Response Of Coral Reefs Research Articles

Multi-Partner Interactions in Corals in the Face of Climate Change

Recent research has explored the possibility that increased sea-surface temperatures and decreasing pH (ocean acidification) contribute to the ongoing decline of coral reef ecosystems. Within corals, a diverse microbiome exerts significant influence on biogeochemical and ecological processes, including food webs, organismal life cycles, and chemical and nutrient cycling. Microbes on coral reefs play a critical role in regulating larval recruitment, bacterial colonization, and pathogen abundance under ambient conditions, ultimately governing the overall resilience of coral reef systems. As a result, microbial processes may be involved in reef ecosystem-level responses to climate change. Developments of new molecular technologies, in addition to multidisciplinary collaborative research on coral reefs, have led to the rapid advancement in our understanding of bacterially mediated reef responses to environmental change. Here we review new discoveries regarding (1) the onset of coral-bacterial associations; (2) the functional roles that bacteria play in healthy corals; and (3) how bacteria influence coral reef response to environmental change, leading to a model describing how reef microbiota direct ecosystem-level response to a changing global climate.

Recent massive coral mortality events in the South China Sea: Was global warming and ENSO variability responsible?

The key to predicting the possible response(s) of coral reefs to hypothesized future global warming is the knowledge of their response(s) to previous extreme climatic events. We obtained 80 high-precision U/Th ages for 78 dead massive Porites colonies from the Nansha Islands, South China Sea, with an aim to understand the long-term history, frequency, timing and causes of local coral mortality. Our results reveal a number of significant episodes of coral mortality since 1860AD, with an apparent increase in frequency and severity since 1930AD, and more recently since 1970, especially when only synchronized mortality events found to have occurred on both reefs were compared. The synchronized mortality events centered around 1865–1875, 1895–1900, 1910–1920, 1930–1945, 1970–1985 and 1990–2005AD, which imply regional common causes, were found to correlate well with the warm phases of the El Niño Southern Oscillation (ENSO) (i.e. El Niño years, e.g. some recent ones during 1972–1973, 1982–1983, 1991–1994, 1997–1998AD) on inter-annual time scale, as well as with positive phases of the Pacific Decadal Oscillation (PDO) or its sharp phase transitions on inter-decadal time scale. Synchronized coral mortality was absent during prolonged negative PDO phases, e.g. during 1950–1970AD. Overall, the data shows that immediately before, and at the time of, modern local ecological monitoring, the region's coral communities had already experienced several recent episodes of stress. The increased frequency distribution of mortality ages since 1930AD and more recently since 1970AD appears to coincide with progressively warmer sea-surface temperatures, both regionally and globally. Our data highlight the vulnerability of local coral communities in the face of present and predicted future warming.

Modeling regional coral reef responses to global warming and changes in ocean chemistry: Caribbean case study

Climatic change threatens the future of coral reefs in the Caribbean and the important ecosystem services they provide. We used a simulation model [Combo (“COral Mortality and Bleaching Output”)] to estimate future coral cover in the part of the eastern Caribbean impacted by a massive coral bleaching event in 2005. Combo calculates impacts of future climate change on coral reefs by combining impacts from long-term changes in average sea surface temperature (SST) and ocean acidification with impacts from episodic high temperature mortality (bleaching) events. We used mortality and heat dose data from the 2005 bleaching event to select historic temperature datasets, to use as a baseline for running Combo under different future climate scenarios and sets of assumptions. Results suggest a bleak future for coral reefs in the eastern Caribbean. For three different emissions scenarios from the Intergovernmental Panel on Climate Change (IPCC; B1, A1B, and A1FI), coral cover on most Caribbean reefs is projected to drop below 5% by the year 2035, if future mortality rates are equivalent to some of those observed in the 2005 event (50%). For a scenario where corals gain an additional 1–1.5°C of heat tolerance through a shift in the algae that live in the coral tissue, coral cover above 5% is prolonged until 2065. Additional impacts such as storms or anthropogenic damage could result in declines in coral cover even faster than those projected here. These results suggest the need to identify and preserve the locations that are likely to have a higher resiliency to bleaching to save as many remnant populations of corals as possible in the face of projected wide-spread coral loss.

