Changes In Benthic Cover Research Articles

Spatial structure and potential processes linking fish and benthic communities in a protected reef ecosystem in SE Brazil

Top-down and bottom-up processes can mediate the structuring of biological communities. Several studies have provided separate support for those processes in pelagic and benthic compartments of reef systems but studies focusing on whole-reef processes are less common. Here, we sampled 3 ‘reef compartments’ in the Alcatrazes Archipelago in Southeastern Brazil: benthic cover of colonial organisms, solitary organisms, and reef fish (i.e. pelagic) to identify the groups responsible for spatial community structure among reefs. A dynamic mosaic structure best defines the assemblages of each compartment, with substantial changes observed among sites over 2 consecutive years and at 2 depth strata, separated by a sharp thermocline. Changes in benthic cover of colonial organisms are largely due to the extent of the blooming of Sargassum canopies, algal turfs, and the zoanthid Palythoa caribaeorum. Solitary organisms show a consistent monotonic change in the abundance of sea urchins, ascidians, and snails. Fish assemblage structure differed among reefs; however, we observed that large invertivore fish tended to concentrate in warmer water above the thermocline and small fish of different feeding habits were found in deeper and colder water in all reefs. We observed a potential strong link between the reefscape and solitary organisms, with a negative relationship between the abundance of urchins and the cover of Sargassum spp. suggesting top-down control. A second link is indicated by a positive relationship between low-lying cover composed of articulated turf and P. caribaeorum and larger invertivorous fish, suggesting bottom-up control through the provision of favorable foraging grounds.

Divergent recovery trajectories of intertidal and subtidal coral communities highlight habitat-specific recovery dynamics following bleaching in an extreme macrotidal reef environment.

Coral reefs face an uncertain future punctuated by recurring climate-induced disturbances. Understanding how reefs can recover from and reassemble after mass bleaching events is therefore important to predict their responses and persistence in a rapidly changing ocean. On naturally extreme reefs characterized by strong daily temperature variability, coral heat tolerance can vary significantly over small spatial gradients but it remains poorly understood how this impacts bleaching resilience and recovery dynamics, despite their importance as resilience hotspots and potential refugia. In the macrotidal Kimberley region in NW Australia, the 2016 global mass bleaching event had a strong habitat-specific impact on intertidal and subtidal coral communities at our study site: corals in the thermally variable intertidal bleached less severely and recovered within six months, while 68% of corals in the moderately variable subtidal died. We therefore conducted benthic surveys 3.5 years after the bleaching event to determine potential changes in benthic cover and coral community composition. In the subtidal, we documented substantial increases in algal cover and live coral cover had not fully recovered to pre-bleaching levels. Furthermore, the subtidal coral community shifted from being dominated by branching Acropora corals with a competitive life history strategy to opportunistic, weedy Pocillopora corals which likely has implications for the functioning and stress resilience of this novel coral community. In contrast, no shifts in algal and live coral cover or coral community composition occurred in the intertidal. These findings demonstrate that differences in coral heat tolerance across small spatial scales can have large consequences for bleaching resilience and that spatial patchiness in recovery trajectories and community reassembly after bleaching might be a common feature on thermally variable reefs. Our findings further confirm that reefs adapted to high daily temperature variability play a key role as resilience hotspots under current climate conditions, but their ability to do so may be limited under intensifying ocean warming.

