Mass transfer limitation of photosynthesis of coral reef algal turfs

Algal turfs are the major primary producing component on many coral reefs and this production supports higher levels in the complex reef trophic web. Rates of metabolism of algal turfs are related positively to water motion, consistent with limitation by the diffusion of a substance through a boundary layer. Based on engineering mass transfer theory, we hypothesized that photosynthesis of algal turfs is controlled by rates of mass transfer and responses of photosynthesis to increasing flow speed should be predicted by engineering correlations. This hypothesis was tested in ten experiments where photosynthesis was estimated in a flume/respirometer from changes in dissolved oxygen at eight flow speeds between 0.08 and 0.52 m/s. Flow in the flume and over the reef at Kaneohe Bay, Oahu, Hawaii was estimated using hot-film thermistor and electromagnetic current meters. Rates of photosynthesis were related positively to flow in all experiments and plots of the log of the average Sherwood number (Sh _meas) versus log Reynolds number (Re _D) for each experiment are lower than predicted for mass transfer through a turbulent boundary layer. Algal turf-covered plates are characterized as hydrodynamically transitional to fully rough surfaces and the lower than predicted slopes suggest that roughness reduces rates of mass transfer. A negative correlation between algal turf biomass and slopes of the log Sh _meas−log Re _D plots suggests that mass transfer to algal turfs is affected significantly by the physical structure of the algal community. Patterns of photosynthesis based on changes in dissolved oxygen and dissolved inorganic carbon concentrations (DIC) indicate that the flow speed effect is not the result of increased flux of oxygen from the algal turfs, and combined with the short response time to flow speed, suggest that DIC may limit rates of photosynthesis. Although there are differences between flow in the flume and flow over algal turfs on the reef, these results suggest that photosynthesis is controlled, at least in part, by mass transfer. The chemical engineering approach provides a framework to pose further testable hypotheses about how algal canopy height, flow oscillation, turbulence, and substratum roughness may modulate rates of metabolism of coral reef algal turfs.     

Diversity and community structure of symbiotic dinoflagellates from Caribbean coral reefs

A community ecology approach to the study of the most common group of zooxanthellae, dinoflagellates in the genus Symbiodinium, was applied to symbiotic invertebrate assemblages on coral reefs in the western Caribbean, off the Yucatan peninsula (Puerto Morelos, Mexico) and over 1000 km away in the northeastern Caribbean, at Lee Stocking Island, Bahamas. Sequence differences and intragenomic variation, as determined by denaturing gradient gel electrophoresis and sequencing of the internal transcribed spacer 2 (ITS 2) region, were used to classify these symbionts. Twenty-eight genetically distinct Symbiodinium types were identified, eleven of which were found in hosts from both Caribbean locations. A single symbiont population was detected in 72% of hosts from the Yucatan and 92% of hosts from the Bahamas. The reef-wide community distribution of these symbionts is dominated by a few types found in many different host taxa, while numerous rare types appear to have high specificity for a particular host species or genus. Clade or lineage A Symbiodinium spp. was restricted to compatible hosts located within 3-4 m of the surface, while Symbiodinium spp. types from other lineages displayed differences in vertical zonation correlated with ITS type but were independent of clade designation. A comparison of the symbiont types found in field-collected hosts with types previously cultured from these hosts indicates the existence of low density or "background"-symbiont populations and cryptic, potentially non-mutualistic types in some hosts.     

