Environmental conditions and herbivore biomass determine coral reef benthic community composition: implications for quantitative baselines

Coral Reefs - Our ability to understand natural constraints on coral reef benthic communities requires quantitative assessment of the relative strengths of abiotic and biotic processes across large...     

Determinants of community structure for coral reef fishes in isolated coral heads at lagoonal and reef slope sites

Summary Ten small isolated corals were selected as units, of habitat in each of two nearby reef sites-a lagoon and a reef slope. On six occasions over two years we collected all fishes resident in each of these corals. Collections yielded 827 fishes of 64 species from the lagoon and 525 fishes of 66 species from the slope, but at each site 12 common species comprised over 80% of the fishes collected. We examined the distribution of species of fishes among units of habitat to assess the extent to which partitioning of habitat was being carried out. Results are compared with others previously reported from a reef flat site. Species discriminated among different types of habitat offered, but to a different degree in each site. Discrimination was most pronounced at the slope site where 7 of the 12 commonest species did not occur in all three types of habitat offered, and least at the lagoon site where no common species failed to occupy both types of habitat offered. No temporal partitioning of habitat could be demonstrated. Fish did not distribute themselves among units of habitat of one type by means of precise microhabitat discrimination. No pair of species in either site could be shown to mutually avoid, or exclude one another from habitat units. At all three sites, chance patterns of recruitment and loss overwhelmingly determined species composition of the groups of fishes coexisting in single habitat units. The significance of these results for our understanding of the ecology of coral reef fishes is discussed.     

The effect of warming and benthic community acclimation on coral reef carbonate sediment metabolism and dissolution

Coral Reefs - Global warming (and the consequent increase in sea surface temperature) is expected to modify rates of gross primary production (GPP), respiration (R), and net calcium carbonate...     

Near‐term impacts of coral restoration on target species,coral reef community structure,and ecological processes

The global decline of corals has created an urgent need for effective, science‐based methods to augment coral populations and restore important ecosystem functions. To meet this challenge, the field of coral restoration has rapidly evolved over the past decade. However, despite widespread efforts to outplant corals and monitor survivorship, there is a shortage of information on the effects of coral restoration on reef communities or important ecosystem functions. To fill this knowledge gap, we examined the effects of restoration on three major criteria: diversity, community structure, and ecological processes. We conducted surveys of four restored sites in the Florida Keys ranging in restoration effort (500–2,300 corals outplanted) paired with surveys of nearby, unmanipulated control sites. Coral restoration successfully enhanced coral populations, increasing coral cover 4‐fold, but manifested in limited differences in coral and fish communities. Some restored sites had higher abundance of herbivorous fish, rates of herbivory, or more juvenile‐sized corals, but these effects were limited to individual reefs. Damselfish were consistently more abundant at restored compared to control sites. Despite augmenting target coral populations, 3 years of coral restoration has not facilitated many of the positive feedbacks that help reinforce coral success. In a time of increasingly frequent disturbances, it is urgent we hasten the speed at which reefs recover important ecological processes, such as herbivory and nutrient cycling, that make reefs more resistant and resilient if we are to achieve long‐term restoration success.     

Distributional patterns and community structure of Caribbean coral reef fishes within a river-impacted bay

This study examined how riverine inputs, in particular sediment, influenced the community structure and trophic composition of reef fishes within Rio Bueno, north Jamaica. Due to river discharge a distinct gradient of riverine inputs existed across the study sites. Results suggested that riverine inputs (or a factor associated with them) had a structuring effect on fish community structure. Whilst fish communities at all sites were dominated by small individuals (<20 cm), diversity and total biomass were reduced with increased proximity to the river mouth. The abundance of all fishes, but particularly small-bodied, juvenile and herbivorous fishes was reduced in turbid water when compared to clear-water sites. Results strongly suggest that fluvial sediment inputs may play an important role in structuring fish assemblages even under intense fishing pressure.     

