A bioindicator system for water quality on inshore coral reefs of the Great Barrier Reef

Responses of bioindicator candidates for water quality were quantified in two studies on inshore coral reefs of the Great Barrier Reef (GBR). In Study 1, 33 of the 38 investigated candidate indicators (including coral physiology, benthos composition, coral recruitment, macrobioeroder densities and FORAM index) showed significant relationships with a composite index of 13 water quality variables. These relationships were confirmed in Study 2 along four other water quality gradients (turbidity and chlorophyll). Changes in water quality led to multi-faceted shifts from phototrophic to heterotrophic benthic communities, and from diverse coral dominated communities to low-diversity communities dominated by macroalgae. Turbidity was the best predictor of biota; hence turbidity measurements remain essential to directly monitor water quality on the GBR, potentially complemented by our final calibrated 12 bioindicators. In combination, this bioindicator system may be used to assess changes in water quality, especially where direct water quality data are unavailable.     

Water quality in the inshore Great Barrier Reef lagoon: Implications for long-term monitoring and management

Coastal and inshore areas of the Great Barrier Reef lagoon receive substantial amounts of material from adjacent developed catchments, which can affect the ecological integrity of coral reefs and other inshore ecosystems. A 5-year water quality monitoring dataset provides a 'base range' of water quality conditions for the inshore GBR lagoon and illustrates the considerable temporal and spatial variability in this system. Typical at many sites were high turbidity levels and elevated chlorophyll a and phosphorus concentrations, especially close to river mouths. Water quality variability was mainly driven by seasonal processes such as river floods and sporadic wind-driven resuspension as well as by regional differences such as land use. Extreme events, such as floods, caused large and sustained increases in water quality variables. Given the highly variable climate in the GBR region, long-term monitoring of marine water quality will be essential to detect future changes due to improved catchment management.     

Agricultural lands are hot-spots for annual runoff polluting the southern Great Barrier Reef lagoon

The world’s largest coral reef ecosystem, the Great Barrier Reef (GBR), continues to be degraded from land-based pollution. Information about the source of pollutants is critical for catchment management to improve GBR water quality. We report here on an 11-year source to sea study of pollutant delivery in runoff from the Fitzroy River Basin (FRB), the largest GBR catchment. An innovative technique that relates land use to pollutant generation is presented. Study results indicate that maximum pollutant concentrations at basin and sub-catchment scales are closely related to the percentage area of croplands receiving heavy rain. However, grazing lands contribute the majority of the long-term average annual load of most common pollutants. Findings suggest improved land management targets, rather than water quality targets should be implemented to reduce GBR pollution. This study provides a substantial contribution to the knowledge base for the targeted management of pollution ‘hot-spots’ to improve GBR water quality.     

Terrestrial pollutant runoff to the Great Barrier Reef: An update of issues, priorities and management responses

The Great Barrier Reef (GBR) is a World Heritage Area and contains extensive areas of coral reef, seagrass meadows and fisheries resources. From adjacent catchments, numerous rivers discharge pollutants from agricultural, urban, mining and industrial activity. Pollutant sources have been identified and include suspended sediment from erosion in cattle grazing areas; nitrate from fertiliser application on crop lands; and herbicides from various land uses. The fate and effects of these pollutants in the receiving marine environment are relatively well understood. The Australian and Queensland Governments responded to the concerns of pollution of the GBR from catchment runoff with a plan to address this issue in 2003 (Reef Plan; updated 2009), incentive-based voluntary management initiatives in 2007 (Reef Rescue) and a State regulatory approach in 2009, the Reef Protection Package. This paper reviews new research relevant to the catchment to GBR continuum and evaluates the appropriateness of current management responses.     

