Isolated Patch Reefs Research Articles

Late Cretaceous–Cenozoic sequence stratigraphic and paleogeographic controls on petroleum system elements of the Nicaraguan platform, western Caribbean Sea

Late Cretaceous–to–present-day mixed carbonate–clastic deposition along the Nicaraguan platform, western Caribbean Sea, has evolved from a tectonically controlled, rifted upper Eocene shallow–to–deep-marine carbonate–siliciclastic shelf to an upper Miocene–to–present-day tectonically stable shallow-marine carbonate platform and passive margin. By integrating subsurface data of 287 two-dimensional seismic lines and 27 wells, we interpret the Cenozoic stratigraphic sequence as 3 cycles of transgression and regression beginning with an upper Eocene rhodolitic–algal carbonate shelf that interfingered with marginal siliciclastic sediments derived from exposed areas of Central America bordering the margin to the west. During the middle Eocene, a carbonate platform was established with both rimmed reefs and isolated patch reefs. A late Eocene forced regression produced widespread erosion and subaerial exposure across much of the platform and was recorded by a regional unconformity. The Oligocene–upper Miocene sedimentary record includes a southeastward prograding delta of the proto-Coco river, which drained the emergent area of what is now northern Nicaragua. The late Miocene–to–present-day period marks a period of strong subsidence with the development of small pinnacle reefs. We describe favorable petroleum system elements of the Nicaraguan platform that include (1) Eocene fossiliferous limestone source rocks documented as thermally mature in vintage exploration wells and seen as active gas chimneys emanating from inferred carbonate reservoirs; (2) upper–to–middle Eocene reservoirs in patch and pinnacle reefs, middle Eocene calcareous slumps, and Oligocene fluvial-deltaic facies documented in wells; and (3) regional seal intervals that consist of both regional unconformities and Eocene–Oligocene intraformational shale.

An exception to the rule: top-down control of a coral reef macroinvertebrate community by a tropical spiny lobster

In temperate habitats, predation by lobsters creates trophic cascades that reverberate through benthic communities. However, the influence of lobsters on tropical benthic communities is thought to be minimal due to higher prey diversity and the mobility and broad diet of lobsters. We hypothesized that this would not be the case for the spotted spiny lobster (Panulirus guttatus Latreille, 1804), an obligate, philopatric dweller of Caribbean coral reefs. Our study took place in the Florida Keys (USA), where we examined the gut contents of wild lobsters, conducted laboratory experiments on lobster prey selection and foraging effectiveness in rubble, and manipulated lobster density on isolated patch reefs to examine the effects of lobster predation on macroinvertebrate prey over a 3-yr period. We found P. guttatus to be a generalist carnivore that consumed a suite of macroinvertebrates, including important reef herbivores (e.g., herbivorous crabs, gastropods, and urchins) that comprised >60% of its diet. Higher densities of P. guttatus exerted strong top-down control over the abundance and composition of macroinvertebrate prey in rubble substrates on patch reefs, but not prey living in substrates dominated by macroalgae. Lobster foraging success in rubble also depended on rubble size; lobsters extracted prey from and overturned rubble more easily when rubble was smaller. The abundance and high site fidelity of predatory P. guttatus appears to provide the necessary conditions for top-down control of prey in coral reef rubble zones, an effect similar to the powerful influence of lobster predation observed in temperate ecosystems.

A field test of the effects of mesopredators and landscape setting on juvenile oyster, Crassostrea virginica, consumption on intertidal reefs