Coral reef ecosystems and anthropogenic climate change

Coral reef ecosystems are among the most biologically diverse ecosystems on the planet. In addition to their value in terms of biodiversity, coral reefs provide food and resources for over 500 million people. Despite their importance, coral reefs are declining at a rapid rate (1–2% per year) as a result of a range of local (e.g., overexploitation of fisheries, declining water quality) and global (e.g., global warming and ocean acidification) drivers. Extensive experimental and field evidence suggests that atmospheric carbon dioxide concentrations of 450 ppm will lead to the loss of coral-dominated reef systems, with the prospect that dangerous levels of atmospheric carbon dioxide for coral reefs were exceeded in 1979 when mass coral bleaching was reported for the first time. The exact response of coral reefs remains uncertain although it is highly unlikely that coral-dominated reef systems will be present in future oceans at the current rate of warming and acidification of the world’s tropical oceans. The loss of these important coastal ecosystems will diminish the resources available to hundreds of millions of people along tropical coastlines. Understanding the impacts on people and industry is an imperative if we are to devise effective systems by which tropical coastal communities are to adapt to rapidly changing tropical coastal environments. Our current understanding of these important issues, however, is in a relatively undeveloped state and must be a priority of future research.

Reserve design for uncertain responses of coral reefs to climate change

Rising sea temperatures cause mass coral bleaching and threaten reefs worldwide. We show how maps of variations in thermal stress can be used to help manage reefs for climate change. We map proxies of chronic and acute thermal stress and develop evidence-based hypotheses for the future response of corals to each stress regime. We then incorporate spatially realistic predictions of larval connectivity among reefs of the Bahamas and apply novel reserve design algorithms to create reserve networks for a changing climate. We show that scales of larval dispersal are large enough to connect reefs from desirable thermal stress regimes into a reserve network. Critically, we find that reserve designs differ according to the anticipated scope for phenotypic and genetic adaptation in corals, which remains uncertain. Attempts to provide a complete reserve design that hedged against different evolutionary outcomes achieved limited success, which emphasises the importance of considering the scope for adaptation explicitly. Nonetheless, 15% of reserve locations were selected under all evolutionary scenarios, making them a high priority for early designation. Our approach allows new insights into coral holobiont adaptation to be integrated directly into an adaptive approach to management.

Sealevel history recorded in the Pleistocene carbonate sequence in IODP Hole 310‐M0005D, off Tahiti

AbstractMaterial cored during the Integrated Ocean Drilling Program (IODP) Expedition 310 ‘Tahiti Sea Level’ revealed that the fossil reef systems around Tahiti are composed of two major stratigraphic sequences: (i) a last deglacial sequence; and (ii) an older Pleistocene sequence. The older Pleistocene carbonate sequence is composed of reef deposits associated with volcaniclastic sediments and was preserved in Hole 310‐M0005D drilled off Maraa. Within an approximately 70‐m‐thick older Pleistocene sequence (33.22–101.93 m below seafloor; 92.85–161.56 m below present sealevel) in this hole, 11 depositional units are defined by lithological changes, sedimentological features, and paleontological characteristics and are numbered sequentially from the top of the hole downward (Subunits P1–P11). Paleowater depths inferred from nongeniculate coralline algae, combined with those determined by using corals and larger foraminifers, suggest two major sealevel rises during the deposition of the older Pleistocene sequence. Of these, the second sealevel rise is associated with an intervening sealevel drop. It is likely that the second sealevel rise corresponds to that during Termination II (TII, the penultimate deglaciation, from Marine Isotope Stages 6 to 5e). Therefore, the intervening sealevel drop can be correlated with that known as the ‘sealevel reversal’ during TII. Because there are limited data on the Pleistocene reef systems in the tropical South Pacific Ocean, this study provides important information about Pleistocene sealevel history, the evolution of coral reef ecosystems, and the responses of coral reefs to Quaternary climate changes.

Response of coral reefs to climate change: Expansion and demise of the southernmost Pacific coral reef

Coral reefs track sea level and are particularly sensitive to changes in climate. Reefs are threatened by global warming, with many experiencing increased coral bleaching. Warmer sea surface temperatures might enable reef expansion into mid latitudes. Here we report multibeam sonar and coring that reveal an extensive relict coral reef around Lord Howe Island, which is fringed by the southernmost reef in the Pacific Ocean. The relict reef, in water depths of 25–50 m, flourished in early Holocene and covered an area more than 20 times larger than the modern reef. Radiocarbon and uranium‐series dating indicates that corals grew between 9000 and 7000 years ago. The reef was subsequently drowned, and backstepped to its modern limited extent. This relict reef, with localised re‐establishment of corals in the past three millennia, could become a substrate for reef expansion in response to warmer temperatures, anticipated later this century and beyond, if corals are able to recolonise its surface.