Sea‐weeding: Manual removal of macroalgae facilitates rapid coral recovery

Abstract Coral reef ecosystems globally are under threat, leading to declining coral cover and macroalgal proliferation. Manually removing macroalgae (i.e. ‘sea‐weeding’) may promote local‐scale coral recovery by reducing a biological barrier, though the impact of removal on community composition of benthic reef organisms has not been quantified. In this three‐year study (2018–2021), fleshy macroalgae (predominantly Sargassum spp.) were periodically removed from 25 m2 experimental plots on two inshore fringing reefs of Yunbenun (Magnetic Island) in the central Great Barrier Reef. By the end of the study, coral cover in removal plots (n = 12 plots) assessed through in‐field transects increased by at least 47% (2019 mean: 25.5%, 2021 mean: 37.4%), and macroalgal cover decreased by more than half. In contrast, in control plots (n = 12 plots), there was no change in macroalgal cover while coral cover remained stable (2019 mean: 16.4%, 2021 mean: 13.6%). Changes in benthic cover were supported by photoquadrat data, with Bayesian probability modelling indicating a 100% likelihood that coral cover more than doubled in removal plots over the study period, compared to only a 29% chance of increased coral cover in control plots. Synthesis and applications. Manual macroalgal removal can provide rapid benefits and enhance inshore coral reef recovery. Through involvement of community groups and citizen scientists, larger scale removal of macroalgae is a low‐tech, high‐impact, and achievable method for local reef management.

The effect of artificial reef design on the attraction of herbivorous fish and on coral recruitment, survival and growth

Fish assemblages of different types of artificial reefs can differ greatly in abundance, biomass and composition, with some reef types harboring over five times more herbivores than others. It is assumed that higher herbivorous fish abundance results in a higher grazing intensity, affecting the benthic community by means of enhanced coral recruitment, survival and growth. Territorial fish species might affect this process by chasing away other fish, especially herbivores. In this study we compared the fish assemblage, territorial behavior and grazing intensity by fish on two artificial reef types: reef balls and layered cakes, differing greatly in their fish assemblage during early colonization. In addition, the effect of artificial reef type on benthic development and coral recruitment, survival and growth, was investigated. Although layered cakes initially harbored higher herbivorous fish biomass, this effect was lost during consecutive monitoring events. This seems to be the result of the higher territorial fish abundance around the layered cakes where almost four times more chasing behavior was recorded compared to the reef balls. This resulted in a more than five times lower fish grazing intensity compared to the reef-ball plots. Although macroalgae were effectively controlled at both reefs, the grazing intensity did not differ enough to cause large enough structural changes in benthic cover for higher coral recruitment, survival or growth. The high turf algae cover, combined with increasing crustose coralline algae and sponge cover likely explained reduced coral development. We recommend further research on how to achieve higher grazing rates for improved coral development on artificial reefs, for example by facilitating invertebrate herbivores.

3D assessment of a coral reef at Lalo Atoll reveals varying responses of habitat metrics following a catastrophic hurricane

Extreme disturbances such as hurricanes can cause reductions in coral cover and three-dimensional (3D) structural complexity of coral reefs. We examined changes in structural complexity utilizing 3D reconstruction of a coral-reef site before and after Hurricane Walaka passed through Lalo of the Northwestern Hawaiian Islands. This event resulted in complete destruction of the coral-reef habitat, with dramatic changes in benthic cover from pre-hurricane tabulate coral to post-hurricane rubble. Rugosity and mean slope decreased after the hurricane, while structural complexity, captured by vector ruggedness measure (VRM), showed resolution-specific responses. This metric captured the structural complexity of rubble at a high raster resolution of 1 cm and that of tabulate coral at lower resolutions, resulting in decreases in mean VRM values at 2- and 4-cm resolutions but an increase at 1-cm resolution. Variability in profile and planform curvature was reduced after the hurricane due to a disappearance of extreme curvature values created by the tabulate coral after the hurricane. This study highlights the varying responses of habitat complexity metrics to the complete destruction of a coral reef and provides us with insights into how choices of habitat complexity metrics can affect quantitative assessments of 3D habitat structure.