Oyster populations are often structured by both biotic interactions and abiotic stresses. Juvenile oysters, i.e., those most vulnerable to predation, face a wide range of predatory characteristics (size, mobility) such that predator identity might exert a strong influence on oyster populations. Likewise, oyster reef location, either as isolated patch reefs or saltmarsh-fringing reefs, can strongly influence the ecological processes impacting oyster populations. Therefore, this study sought to quantify the contribution of predator identity, in particular mesopredators, to oyster survival in the field, while also examining the role of landscape setting in predation. Using a multiple mesh size cage design, oyster survival was measured by excluding access to different groups of predators at both patch and fringing reef and reference sites in Hewlett’s Creek, Wilmington, North Carolina, in August/September 2013, while also monitoring settlement and recruitment at both reef locations from May to August 2013. The results indicated a significant cage by location interaction, indicating that the predator identity or modality was not the same across all sites. Despite this, at either reef location, oyster survival did not differ between 37-mm mesh cages, which allowed access only by mesopredators, and no-cage treatments, which allowed access to all predators, while survival was reduced by >20 % on fringing reefs relative to patch reefs. This study demonstrates the significant contribution of mesopredators, likely xanthid crabs, to oyster predation in the field at ambient predator densities and suggests that the differences in oyster abundance between patch and fringing reef locations are likely due to differential predation.

Two's company, three's a crowd: Food and shelter limitation outweigh the benefits of group living in a shoaling fish

Identifying how density and number-dependent processes regulate populations is important for predicting population response to environmental change. Species that live in groups, such as shoaling fish, can experience both direct density-dependent mortality through resource limitation and inverse number-dependent mortality via increased feeding rates and predator evasion in larger groups. To investigate the role of these processes in a temperate reef fish population, we manipulated the density and group size of the shoaling species Trachinops caudimaculatus on artificial patch reefs at two locations with different predator fields in Port Phillip Bay, Australia. We compared mortality over four weeks to estimates of predator abundance and per capita availability of refuge and food to identify mechanisms for density or number dependence. Mortality was strongly directly density dependent throughout the experiment, regardless of the dominant predator group; however, the limiting resource driving this effect changed over time. In the first two weeks when densities were highest, density-dependent mortality was best explained by refuge competition and the abundance of benthic predators. During the second two weeks, food competition best explained the pattern of mortality. We detected no effect of group size at either location, even where pelagic-predator abundance was high. Overall, direct density effects were much stronger than those of group size, suggesting little survival advantage to shoaling on isolated patch reefs where resource competition is high. This study is the first to observe a temporal shift in density-dependent mechanisms in reef fish, and the first to observe food limitation on short temporal scales. Food competition may therefore be an important regulator of postsettlement reef fish cohorts after the initial intense effects of refuge limitation and predation.

Dynamics of fish assemblages on a continuous rocky reef and adjacent unconsolidated habitats at Fernando de Noronha Archipelago, tropical western Atlantic

In recent years, many studies investigated how density-dependent factors, such as shortages in microhabitat and food availability influence the structure of reef fish assemblages. Most of what is currently known, however, comes from comparisons of isolated patch reefs and from correlations between fish abundance and one or few microhabitat variables. In addition, most studies were done in the Caribbean and Indo-Pacific regions, whereas the South Atlantic region has been, to date, understudied. The present study evaluated spatial and temporal variations in reef fish abundance and species richness in a continuous rocky reef and adjacent unconsolidated habitats in a Southwestern Atlantic reef, using underwater techniques to assess both fish numbers and microhabitat variables (depth, rugosity, number of crevices and percent cover of live benthic organisms, bare rock, sand, and limestone). Higher species richness was observed at consolidated substratum stations on both sampling periods (May and October), but fish abundance did not show a significant spatial variation. Topographical complexity and percent cover of algae (except coralline algae) were amongst the most important determinants of species richness, and correlations between fish size and refuge crevice size were observed. The non-random patterns of spatial variation in species richness, and to a lesser extent, fish abundance, were related to differences in substratum characteristics and the inherent characteristics of fishes (i.e. habitat preferences) and not to geographical barriers restraining fish movement. This study highlights the importance of concomitantly assessing several microhabitat variables to determine their relative influence in reef fish assemblages.

