reef functional structures, we characterized both taxonomic and functional beta diversities, and their turnover and nestedness-resultant components. Within the three reefs, we observed strong taxonomic and functional dissimilarities across sampling sites, mainly determined by turnover. Null models highlighted several processes, which structured macroalgal assemblages across sites: a combined effect of environmental variables (geomorphology and wave exposure), limiting similarity and stochastic effects. At the inter-reef scale, the three reefs only shared a small number of species, but the functional beta diversity between Glorioso and Juan de Nova was weak. This suggested that although assemblages were different, fairly similar environmental conditions may have homogenized macroalgae functions through both ecological and evolutionary scale processes. Our results support the idea that macroalgal assemblages can provide similar functional trait portfolios, despite distinct species composition. We stress the need to focus on macroalgae life-history traits for a better understanding of the processes structuring their communities.

Innovation and technology in marine science: AIMS' North West Shoals to Shore Research Program – an update

In 2017, the Australian Institute of Marine Science and its partners commenced the North West Shoals to Shore Research Program. The program is designed to address significant scientific and environmental knowledge gaps pertinent to the management of the offshore petroleum industry, a key stakeholder in this ecologically and commercially important region of Australia. The program comprises four themes. 1. Marine noise monitoring and impacts: includes two seismic source (2600 cubic inch air-gun array) exposure experiments have been conducted to investigate selected responses by demersal fishes and pearl oysters across different spatial and temporal scales. 2. Seabed habitats and demersal biodiversity: seeks to understand the physical and biological characteristics of the ancient coastline key ecological feature around the 125 m depth contour and pearl oyster habitats offshore from Eighty Mile Beach. The work examines the ecological processes that maintain benthic communities on both ancient and contemporary coastlines 3. Protected and iconic species movement, distribution and threats: uses innovative sampling techniques to confirm biologically important areas for pygmy blue whales, hawksbill and green turtles. This will assist the quantification and mitigation of the risks vessel movements, industrial infrastructure and activities pose to marine megafauna on the Northwest Shelf. 4. Spatial dynamics of isolated coral reef atolls: develops a habitat model and adaptive monitoring program that informs the future condition of these remote coral reef atolls. Significant progress has been made by the program in 2018, including the development of innovative and technical approaches to sampling.

Cryptic lineages in the Wolf Cardinalfish living in sympatry on remote coral atolls

Coral reef health and biodiversity is under threat worldwide due to rapid climate change. However, much of the inter- and intra-specific diversity of coral reefs are undescribed even in well studied taxa such as fish. Delimiting previously unrecognised diversity is important for understanding the processes that generate and sustain biodiversity in coral reef ecosystems and informing strategies for their conservation and management. Many taxa that inhabit geographically isolated coral reefs rely on self-recruitment for population persistence, providing the opportunity for the evolution of unique genetic lineages through divergent selection and reproductive isolation. Many such lineages in corals and fish are morphologically similar or indistinguishable. Here, we report the discovery and characterisation of cryptic lineages of the Wolf Cardinalfish, Cheilodipterus artus, from the coral atolls of northwest Australia using multiple molecular markers from mitochondrial (CO1 and D-loop) and nuclear (microsatellites) DNA. Concordant results from all markers identified two highly divergent lineages that are morphologically cryptic and reproductively isolated. These lineages co-occurred at daytime resting sites, but the relative abundance of each lineage was strongly correlated with wave exposure. It appears, therefore, that fish from each lineage are better adapted to different microhabitats. Such cryptic and ecologically based diversity appears to be common in these atolls and may well aid resilience of these systems. Our results also highlight that underwater surveys based on visual identification clearly underestimate biodiversity, and that a taxonomic revision of the Cheilodipterus genus is necessary.

Nutrient fluxes into an isolated coral reef atoll by tidally driven internal bores

AbstractScott Reef is a remote yet biodiverse coral reef atoll, rising from depths > 1000 m on the edge of the continental shelf of northwestern Australia. An intensive 2‐week field study was conducted to assess how physical oceanographic processes influence reef communities in South Scott's deep (~ 50 m) and expansive (~ 300 km2) semi‐enclosed lagoon. The study covered a spring–neap tidal cycle, during which moored instruments measured temperature and velocities at a lagoon entrance and ship‐based vertical profilers measured water quality parameters. Tidally driven internal bores advected cooler and deeper offshore waters (up to 4.5°C, mean 2.4°C) into the lagoon, which were also richer in nutrients and chlorophyll a than lagoon waters. This offshore water originated from approximately 75 m depth. We used the strong observed relationship between in situ measurements of nitrate and temperature to estimate nitrate concentrations from high temporally and spatially resolved temperature measurements at the lagoon entrance, and then estimated the horizontal nitrate fluxes into the lagoon using the velocity measurements. The depth‐integrated, time‐averaged nitrate flux during spring tides (1.46 kg m−1 d−1) was over three times larger than during neap tides (0.46 kg m−1 d−1), despite a passing tropical storm that temporarily deepened the offshore thermocline and reduced the magnitude of the fluxes. Our observations indicate that colder, deeper, and nutrient‐rich water associated with internal tidal bores during spring tide is likely to be the primary mechanism through which allochtonous nutrients are delivered to benthic and pelagic communities within the lagoon of this isolated atoll.