Coral reef resilience differs among islands within the Gulf of Mannar, southeast India, following successive coral bleaching events

We used a 12-yr data set of benthic cover (2005–2017), spanning two bleaching events, to assess changes in benthic cover and coral community composition along 21 islands within Gulf of Mannar (GoM), southeast India. Overall, between 2005 and 2017 reefs had a simultaneous decrease in relative coral cover (avg. = − 36%) and increase in algal cover (avg. = + 45%). Changes in benthic cover were not consistent among islands, ranging from − 34 to + 5% for coral cover and from − 0.3 to + 50% for algae. There was a spatial gradient in coral mortality, which increased among islands from west to east. However, there was a disconnect between coral loss and subsequent increases in algae. Algal cover increased more on islands in west GoM where coral loss was minimal. Environmental co-factors (coral cover, percent bleaching, degree heating weeks, fish densities, Chl-a, pollution) explained > 50% of the benthic cover responses to successive bleaching. Coral survival was favored on islands with higher fish densities and chlorophyll-a levels, and increases in algal cover were associated with higher measures of pollution from terrestrial runoff. Coral morphotypes differed in their response following successive bleaching resulting in changes in the relative abundance of different coral morphotypes. Existing climate projections (RCP8.5) indicate a 22-yr gap in the onset of annual severe bleaching (ASB) for reefs in the east versus west GoM, and ASB was ameliorated for all reefs under the RCP4.5 projections. There is limited knowledge of the resilience of GoM reefs, and this study identifies coral morphotypes and reefs that are most likely to recover or decline from successive bleaching, in the context of forecasts of the frequency of future bleaching events in GoM.

Changes in Benthic Cover in the South Atoll of Tubbataha Reefs due to Possible Eutrophication

The Tubbataha Reefs Natural Park is of global and local importance and is a benchmark and aspirational model for the condition of reefs within an MPA. Recently, small but significant overall declines in hard coral cover (HCC) and increases in algae were detected in Tubbataha. Changes were most consistent in Site 4 at the South Atoll, where lagoon waters drain through during ebb tide. Space available to corals here was covered by cyanobacteria in 2018, and then by sponges in 2019. These sponges have begun to overgrow coral. These changes in Site 4 are likely due to an increase in guano-derived nutrients in the lagoon waters of the South Atoll. They may initiate a continuing decline in corals and, thus, require enhanced monitoring.

Spatial patterns and drivers of fish and benthic reef communities at São Tomé Island, Tropical Eastern Atlantic

AbstractMarine communities vary across spatial scales due to changes in habitat structure, wave exposure, depth and anthropogenic activities. Here, we quantitatively characterized, for the first time, the reef fish assemblages and benthic communities in six sites around São Tomé Island, in the Tropical Eastern Atlantic region. We performed 139 underwater visual census and 112 photo‐quadrats across six sites around the island to explore the relative importance of exposure, depth and topographic complexity as drivers of fish and benthic reef communities. Planktivores were the most important trophic group, in terms of both fish abundance and biomass. Small fishes (0–7 cm) dominated the abundance, whereas biomass was mainly concentrated in 8–15 and 16–30 cm body size classes. About 30% of the total benthic cover was composed of turf algae and circa of 28% of calcareous coralline algae, whereas corals were comparably less abundant (only 8%). Among these, Montastraea cavernosawas the most abundant coral (46% of the coral cover and 4% of total benthic cover). Wave exposure was the most important driver of the reef fish assemblages and was also important for benthic communities. Also, fish species composition seemed to respond, although weakly, to benthic composition. Topographic complexity had little effect on the indicators of fish assemblages but seemed to drive some changes in benthic cover. Overall, our results evidence the dominance of small planktivorous reef fish assemblages and of turf algal communities in São Tomé. Taken together, they support the idea of benthic communities shaped by wave action, with minor importance of corals, and of important energetic contribution from planktonic material. We also suggest that the relatively low biomass of medium and large fish species reflects the long‐term fishing pressure in São Tomé Island.