Coral reef evolution on rapidly subsiding margins

A series of well-developed submerged coral reefs are preserved in the Huon Gulf (Papua New Guinea) and around Hawaii. Despite different tectonics settings, both regions have experienced rapid subsidence (2–6 m/ka) over the last 500 ka. Rapid subsidence, combined with eustatic sea-level changes, is responsible for repeated drowning and backstepping of coral reefs over this period. Because we can place quantitative constraints on these systems (i.e., reef drowning age, eustatic sea-level changes, subsidence rates, accretion rates, basement substrates, and paleobathymetry), these areas represent unique natural laboratories for exploring the roles of tectonics, reef accretion, and eustatic sea-level changes in controlling the evolution of individual reefs, as well as backstepping of the entire system. A review of new and existing bathymetric, radiometric, sedimentary facies and numerical modeling data indicate that these reefs have had long, complex growth histories and that they are highly sensitive, recording drowning not only during major deglaciations, but also during high-frequency, small-amplitude interstadial and deglacial meltwater pulse events. Analysis of five generalized sedimentary facies shows that reef drowning is characterized by a distinct biological and sedimentary sequence. Observational and numerical modeling data indicate that on precessional (20 ka) and sub-orbital timescales, the rate and amplitude of eustatic sea-level changes are critical in controlling initiation, growth, drowning or sub-aerial exposure, subsequent re-initiation, and final drowning. However, over longer timescales (> 100–500 ka) continued tectonic subsidence and basement substrate morphology influence broad scale reef morphology and backstepping geometries. Drilling of these reefs will yield greatly expanded stratigraphic sections compared with similar reefs on slowly subsiding, stable and uplifting margins, and thus they represent a unique archive of sea-level and climate changes, as well as a record of the response of coral reefs to these changes over the last six glacial cycles.

Assessing coral reef health across onshore to offshore stress gradients in the US Virgin Islands

Managing the effects of anthropogenic disturbance on coral reefs is highly dependant on effective strategies to assess degradation and recovery. We used five years of field data in the US Virgin Islands to investigate coral reef response to a potential gradient of stress. We found that the prevalence of old partial mortality, bleaching, and all forms of coral health impairment (a novel category) increased with nearshore anthropogenic processes, such as a five-fold higher rate of clay and silt sedimentation. Other patterns of coral health, such as recent partial mortality, other diseases, and benthic cover, did not respond to this potential gradient of stress or their response could not be resolved at the frequency or scale of monitoring. We suggest that persistent signs of disturbance are more useful to short-term, non-intensive (annual) coral reef assessments, but more intensive (semi-annual) assessments are necessary to resolve patterns of transient signs of coral health impairment.

A modeling tool to evaluate regional coral reef responses to changes in climate and ocean chemistry

We developed a spreadsheet‐based model for the use of managers, conservationists, and biologists for projecting the effects of climate change on coral reefs at local‐to‐regional scales. The COMBO (Coral Mortality and Bleaching Output) model calculates the impacts to coral reefs from changes in average SST and CO2 concentrations, and from high temperature mortality (bleaching) events. The model uses a probabilistic assessment of the frequency of high temperature events under a future climate to address scientific uncertainties about potential adverse effects. COMBO offers data libraries and default factors for three selected regions (Hawai'i, Great Barrier Reef, and Caribbean), but it is structured with user‐selectable parameter values and data input options, making possible modifications to reflect local conditions or to incorporate local expertise. Preliminary results from sensitivity analyses and simulation examples for Hawai'i demonstrate the relative importance of high temperature events, increased average temperature, and increased CO2 concentration on the future status of coral reefs; illustrate significant interactions among variables; and allow comparisons of past environmental history with future predictions.