Relative impact of a seagrass bed and its adjacent epilithic algal community in consumer diets

The aim of this work was to identify and compare, using nitrogen and carbon stable isotope data, the food sources supporting consumer communities in a Mediterranean seagrass bed (Gulf of Calvi, Corsica) with those in an adjacent epilithic alga-dominated community. Isotopic data for consumers are not significantly different in the two communities. Particulate matter and algal material (seagrass epiflora and dominant epilithic macroalgae) appear to be the main food sources in both communities. Generally, the δ¹³C of animals suggests that the seagrass Posidoniaoceanica (L.) Delile represents only a minor component of their diet or of the diet of their prey, but the occurrence of a mixed diet is not excluded. P. oceanica dominates the diet of only of few species, among which holothurians appear as key components in the cycling of seagrass material. Received: 30 July 1999 / Accepted: 17 January 2000     

Habitat biodiversity as a determinant of fish community structure on coral reefs

Increased habitat diversity is often predicted to promote the diversity of animal communities because a greater variety of habitats increases the opportunities for species to specialize on different resources and coexist. Although positive correlations between the diversities of habitat and associated animals are often observed, the underlying mechanisms are only now starting to emerge, and none have been tested specifically in the marine environment. Scleractinian corals constitute the primary habitat-forming organisms on coral reefs and, as such, play an important role in structuring associated reef fish communities. Using the same field experimental design in two geographic localities differing in regional fish species composition, we tested the effects of coral species richness and composition on the diversity, abundance, and structure of the local fish community. Richness of coral species overall had a positive effect on fish species richness but had no effect on total fish abundance or evenness. At both localities, certain individual coral species supported similar levels of fish diversity and abundance as the high coral richness treatments, suggesting that particular coral species are disproportionately important in promoting high local fish diversity. Furthermore, in both localities, different microhabitats (coral species) supported very different fish communities, indicating that most reef fish species distinguish habitat at the level of coral species. Fish communities colonizing treatments of higher coral species richness represented a combination of those inhabiting the constituent coral species. These findings suggest that mechanisms underlying habitat-animal interaction in the terrestrial environment also apply to marine systems and highlight the importance of coral diversity to local fish diversity. The loss of particular key coral species is likely to have a disproportionate impact on the biodiversity of associated fish communities.     

Catastrophe and the life span of coral reefs

A strong earthquake in the western Caribbean in 2009 had a catastrophic impact on uncemented, unconsolidated coral reefs in the central sector of the shelf lagoon of the Belizean barrier reef. In a set of 21 reef sites that had been observed prior to the earthquake, the benthic assemblages of 10 were eradicated, and one was partially damaged, by avalanching of their slopes. Ecological dynamics that had played out over the previous 23 years, including the mass mortalities of two sequentially dominant coral species and a large increase in the cover of an encrusting sponge, were instantaneously rendered moot in the areas of catastrophic reef-slope failure. Because these prior dynamics also determined the benthic composition and resilience of adjacent sections of reef that remained intact, the history of disturbance prior to the earthquake will strongly influence decadal-scale recovery in the failed areas. Geological analysis of the reef framework yielded a minimum return time of 2000-4000 years for this type of high-amplitude event. Anthropogenic degradation of ecosystems must be viewed against the backdrop of long-period, natural catastrophes, such as the impact of strong earthquakes on uncemented, lagoonal reefs.     

Modeling the beta diversity of coral reefs

Quantifying the beta diversity (species replacement along spatiotemporal gradients) of ecosystems is important for understanding and conserving patterns of biodiversity. However, virtually all studies of beta diversity focus on one-dimensional transects orientated along a specific environmental gradient that is defined a priori. By ignoring a second spatial dimension and the associated changes in species composition and environmental gradients, this approach may provide limited insight into the full pattern of beta diversity. Here, we use remotely sensed imagery to quantify beta diversity continuously, in two dimensions, and at multiple scales across an entire tropical marine seascape. We then show that beta diversity can be modeled (0.852 > or = r2 > or = 0.590) at spatial scales between 0.5 and 5.0 km2, using the environmental variables of mean and variance of depth and wave exposure. Beta diversity, quantified within a "window" of a given size, is positively correlated to the range of environmental conditions within that window. For example, beta diversity increases with increasing variance of depth. By analyzing such relationships across seascapes, this study provides a framework for a range of disparate coral reef literature including studies of zonation, diversity, and disturbance. Using supporting evidence from soft-bottom communities, we hypothesize that depth will be an important variable for modeling beta diversity in a range of marine systems. We discuss the implications of our results for the design of marine reserves.     