Population connectivity among shallow and mesophotic Montastraea cavernosa corals in the Gulf of Mexico identifies potential for refugia

Successful management of spatially isolated coral reefs is contingent on an understanding of ecological connections across populations. To investigate genetic connectivity of the depth-generalist coral species Montastraea cavernosa, populations from both shallow (15–30 m) and mesophotic coral ecosystems (30–70 m) in the Gulf of Mexico (GOM) were analyzed with microsatellite genotyping. A series of upstream and downstream sites were chosen in marine protected areas including Carrie Bow Cay, Belize; Flower Garden Banks and nearby mesophotic bank habitats; Pulley Ridge; and Dry Tortugas. Patterns of genetic diversity within the northwest GOM supported relatively open coral populations with high levels of gene flow between shallow and mesophotic depth zones, consistent with strong oceanographic patterns and hypothesized availability of coral reef habitats in the GOM. Conversely, genetic differentiation within Belize and the southeast GOM indicate relative isolation of shallow and mesophotic M. cavernosa populations in these regions. Structure analysis showed dominant genetic clusters within each region that did not correlate strongly with depth zones, and identified a cluster of unknown origin contributing to high differentiation at Pulley Ridge. Migration modeling predicted historical region-wide panmixia for most regions, with Pulley Ridge appearing to be a potential sink population. The GOM appears to demonstrate stronger evidence of vertical connectivity compared to elsewhere in the Tropical Western Atlantic, which may be the result of oceanographic variability and/or lack of local selection at depth. These findings are consistent with previous studies identifying genetic connectivity of broadcast-spawning corals across broad spatial scales and highlight the potential importance of mesophotic habitats in the GOM as larval sources to geographically distant populations.

Geography and island geomorphology shape fish assemblage structure on isolated coral reef systems.

We quantify the relative importance of multi‐scale drivers of reef fish assemblage structure on isolated coral reefs at the intersection of the Indian and Indo‐Pacific biogeographical provinces. Large (>30 cm), functionally‐important and commonly targeted species of fish, were surveyed on the outer reef crest/front at 38 coral reef sites spread across three oceanic coral reef systems (i.e. Christmas Island, Cocos (Keeling) Islands and the Rowley Shoals), in the tropical Indian Ocean (c. 1.126 x 106 km2). The effects of coral cover, exposure, fishing pressure, lagoon size and geographical context, on observed patterns of fish assemblage structure were modelled using Multivariate Regression Trees. Reef fish assemblages were clearly separated in space with geographical location explaining ~53 % of the observed variation. Lagoon size, within each isolated reef system was an equally effective proxy for explaining fish assemblage structure. Among local‐scale variables, ‘distance from port’, a proxy for the influence of fishing, explained 5.2% of total variation and separated the four most isolated reefs from Cocos (Keeling) Island, from reefs with closer boating access. Other factors were not significant. Major divisions in assemblage structure were driven by sister taxa that displayed little geographical overlap between reef systems and low abundances of several species on Christmas Island corresponding to small lagoon habitats. Exclusion of geographical context from the analysis resulted in local processes explaining 47.3% of the variation, highlighting the importance of controlling for spatial correlation to understand the drivers of fish assemblage structure. Our results suggest reef fish assemblage structure on remote coral reef systems in the tropical eastern Indian Ocean reflects a biogeographical legacy of isolation between Indian and Pacific fish faunas and geomorphological variation within the region, more than local fishing pressure or reef condition. Our findings re‐emphasise the importance that historical processes play in structuring contemporary biotic communities.