Effect of aragonite saturation, temperature, and nutrients on the community calcification rate of a coral reef

In this study we investigated the relations between community calcification of an entire coral reef in the northern Red Sea and annual changes in temperature, aragonite saturation and nutrient loading over a two year period. Summer (April–October) and winter (November–March) average calcification rates varied between 60 ± 20 and 30 ± 20 mmol·m−2·d−1, respectively. In general, calcification increased with temperature and aragonite saturation state of reef water with an apparent effect of nutrients, which is in agreement with most laboratory studies and in situ measurements of single coral growth rates. The calcification rates we measured in the reef correlated remarkably well with precipitation rates of inorganic aragonite calculated for the same temperature and degree of saturation ranges using empirical equations from the literature. This is a very significant finding considering that only a minute portion of reef calcification is inorganic. Hence, these relations could be used to predict the response of coral reefs to ocean acidification and warming.

Numerical modeling of the growth and drowning of Hawaiian coral reefs during the last two glacial cycles (0–250 kyr)

Drowned coral reefs on rapidly subsiding margins possess a unique archive of sea level and climate changes, generally unavailable from stable or uplifting margins. Using available field observations and sedimentary, radiometric age, and numerical modeling data, we propose a new model of submerged reef development around Hawaii during the last two glacial cycles (250 kyr). This model provides a quantitative predictive stratigraphy for the reefs that we argue, if drilled, will yield new information on sea level and climate changes, as well as coral reef response over the last 250 kyr. Comparing the observational and numerical modeling data, combined with sensitivity testing, we present our “best case” scenario for the evolution of the drowned lowstand reefs now at −400 (H2) and −150 m (H1). We find that growth rates of 2.5–2.85 m/kyr for the main shallow reef building facies, a subsidence rate of 2.5 m/kyr, and a variable basement substrate configuration best explain the observational data. Modeling of the internal stratigraphic succession of the reefs shows that the number and thickness of shallow reef units, as well as the frequency and duration of subaerial exposure and reef‐drowning events, are sensitive to the frequency and amplitude of eustatic sea level variations but not the rate of subaerial erosion. H2 and H1 initiated growth during stable eustatic sea level conditions during highstands circa 222 ka (MIS7) and circa 126 ka (MIS5e), respectively. Both H2 and H1 have a long and complex growth history, growing episodically for ∼90 kyr. Precessional (∼20 kyr) and higher‐frequency, suborbital eustatic sea level fluctuations dominate, with each reef experiencing repeated but brief (<5–10 kyr) drowning and subaerial exposure, producing a complex layer cake stratigraphy of shallow (0–30 m) coral reef units separated by either subaerial exposure horizons or thin, intermediate (30–60 m) coralgal units. Final drowning of H2 and H1 occurs during the penultimate (133–134 ka) and last deglaciation (12–14 ka). These findings are consistent with available age data and qualitative predictions of previous studies around Hawaii.

Introductory perspective on the COREF Project

Abstract Coral reefs are tropic to subtropic, coastal ecosystems comprising very diverse organisms. Late Quaternary reef deposits are fossil archives of environmental, tectonic and eustatic variations that can be used to reconstruct the paleoclimatic and paleoceanographic history of the tropic surface oceans. Reefs located at the latitudinal limits of coral‐reef ecosystems (i.e. those at coral‐reef fronts) are particularly sensitive to environmental changes – especially those associated with glacial–interglacial changes in climate and sealevel. We propose a land and ocean scientific drilling campaign in the Ryukyu Islands (the Ryukyus) in the northwestern Pacific Ocean to investigate the dynamic response of the corals and coral‐reef ecosystems in this region to Late Quaternary climate and sealevel change. Such a drilling campaign, which we call the COREF (coral‐reef front) Project, will allow the following three major questions to be evaluated: (i) What are the nature, magnitude and driving mechanisms of coral‐reef front migration in the Ryukyus? (ii) What is the ecosystem response of coral reefs in the Ryukyus to Quaternary climate changes? (iii) What is the role of coral reefs in the global carbon cycle? Subsidiary objectives include (i) the timing of coral‐reef initiation in the Ryukyus and its causes; (ii) the position of the Kuroshio current during glacial periods and its effects on coral‐reef formation; and (iii) early carbonate diagenetic responses as a function of compounded variations in climate, eustacy and depositional mineralogies (subtropic aragonitic to warm‐temperate calcitic). The geographic, climatic and oceanographic settings of the Ryukyu Islands provide an ideal natural laboratory to address each of these research questions.