A Paddock to reef monitoring and modelling framework for the Great Barrier Reef: Paddock and catchment component

Targets for improvements in water quality entering the Great Barrier Reef (GBR) have been set through the Reef Water Quality Protection Plan (Reef Plan). To measure and report on progress towards the targets set a program has been established that combines monitoring and modelling at paddock through to catchment and reef scales; the Paddock to Reef Integrated Monitoring, Modelling and Reporting Program (Paddock to Reef Program). This program aims to provide evidence of links between land management activities, water quality and reef health. Five lines of evidence are used: the effectiveness of management practices to improve water quality; the prevalence of management practice adoption and change in catchment indicators; long-term monitoring of catchment water quality; paddock & catchment modelling to provide a relative assessment of progress towards meeting targets; and finally marine monitoring of GBR water quality and reef ecosystem health. This paper outlines the first four lines of evidence.     

Excess seawater nutrients,enlarged algal symbiont densities and bleaching sensitive reef locations: 2. A regional-scale predictive model for the Great Barrier Reef,Australia

A spatial risk assessment model is developed for the Great Barrier Reef (GBR, Australia) that helps identify reef locations at higher or lower risk of coral bleaching in summer heat-wave conditions. The model confirms the considerable benefit of discriminating nutrient-enriched areas that contain corals with enlarged (suboptimal) symbiont densities for the purpose of identifying bleaching-sensitive reef locations. The benefit of the new system-level understanding is showcased in terms of: (i) improving early-warning forecasts of summer bleaching risk, (ii) explaining historical bleaching patterns, (iii) testing the bleaching-resistant quality of the current marine protected area (MPA) network (iv) identifying routinely monitored coral health attributes, such as the tissue energy reserves and skeletal growth characteristics (viz. density and extension rates) that correlate with bleaching resistant reef locations, and (v) targeting region-specific water quality improvement strategies that may increase reef-scale coral health and bleaching resistance.     

Changes in algal, coral and fish assemblages along water quality gradients on the inshore Great Barrier Reef

Macroalgae, hard corals, octocorals, and fish were surveyed on 10 to 13 inshore coral reefs of the Great Barrier Reef, along a water quality gradient in two regions with contrasting agricultural land use. A water quality index was calculated for each reef based on available data of particulate and dissolved nutrients, chlorophyll and suspended solids. Strong gradients in ecological attributes occurred along the water quality gradient. Macroalgae of the divisions Rhodophyta and Chlorophyta increased with increasing nutrients, while Phaeophyta remained similar. Octocoral richness and abundances of many hard coral and octocoral taxa decreased, and none of the hundreds of species increased. At reefs in higher nutrient environments, hard coral and octocoral assemblages were composed of subsets of the many species found in lower nutrient environments, whereas fish and macroalgal assemblages consisted of contrasting suites of species. The study identifies species groups that are likely to increase or decrease in abundance with changing water quality.     

Water quality and coral bleaching thresholds: Formalising the linkage for the inshore reefs of the Great Barrier Reef, Australia

The threats of wide-scale coral bleaching and reef demise associated with anthropogenic climate change are widely known. Here, the additional role of poor water quality in lowering the thermal tolerance (i.e. bleaching ‘resistance’) of symbiotic reef corals is considered. In particular, a quantitative linkage is established between terrestrially-sourced dissolved inorganic nitrogen (DIN) loading and the upper thermal bleaching thresholds of inshore reefs on the Great Barrier Reef, Australia. Significantly, this biophysical linkage provides concrete evidence for the oft-expressed belief that improved coral reef management will increase the regional-scale survival prospects of corals reefs to global climate change. Indeed, for inshore reef areas with a high runoff exposure risk, it is shown that the potential benefit of this ‘local’ management imperative is equivalent to ∼2.0-2.5 °C in relation to the upper thermal bleaching limit; though in this case, a potentially cost-prohibitive reduction in end-of-river DIN of >50-80% would be required. An integrated socio-economic modelling framework is outlined that will assist future efforts to understand (optimise) the alternate tradeoffs that the water quality/coral bleaching linkage presents.     