Food supply influences offspring provisioning but not density-dependent fecundity in a marine fish

Replenishment of many marine populations occurs through the entry of juveniles to adult populations following a pelagic larval stage. Because mortality during the pelagic stage is thought to be high and density independent, larval abundance and traits of individual larvae should have strong effects on overall population dynamics in marine organisms. Surprisingly, few experiments have tested how localized interactions among breeding adults affect the quantity and phenotypic traits of larvae they produce. Here I experimentally test for the influence of food competition, mate limitation, and population density on somatic growth, fecundity, and offspring provisioning (larval length and energy reserves) in a planktivorous, territorial coral reef damselfish, Stegastes partitus. I manipulated food supply and adult S. partitus density on isolated patch reefs in the Bahamas and also made behavioral observations of S. partitus occurring on nearby natural reefs at a range of population densities. On the experimental reefs, females experienced density-dependent growth and fecundity; male reproductive success was density dependent, but male growth was not. Density-dependent growth and reproduction were not moderated by food supplementation, and density-dependent reproduction was not influenced by mate availability. On natural reefs, the frequency of aggressive interactions, particularly involving females, increased with population density, implicating aggression-related energetic costs as the source of both forms of density dependence in the experiments. Food supplementation increased female somatic growth and larval energy reserves, suggesting that females allocated surplus energy to future reproductive potential and enhanced offspring quality. Neither experimental treatment affected larval length. By altering patterns of reproduction, the interplay between spatial variation in food availability and population density may drive population dynamics in a broad range of benthic marine organisms.

Indirect effects of heterospecific interactions on progeny size through maternal stress

Maternal effects are increasingly being recognized as an important pre‐natal source of life history variation in the next generation. The present study uses a field experiment to explore the influence of heterospecific interactions on the reproductive output and offspring characteristics of a common Indo‐Pacific damselfish, Pomacentrus amboinensis. On the Great Barrier Reef pairs of breeding P. amboinensis were placed on isolated patch reefs and to half of the pairs resource competitors (other planktivorous damselfishes), and predators of eggs and juveniles were added. Females inhabiting patches with heterospecifics had more aggressive interactions and higher levels of the stress hormone, cortisol. Neither the number of clutches nor number of eggs produced differed among treatments. The size of larvae at hatching was found to be reduced as a result of the stress associated with increased interactions with heterospecific and the transfer of cortisol to offspring. This stress‐associated mechanism appears to be an important and directional source of life history variability, but the individual nature of the maternal response is likely to result in a conclusion of a diversified bet hedging reproductive strategy when viewed at the local population level. These findings highlight the complex determinants of individual success and the important role of parental well‐being in the population dynamics of the next generation.

Habitat patchiness and predation modify the distribution of a coral-dwelling damselfish

Fish abundance is often better predicted by microhabitat variables on continuous reefs than on isolated patch reefs. Although this was suggested to stem from reduced post-recruitment relocation, this has not been shown experimentally. We found the relationship between the presence of a coral-dwelling fish, Dascyllus marginatus, and the size of its coral host to differ between corals on continuous reefs and the sparsely distributed corals on sandy bottoms. Empty transplanted corals were colonized exclusively by new recruits when on the sandy bottom, and both by new recruits and post-recruitment dispersal of adults when on the continuous reef. New recruits settled predominantly into small corals, although analyses of recruitment patterns were confounded by low recruitment in the studied years. Both tank experiments and field survey data suggest that the presence of recruits in small corals is at least partially driven by predation by the dottyback, Pseudochromis olivaceus, which lives predominantly in large corals within both habitats. Consequently, we suggest that the relationship between fish presence and coral size differs between the habitats due to coral size dependent predation on recruits and variability in the importance of direct recruitment to replenish fish populations.