Detection of changes in shallow coral reefs status: Towards a spatial approach using hyperspectral and multispectral data

Coral reef degradation due to environmental change, including anthropogenic disturbances, is a major concern worldwide. Detecting and assessing both temporal and spatial changes in benthic cover is a crucial requirement to inform policy makers and guide conservation measures. Here, we introduce a spatial approach based on high resolution multispectral and hyperspectral image analysis, developed in order to detect and quantify changes in benthic cover in a highly heterogeneous shallow coral reef flat in Reunion Island in the South-West Indian Ocean. We propose a new index called HCAI (Hyperspectral Coral to Algae Index), defined as the ratio of living coral cover to the sum of living coral and algal covers. Benthic cover estimates were derived from airborne hyperspectral image processing using water column correction and unmixing models implemented with the four main coral reef benthic components: corals, algae, seagrass and sand. Ground truth and LIDAR data acquired simultaneously were used to validate processing accuracy. A significant positive correlation (adjusted R2 = 0.72, p-value < 0.001) was obtained between coral cover recorded in situ and estimated from image analysis. Moreover, 13 habitat classes based on the four main benthic component covers were mapped at a scale of an entire reef. Diachronic analyses of hyperspectral images between 2009 and 2015 revealed an overall decrease of the HCAI index and a decrease in the area of all the dominant coral classes along the reef (−28.24% for the coral class for example), while the area of habitat classes dominated by algae strongly increased during the same period. Moreover, we detected and documented the spatial and temporal evolutions of coral geomorphological features composed with coral rubble deposits called rubble tongues (RTs) using different available sensors (i.e. hyperspectral, satellite, and orthophotography). Since 2003, four detected (RTs) have spread shoreward at a mean rate of 8.4 m.y−1 including significant loss of reef structural complexity and heterogeneity, a spreading pattern which was confirmed by 2009 and 2015 hyperspectral data. Remote sensing and more specifically airborne hyperspectral approaches open new perspectives for coral reef monitoring, at high temporal and spatial resolutions.

Herbivore biocontrol and manual removal successfully reduce invasive macroalgae on coral reefs.

Invasive macroalgae pose a serious threat to coral reef biodiversity by monopolizing reef habitats, competing with native species, and directly overgrowing, and smothering reef corals. Several invasive macroalgae (Eucheuma clade E, Kappaphycus clade A and B, Gracilaria salicornia, and Acanthophora spicifera) are established within Kāne‘ohe Bay (O‘ahu, Hawai‘i, USA), and reducing invasive macroalgae cover is a coral reef conservation and management priority. Invasive macroalgae control techniques, however, are limited and few successful large-scale applications exist. Therefore, a two-tiered invasive macroalgae control approach was designed, where first, divers manually remove invasive macroalgae (Eucheuma and Kappaphycus) aided by an underwater vacuum system (“The Super Sucker”). Second, hatchery-raised juvenile sea urchins (Tripneustes gratilla), were outplanted to graze and control invasive macroalgae regrowth. To test the effectiveness of this approach in a natural reef ecosystem, four discrete patch reefs with high invasive macroalgae cover (15–26%) were selected, and macroalgae removal plus urchin biocontrol (treatment reefs, n = 2), or no treatment (control reefs, n = 2), was applied at the patch reef-scale. In applying the invasive macroalgae treatment, the control effort manually removed ∼19,000 kg of invasive macroalgae and ∼99,000 juvenile sea urchins were outplanted across to two patch reefs, totaling ∼24,000 m2 of reef area. Changes in benthic cover were monitored over 2 years (five sampling periods) before-and-after the treatment was applied. Over the study period, removal and biocontrol reduced invasive macroalgae cover by 85% at treatment reefs. Our results show manual removal in combination with hatchery raised urchin biocontrol to be an effective management approach in controlling invasive macroalgae at reef-wide spatial scales and temporal scales of months to years.

Human activities influence benthic community structure and the composition of the coral-algal interactions in the central Maldives

Competitive processes and their outcomes, such as interactions between scleractinian corals and macroalgae, are important drivers of the structure and function of coral reef ecosystems. Human communities can alter the dynamics of coral-algal interactions by changing species abundance and by affecting competitive ability. Here, we investigated how a natural human population gradient in the Maldives influences the relative abundance of benthic organisms, and if changes in benthic cover can influence the diversity, frequency and outcomes of coral-algal interactions. We observed a decline in some coral assemblages and an increase in coral mortality and filamentous algae on reefs with the highest human population pressures. At the highest level of human population, the diversity of coral-algal interactions was significantly reduced, with some genera of plating corals locally sparse. Human population pressures did not increase the frequency of coral-algal interactions or the competitive ability of macroalgal types. Regardless of human population, interactions between filamentous algae and cyanobacteria were the most damaging to competing corals. Interactions between crustose coralline algae and Halimeda were not only the most common and least harmful to coral, but were also positively correlated with coral cover, emphasizing the role that positive species interactions can play in regulating community structure and function.