Bathymetry, biota and sediments on the Hirota Reef, Tane-ga-shima – the northernmost coral reef in the Ryukyu Islands

Abstract Investigations were conducted on bathymetry, reef biota and sediments on the Hirota Reef, Tane‐ga‐shima, North Ryukyus, near the northern limit for coral‐reef formation. A bathymetric profile from shore to the reef edge was depicted along an approximately 420‐m transect on the Hirota Coast of this island. A total of 20 quadrats (1 m × 1 m) were analyzed along the profile at 10‐ or 20‐m intervals to clarify distribution of macrobenthos inhabiting the reef. The Hirota Reef is divided into four geomorphologic zones according to their depth, gradient, surface roughness, substrate and characteristic macrobenthos. They are, from shore to offshore, shallow lagoon, seaward reef flat, reef edge and reef slope. The shallow lagoon comprises a shoreward depression (∼160 m wide on the transect) with a sand/gravel bottom that inclines gently toward offshore, and a seaward patch zone (∼70 m wide). The patches (<2 m high) are covered with fleshy algae, coralline algae and hermatypic corals. The seaward reef flat (∼190 m wide) is a flat plane that is constructed by biogenic carbonates and is covered with turf algae, with hermatypic corals scattered. Although the seaward reef flat of the Hirota Reef cannot be differentiated into different geomorphologic zones, similar seaward reef flat areas in the Central and South Ryukyus can be clearly subdivided into inner reef flat, reef crest and outer reef flat. This difference may be attributed to a lower reef growth rate and/or the later reef formation of the Hirota Reef in Holocene time than the southern examples. The coral fauna on the Hirota Reef is delineated by low diversity and characterized by taxa typical of high‐latitude, non‐reefal communities. The algal flora consists of tropical to subtropical species associated with warm‐temperate species. These faunal and floral characteristics may be related largely to lower water temperature in Tane‐ga‐shima than those in typical coral‐reef regions.     

Cyclone-induced net sediment transport pathway on the continental shelf of tropical Australia inferred from reef talus deposits

Tropical cyclones affect storm-dominated sediment transport processes that characterise Holocene shelf deposits in many shelf environments. A summary of cyclone-associated deposits in the Great Barrier Reef published by Larcombe and Carter [2004. Cyclone pumping, sediment partitioning and the development of the Great Barrier Reef shelf system: a review. Quaternary Science Reviews 23, pp. 107–135 indicates a pervasive northwards orientation of deposits on the lee side of reefs and other obstacles. In this paper, we describe the geomorphology of reef talus deposits found in the Gulf of Carpentaria and Arafura Sea, Australia, that we attribute to tropical cyclones. The orientation of these deposits is also indicative of a consistent, along-coast transport pathway. The deposits are located on the leeward side of submerged coral reefs; they are up to 10 m in thickness, comprised of re-worked carbonate sand and gravel and radiocarbon dating indicates that they are of Holocene age. An explanation for the consistent along-coast cyclone transport pathway is presented based on previously published hydrodynamic modelling results. These models illustrate how currents generated by the passage of a cyclone are asymmetric in plan view, such that stronger flows are generated between the eye of the cyclone and the coast. The result of the passage of many cyclones over geologic timescales is a net along-coast sediment transport pathway located on the inner- to mid-shelf, possibly extending over the entire length of northern Australia's coastline. This process provides an explanation for the observed sediment transport patterns on modern tropical continental shelves, as well as a basis for the interpretation of ancient tropical shelf deposits.     

Linear and nonlinear computations of the 1992 Nicaragua earthquake tsunami

Numerical computations of tsunamis are made for the 1992 Nicaragua earthquake using different governing equations, bottom frictional values and bathymetry data. The results are compared with each other as well as with the observations, both tide gauge records and runup heights. Comparison of the observed and computed tsunami waveforms indicates that the use of detailed bathymetry data with a small grid size is more effective than to include nonlinear terms in tsunami computation. Linear computation overestimates the amplitude for the later phase than the first arrival, particularly when the amplitude becomes large. The computed amplitudes along the coast from nonlinear computation are much smaller than the observed tsunami runup heights; the average ratio, or the amplification factor, is estimated to be 3 in the present case when the grid size of 1 minute is used. The factor however may depend on the grid size for the computation.     