INDIRECT EFFECTS OF AN EXPLOITED PREDATOR ON RECRUITMENT OF CORAL-REEF FISHES

The more ecologists examine the role of trait-mediated indirect interactions (TMIIs), especially in regulating predator-prey interactions, the more we recognize their fundamental role in structuring food webs. However, most empirical evidence for TMIIs comes from studies that are either conducted in laboratory or mesocosm venues or are restricted to simple food webs involving lower trophic-level animals. Here, I quantified the direct and indirect effects of interactions between high-level vertebrate predators on their vertebrate prey using a field experiment. Specifically, I tested how varying densities of a large-bodied, top predator (Nassau grouper; Epinephelus striatus) affected persistence, growth, and behavior of two smaller-bodied, intermediate predators (coney and graysby groupers; Cephalopholis fulva and C. cruentata) on 20 isolated patch reefs in the Bahamas. Large-bodied groupers are capable of consuming their smaller-bodied counterparts, and previous observational studies have indicated that local abundances of these groupers are negatively correlated. I measured the effects of interactions among groupers on lower trophic-level prey by quantifying recruitment of coral-reef fishes to the reefs. The field experiment demonstrated a strong trophic cascade that was entirely mediated by modified behavior of the intermediate predators. These results indicate that indirect, nonlethal interactions in natural systems can have strong cascading effects even at high trophic levels and in high-diversity food webs. Incorporating the complexity of such indirect effects into fisheries management may improve the sustainability of fished populations and strengthen marine conservation efforts; however these results also indicate that the effects of fishing are complex and difficult to predict.

Microbial–metazoan reefs of the terminal Proterozoic Nama Group (c. 550–543 Ma), Namibia

Thrombolite and stromatolite reefs occur at several stratigraphic levels within the terminal Proterozoic Nama Group (c. 550–543 Ma) of central and southern Namibia. The reefs form integral parts of several carbonate platforms within the Nama Group, including the Kuibis platform of the northern Nama Basin (Zaris subbasin), and Huns platform (Witputs subbasin) of the southern Nama Basin. The reefs are composed of both thrombolites and stromatolites that form laterally continuous biostromes, isolated patch reefs, and isolated pinnacle reefs ranging in scale from a metre to several kilometres in width. In the majority of cases, the reefs occur stratigraphically as an integral facies within the transgressive systems tracts of sequences making up the Kuibis and Huns platforms. This suggests that a regime of increasing accommodation was required to form well-developed reefs, though reefs also occur sporadically in highstand systems tract settings. Within a given transgressive systems tract, a regime of increasing accommodation through time favours the transition from sheet-like biostromal geometries to more isolated patch and pinnacle biohermal geometries. Similarly, increasing accommodation in space, such as a transect down depositional dip, shows a similar transition from more sheet-like geometries in updip positions to more isolated geometries in downdip positions. Reefal facies consist of thrombolitic domes, columns and mounds with well-developed internal clotted textures, in addition to stromatolitic domes, columns and mounds, with crudely to moderately well-developed internal lamination. Stromatolites are better developed in conditions of relatively low accommodation, and updip locations, under conditions of higher current velocities and greater sediment influx. Thrombolites are better developed in conditions of relatively high accommodation and low sediment influx. Both types of microbialites are intimately associated with the first calcifying metazoan organisms, which may have attached themselves to the sediment surface or otherwise lived within sheltered depressions within the rough topography created by ecologically complex mats. The appearance of thrombolitic textures during terminal Proterozoic time is consistent with colonization of cyanobacterial mats by higher algae and metazoans, which would have been an important process in generating clotted textures. Fabrics in the Nama thrombolites are well preserved and show evidence of thrombolitic mesoclots being overgrown by fibrous marine carbonate, interpreted as former aragonite. This was followed by emplacement of geopetal micrite fills, and precipitation of dolomite as an isopachous rim cement, followed by occlusion of remaining porosity by blocky calcite spar.