Resolving high-frequency internal waves generated at an isolated coral atoll using an unstructured grid ocean model

We apply the unstructured grid hydrodynamic model SUNTANS to investigate the internal wave dynamics around Scott Reef, Western Australia, an isolated coral reef atoll located on the edge of the continental shelf in water depths of 500,m and more. The atoll is subject to strong semi-diurnal tidal forcing and consists of two relatively shallow lagoons separated by a 500 m deep, 2 km wide and 15 km long channel. We focus on the dynamics in this channel as the internal tide-driven flow and resulting mixing is thought to be a key mechanism controlling heat and nutrient fluxes into the reef lagoons. We use an unstructured grid to discretise the domain and capture both the complex topography and the range of internal wave length scales in the channel flow. The model internal wave field shows super-tidal frequency lee waves generated by the combination of the steep channel topography and strong tidal flow. We evaluate the model performance using observations of velocity and temperature from two through water-column moorings in the channel separating the two reefs. Three different global ocean state estimate datasets (global HYCOM, CSIRO Bluelink, CSIRO climatology atlas) were used to provide the model initial and boundary conditions, and the model outputs from each were evaluated against the field observations. The scenario incorporating the CSIRO Bluelink data performed best in terms of through-water column Murphy skill scores of water temperature and eastward velocity variability in the channel. The model captures the observed vertical structure of the tidal (M2) and super-tidal (M4) frequency temperature and velocity oscillations. The model also predicts the direction and magnitude of the M2 internal tide energy flux. An energy analysis reveals a net convergence of the M2 energy flux and a divergence of the M4 energy flux in the channel, indicating the channel is a region of either energy transfer to higher frequencies or energy loss to dissipation. This conclusion is supported by the mooring observations that reveal high frequency lee waves breaking on the turning phase of the tide.

Coral reef fish assemblages at Clipperton Atoll (Eastern Tropical Pacific) and their relationship with coral cover

Clipperton Atoll, one of the most isolated coral reefs worldwide, is of great scientific interest due to its geomorphology and high levels of endemism. This study explored the reef fish assemblage structure of Clipperton Atoll and its relationship with live coral cover. Nine stations were sampled at three sites and three depths (6, 12 and 20 m) around the reef, measuring fish species richness and biomass and hermatypic coral cover (at genus level). We evaluated variation in species richness, biomass and diversity of fish assemblages among sites and depths, as well as the relationship between the entire fish assemblage composition and live coral cover. The results showed that species richness and biomass were similar among sites, but differed across depths, increasing with depth. In contrast, diversity differed among sites but not among depths. Multivariate analyses indicated that fish assemblage composition differed among sites and depths in relation to changes in cover of coral of the genera Pocillopora, Porites, Pavona and Leptoseris, which dominate at different depths. The results showed that fish species richness and diversity were low at Clipperton Atoll and that, in isolated coral reefs with a low habitat heterogeneity and low human disturbance, live coral cover has a significant influence on the spatial variation of the reef fish assemblages. This study highlights the importance of coral habitat structure in shaping coral reef fish assemblages.

Mapping of selected coral reefs in Southern, Sri Lanka using remote sensing methods

Most parts of the Southern coastal belt of Sri Lanka are fringed with shallow water coral reefs of high economic, ecological and socioeconomic attributes. Polhena, Madiha and Dondra are some of the places where the isolated coral reefs are located in this region. Descriptive details such as the extent, reef margins, and coral compositions of these reefs are lacking due to the difficulties of accessibility, harsh conditions and the lacking of trained expertise for reef mapping. Remote sensing can be used as a tool to identify the reef locations, spatial distribution, bottom nature, present status of the reef area for their effective management and conservation. In this research, bathymetry maps were produced using ‘Depth of Penetration’ method for Landsat7 ETM+ satellite images. Image band 1 (0.45-0.515μm), band2 (0.525-0.605μm), band3 (0.63-0.69μm) and band4 (0.75-0.9μm) have been used as main inputs of bathymetry mapping. Colour composites, classified images and manually digitized images were used to map the spatial pattern of the reefs. The image processing has been carried out with ILWIS 3.3 and ArcGIS 9.3 software. Created maps were validated using ground truth data which were taken during the field survey. The Pearson correlation coefficient (r=0.921) indicates that there is a high correlation between true depth and the calculated depth from the map. The standard error of the estimate is 0.85. According to the created GIS maps, maximum depth (1.83±0.79m) was recorded in Dondra reef while lowest depth (1.46±1.23 m) was recorded in Polhena reef. Depth of the madiha reef was 1.81±0.99 m. Highest reef area represented by the Polhena reef (240300 m 2 ) while lowest area represented by the Madiha reef (51300 m 2 ). Area of the Dondra reef was 85500 m 2 . Band 4 is the best for shallow ( DOI: http://dx.doi.org/10.4038/sljas.v19i0.7450 Sri Lanka J. Aquat. Sci. 19 (2014): 41-55