Water quality in the Great Barrier Reef region: responses of mangrove, seagrass and macroalgal communities

Marine plants colonise several interconnected ecosystems in the Great Barrier Reef region including tidal wetlands, seagrass meadows and coral reefs. Water quality in some coastal areas is declining from human activities. Losses of mangrove and other tidal wetland communities are mostly the result of reclamation for coastal development of estuaries, e.g. for residential use, port infrastructure or marina development, and result in river bank destabilisation, deterioration of water clarity and loss of key coastal marine habitat. Coastal seagrass meadows are characterized by small ephemeral species. They are disturbed by increased turbidity after extreme flood events, but generally recover. There is no evidence of an overall seagrass decline or expansion. High nutrient and substrate availability and low grazing pressure on nearshore reefs have lead to changed benthic communities with high macroalgal abundance. Conservation and management of GBR macrophytes and their ecosystems is hampered by scarce ecological knowledge across macrophyte community types.     

Hydrodynamic response of a fringing coral reef to a rise in mean sea level

Ningaloo Reef, located along the northwest coast of Australia, is one of the longest fringing coral reefs in the world extending ~300 km. Similar to other fringing reefs, it consists of a barrier reef ~1–6 km offshore with occasional gaps, backed by a shallow lagoon. Wave breaking on the reef generates radiation stress gradients that produces wave setup across the reef and lagoon and mean currents across the reef. A section of Ningaloo Reef at Sandy Bay was chosen as the focus of an intense 6-week field experiment and numerical simulation using the wave model SWAN coupled to the three-dimensional circulation model ROMS. The physics of nearshore processes such as wave breaking, wave setup and mean flow across the reef was investigated in detail by examining the various momentum balances established in the system. The magnitude of the terms and the distance of their peaks from reef edge in the momentum balance were sensitive to the changes in mean sea level, e.g. the wave forces decreased as the mean water depth increased (and hence, wave breaking dissipation was reduced). This led to an increase in the wave power at the shoreline, a slight shift of the surf zone to the lee side of the reef and changes in the intensity of the circulation. The predicted hydrodynamic fields were input into a Lagrangian particle tracking model to estimate the transport time scale of the reef-lagoon system. Flushing time of the lagoon with the open ocean was computed using two definitions in renewal of semi-enclosed water basins and revealed the sensitivity of such a transport time scale to methods. An increase in the lagoon exchange rate at smaller mean sea-level rise and the decrease at higher mean sea-level rise was predicted through flushing time computed using both methods.     

A relic coral fauna threatened by global changes and human activities, Eastern Brazil

Coral species composition of drilled cores from emergent bank reefs, and coral cover of the surface of old and living reefs located along the coast of the state of Bahia, Eastern Brazil, revealed that there is a marked change in the occurrence of the major building coral species in different time intervals of the reef structure, as well as in the living surface of reefs located in two different geographical sites. Holocene core sections from two reef areas (12 degrees 40'S-38 degrees 00'W and 18 degrees 00'S-39 degrees 00'W) have as major reef builders, on its topmost core interval (3 to 4 ky old), the endemic coral Mussismilia braziliensis Verrill, 1868, which also dominate on the 2.5-3.5 ky old surfaces of truncated reef tops. At the base of the cores (the 2m lower interval, older than 4 ky BP), another endemic coral Mussismilia harttii Verrill, 1868 is the dominant reef component. The relative abundance of M. braziliensis on the living surfaces of shallow reefs from both areas, shows that in the southern area, it is up to 98% on reefs located 60 km off the coast, in depths between 3 and 4m, but do not exceed 1.3% on the surface of the northern reefs located 1-2 km off the coast in depths 4-5m. The Holocene falling sea level that occurred along the coast of Brazil since 5.1 ky BP, causes an increasing runoff into the area of coastal reefs. This phenomenon may have affected the nearshore reef building fauna, replacing a more susceptive coral fauna with one better adapted to low light levels and higher sediment influx. The high turbidity associated with early Holocene shelf flooding, should also be responsible for the absence of M. braziliensis during the initial stages of reef buildup in Brazil. At the present time, the rapidly increasing human pressure, due to changes in land uses of the coastal zone (increasing sedimentation rate, nutrification of coastal waters, industrial pollution) and underwater practices, such as overfishing and an intense tourism, is aggravating the recovery capacity of this already naturally threatened coral community. If this situation coupled with increasing sea surface temperature persists, modern coral reef growth, in Brazil cannot be maintained and the major reef building coral species of the reefs in Bahia, a remnant endemic coral fauna will very soon appear in the list of endangered species.     