The effect of micrograzers on algal community structure in a coral reef microcosm

The effect of amphipod grazing on algal community structure was studied within a 75 l refuge tank connected to a 6500 l closed-system, coral reef microcosm. When amphipods (Ampithoe ramondi) were absent or present in low numbers, a high biomass of mostly filamentous algal species resulted, including Bryopsis hypnoides, Centroceras clavulatum, Ceramium flaccidum, Derbesia vaucheriaeformis, Enteromorpha prolifera, Giffordia rallsiae, and Polysiphonia havanensis. These microalgae disappeared when amphipod density increase beyond approximately 1 individual cm^-2 of tank surface. The macroalga Hypnea spinella germinated in the system in association with amphipod tube sites. H. spinella plants remained rare until filamentous species were eliminated by amphipod grazing. Feeding trials confirmed that H. spinella was protected from grazing by its size rather than a chemical defense strategy. The H. spinella community we observed is similar to the flora described on algal ridges where physical conditions exclude fish grazing. We suggest that amphipods and similar micrograzers are responsible for the algal community structure of these ridges. Caging experiments may be subject to similar effects from increased amphipod grazing on the algae. Introduction of fish that are amphipod predators into the refuge tank caused an increase in algal species diversity but total H. spinella growth rates fell from 25 g dry wt month^-1 to less than 8 g dry wt month^-1. We describe amphipod behavior in relation to changes in population density and food supply, and we stress the potential for increasing the productivity of commercial seaweeds through maintenance of appropriate amphipod species in mariculture facilities.     

Development of contemporary Eastern Pacific coral reefs

An overview of oceanographic conditions prevailing in the tropical Eastern Pacific Ocean suggests that the entire region is environmentally marginal for coral-reef development. The principal features of this environment are a strong, permanent, shallow thermocline and an annual north-south migration of the Intertropical Convergence resulting in wet and dry seasons. Along tropical Eastern Pacific continental margins structural coral reefs are best developed in the Gulf of Chiriquí off western Panamá. These reefs are relatively small, with reef formation taking place at a maximum depth of roughly 10 m. All of the reefs are judged to have formed since sea level approached its present height some 5,000 years ago. A study of the physical environment in the Gulf of Chiriquí revealed the following. Seasonal differences in surface temperatures were small but significant (P0.01), with the dry-season median (Md) of 28.9°C higher than the rainy season one of 28.0°C. At all times, surface-water temperatures were within the range considered optimal for coral growth. There were also significant (P<0.05) seasonal differences in the depths of the 25°, 20°, and 18°C isotherms. The first was shallowest (Md=18.5 m) during the rainy season due to vertical mixing, while the latter two were shallowest (Mds=31.5 and 33.0 m, respectively) during the dry season due to a generalized shoaling of the thermocline. All three isotherms are closely associated with the thermocline and showed remarkable variability in depth, most likely connected with internal waves. Salinities were reduced down to depths greater than 20 m and for distances of more than 50 km from the coast. Seasonal differences were slight (0.7% S) but statistically significant (P<0.01). Turbidity during the rainy season reduces the amount of light reaching the bottom at 10 m depth roughly by a factor of three compared to the dry season. Even the dry season amount is only about one half as much as would be expected to reach the same depth on the seaward reef of a Westen Pacific atoll. These conditions of cool water a short distance below the surface, reduced salinities, and high seasonal turbidity combine to make the region a poor one for coral-reef formation. The history of the Eastern Pacific coral fauna is traced from the Cretaceous to the Holocene. The present fauna is of Indo-Western Pacific origin, having become established following (1) the final closure of the connections between the Caribbean and Eastern Pacific (Pliocene), (2) movement of the northern Line Islands by sea-floor spreading into the path of the North Pacific Equatorial Countercurrent (Pliocene), and (3) the loss of all Eastern Pacific hermatypes during the Pleistocene.     

