Three to six-month-old juveniles of Acropora tenuis, A. millepora and Pocillopora acutawere experimentally co-exposed to nutrient enrichment and suspended sediments (without light attenuation or sediment deposition) for 40 days. Suspended sediments reduced survivorship of A. millepora strongly, proportional to the sediment concentration, but not in A. tenuis or P. acuta juveniles. However, juvenile growth of the latter two species was reduced to less than half or to zero, respectively. Additionally, suspended sediments increased effective quantum yields of symbionts associated with A. milleporaand A. tenuis, but not those associated with P. acuta. Nutrient enrichment did not significantly affect juvenile survivorship, growth or photophysiology for any of the three species, either as a sole stressor or in combination with suspended sediments. Our results indicate that exposure to suspended sediments can be energetically costly for juveniles of some coral species, implying detrimental longer-term but species-specific repercussions for populations and coral cover.
Refugia can facilitate the persistence of biodiversity under changing environmental conditions, such as anthropogenic climate change, and therefore constitute the best chance of survival for many coral species in the wild. Despite an increasing amount of literature, the concept of coral reef refugia remains poorly defined; so that climate change refugia have been confused with other phenomena, including temporal refuges, pristine habitats and physiological processes such as adaptation and acclimatization. We propose six criteria that determine the capacity of refugia to facilitate species persistence, including long-term buffering, protection from multiple climatic stressors, accessibility, microclimatic heterogeneity, size, and low exposure to non-climate disturbances. Any effective, high-capacity coral reef refugium should be characterized by long-term buffering of environmental conditions (for several decades) and multi-stressor buffering (provision of suitable environmental conditions with respect to climatic change, particularly ocean warming and acidification). Although not always essential, the remaining criteria are important for quantifying the capacity of potential refugia.
Urban shoreline erosion mitigation through beach renourishment has often been dismissed as environmentally insignificant. Given predicted impacts of sea level rise (SLR) and increased shoreline erosion, such activities might become a common practice in the future. But its long-term impacts on adjacent coral reefs have remained poorly documented. Benthic community trajectories were addressed during a period of twelve years across a spatial gradient of sediment burial impacts by beach renourishment on a high-energy urban coral reef at La Marginal Beach, Arecibo, Puerto Rico. Impacts associated to beach renourishment, followed by long-term, slowly-evolving impacts associated to sediment bedload, increased turbidity, increased Arecibo River streamflow, urban polluted runoff discharges, high particulate organic carbon (POC) concentration, and coral mortality following massive coral bleaching in 2005 were addressed through long-term monitoring. There was an initial catastrophic loss in coral species richness, diversity index and percent living coral cover, and a rapid regime shift favoring dominance by macroalgae and other non-reef building taxa. Long-term chronic impacts arrested high impact sites to an early successional stage, and drove moderate and low impact sites to a similar stage of very low species diversity, colony abundance and reef growth. Such chronic changes in community trajectories represent a glimpse into potential future impacts of shoreline erosion, sediment bedload, increasing turbidity and coastal water quality decline associated to SLR. The combination of chronic coral reef decline resulting from beach renourishment, coastal pollution, turbidity, and sediment bedload may have critical long-term ecological implications for urban coral reef resilience, functions and benefits.
Sedimentation is a critical threat to coral reefs worldwide. Major land use alteration at steep, highly erodible semi-arid islands accelerates the potential of soil erosion, runoff, and sedimentation stress to nearshore coral reefs during extreme rainfall events. The goal of this study was to assess spatio-temporal variation of sedimentation dynamics across nearshore coral reefs as a function of land use patterns, weather and oceanographic dynamics, to identify marine ecosystem conservation strategies. Sediment was collected at a distance gradient from shore at Bahia Tamarindo (BTA) and Punta Soldado (PSO) coral reefs at Culebra Island, Puerto Rico. Sediment texture and composition were analyzed by dry sieving and loss-on-ignition techniques, and were contrasted with environmental variables for the research period (February 2014 to April 2015). Rainfall and oceanographic data were analyzed to address their potential role on affecting sediment distribution with BEST BIO-ENV, RELATE correlation, and linear regression analysis. A significant difference in sedimentation rate was observed by time and distance from shore (PERMANOVA, p < 0.0100), mostly attributed to higher sediment exposure at reef zones closer to shore due to strong relationships with coastal runoff. Sedimentation rate positively correlated with strong rainfall events (Rho = 0.301, p = 0.0400) associated with storms and rainfall intensity exceeding 15 mm/h. At BTA, sediment deposited were mostly composed of sand, suggesting a potential influence of resuspension produced by waves and swells. In contrast, PSO sediments were mostly composed of silt-clay and terrigenous material, mainly attributed to a deforestation event that occurred at adjacent steep sub-watershed during the study period. Spatial and temporal variation of sedimentation pulses and terrigenous sediment input implies that coral reefs exposure to sediment stress is determined by local land use patterns, weather, and oceanographic dynamics. Comprehensive understanding of sediment dynamics and coastal ecosystem interconnectivity is fundamental to implement integrated and adaptive management strategies aimed to promote sustainable development at watershed and island wide-scale to fully mitigate terrigenous sediment impact to marine ecosystems. Furthermore, decision-making processes and policy needs to address sedimentation stress in the context of future climate to reduce land-based threats and strengthen coral reef resilience.