The anomalous tides near Broad Sound

Observations of tidal current and height, in conjunction with theoretical mathematical models are used to investigate the propagation of the tide near Broad Sound, a narrowing estuary situated on a wide section of continental shelf toward the southern end of the Great Barrier Reef. The observations indicate that the dense offshore reefs severely inhibit tidal flow, with the result that tides flood toward Broad Sound from the north and from the south, along the main lagoon. There is a local magnification of the semi-diurnal tides within Broad Sound itself.Models of flow across reefs confirm the effectiveness of dense, shallow, and broad reefs in acting as a barrier to the tide. The diffraction of tides through large gaps in the reef is modelled using conformal mapping techniques and with the inclusion of energy leakage, the diffraction model predicts magnification of the semi-diurnal tidal heights by a factor of about 4 and a phase lag of 3 h on the shelf near Broad Sound, these values being consistent with observation. The observed convergence of the tide close to, and within Broad Sound itself is consistent with the proximity of the semi-diurnal tidal period to the natural period for flow in Broad Sound, considered as a narrowing estuary. This results in further amplification, by an additional factor of about 1.5, so that the tides in Broad Sound are increased by a factor of between 5 and 6, altogether, compared with those elsewhere on the east Australian coast.     

Circulation in the Great Barrier Reef Lagoon using numerical tracers and in situ data

Numerical hydrodynamic models of the northeastern Queensland shelf, forced by regional winds and modelled boundary currents in the northern Coral Sea, are used to provide improved estimates of general flow trajectories and water residence times within the Great Barrier Reef (GBR) shelf system. Model performance was checked against a limited set of current metre records obtained at Lark Reef (16°S) and the Ribbon Reefs (15.5°S). Estimates of water parcel trajectories are derived from a series of numerical tracer experiments, with daily releases of neutrally buoyant, un-reactive particles at 320 sites along the coast between Cape York (10.7°S) and Hervey Bay (25°S). Flow trajectories and residence times for tracer particles introduced to the GBR lagoon in the southern—ca. 22°S, central—19°S, and northern reef—14°S are emphasised. For purposes of the analysis, the year was divided into two seasons based on mean alongshore current direction. Most coastal sourced tracers entering the central GBR lagoon between 16° and 20°S during the northward-current season (January–August) primarily encounter the outer-shelf reef matrix after exiting the lagoon at its northern “head” (nominally 16°S), after 50–150 days. Up to 70% of tracer particles entering in the southward-current season (August–December) eventually crossed the lagoon to the outer-shelf reef matrix, with median crossing times between 20 and 330 days. During favourable wind conditions, tracers introduced at the coast may move rapidly across the lagoon into the reef matrix. The tracer experiments indicate that most coastal-sourced tracers entering the GBR lagoon remain near the coast for extended periods of time, moving north and south in a coastal band. Residence times for conservative tracer particles (and implied residence times for water-borne materials) within the GBR shelf system ranged from ca. 1 month to 1 year—time frames that are very long relative to development times of planktonic larvae and cycling times for nutrient materials in the water column, implying they are transformed long before reaching the outer reef matrix.     