Coral bleaching, reef fish community phase shifts and the resilience of coral reefs

The 1998 global coral bleaching event was the largest recorded historical disturbance of coral reefs and resulted in extensive habitat loss. Annual censuses of reef fish community structure over a 12-year period spanning the bleaching event revealed a marked phase shift from a prebleach to postbleach assemblage. Surprisingly, we found that the bleaching event had no detectable effect on the abundance, diversity or species richness of a local cryptobenthic reef fish community. Furthermore, there is no evidence of regeneration even after 5–35 generations of these short-lived species. These results have significant implications for our understanding of the response of coral reef ecosystems to global warming and highlight the importance of selecting appropriate criteria for evaluating reef resilience.     

Impact of cyclones on hard coral and metapopulation structure,connectivity and genetic diversity of coral reef fish

Cyclones have one of the greatest effects on the biodiversity of coral reefs and the associated species. But it is unknown how stochastic alterations in habitat structure influence metapopulation structure, connectivity and genetic diversity. From 1993 to 2018, the reefs of the Capricorn Bunker Reef group in the southern part of the Great Barrier Reef were impacted by three tropical cyclones including cyclone Hamish (2009, category 5). This resulted in substantial loss of live habitat-forming coral and coral reef fish communities. Within 6–8 years after cyclones had devastated, live hard corals recovered by 50–60%. We show the relationship between hard coral cover and the abundance of the neon damselfish (Pomacentrus coelestis), the first fish colonizing destroyed reefs. We present the first long-term (2008–2015 years corresponding to 16–24 generations of P. coelestis) population genetic study to understand the impact of cyclones on the meta-population structure, connectivity and genetic diversity of the neon damselfish. After the cyclone, we observed the largest change in the genetic structure at reef populations compared to other years. Simultaneously, allelic richness of genetic microsatellite markers dropped indicating a great loss of genetic diversity, which increased again in subsequent years. Over years, metapopulation dynamics were characterized by high connectivity among fish populations associated with the Capricorn Bunker reefs (2200 km²); however, despite high exchange, genetic patchiness was observed with annual strong genetic divergence between populations among reefs. Some broad similarities in the genetic structure in 2015 could be explained by dispersal from a source reef and the related expansion of local populations. This study has shown that alternating cyclone-driven changes and subsequent recovery phases of coral habitat can greatly influence patterns of reef fish connectivity. The frequency of disturbances determines abundance of fish and genetic diversity within species.

     

Storm-induced coral rubble deposition: Pleistocene records of natural reef disturbance and community response.

Shallow-water Pleistocene coral reef facies in Barbados (dominated by Acropora palmata rubble) record evidence of deposition under contrasting non-catastrophic - (fair-weather?) and storm-induced conditions. Depositional styles are interpreted on the basis of coral rubble fabrics and calcareous encruster successions. Terrace exposures on the west of the island comprise uniform (3-4 m thick) depositional sequences. Individual coral samples exhibit similarities in encruster community composition and thickness, and a transition from photophilic to sciaphilic encrusting forms. These are indicative of colonization during gradual burial in an accumulating rubble pile. By contrast, NE coast sites comprise repetitive sequences of discrete (0.4-1 m thick) depositional units. Rubble colonization within each unit is characterized by a vertical succession from thin (1-2 mm), sciaphilic encruster-dominated sequences at the base, to progressively thicker (up to 20 mm), photophilic encruster-dominated sequences at the top. These are interpreted as multiple storm deposits, with upper surfaces colonized by opportunist coral species (primarily Agaricia agaricites). In contrast to many modern hurricane-impacted reef systems, however, there is no evidence of long-term shifts in coral community composition following physical disturbance. Colony sizes of opportunist corals at the tops of storm units are consistent with growth over timescales of <10 years. These are overlain on each occasion by a new A. palmata rubble pile, indicating successful recovery over successive physical disturbance cycles.     