Responses of coral reefs to increased amplitude of sea-level changes at the Mid-Pleistocene Climate Transition

We show responses of coral reefs to increased amplitude of sea-level changes at the Mid-Pleistocene Climate Transition (MPT) based on lithostratigraphic, sedimentologic and calcareous nannofossil biostratigraphic investigations on Pleistocene reef-complex deposits (Ryukyu Group) on the Motobu Peninsula, Okinawa-jima, Central Ryukyus. Our data show that reef growth started in earliest Quaternary time (1.45–1.65 Ma) and that extensive reef formation dates back to ∼ 0.8 Ma. The mode of Quaternary sedimentation changed at ∼ 0.8 Ma in the study area. Before this time, thick siliciclastics and mixed carbonate-siliciclastics accumulated, which were followed by the deposition of bioclastic sediments (detrital limestone). No indications have been found of episodic subaerial exposures in these deposits and no calcareous nannofossil biozones are lacking. Since the detrital limestone includes biogenic components characterizing fore-reef to shelf environments, the coastal areas of the northern Motobu Peninsula mostly lay in fore-reef to shelf environments for > 0.6 million years (between ∼ 0.8 Ma and 1.45–1.65 Ma), when the sediments had not been subaerially exposed due to sea-level changes characterized by relatively small amplitudes. Coral limestone that formed in the latest Early to Middle Pleistocene between 0.4 Ma and 0.8 Ma extends over the study area, ranging in elevation from 0 to 70 m. This coral limestone grades upward into fore-reef to shelf carbonates (rhodolith, Cycloclypeus-Operculina, and detrital limestones) which is in turn overlain by coral limestone. This succession, combined with configuration of the lithofacies and paleobathymetry inferred from lithology and biogenic components, implies that the reef-complex deposits formed responding to sea-level changes with amplitude of > 60 m. Consequently, we suggest that the change in the mode of sedimentation results from increased amplitude of sea-level fluctuations at ∼ 0.8 Ma. This timing corresponds roughly to the timing of the Mid-Pleistocene Climate Transition (MPT).

Detriments to post-bleaching recovery of corals

Predicting the response of coral reefs to large-scale mortality induced by climate change will depend greatly on the factors that influence recovery after bleaching events. We experimentally transplanted hard corals from a shallow reef with highly variable seawater temperature (23–36°C) to three unfished marine parks and three fished reefs with variable coral predator abundance and benthic cover. The transplanted corals were fragmented colonies collected from a reef that was relatively undisturbed by the 1997–1998 warm-water temperature anomaly, one of the most extreme thermal events of the past century, and it was assumed that they would represent corals likely to succeed in the future temperature environment. We examined the effects of four taxa, two fragment sizes, an acclimation period, benthic cover components, predators and tourists on the survival of the coral fragments. We found the lowest survival of transplants occurred in the unfished marine parks and this could be attributed to predation and not tourist damage. The density of small coral recruits approximately 6 months after the spawning season was generally moderate (~40–60/m2), and not different on fished and unfished reefs. Coral recovery between 1998 and 2002 was variable (0–25%), low (mean of 6.5%), and not different between fished and unfished reefs. There was high variability in coral mortality among the three unfished areas despite low variation in estimates of predator biomass, with the highest predation occurring in the Malindi MNP, a site with high coralline algal cover. Stepwise multiple regression analysis with 14 variables of coral predators and substratum showed that coralline algae was positively, and turf algae negatively associated with mortality of the transplants, with all other variables being statistically insignificant. This suggests that alternate food resources and predator choices are more important than predator biomass in determining coral survival. Nonetheless, large predatory fish in areas dominated by coralline algae may considerably retard recovery of eurythermal corals. This will not necessarily retard total hard coral recovery, as other more predator-tolerant taxa can recover. Based on the results, global climate change will not necessarily favor eurythermal over stenothermal coral taxa in remote or unfished reefs, where predation is a major cause of coral mortality.

Coral reefs: corals' adaptive response to climate change.

The long-term response of coral reefs to climate change depends on the ability of reef-building coral symbioses to adapt or acclimatize to warmer temperatures, but there has been no direct evidence that such a response can occur. Here we show that corals containing unusual algal symbionts that are thermally tolerant and commonly associated with high-temperature environments are much more abundant on reefs that have been severely affected by recent climate change. This adaptive shift in symbiont communities indicates that these devastated reefs could be more resistant to future thermal stress, resulting in significantly longer extinction times for surviving corals than had been previously assumed.