FISH AGGREGATION RESULTS IN INVERSELY DENSITY-DEPENDENT PREDATION ON CONTINUOUS CORAL REEFS

Spatially density-dependent predation is a leading hypothesis describing mechanisms of population regulation in coral reef fish. However, studies supporting this hypothesis predominantly have been conducted on small, isolated patch reefs. Here, we searched for evidence of spatially density-dependent predation on the continuous reefs of the Netherlands Antilles in a study of a dominant planktivore, the blue chromis (Chromis cyanea). Across space, we quantified both the patterns of loss from site-attached aggregations of C. cyanea through time and the behavioral reaction of predators to these aggregations. Looking across C. cyanea densities, we found that loss from aggregations was not characteristic of direct density dependence, but instead was commonly inversely related to density. Individual C. cyanea in larger aggregations were less likely to be lost from the group than were individuals in smaller aggregations. Thus, the observed density dependence increased spatial heterogeneity of C. cyanea. Predators showed behaviors that were consistent with these demographic patterns. Using remote videography, we quantified predator visitation and strike rates across a range of C. cyanea aggregation sizes. Predators consistently visited and struck at individuals in C. cyanea aggregations in a pattern that was strongly inversely density dependent, suggesting that aggregation is an effective means of minimizing per capita risk of predation for prey reef fish. Differences in spatial distribution of resources for predators (i.e., prey fish) between continuous and patch reef habitats may explain the difference between these results and those of previous studies on patch reefs.

PRIORITY EFFECTS IN CORAL REEF FISH COMMUNITIES OF THE GREAT BARRIER REEF

Priority effects occur when established residents influence the colonization of individuals entering the community and thus provide insight into mechanisms underlying spatial differences and temporal changes in community composition. Using 20 spatially isolated patch reefs, I factorially manipulated the presence and absence of resident predators (groupers and dottybacks) and potential competitors (damselfishes) to determine whether and how they affect subsequent recruitment and mortality of newly settled fishes. During the 50-day experiment at Lizard Island (Great Barrier Reef, western Pacific), prior residency by predators dramatically reduced recruitment of damselfish, surgeonfish, butterflyfish, and rabbitfish and increased damselfish recruit mortality. In contrast, prior residency by potential competitors only reduced recruitment of damselfish and rabbitfish and did not affect recruit mortality. Effects of competitors were likely due to aggressive interactions between competitors and recruits that increased susceptibility of recruits to predators. Effects of residents were strongest within 48 hours of settlement, resulting in rapid establishment of patterns that persisted to the conclusion of the experiment. These results are similar to those from a comparable experiment in the Bahamas (western Atlantic), suggesting that priority effects may be a generally important cause of temporal and spatial variability in the composition of reef fish communities.

Architecture and growth history of a Miocene carbonate platform from 3D seismic reflection data; Luconia province, offshore Sarawak, Malaysia

Using 3D seismic reflection data and wireline logs we reconstruct in detail the architecture and growth history of a Miocene carbonate platform in the Luconia province, offshore Sarawak, Malaysia. Platform growth started in the Late Oligocene to Early Miocene, by coalescence of isolated patch reefs. The growth history includes phases of progradation, backstepping and occasional collapsing of platform flanks, guided by syndepositional faulting. The most pronounced seismic reflections in the platform correspond to flooding events (thin transgressive systems tracts). Subaerial exposure preceding the flooding could be demonstrated in only one case. Platform growth was terminated by gradual submergence (drowning) indicated by smooth, concentric seismic reflectors forming a convex mound.

Depositional history and origin of porosity in a Miocene carbonate platform of Central Luconia, offshore Sarawak

Recently acquired 3-D seismic reflection data, wireline logs and selected core samples were used to reconstruct the growth history of the platform as well as the sequence of diagenetic events and origin of porosity. Platform growth started by coalescence of isolated patch reefs. Growth history includes phase of progradation, backstepping and collapse of the platform flanks. The most pronounced seismic reflectors in the platform correspond to flooding events (transgressive systems tracts), occasionally preceded by lowstands. Platform growth was terminated by gradual submergence (drowning) indicated by smooth, concentric seismic reflections forming a convex mound. Syndepositionallow-offset normal faults controlled the geometry of the platform and triggered large landslides that can be seismically traced about 5,000 ft (1.5 km) in the basin. Three different processes have significantly contributed to porosity in the carbonate rocks: selective leaching during exposure, dolomitization and leaching during deep burial, probably related to warm fluids rising from depth. Quantitatively, the last group is the most important. As most of the carbonate porosity formed by carbonate dissolution under deep burial, the slide masses and related turbidites may contain highly porous rocks in the basins between platforms. These bodies can be separated by the platforms where their upslope ends are enveloped in clay-rich hemipelagic sediment. In other places, the porous slides and turbidites may establish connections between neighbouring platforms.