Depth distributions of coral reef fishes: the influence of microhabitat structure,settlement, and post-settlement processes

Many coral reef fishes have restricted depth ranges that are established at settlement or soon after, but the factors limiting these distributions are largely unknown. This study examines whether the availability of microhabitats (reef substrata) explains depth limits, and evaluates whether juvenile growth and survival are lower beyond these limits. Depth-stratified surveys of reef fishes at Kimbe Bay (Papua New Guinea) showed that the abundance of new settlers and the cover of coral substrata differed significantly among depths. A field experiment investigated whether settling coral reef fishes preferred particular depths, and whether these depth preferences were dependent on microhabitat. Small patch reefs composed of identical coral substrata were set up at five depths (3, 6, 10, 15 and 20 m), and settlement patterns were compared to those on unmanipulated reef habitat at the same five depths. For all species, settlement on patch reefs differed significantly among depths despite uniform substratum composition. For four of the six species tested, depth-related settlement patterns on unmanipulated habitat and on patch reefs did not differ, while for the other two, depth ranges were greater on the patch reefs than on unmanipulated habitat. A second experiment examined whether depth preferences reflected variation in growth and survival when microhabitat was similar. Newly settled individuals of Chrysiptera parasema and Dascyllus melanurus were placed, separately, on patch reefs at five depths (as above) and their survival and growth monitored. D. melanurus, which is restricted to shallow depths, had highest survival and growth at the shallowest depth. Depth did not affect either survival or growth of C. parasema, which has a broader depth range than D. melanurus (between 6 and 15 m). This suggests that the fitness costs potentially incurred by settling outside a preferred depth range may depend on the strength of the depth preference.     

The role of the reef flat in coral reef trophodynamics: Past,present, and future

The reef flat is one of the largest and most distinctive habitats on coral reefs, yet its role in reef trophodynamics is poorly understood. Evolutionary evidence suggests that reef flat colonization by grazing fishes was a major innovation that permitted the exploitation of new space and trophic resources. However, the reef flat is hydrodynamically challenging, subject to high predation risks and covered with sediments that inhibit feeding by grazers. To explore these opposing influences, we examine the Great Barrier Reef (GBR) as a model system. We focus on grazing herbivores that directly access algal primary productivity in the epilithic algal matrix (EAM). By assessing abundance, biomass, and potential fish productivity, we explore the potential of the reef flat to support key ecosystem processes and its ability to maintain fisheries yields. On the GBR, the reef flat is, by far, the most important habitat for turf‐grazing fishes, supporting an estimated 79% of individuals and 58% of the total biomass of grazing surgeonfishes, parrotfishes, and rabbitfishes. Approximately 59% of all (reef‐wide) turf algal productivity is removed by reef flat grazers. The flat also supports approximately 75% of all grazer biomass growth. Our results highlight the evolutionary and ecological benefits of occupying shallow‐water habitats (permitting a ninefold population increase). The acquisition of key locomotor and feeding traits has enabled fishes to access the trophic benefits of the reef flat, outweighing the costs imposed by water movement, predation, and sediments. Benthic assemblages on reefs in the future may increasingly resemble those seen on reef flats today, with low coral cover, limited topographic complexity, and extensive EAM. Reef flat grazing fishes may therefore play an increasingly important role in key ecosystem processes and in sustaining future fisheries yields.     

Community structure,social organization and ecological requirements of coral reef fish (Pomacentridae)

In the Gulf of Aqaba (Red Sea), 25 species of damselfish (Pomacentridae) which form typical interspecific communities were studied. Several species are typical for different depth zones, others are ubiquitous. Damselfish are variously specialized in feeding (herbivore, omnivore, plankton-carnivore); habitat (bottom-dwelling, midwater); home-and-shelter site (selective or unselective). Environmental factors and social structures were found to be interlinked. Species with the same social structure are similarly specialized ecologically. A hypothetical flow diagram shows the effects of abiotic and biotic factors on group size, available food, and social structure. Damselfish are extremely flexible to environmental change, showing intraspecific modification of group structure, feeding habits, reproductive strategy etc. Experimental results suggest a strong selection against redundant males. This research is considered a pilot study for future field work, aimed at a prediction of environmental influences on behaviour.Supported by the Deutsche Forschungsgemeinschaft, Bad Godesberg, and the Max-Planck-Institut für Verhaltensphysiologie, Seewiesen.     