Interest in restoring staghorn coral Acropora cervicornis has grown following the widespread decline of this species in recent decades. To date, thousands of nursery-reared A. cervicornis have been outplanted to restore degraded reefs, but survivorship and growth among outplanted colonies can be spatially variable. In particular, data on distribution of remnant wild populations and outplant performance in varying reef zones is lacking. To address this gap, we conducted a study to characterize existing wild populations and assess performance of nursery-reared, outplanted A. cervicornis among three reef zones of varying depth at Little Cayman Island: the shallow back reef (0–3 m), the intermediate spur-and-groove reef (8–15 m), and the deep reef terrace (>15 m). Wild populations of A. cervicorniswere present in each reef zone, and colony height and prevalence of predation by Stegastesspp. were highest in the intermediate zone. For outplanted A. cervicornis, survivorship differed among sites and was lowest for outplants in the deep zone during the 85-day observation period. Post-outplant growth and branching was lowest among outplants in the shallow zone due to high rates of colony breakage. Following the conclusion of the study, a mortality event occurred in which 90% of outplants at the shallow plots died during a period of elevated sea temperature. The information provided in this study suggests that intermediate spur-and-groove reefs are optimal for outplanting activities in Little Cayman using existing restoration methods. These data could be useful for coral restoration practitioners and government agencies in the Caribbean, particularly the Cayman Islands, which is actively expanding its coral nursery program. New strategies must be developed to improve restoration outcomes in shallow and deep zones.
Deep-sea scleractinian coral reefs are protected ecologically and biologically significant areas that support global fisheries. The absence of observations of deep-sea scleractinian reefs in the Central and Northeast Pacific, combined with the shallow aragonite saturation horizon (ASH) and high carbonate dissolution rates there, fueled the hypothesis that reef formation in the North Pacific was improbable. Despite this, we report the discovery of live scleractinian reefs on six seamounts of the Northwestern Hawaiian Islands and Emperor Seamount Chain at depths of 535–732 m and aragonite saturation state (Ωarag) values of 0.71–1.33. Although the ASH becomes deeper moving northwest along the chains, the depth distribution of the reefs becomes shallower, suggesting the ASH is having little influence on their distribution. Higher chlorophyll moving to the northwest may partially explain the geographic distribution of the reefs. Principle Components Analysis suggests that currents are also an important factor in their distribution, but neither chlorophyll nor the available current data can explain the unexpected depth distribution. Further environmental data is needed to elucidate the reason for the distribution of these reefs. The discovery of reef-forming scleractinians in this region is of concern because a number of the sites occur on seamounts with active trawl fisheries.
Shallow marine ecosystems naturally experience fluctuating physicochemical conditions across spatial and temporal scales. Widespread coral-bleaching events, induced by prolonged heat stress, highlight the importance of how the duration and frequency of thermal stress influence the adaptive physiology of photosymbiotic calcifiers. Large benthic foraminifera harboring algal endosymbionts are major tropical carbonate producers and bioindicators of ecosystem health. Like corals, they are sensitive to thermal stress and bleach at temperatures temporarily occurring in their natural habitat and projected to happen more frequently. However, their thermal tolerance has been studied so far only by chronic exposure, so how they respond under more realistic episodic heat-event scenarios remains unknown. Here, we determined the physiological responses of Amphistegina gibbosa, an abundant western Atlantic foraminifera, to four different treatments––control, single, episodic, and chronic exposure to the same thermal stress (32°C)––in controlled laboratory cultures. Exposure to chronic thermal stress reduced motility and growth, while antioxidant capacity was elevated, and photosymbiont variables (coloration, oxygen-production rates, chlorophyll a concentration) indicated extensive bleaching. In contrast, single- and episodic-stress treatments were associated with higher motility and growth, while photosymbiont variables remained stable. The effects of single and episodic heat events were similar, except for the presumable occurrence of reproduction, which seemed to be suppressed by both episodic and chronic stress. The otherwise different responses between treatments with thermal fluctuations and chronic stress indicate adaptation to thermal peaks, but not to chronic exposure expected to ensue when baseline temperatures are elevated by climate change. This firstly implies that marine habitats with a history of fluctuating thermal stress potentially support resilient physiological mechanisms among photosymbiotic organisms. Secondly, there seem to be temporal constraints related to heat events among coral reef environments and reinforces the importance of temporal fluctuations in stress exposure in global-change studies and projections.