Significance of relic carbonate deposits along the central and southwestern margin of India for late Quaternary environmental and sea level changes

Environmental and sea level indicators were investigated using dredge samples from late Quaternary carbonate deposits along the shelf break between Goa and Cape Comorin, India. Geomorphic features in the area were identified from sonar profiles and included isolated patch reefs with a relief of up to 10 m, and linear reefs with reliefs between 2 and 15 m. The main clast types recovered from these features include fragmented corals and carbonate nodules dominated by either encrusted foraminifera or coralline algae. Some of these clast types are clearly of shallow-water origin. Fragments of reef-forming Poritid corals, for example, were collected off Mangalore at depths of 110–105 m and dated between 11,520 and 12,610 14C years BP (13.42–14.77 ka). Nodules of similar age dominated by Lithothamnium and capped by foraminiferal veneers were also collected at −90 m off Cape Comorin. Their altered algal tissues are consistent with formation in shallow water, high-energy conditions. In contrast, nodules recovered off Kochi and Mangalore–Goa are of deeper water origin, younger in age (10,980–7350 14C years BP), and are dominated by Gypsina encrustations with volumetrically less algal encrustation. They show cyclic succession of foraminiferal–algal, or foraminiferal–algal–coral laminations in which the algal species are typical of deeper waters. The age and elevation of corals and shallow-water nodules are both consistent with published glacio-eustatic sea-level curves. In addition, the alternate micro-encrustations of foraminifera, algae and encrusting corals could indicate changing conditions from nutrient-rich and turbid to nutrient-poor and clear water that may be attributable to seasonal variations in sediment flux caused by monsoons.

Regulation of local populations of a coral reef fish via joint effects of density- and number-dependent mortality.

Density-dependent mortality can regulate local populations - effectively minimizing the likelihood of local extinctions and unchecked population growth. It is considered particularly important for many marine reef organisms with demographically open populations that lack potential regulatory mechanisms tied to local reproduction. While density-dependent mortality has been documented frequently for reef fishes, few studies have explored how the strength of density-dependence varies with density, or how density-dependence may be modified by numerical effects (i.e., number-dependent mortality). Both issues can have profound effects on spatial patterns of abundance and the regulation of local populations. I address these issues through empirical studies in Moorea, French Polynesia, of the six bar wrasse (Thalassoma hardwicke), a reef fish that settles to isolated patch reefs. Per capita mortality rates of newly settled wrasse increased as a function of density and were well approximated by the Beverton-Holt function for both naturally formed and experimentally generated juvenile cohorts. Average instantaneous mortality rates were a decelerating function of initial densities, indicating the per capita strength of density-dependence decreased with density. Results of a factorial manipulation of density and group size indicate that per capita mortality rates were simultaneously density- and number-dependent; fish at higher densities and/or in groups had higher probabilities of disappearing from patch reefs compared with fish that were solitary and/or at lower densities. Mortality rates were ~30% higher for fish at densities of 0.5fish/m2 than at 0.25fish/m2. Similarly, mortality rates increased by ~45% when group size was increased from 1 to 2 individuals per patch, even when density was kept constant. These observations suggest that the number of interacting individuals, independent of patch size (i.e., density-independent effects) can contribute to regulation of local populations. Overall, this work highlights a greater need to consider numerical effects in addition to density effects when exploring sources of population regulation.