Complex migration and the development of genetic structure in subdivided populations: an example from Caribbean coral reef ecosystems

A matrix‐based projection model is used in conjunction with the results of a coupled bio‐physical dispersal model to examine the spread of alleles through subdivided populations over time, and the associated development of genetic structural patterns. Applying this approach, it becomes possible to quantitatively evaluate the contribution of spatially explicit migration towards patterns of genetic structure observed in the field. To provide a concrete example, the model was used to examine genetic dispersal between coral reef patches of the Caribbean. Using generic life‐history parameters, the model shows the formation of a strong genetic break between eastern and western patches, as well as the development of a gradient along the length of the Bahamian archipelago, corresponding with evidence previously collected for coral and fish species. The data also suggest that Jamaica and the Cayman Islands are important stepping stones between the reefs of the northern Caribbean (Hispaniola and the Bahamas) and the Mesoamerican Barrier Reef. The model provides an effective means of evaluating regional‐scale genetic connectivity through time and identifying natural clusters of genetic exchange.     

Future coral reef habitat marginality: temporal and spatial effects of climate change in the Pacific basin

Marginal reef habitats are regarded as regions where coral reefs and coral communities reflect the effects of steady-state or long-term average environmental limitations. We used classifications based on this concept with predicted time-variant conditions of future climate to develop a scenario for the evolution of future marginality. Model results based on a conservative scenario of atmospheric CO₂ increase were used to examine changes in sea surface temperature and aragonite saturation state over the Pacific Ocean basin until 2069. Results of the projections indicated that essentially all reef locations are likely to become marginal with respect to aragonite saturation state. Significant areas, including some with the highest biodiversity, are expected to experience high-temperature regimes that may be marginal, and additional areas will enter the borderline high temperature range that have experienced significant ENSO-related bleaching in the recent past. The positive effects of warming in areas that are presently marginal in terms of low temperature were limited. Conditions of the late 21st century do not lie outside the ranges in which present-day marginal reef systems occur. Adaptive and acclimative capabilities of organisms and communities will be critical in determining the future of coral reef ecosystems.Electronic supplementary material is available for this article if you access the article at . A link in the frame on the left on that page takes you directly to the supplementary material.     

Exploring the nature of ecological specialization in a coral reef fish community: morphology,diet and foraging microhabitat use

Patterns of ecological specialization offer invaluable information about ecosystems. Yet, specialization is rarely quantified across several ecological niche axes and variables beyond the link between morphological and dietary specialization have received little attention. Here, we provide a quantitative evaluation of ecological specialization in a coral reef fish assemblage (f. Acanthuridae) along one fundamental and two realized niche axes. Specifically, we examined ecological specialization in 10 surgeonfish species with regards to morphology and two realized niche axes associated with diet and foraging microhabitat utilization using a recently developed multidimensional framework. We then investigated the potential relationships between morphological and behavioural specialization. These relationships differed markedly from the traditional ecomorphological paradigm. While morphological specialization showed no relationship with dietary specialization, it exhibited a strong relationship with foraging microhabitat specialization. However, this relationship was inverted: species with specialized morphologies were microhabitat generalists, whereas generalized morphotypes were microhabitat specialists. Interestingly, this mirrors relationships found in plant–pollinator communities and may also be applicable to other ecosystems, highlighting the potential importance of including niche axes beyond dietary specialization into ecomorphological frameworks. On coral reefs, it appears that morphotypes commonly perceived as most generalized may, in fact, be specialized in exploiting flat and easily accessible microhabitats.     