Residency patterns and movements of grey reef sharks (Carcharhinus amblyrhynchos) in semi-isolated coral reef habitats

The degree of reef isolation may limit the frequency of long-range dispersals in reef-associated sharks. Therefore, understanding how the behaviour and spatial ecology of a species differs across reef habitats is essential for developing sound conservation approaches. The present study examined the residency, movement and activity space of grey reef sharks (Carcharhinus amblyrhynchos) in the central Great Barrier Reef (GBR). An array of 56 acoustic receivers covering 17 semi-isolated coral reefs across 150 km was used to monitor shark movements. Forty C. amblyrhynchos were tagged with acoustic transmitters and monitored from 251 to 821 days. Most sharks were detected on a single reef; however, some individuals (4 females; 10 males) moved to up to five reefs. Residency index ranged from 0.02 to 1.0, with a mean ± SD of 0.78 ± 0.26. Mixed-effect models showed that weekly and monthly residency was mainly influenced by shark size, with little or no effect of environmental parameters. Although C. amblyrhynchos were present year-round, juvenile sharks had lower residency to their tagging reef than adults. In addition, mature females were detected less between November and mid-February, which coincides with reported parturition in the central GBR. Long-term monitoring data revealed that C. amblyrhynchos exhibited high residency to their tagging reef, and therefore, even in systems with semi-isolated reefs such as the GBR, this species may benefit from spatial management approaches at the reef level. However, behavioural differences between sexes and life-stages of C. amblyrhynchos reported in this study suggest marine reserves may provide lower protection relative to more remote and isolated coral reefs.

Emergent patterns of population genetic structure for a coral reef community.

What shapes variation in genetic structure within a community of codistributed species is a central but difficult question for the field of population genetics. With a focus on the isolated coral reef ecosystem of the Hawaiian Archipelago, we assessed how life history traits influence population genetic structure for 35 reef animals. Despite the archipelago's stepping stone configuration, isolation by distance was the least common type of genetic structure, detected in four species. Regional structuring (i.e. division of sites into genetically and spatially distinct regions) was most common, detected in 20 species and nearly in all endemics and habitat specialists. Seven species displayed chaotic (spatially unordered) structuring, and all were nonendemic generalist species. Chaotic structure also associated with relatively high global FST. Pelagic larval duration (PLD) was not a strong predictor of variation in population structure (R2=0.22), but accounting for higher FST values of chaotic and invertebrate species, compared to regionally structured and fish species, doubled the power of PLD to explain variation in global FST (adjusted R2=0.50). Multivariate correlation of eight species traits to six genetic traits highlighted dispersal ability, taxonomy (i.e. fish vs. invertebrate) and habitat specialization as strongest influences on genetics, but otherwise left much variation in genetic traits unexplained. Considering that the study design controlled for many sampling and geographical factors, the extreme interspecific variation in spatial genetic patterns observed for Hawaìi marine species may be generated by demographic variability due to species-specific abundance and migration patterns and/or seascape and historical factors.

Recovery of an Isolated Coral Reef System Following Severe Disturbance

Coral reef recovery from major disturbance is hypothesized to depend on the arrival of propagules from nearby undisturbed reefs. Therefore, reefs isolated by distance or current patterns are thought to be highly vulnerable to catastrophic disturbance. We found that on an isolated reef system in north Western Australia, coral cover increased from 9% to 44% within 12 years of a coral bleaching event, despite a 94% reduction in larval supply for 6 years after the bleaching. The initial increase in coral cover was the result of high rates of growth and survival of remnant colonies, followed by a rapid increase in juvenile recruitment as colonies matured. We show that isolated reefs can recover from major disturbance, and that the benefits of their isolation from chronic anthropogenic pressures can outweigh the costs of limited connectivity.

Mate choice and sperm limitation in the spotted spiny lobster, Panulirus guttatus

Mate choice by females is influenced by male size, resource provisioning and other proxies for male quality. In decapod crustaceans, mating dynamics are complicated by size-dependent sperm limitation associated with unusually low sperm :egg ratios. We explored how mate size influences mating dynamics in the spotted spiny lobster (Panulirus guttatus), a philopatric species that dwells on shallow and often isolated coral reefs in the Caribbean where choice of mates can be limited. We varied the availability and size of male and female lobsters in a series of laboratory experiments and then quantified courtship behaviour, mate choice and fertilization success of each mating. We found that large males initiated most interactions with other males and won 99% of those encounters. Large males were also more successful in garnering mates, but males of all sizes attempted to mate with all sizes of females. Females nearly always (92% of trials) chose males larger than themselves. However, if large males were unavailable, females mated with smaller males, which resulted in reduced fertilization success. Thus, for species like P. guttatus that dwell in patchy habitats with limited mate availability, the optimal strategy for mate choice is context-dependent, although not without cost to the largest females.