Biomass of suspended bacteria over coral reefs

The biomass of bacteria suspended in water flowing over coral reefs at Lizard Island and Yonge Reef (Northern Great Barrier Reef) was estimated by measurement of muramic acid. Values ranged from 20 mg C m^-3 in the open water up to about 60 mg C m^-3 over the reef flat. Direct counts of total numbers of free bacteria were made for comparison. Values of around 2.0x10⁹ cells g^-1 muramic acid showed that there was a good agreement between direct counts and muramic acid content of free bacteria in the open water. In samples containing suspended particulate matter, ratios of direct counts to muramic acid concentration were lower, because bacteria on particles could not be counted. Thus, these ratios were used to indicate the proportions of bacteria attached to particles. Changes in the biomass and numbers of bacteria were determined in water masses identified either by a drogue or fluorescein, as they moved across the reefs. In the zone on the outside of the reef, the number of free bacteria decreased compared to open sea water, but total biomass increased, showing that particulate matter containing bacteria was thrown up into suspension. About 50% of bacteria were attached to particles. Water flowing over the reef flats contained much particulate material with bacteria attached. Bacteria constituted between about 5 and 20% of particulate organic carbon.     

Diversity and stability of herbivorous fishes on coral reefs

Biodiversity may provide insurance against ecosystem collapse by stabilizing assemblages that perform particular ecological functions (the "portfolio effect"). However, the extent to which this occurs in nature and the importance of different mechanisms that generate portfolio effects remain controversial. On coral reefs, herbivory helps maintain coral dominated states, so volatility in levels of herbivory has important implications for reef ecosystems. Here, we used an extensive time series of abundances on 35 reefs of the Great Barrier Reef of Australia to quantify the strength of the portfolio effect for herbivorous fishes. Then, we disentangled the contributions of two mechanisms that underlie it (compensatory interactions and differential responses to environmental fluctuations ["response diversity"]) by fitting a community-dynamic model that explicitly includes terms for both mechanisms. We found that portfolio effects operate strongly in herbivorous fishes, as shown by nearly independent fluctuations in abundances over time. Moreover, we found strong evidence for high response diversity, with nearly independent responses to environmental fluctuations. In contrast, we found little evidence that the portfolio effect in this system was enhanced by compensatory ecological interactions. Our results show that portfolio effects are driven principally by response diversity for herbivorous fishes on coral reefs. We conclude that portfolio effects can be very strong in nature and that, for coral reefs in particular, response diversity may help maintain herbivory above the threshold levels that trigger regime shifts.     

Measuring settlement intensity of echinoderms on coral reefs

A pilot study was conducted at Davies Reef on the central Great Barrier Reef between 20 November 1991 and 20 January 1992 to assess the suitability of settlement collectors for measuring larval settlement rates of echinoderms on coral reefs. The collectors were deployed for two months during summer when many echinoderm species are known to spawn. A total of 657 newly settled echinoderms were recovered from just 47 collectors, each having a volume of just 0.005 m³ but with a utilisable surface area of about 1.44 m², indicating that competent-to-settle larvae were very abundant during the sampling period. Echinoids (7.8 sampler^-1) and ophiuroids (5.6 sampler^-1) were the most abundant groups on the collectors. Asteroids (0.2 sampler^-1) and crinoids (0.3 sampler^-1 were less common. The asteroids were all identified to species and included the crown-of-thorns starfish Acanthaster planci, Choriaster granulatus and Culcita novaeguineae. The newly settled echinoids could not be distinguished from each other but included Echinometra mathaei and Mespilia globulus. The abundance of each of the five different classes of echinoderms on the samplers was correlated with their abundance on the natural substratum. Significant spatial variability was found in settlement rates of echinoids over 1000s of metres and ophiuroids over 100s of metres, but not over smaller spatial scales. It is concluded that the collectors can provide a useful tool for monitoring spatial and temporal variability in settlement rates of echinoderms on coral reefs and for testing hypotheses about patterns of larval dispersal.     