Scleractinian corals, the main framework builders of coral reefs, are in serious global decline, although there remains significant uncertainty as to the consequences for individual species and particular regions. We assessed coral species richness and ranked relative abundance across 3075 depth-stratified survey sites, each < 0.5 ha in area, using a standardized rapid assessment method, in 31 Indo-West Pacific (IWP) coral ecoregions (ERs), from 1994 to 2016. The ecoregions cover a significant proportion of the ranges of most IWP reef coral species, including main centres of diversity, providing a baseline (albeit a shifted one) of species abundance over a large area of highly endangered reef systems, facilitating study of future change. In all, 672 species were recorded. The richest sites and ERs were all located in the Coral Triangle. Local (site) richness peaked at 224 species in Halmahera ER (IWP mean 71 species Standard Deviation 38 species). Nineteen species occurred in more than half of all sites, all but one occurring in more than 90% of ERs. Representing 13 genera, these widespread species exhibit a broad range of life histories, indicating that no particular strategy, or taxonomic affiliation, conferred particular ecological advantage. For most other species, occurrence and abundance varied markedly among different ERs, some having pronounced “centres of abundance”. Conversely, another 40 species, also with widely divergent life histories, were very rare, occurring in five or fewer sites, 14 species of which are ranked as “Vulnerable” or “Endangered” on the International Union for Conservation of Nature (IUCN) Red List. Others may also qualify in these Threatened categories under criteria of small geographic range and population fragmentation, the utility of which is briefly assessed.
The coastal and upslope terrains of West Maui have had a long history of impacts owing to more than a century of human activities. Resource extraction, agriculture, as well as residential and resort development have caused land-based pollution that impairs water quality and adversely impact the adjacent marine ecosystem. Today, West Maui’s coral reefs are chronically impacted by the effects of land-based pollution, mainly sedimentation and nutrients, with documented losses of 30 – 75% in coral cover over the last 20 years. Nonetheless, despite their current status and levels of environmental impact, these coral reef communities represent a key local resource and a counterpoint to the overall low coral reef development levels both island- and state-wide. This is of high relevance because the occurrence of coral-rich assemblages and accreted reef complexes statewide is sparse. Only limited segments along the coastlines of Maui, Hawai‘i, Lana‘i, Moloka‘i, and Kaho‘olawe, harbor mature, fringing coral reefs; and unfortunately, many of them are seriously threatened by terrestrial runoff.
This report describes the results of baseline assessment surveys of coral reef benthic structure, coral community demographics, and coral condition. These surveys are intended to provide benchmarks for continued monitoring efforts and provide a gauge for comparing and evaluating the effectiveness of management actions to reduce land-based sources of pollution in priority watersheds on West Maui. Within this context, 12 permanent, long-term monitoring sites were strategically established adjacent to the 7 primary stream drainages (Wahikuli, Honokōwai, Mahinahina, Kahana/Ka‘opala, Honokeana, Honokahua, and Honolua) within the five priority watersheds (Wahikuli, Honokōwai, Kahana, Honokahua, and Honolua). Herein, benthic cover and composition, coral demographics, and coral condition of the monitoring sites are described and contrasted in the “Benthic Characterization” and “Synthesis and Discussion” sections of this report.
The baseline assessments revealed that although some areas harbor prominent coral reef structures with high live coral cover and multispecies assemblages, others are characterized by sediment-impacted corals in impoverished and species-poor communities. Mean coral cover varied widely, from 49% at Wahikuli-shallow to 4.6% at Mahinahina-shallow. Similarly, coralline algal cover averaged 12.7% at Ka‘opala and Honokeana-north, but was altogether absent at the Mahinahina sites. Macroalgae was a minor component of the benthos across all study sites, representing only up to 2.3% at Mahinahina-south, while turf algae varied considerably, from 41% at Honokeana-north to 84% at the Honokahua site. Consequently, the Benthic Substrate Ratio (BSR) also varied considerably region wide, with the highest values (≥ 1), suggesting a healthier reef condition reported for the Wahikuli, Honokeana, and Honokōwai sites; and the lowest (≤ 0.5), suggesting impairment in structure and function, recorded at the Honolua and Honokahua sites. Adult colony densities were the highest at the Wahikuli (27 col/m2) but lowest at the Ka‘opala (7 col/m2 ) site. And, colony partial mortality peaked at the Ka‘opala (33%) and was the lowest at the Honokeana Bay (12%). Moreover, in-situ and derived estimates of water turbidity and sediment loading revealed that the Ka‘opala and Wahikuli stream sites ranked the highest for turbidity, whereas the Honokōwai and Ka‘opala sites ranked highest for sediment loading.
Chronic and episodic terrestrial sediment stress has resulted in coral reef community demise, clearly illustrated at the Honolua, Honokahua, and Ka‘opala sites, where coral benthic cover and colony abundances ranked the lowest and levels of turf algae ranked among the highest. Left unattended, land-based pollution impacts will continue to negatively affect the coral reef communities of West Maui. And, under the current turbidity and sediment loading conditions, the coral-rich habitats in the Wahikuli and Honōkowai Watersheds are probably at greatest risk, given they harbor the most prominent and well-developed reefs in the region, characterized by the highest coral cover, colony densities, and structural complexity.
In this article, we examine the problem of coral reef destruction and discuss various stakeholders who suffer losses from the destruction. We then postulate a stakeholder versus threats matrix and outline an algorithm where public authorities can streamline policy based on expected losses. We also formulate, using local data, divergence between public good and individual benefits and examine the agent behaviour under monitoring. Our examples, using previous estimations on net benefits, give guidelines on how to form public policy and management strategies.