Effects of Protection and Sediment Stress on Coral Reefs in Saint Lucia.

The extent to which Marine Protected Areas (MPAs) benefit corals is contentious. On one hand, MPAs could enhance coral growth and survival through increases in herbivory within their borders; on the other, they are unlikely to prevent disturbances, such as terrestrial runoff, that originate outside their boundaries. We examined the effect of spatial protection and terrestrial sediment on the benthic composition of coral reefs in Saint Lucia. In 2011 (10 to 16 years after MPAs were created), we resurveyed 21 reefs that had been surveyed in 2001 and analyzed current benthic assemblages as well as changes in benthic cover over that decade in relation to protection status, terrestrial sediment influence (measured as the proportion of terrigenous material in reef-associated sediment) and depth. The cover of all benthic biotic components has changed significantly over the decade, including a decline in coral and increase in macroalgae. Protection status was not a significant predictor of either current benthic composition or changes in composition, but current cover and change in cover of several components were related to terrigenous content of sediment deposited recently. Sites with a higher proportion of terrigenous sediment had lower current coral cover, higher macroalgal cover and greater coral declines. Our results suggest that terrestrial sediment is an important factor in the recent degradation of coral reefs in Saint Lucia and that the current MPA network should be complemented by measures to reduce runoff from land.

The influence of a new artificial structure on fish assemblages of adjacent hard substrata

Deployed for diverse purposes, artificial substrata can, theoretically, act as fish attractors or as fish producers. We tested these hypotheses by comparing fish assemblages on two artificial hard substrata, the Ca’ Roman and Sottomarina jetties, before (1994 and 2003) and after (2005 and 2006) the deployment of a new breakwater in the area. Our results support the fish production hypothesis. A remarkable increase in richness and abundance of sedentary species was observed on the extant substrata with the presence of the new barrier. More mobile species showed less definite trends. The net result has been a total increase in abundance and richness in the area. While fish assemblage showed important temporal variation in composition before deployment (Ca’ Roman, comparison between 1994 and 2003), clear trends of increasing abundance were concurrent with the presence of the new barrier. This increase in abundance, particularly of sedentary species, may be attributed to enhanced connectivity and/or an improvement in larval retention due to the changes in water circulation connected with the new breakwater, or to an increase in juveniles and/or adults promoted by changes in environmental conditions. Indeed, changes in benthic cover were recorded, with an increase of uncovered rock and the appearance of previously unrecorded algae after breakwater deployment.

Recent Region-wide Declines in Caribbean Reef Fish Abundance

Profound ecological changes are occurring on coral reefs throughout the tropics, with marked coral cover losses and concomitant algal increases, particularly in the Caribbean region. Historical declines in the abundance of large Caribbean reef fishes likely reflect centuries of overexploitation. However, effects of drastic recent degradation of reef habitats on reef fish assemblages have yet to be established. By using meta-analysis, we analyzed time series of reef fish density obtained from 48 studies that include 318 reefs across the Caribbean and span the time period 1955-2007. Our analyses show that overall reef fish density has been declining significantly for more than a decade, at rates that are consistent across all subregions of the Caribbean basin (2.7% to 6.0% loss per year) and in three of six trophic groups. Changes in fish density over the past half-century are modest relative to concurrent changes in benthic cover on Caribbean reefs. However, the recent significant decline in overall fish abundance and its consistency across several trophic groups and among both fished and nonfished species indicate that Caribbean fishes have begun to respond negatively to habitat degradation.