Observations on organic aggregates in the vicinity of coral reefs

The abundance of organic aggregates is reported for a variety of coral-reef sites and surrounding waters. There is no net increase of aggregates forming from dissolved organic matter and streaming off the reefs into the lagoons, though the organic components and the aeration of this water might be conducive to aggregate formation. Also, although the waters flowing from the surrounding oceans across the reefs and into the lagoons contribute organic components, the concentration of organic aggregates in the latter habitats is not in excess of quantities in surrounding oceanic waters.     

Parrotfish predation on cryptic sponges of Caribbean coral reefs

Some sponge species that live in crevices in the reef frame appear to be restricted to their cryptic habitat by predation. When cryptic sponges were excavated, on Guigalotupo reef, San Blas, Panama, exposing them to potential predators, they were eaten by fishes that are generally considered to be herbivores, primarily parrotfishes of the genus Sparisoma: S.aurofrenatum (Cuvier & Valenciennes), S.viride (Bonnaterre), and S.chrysopterum (Bloch & Schneider). Of the 9150 bites observed to be taken by these species during paired (i.e., with sponges versus without sponges) trials conducted in defined feeding areas during 1986, 1987, and 1988, 72% (i.e. 6581 bites) were on cryptic sponges, even though these were only offered during half of the total observation time and never constituted more than 7% of the cover of the feeding observation areas. Individual parrotfish returned over and over to take bites of the exposed cryptic sponges until they were entirely consumed. They vigorously chased each other away from the sponges, but exhibited no such defense of their usual algal foods. A total of 18 sponge species were tested. Of the cryptic and semi-cryptic sponge species tested, only one of six was rejected by the parrotfish. Two of these six sponge species were consistently consumed entirely, and two were consumed entirely whenever their surfaces were sliced off with a razor blade, demonstrating that these sponges concentrate defenses against predators in their surfaces. One semi-cryptic species and one semi-exposed species were fed upon, but not entirely consumed. By contrast, 11 of 12 of the exposed and semi-exposed species were rejected. Cryptic sponges grew out of their cavities in the reef only when protected by seasonally thick mats of macroalgae or by cages that excluded fish. Received: 10 January 1997 / Accepted: 4 February 1997     

The trophic status of herbivorous fishes on coral reefs

Dietary analyses of 17 species of nominally herbivorous fishes on the northern Great Barrier Reef were carried out to resolve the questions to what extent diet in nominally herbivorous reef fishes is dominated by living plant material and whether short-chain fatty acid profiles in the alimentary tract predict diet. The fishes included members of the families Acanthuridae, Scaridae and Kyphosidae. The analyses revealed consumption of a wide range of food resources including macroscopic algae, turfing algae, pelagic and planktonic animal matter, organic detritus and sediments. In only 7 of the 17 species was living algae the dominant dietary item. Gastrointestinal short-chain fatty acids (SCFA) profiles reflecting the fermentative activities of gut symbionts were used as a framework for the initial analysis of dietary patterns. The feeding patterns implied by the SCFA profiles were validated by direct observation of food material through gut content analysis. We identified four main trophic groupings differentiated by fermentation profiles and diet. These were: (1) species with low levels of SCFA in gut samples (mean mM 9.2ǂ.8) but a high proportion of isovalerate (mean 15.4%dž.7): Acanthurus nigricauda, A. olivaceus, Ctenochaetus striatus, Chlorurus microrhinos, C.sordidus, Scarus schlegeli; (2) species that fed on turfing, filamentous and small thallate algae and displayed moderate levels of SCFA (mean mM 21.9DŽ.9): Acanthurus nigricans, A.lineatus, Naso tuberosus; (3) species that fed on large thallate (macroscopic) algae with high levels of SCFA (mean mM 41.2Dž.5) with very low proportions of isovalerate (mean 0.5%ǂ.1): Kyphosus cinerascens, K.vaigiensis, Naso unicornis, Zebrasoma scopas; and (4) species with diets dominated by planktonic animal material with moderate levels of SCFA (mean mM 21.3ǃ.5): Naso annulatus, N. brevirostris, N. hexacanthus, N. vlamingii. We conclude that a wide range of dietary items are taken by the 17 nominal herbivores, and that dietary groupings do not reflect taxonomic relationships. There was evidence of convergence in feeding modes and diet between phylogenetically distinct taxa and divergence within particular lineages.     