Uptake of picophytoplankton,bacterioplankton and virioplankton by a fringing coral reef community (Ningaloo Reef,Australia)

We examined the importance of picoplankton and virioplankton to reef trophodynamics at Ningaloo Reef, (north-western Australia), in May and November 2008. Picophytoplankton (Prochlorococcus, Synechococcus and picoeukaryotes), bacterioplankton (inclusive of bacteria and Archaea), virioplankton and chlorophyll a (Chl a) were measured at five stations following the consistent wave-driven unidirectional mean flow path of seawater across the reef and into the lagoon. Prochlorococcus, Synechococcus, picoeukaryotes and bacterioplankton were depleted to similar levels (~40% on average) over the fore reef, reef crest and reef flat (=‘active reef’), with negligible uptake occurring over the sandy bottom lagoon. Depletion of virioplankton also occurred but to more variable levels. Highest uptake rates, m, of picoplankton occurred over the reef crest, while uptake coefficients, S (independent of cell concentration), were similarly scaled over the reef zones, indicating no preferential uptake of any one group. Collectively, picophytoplankton, bacterioplankton and virioplankton accounted for the uptake of 29 mmol C m⁻² day⁻¹, with Synechococcus contributing the highest proportion of the removed C. Picoplankton and virioplankton accounted for 1–5 mmol N m⁻² day⁻¹ of the removed N, with bacterioplankton estimated to be a highly rich source of N. Results indicate the importance of ocean–reef interactions and the dependence of certain reef organisms on picoplanktonic supply for reef-level biogeochemistry processes.     

Long-term monitoring of benthic macroinvertebrate community structure: a perspective from a Colorado river*

Temporal and spatial trends were examined in benthic macroinvertebrate and physical-chemical data collected for at least ten years at ten sites along the plains reach of the Cache la Poudre River, Colorado, USA. A distinct longitudinal environment gradient was found as many of the water chemistry parameter levels changed downstream from the reference site. Seasonal Kendall analyses on individual sites indicated that several chemical parameters, including conductivity, un-ionized NH₃-N and NO₂-N have increased since the beginning of the study within most sites. Levels of some parameters (e.g., dissolved oxygen, un-ionized NH₃-N) violated aquatic life standards a few times during the study.Over 175 taxa of macroinvertebrates (primarily insects) were collected in the study reach from 1981–1996. Results from detrended correspondence analyses (DCA) on macroinvertebrate data indicated that this stretch of the river exhibited little longitudinal change beyond the two farthest upstream sites. There was a decline in macroinvertebrate density and total number of taxa within most individual sites during 1983–1984, corresponding with the highest recorded discharge in 75 years (1983) and a prolonged, heavy spring runoff in 1984. Taxa richness and density recovered to pre-1983 levels within a few months to a year following the high flows at most sites. These findings suggested that the macroinvertebrate assemblages had low resistance to disturbance, but high resilience. However, the results from DCAs and Kendall's Coefficient of Concordance (W) on individual sites for the entire study period suggested a similar macroinvertebrate community structure through time. It would appear that the composition and abundance of the lotic macroinvertebrate assemblages in the Poudre River has remained relatively constant over the long-term. This has occurred even with some potentially negative changes in water chemistry and increased urban development.     

Species composition and structure of coral community of a platform reef at Bach Long Vi Island in the South China Sea

A platform reef at Bach Long Vi Island (Gulf of Tonkin in the South China Sea) was investigated for the first time. In all, 264 species of corals and their accompanying species of macrobenthos were found. Among the scleractinian corals, acroporids, poritids, and mussids dominated. Monospecific aggregations of alcyonarians Sinularia and Lobophytum and the hydroid Millepora were rather numerous. Based on its geomorphological characteristics, coral species diversity and zonal distribution, the investigated reef is comparable with ribbon and platform reefs on the Great Barrier Reef in Australia and in the Indian Ocean.