Calcified metazoans in thrombolite-stromatolite reefs of the terminal Proterozoic Nama Group, Namibia

Reefs containing abundant calcified metazoans occur at several stratigraphic levels within carbonate platforms of the terminal Proterozoic Nama Group, central and southern Namibia. The reef-bearing strata span an interval ranging from approximately 550 Ma to 543 Ma. The reefs are composed of thrombolites (clotted internal texture) and stromatolites (laminated internal texture) that form laterally continuous biostromes, isolated patch reefs, and isolated pinnacle reefs ranging in scale from a meter to several kilometers in width. Stromatolite-dominated reefs occur in depositionally updip positions within carbonate ramps, whereas thrombolite-dominated reefs occur broadly across the ramp profile and are well developed as pinnacle reefs in downdip positions.The three-dimensional morphology of reef-associated fossils was reconstructed by computer, based on digitized images of sections taken at 25-micron intervals through 15 fossil specimens and additionally supported by observations of over 90 sets of serial sections. Most variation observed in outcrop can be accounted for by a single species of cm-scale, lightly calcified goblet-shaped fossils herein described asNamacalathus hermanastesgen. et sp. nov. These fossils are characterized by a hollow stem open at both ends attached to a broadly spheroidal cup marked by a circular opening with a downturned lip and six (or seven) side holes interpreted as diagenetic features of underlying biological structure. The goblets lived atop the rough topography created by ecologically complex microbial-algal carpets; they appear to have been sessile benthos attached either to the biohermal substrate or to soft-bodied macrobenthos such as seaweeds that grew on the reef surface. The phylogenetic affinities ofNamacalathusare uncertain, although preserved morphology is consistent with a cnidarian-like bodyplan. In general aspect, these fossils resemble some of the unmineralized, radially symmetric taxa found in contemporaneous sandstones and shales, but do not appear to be closely related to the well-skeletonized bilaterian animals that radiated in younger oceans. Nama reefs demonstrate that biohermal associations of invertebrates and thrombolite-forming microorganisms antedate the Cambrian Period.

Effects of experimental coral disturbance on the population dynamics of fishes on large patch reefs

The effects of disturbance to coral on the abundances of fishes were investigated experimentally on the Great Barrier Reef. Fishes on ten large (108–267 m 2) isolated patch reefs or `bommies' were monitored at 1–2 month intervals for a year. The hard corals on five of these were disturbed by breaking the colonies into small pieces with a mallet. Fishes were counted for the next year. Cover of coral on the damaged bommies decreased from ≈66% to ≈29% by the end of the study, but increased by 5% at the control bommies. Linear mixed-effects models were used to examine the numbers of 53 species of fishes (from a total pool of 150 species). Significant declines in abundance were detected in five species, all of which associate closely with live coral: an apogonid ( Cheilodipterus artus), a chaetodontid ( Chaetodon rainfordi) and three pomacentrids (the species complex Chromis atripectoralis-viridis, Dascyllus reticulatus, and Pomacentrus moluccensis). One herbivorous pomacentrid ( Pomacentrus grammorhynchus) significantly increased in abundance. Each of the affected species showed a different temporal response to the disturbance, ranging from emigration and local extinction within one month, to changes in recruitment six months later. This study shows that many species of fishes are robust to physical disturbances which substantially reduce cover of coral and modify the fine-scale (1 m) heterogeneity provided by the veneer of live corals on the surface of a reef. The few negatively affected species frequently sheltered in live coral; the one positively affected species was herbivorous. Many coral-associated species and many herbivores were not affected, so it is not yet possible to make precise predictions about which species will be affected by disturbance to coral.