Large-scale spatial variation in epilithic algal matrix cryptofaunal assemblages on the Great Barrier Reef

Spatial variation in the epilithic algal matrix (EAM) cryptofauna was investigated at three locations on the Great Barrier Reef: two inner shelf—Orpheus Island and the Turtle Island group—and a mid-shelf location, Lizard Island. Although the EAM appears to be a relatively simple and consistent habitat, significant differences in cryptofaunal assemblages were found between locations. EAM assemblages from Orpheus Island were markedly different. This appears to be a function of the sediment profile characteristics (grain size >60 µm) at Orpheus Island, as many cryptofaunal taxa displayed positive relationships with sediment volume. However, sediment volumes did not differ significantly between the three locations, highlighting the possibility of cyclonic activity affecting the sediment profile at Orpheus Island in the months preceding the study, in addition to the nutrient input from major terrigenous sources. The results of this study show that EAM cryptofaunal assemblages are not uniform across the Great Barrier Reef and suggest that dissolved nutrients, sediment loads and distance from river systems may be significant drivers of cryptobenthic faunal compositions.     

The trophic status of herbivorous fishes on coral reefs

Estimates of feeding rates, alimentary tract structure and temporal patterns of food processing obtained from twelve species of nominally herbivorous fishes on the northern Great Barrier Reef were compared. These included members of the families Acanthuridae, Scaridae and Kyphosidae. Based on an analysis of diet and short-chain fatty acid (SCFA) profiles from a previous study we initially partitioned the twelve species into four dietary categories, as follows: (a) Category 1: herbivores with a diet of macroscopic brown algae and high SCFA profiles in the hindgut region (Naso unicornis, Kyphosus vaigiensis); (b) Category 2: herbivores feeding on turfing and filamentous red and green algae with moderate SCFA profiles in the hindgut region (N. tonganus, K. cinerascens, Zebrasoma scopas, Acanthurus lineatus); (c) Category 3: zooplankton feeders with moderate SCFA profiles (N. vlamingii, N. brevirostris); (d) Category 4: species feeding on detrital and sedimentary materials with low levels of SCFA (Chlorurus microrhinos, Scarus schlegeli, Ctenochaetus striatus, A. olivaceus). The purpose of this comparison was to determine whether measures of feeding activity, alimentary tract structure, and food processing were concordant with diet. A dichotomy in feeding rates was observed. Species with a diet of algae and zooplankton (categories 1–3) had slower feeding rates than those feeding on detrital aggregates and sediment (category 4). The pattern of food processing also followed the same dichotomy with species of categories 1–3 retaining food in the alimentary tract overnight and commencing the feeding day with substantial amounts of food in the intestine and hindgut. Category-4 species commenced the feeding day with empty alimentary tracts suggesting a rapid turnover of gut contents. Within the herbivorous and zooplankton-feeding species neither alimentary tract structure nor food processing mode were predicted by diet or SCFA profiles. A hindgut fermentation chamber was present in K. vaigiensis but not in N. unicornis, a species with high levels of SCFA in the hindgut region and a diet of brown macroscopic algae. In contrast N. vlamingii, with a diet dominated by animal matter, retained large amounts of food material in a hindgut chamber over the entire feeding cycle. In tropical perciform fishes, herbivory and fermentation are not associated with the alimentary tract structures that characterise herbivorous terrestrial vertebrates. Estimates of the abundance of the different groupings of nominally herbivorous fishes indicated that the dominant elements in the reef grazing and browsing fauna were consumers of detrital and sedimentary materials. These could not be classified as herbivores. Members of this group were dominant in all habitats investigated. Explicitly herbivorous taxa were a minority component in all habitats investigated.Communicated by G.F. Humphrey, Sydney     