The role of fringing coral reef in beach protection of Hurghada, Gulf of Suez, Red Sea of Egypt

The coral-reef system fronting most of the coastline of the Red Sea provides natural protection to the aquatic system. Its pronounced morphologic features protect coastal recreation facilities located behind the reef. To provide a basis for evaluating the role of the fringing coral reef in protecting marinas and sandy beaches against waves and currents, a two-dimensional (2D) numerical model “SIMulating COastal PROcess” (SIMCOPRO) was applied with its three main modules including wave, current and sediment. The selected study area is located at Sahl Hasheesh coast south of Hurghada on the west coast of the Red Sea at a site proposed for the construction of a marina. The model offers insight in how the reef system modifies the wave and current fields. The application of the model in the presence of a marina reveals insignificant erosion on the downdrift side of the marina breakwaters (south of the southern breakwater). The erosion is expected to be 1 m after the first year, 2 m after the next 5 years and no further change for a period of 10 years. This is because the rocky reefal beachface will be exposed and erosion of sand will be diminished. The minor local erosion is controlled by the protective response of coral reef, the very limited coarse sand on the beachface (<0.5 m thick), the weak current induced by waves (0.13 m/s) and other topographic protective elements in the region. The submerged/emerged geometric nature of the reefal system, both reef flat and reef crest, allow wave dissipation and thus behaves as a submerged breakwater to protect marinas or artificial beaches in the shelter zone of this reef. An important lesson to be learned from this study is that improper construction practices of building marinas can seriously hurt the environment. The dredging of artificial reef lagoons is one improper practice that would create unexpected beach erosion.

Long-term population dynamics of a coral reef gastropod and responses to disturbance

The age-specific density of the red-lipped stromb Strombus luhuanus (Mollusca: Gastropoda) was monitored over 13 years (1981-1993) at four locations on the intertidal reef flat at Heron Island, Great Barrier Reef. Densities were highly variable, but there were persistent, location-specific differences in population density, age structure and adult body size, the latter indicating that the populations were not extensively linked by adult movement. There was relatively high recruitment at most locations in 1984, 1989 and 1993, each occurring approximately 2 years after El Nino/Southern Oscillation events, although recruit density during these years varied in both space and time. The studied strombs experienced three disturbance events: (i) experimental harvesting at two locations (1984-1985); (ii) siltation from a harbour dredging operation (1987-1988); and (iii) a severe cyclone (1992). Resilience to harvesting at a local scale (0.5-2 ha) was high: density had recovered within a year, due to immigration of adults and older juveniles. Strombus luhuanus responded much more strongly to broad-scale changes to its environment than to localized harvesting. After dredging, there was a progressive density decline coupled with low recruitment at two locations, and a later decline at a third location, followed by a recruitment-driven rebound after the cyclone. Generalized environmental effects of siltation and the cyclone were also reflected in substantial changes in algal cover. Long-term variations in environmental conditions probably cause high temporal variation over large spatial scales through effects on the survival of larvae or recruits. Localized short-term field monitoring of such species would give a misleading picture of key factors affecting population dynamics.

Biological response of coral reefs to sea surface temperature variation: evidence from the raised Holocene reefs of Kikai-jima (Ryukyu Islands, Japan)

The island of Kikai-jima (central Ryukyu Islands, Japan) provides a rare opportunity to define both environmental and biological variations within a reef ecosystem over a geological time frame. This paper documents the palaeo-environmental records archived in the δ18O composition of four Holocene Porites cores collected from the raised reef terraces of Kikai-jima. These coral samples record mean sea surface temperatures of 23.5 °C at 4,220 years B.P., falling to 22.2 °C at 3,790 years B.P. and to a minimum of 21.4 °C at 3,400 years B.P. After this time mean sea surface temperatures rose to 23.5 °C at 1,860 years B.P. before reaching modern-day conditions of 24.9 °C. During the cool water period between 3,790 and 3,400 years B.P., the coral isotope data indicate that sea surface temperatures were below the currently accepted 18 °C minimum temperature for reef development approximately 14% of the time. Biological changes (total coral abundance, colony size, individual coral coverage, genera diversity and evenness) preserved in the raised reef terraces of Kikai-jima can be explained by these variations in Holocene sea surface temperature. The biological data indicate that the coral community at Kikai-jima was ecologically stressed during the cool water period from 3,790 to 3,400 years B.P.; however, in the gross morphological sense the coral community did still constitute a coral reef.