SPATIALLY AND TEMPORALLY PREDICTABLE FISHCOMMUNITIES ON CORAL REEFS

Spatial and temporal variation in the distribution and abundance of site-attached fish species inhabiting small, isolated patches of coral reef has been attributed to variability in larval recruitment. However, the relative importance of settlement and postsettlement processes in determining the structure of fish communities in general, i.e., including non-site-attached species inhabiting large sections of contiguous reef, remains to be determined. Here, we examine the degree of spatial and temporal variation in community structure and population density of fish inhabiting sections of coral reef varying in size and connectivity. To investigate spatial variability in fish community structure and population density, we surveyed 36 sites on contiguous reef and 39 patch-reef sites varying in size and isolation from neighboring patches. Ordination and regression analyses indicated that the structure of fish assemblages inhabiting contiguous reef varied predictably along habitat gradients. In contrast, intrinsic habitat characteristics, such as the shelter availability and the composition of the substratum, were apparently unrelated to the structure of fish assemblages on patch reefs. For sites on contiguous reef, multiple regression models explained a significant proportion of spatial variation in the population density of 10 site-attached and vagile species (including 90% of variation in the density of Pomacentrus moluccensis, a site-attached damselfish). For patch-reef sites, models of spatial variation in population density were significant for six species, five of which were not site attached. The overall pattern across most species was that patch-reef models were characterized by a reduced r2 relative to corresponding models of contiguous-reef populations, but the reduction was substantially less for vagile species than for site-attached species. Ordination analysis of temporal variability in community structure over two years suggested that none of the fish communities at the sites examined was in a consistent state of community succession. For most sites, community structure varied randomly over time, although at some sites, the structure of resident fish communities tended towards a stable position in multivariate community space. Across all sites, temporal change in community structure was significantly higher during periods of recruitment than at other times of the year, although there was little evidence to suggest that recruitment was the sole source of temporal variation. At most sites, the structure of fish assemblages fluctuated considerably during nonrecruitment periods. Patterns of temporal variation in the population density of four site-attached species indicated that population increases corresponding with pulses of recruitment were modified by postsettlement processes. For site-attached and moderately vagile species, overall temporal variability in assemblage structure increased significantly as sites became smaller and more isolated. Temporal variability in assemblages of highly vagile species was unrelated to survey area and connectivity. Overall, the results of the analyses of spatial and temporal variability in fish community structure suggest that species' vagility and reef connectivity strongly influence the relative importance of recruitment and postrecruitment processes in determining local population density. Individuals of highly vagile species are able to move among isolated patches in response to habitat preferences or resource availability. Conversely, the continuous shelter provided by contiguous reef may allow sedentary species to migrate to more favorable areas. We suggest that for many fish species, including vagile species on patchy reef and site-attached species on contiguous reef, patterns in distribution and abundance established at recruitment are modified by postsettlement migration, which enhances the relationship between population density and habitat structure. Thus, while recruitment patterns may explain much of the spatial and temporal variation in populations of site-attached fish on small, isolated patch reefs, this result cannot necessarily be extrapolated to fish communities inhabiting large sections of contiguous reef.

Effects of experimental coral disturbance on the structure of fish communities on large patch reefs

Communit~es of reef fishes on 10 large isolated patch reefs or ' b o m e s ' (108 to 267 m') were monitored at 1 to 2 mo intervals over 1 yr, and then the hard corals on 5 of the bommies were physically disturbed by breaking the colonies into small pieces with a mallet. Monitoring continued for a further year after the disturbance. Coral cover at the impacted bommies decreased from -66 to -29% by the end of the study, while coral cover had increased by 5% in the same period at the control bommies. There were significant declines in species richness and total abundance of fishes on the impacted bommies, but multivariate analyses showed that the disturbance had no significant effect on spatiotemporal patterns of adult relative abundance and larval recruitment at the community level. Effects of disturbance on the abundance and species richness of 6 families and 8 distinct ecological categories of fishes were also exammed. There were significant declines in the abundance of 1 family (the Chaetodontidae) and 1 ecological category (water column/substratum feeding Pomacentridae), and significant declines in the species richness of 2 families (the Apogonidae and Scaridae) and 1 ecological category (invertebratc feeders). Although coral disturbance did effert some elements of the fish con~munities, it appears that patterns of spatial heterogeneity In the underlying hard substrata at each bommie were also important determinants of fish community structure. Hence, historical patterns of coral growth and substrate consolidation, together with the contemporary coral community, can affect the structure of fish communities on these large patch reefs. Further studies of fish-habitat associations, together with field experiments which manipulate both the underlying substratum and the abundance of live corals, will be necessary to determine the relative importance of each of these habitat characteristics to reef fishes.