Increased reproductive opportunity: a potential benefit of seasonal aggregation for a little-gregarious and highly sedentary spiny lobster

The spiny lobster Panulirus guttatus is a highly sedentary, obligate reef-dweller that exhibits a low degree of gregariousness yet reproduces year-round. Previous laboratory studies revealed that these lobsters were significantly attracted to scents released by conspecifics during a ‘high reproductive activity’ period (HRA,~60% of ovigerous females on average) but not during a ‘low reproductive activity’ period (LRA,~20%), suggesting that becoming more closely distributed at the right time may increase reproductive opportunities for these lobsters. We tested this hypothesis in the field. We marked all crevices harbouring P. guttatus lobsters (‘dens’) on two isolated coral reef patches over six consecutive sampling periods (three during an LRA and three during the ensuing HRA) and recorded the number, sex and size of lobsters in each den. Dens marked at a given time were considered as harbouring zero lobsters at previous times. For each site, a model selection based on parameters of the negative binomial distribution showed that m (mean lobsters/den) varied little over time but that k (dispersion parameter) decreased across the LRA and remained low (indicating a more clumped distribution) across the HRA. These trends in aggregation were further confirmed by values of Lloyd's ‘patchiness’, supporting the hypothesis that seasonal aggregation increases reproductive opportunities for P. guttatus. However, the tendency to aggregate appeared to be modulated by density and mean size of lobsters, which differed with site. These findings have potential implications for exploited P. guttatus populations and invite further study of the complex behaviours that characterize sedentary lobsters.

Rapid increase in coral cover on an isolated coral reef, the Ashmore Reef National Nature Reserve, north-western Australia

Against a background of coral reef ecosystem decline, understanding the propensity for coral communities to recover after acute disturbances is fundamental to forecasting and maintaining resilience. It may be expected that offshore reef ecosystems are less affected by anthropogenic disturbances compared with reefs closer to population centres, but that recovery may be slower on isolated reefs following disturbances. To test the hypothesis that community recovery is slow in isolated locations, we measured changes in coral cover and relative abundance of coral genera over a 4 year period (2005–09) at Ashmore Reef, north Western Australia, following severe bleaching. The percent cover of hard coral tripled, from 10.2% (±1.46 s.e.) in 2005 to 29.4% (±1.83 s.e.) in 2009 in all habitats (exposed and lagoonal) and depth zones (2–5 and 8–10 m), and the percent cover of soft corals doubled, from 4.5% (+0.63 s.e.) in 2005 to 8.3% (+1.4 s.e.) in 2009. Significant shifts in the taxonomic composition of hard corals were detected. Our results imply that coral recovery in isolated locations can occur rapidly after an initial delay in recruitment, presumably through the interacting effects of self-recruitment and reduced exposure to additive impacts such as coastal pollution.

Temperature variability in a shallow, tidally isolated coral reef lagoon

Temperature data collected in the shallow, tidally isolated reef flat/lagoon of Lady Elliot Island off Queensland, Australia, show marked variability under solar and tidal forcing. Sea level drops below the height of the protective lagoon rim for a few hours during low tide, effectively isolating the remaining water. Because the lagoon is shallow, its temperature change (from diurnal solar forcing and cooling) is amplified. We develop a simple analytical model to predict the time evolution of mean lagoon temperature, beginning with a well‐mixed control volume. This approach highlights the asymmetric flood/ebb physics of tidally isolated lagoons. After discussing the response of this model, we compare it with results from two idealized numerical simulations that illustrate differing aspects of lagoon temperature variability under “potential flow” and “prevailing current” situations. The conceptual model captures the essence of lagoon temperature variability and underscores the importance of solar‐lunar phasing. However, because of the well‐mixed assumption, it cannot reproduce sudden temperature transitions associated with new incoming water masses. Observations show that a slowly progressing thermal wave inundates the lagoon on rising tides. This wave is similar to our “potential flow” simulation in that it is approximately radially symmetric. On the other hand, it appears to advectively replace resident lagoon water, similar to our “prevailing current” simulations. We attempt to account for this behavior with a simple “frontal” modification to our conceptual model. Results show that this frontal model is able to capture the sudden temperature transitions present in the data and offers improved predictive capabilities over the well‐mixed model.