ENCORE: The Effect of Nutrient Enrichment on Coral Reefs. Synthesis of Results and Conclusions

Coral reef degradation resulting from nutrient enrichment of coastal waters is of increasing global concern. Although effects of nutrients on coral reef organisms have been demonstrated in the laboratory, there is little direct evidence of nutrient effects on coral reef biota in situ. The ENCORE experiment investigated responses of coral reef organisms and processes to controlled additions of dissolved inorganic nitrogen (N) and/or phosphorus (P) on an offshore reef (One Tree Island) at the southern end of the Great Barrier Reef, Australia. A multi-disciplinary team assessed a variety of factors focusing on nutrient dynamics and biotic responses. A controlled and replicated experiment was conducted over two years using twelve small patch reefs ponded at low tide by a coral rim. Treatments included three control reefs (no nutrient addition) and three+N reefs (NH 4Cl added), three+P reefs (KH 2PO 4 added), and three+N+P reefs. Nutrients were added as pulses at each low tide ( ca twice per day) by remotely operated units. There were two phases of nutrient additions. During the initial, low-loading phase of the experiment nutrient pulses (mean dose=11.5 μM NH 4 + ; 2.3 μM PO 4 −3) rapidly declined, reaching near-background levels ( mean=0.9 μM NH 4 + ; 0.5 μM PO 4 −3 ) within 2–3 h. A variety of biotic processes, assessed over a year during this initial nutrient loading phase, were not significantly affected, with the exception of coral reproduction, which was affected in all nutrient treatments. In Acropora longicyathus and A. aspera, fewer successfully developed embryos were formed, and in A. longicyathus fertilization rates and lipid levels decreased. In the second, high-loading, phase of ENCORE an increased nutrient dosage (mean dose=36.2 μM NH 4 + ; 5.1 μM PO 4 −3 declining to means of 11.3 μM NH 4 + and 2.4 μM PO 4 −3 at the end of low tide) was used for a further year, and a variety of significant biotic responses occurred. Encrusting algae incorporated virtually none of the added nutrients. Organisms containing endosymbiotic zooxanthellae (corals and giant clams) assimilated dissolved nutrients rapidly and were responsive to added nutrients. Coral mortality, not detected during the initial low-loading phase, became evident with increased nutrient dosage, particularly in Pocillopora damicornis. Nitrogen additions stunted coral growth, and phosphorus additions had a variable effect. Coral calcification rate and linear extension increased in the presence of added phosphorus but skeletal density was reduced, making corals more susceptible to breakage. Settlement of all coral larvae was reduced in nitrogen treatments, yet settlement of larvae from brooded species was enhanced in phosphorus treatments. Recruitment of stomatopods, benthic crustaceans living in coral rubble, was reduced in nitrogen and nitrogen plus phosphorus treatments. Grazing rates and reproductive effort of various fish species were not affected by the nutrient treatments. Microbial nitrogen transformations in sediments were responsive to nutrient loading with nitrogen fixation significantly increased in phosphorus treatments and denitrification increased in all treatments to which nitrogen had been added. Rates of bioerosion and grazing showed no significant effects of added nutrients. ENCORE has shown that reef organisms and processes investigated in situ were impacted by elevated nutrients. Impacts were dependent on dose level, whether nitrogen and/or phosphorus were elevated and were often species-specific. The impacts were generally sub-lethal and subtle and the treated reefs at the end of the experiment were visually similar to control reefs. Rapid nutrient uptake indicates that nutrient concentrations alone are not adequate to assess nutrient condition of reefs. Sensitive and quantifiable biological indicators need to be developed for coral reef ecosystems. The potential bioindicators identified in ENCORE should be tested in future research on coral reef/nutrient interactions. Synergistic and cumulative effects of elevated nutrients and other environmental parameters, comparative studies of intact vs. disturbed reefs, offshore vs. inshore reefs, or the ability of a nutrient-stressed reef to respond to natural disturbances require elucidation. An expanded understanding of coral reef responses to anthropogenic impacts is necessary, particularly regarding the subtle, sub-lethal effects detected in the ENCORE studies.