Herbivory by the Caribbean king crab on coral patch reefs

Caribbean coral reefs are increasingly dominated by macroalgae instead of corals due to several factors, including the decline of herbivores. Yet, virtually unknown is the role of crustacean macrograzers on coral reef macroalgae. We examined the effect of grazing by the Caribbean king crab (Mithrax spinosissimus) on coral patch reef algal communities in the Florida Keys, Florida (USA), by: (1) measuring crab selectivity and consumption of macroalgae, (2) estimating crab density, and (3) comparing the effect of crab herbivory to that of fishes. Mithrax prefers fleshy macroalgae, but it also consumes relatively unpalatable calcareous algae. Per capita grazing rates by Mithrax exceed those of most herbivorous fish, but Mithrax often occurs at low densities on reefs and its foraging activities are reduced in predator-rich environments. Therefore, the effects of grazing by Mithrax tend to be localized and when at low density contribute primarily to spatial heterogeneity in coral reef macroalgal communities.     

Inhibition of marine algal photosynthesis by heavy metals

The orientation of the opportunist, sublittoral barnacle Balanus trigonus is investigated. Water movement is the primary orientation stimulus for this species, but this effect may be modified by the influence of light acting in opposition to it. The barnacles orientate at right angles to the axis of wave-surge movement, and observations of feeding behaviour showed that in this position the cirral net could be swivelled 90° each way to make best use of both the advancing, and the reversing, water flow. In a steady unidirectional tidal current the barnacles aligned themselves so as to beat with the water flow, and observation suggests that stroking with the flow is more efficient than forcing the cirral net against the motion of the water. It is suggested that, for an opportunist short-lived species such as B. trigonus which is subjected to high predation rates, there is considerable selection pressure for rapid growth and early sexual maturity. To achieve this in a filterfeeding organism for which food is limiting, food collection must be as efficient as possible. B. trigonus has adopted a system of orientation to a variety of water movement regimes that allows efficient food collection over a wide range of conditions.     

A geological perspective on the degradation and conservation of western Atlantic coral reefs

Continuing coral‐reef degradation in the western Atlantic is resulting in loss of ecological and geologic functions of reefs. With the goal of assisting resource managers and stewards of reefs in setting and measuring progress toward realistic goals for coral‐reef conservation and restoration, we examined reef degradation in this region from a geological perspective. The importance of ecosystem services provided by coral reefs—as breakwaters that dissipate wave energy and protect shorelines and as providers of habitat for innumerable species—cannot be overstated. However, the few coral species responsible for reef building in the western Atlantic during the last approximately 1.5 million years are not thriving in the 21st century. These species are highly sensitive to abrupt temperature extremes, prone to disease infection, and have low sexual reproductive potential. Their vulnerability and the low functional redundancy of branching corals have led to the low resilience of western Atlantic reef ecosystems. The decrease in live coral cover over the last 50 years highlights the need for study of relict (senescent) reefs, which, from the perspective of coastline protection and habitat structure, may be just as important to conserve as the living coral veneer. Research is needed to characterize the geological processes of bioerosion, reef cementation, and sediment transport as they relate to modern‐day changes in reef elevation. For example, although parrotfish remove nuisance macroalgae, possibly promoting coral recruitment, they will not save Atlantic reefs from geological degradation. In fact, these fish are quickly nibbling away significant quantities of Holocene reef framework. The question of how different biota covering dead reefs affect framework resistance to biological and physical erosion needs to be addressed. Monitoring and managing reefs with respect to physical resilience, in addition to ecological resilience, could optimize the expenditure of resources in conserving Atlantic reefs and the services they provide.