On the small-scale fractal geometrical structure of a living coral reef barrier

The topographical complexity of coral reefs is of primary importance for a number of hydrodynamical and ecological processes. The present study is based on a series of high-resolution seabottom elevation measurements along the Maupiti Barrier Reef, French Polynesia. Several statistical metrics and spectral analysis are used to characterize the spatial evolution of the coral geometrical structure from the reef crest to the backreef. A consistent fractal-like power law exists in the spectral density of bottom elevation for length scales between 0.1 and 7 m, while at larger scale, the reef structure shows a different pattern. Such a fine characterization of the reef geometrical structure provides key elements to reconstruct the reef history, to improve the representation of reef roughness in hydrodynamical models and to monitor the evolution of coral reef systems in the context of global change. © 2020 John Wiley & Sons, Ltd.     

Exposure of inner-shelf reefs to nutrient enriched runoff entering the Great Barrier Reef Lagoon: post-European changes and the design of water quality targets

We used historical flood plume extent data (modelled) to quantify the typical spatial extent of the summer runoff-seawater mixing zone of the Great Barrier Reef (GBR) lagoon. Spatially explicit analysis of the variability of in situ chlorophyll a concentrations (observed) across the runoff-seawater mixing zone, then allowed us to explore regional differences in the nutrient enrichment impact of runoff events from the various river systems that drain the GBR catchment. We demonstrate the existence of a discernable north-south gradient along the length of the GBR, such that for equivalent runoff:seawater dilutions ratios, lower levels of nutrient enrichment (as indicated by chlorophyll alpha observations) result from the river systems that drain the relatively undisturbed northern areas of the GBR catchment, compared to more human-impacted central and south areas. We identify a strong correlation between this north-south enrichment gradient and the flood concentration of dissolved inorganic nitrogen (DIN) entrained by the various river systems. By substituting the nutrient enrichment characteristics of the human-impacted river discharges with those of the undisturbed northern rivers, we provide a means to compare the short-term enriching 'footprint' for existing runoff intrusions with those that are likely to have occurred under pre-European catchment conditions. We demonstrate that under pre-European conditions, the nutrient enriching impact from river runoff was likely to have been largely constrained within 1-2 km of the coast, whereas existing conditions support the impact of reefs some 20-30 km off the coast. By using the developed spatial relations, we show that for the heavily human-impacted river systems, reductions in the end-of-river concentrations of DIN in the order 50-80% are needed in order to restore parity with pre-European conditions. We discuss these results in regard to developing end-of-catchment water quality targets for the region.     

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 (NH4Cl added), three + P reefs (KH2PO4 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 microM NH4+; 2.3 microM PO4(-3)) rapidly declined, reaching near-background levels (mean = 0.9 microM NH4+; 0.5 microM PO4(-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 microM NH4+; 5.1 microM PO4(-3)) declining to means of 11.3 microM NH4+ and 2.4 microM PO4(-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.     

Is proximity to land-based sources of coral stressors an appropriate measure of risk to coral reefs? An example from the Florida Reef Tract

Localized declines in coral condition are commonly linked to land-based sources of stressors that influence gradients of water quality, and the distance to sources of stressors is commonly used as a proxy for predicting the vulnerability and future status of reef resources. In this study, we evaluated explicitly whether proximity to shore and connections to coastal bays, two measures of potential land-based sources of disturbance, influence coral community and population structure, and the abundance, distribution, and condition of corals within patch reefs of the Florida Reef Tract. In the Florida Keys, long-term monitoring has documented significant differences in water quality along a cross-shelf gradient. Inshore habitats exhibit higher levels of nutrients (DIN and TP), TOC, turbidity, and light attenuation, and these levels decrease with increasing distance from shore and connections to tidal bays. In clear contrast to these patterns of water quality, corals on inshore patch reefs exhibited significantly higher coral cover, higher growth rates, and lower partial mortality rates than those documented in similar offshore habitats. Coral recruitment rates did not differ between inshore and offshore habitats. Corals on patch reefs closest to shore had well-spread population structures numerically dominated by intermediate to large colonies, while offshore populations showed narrower size-distributions that become increasingly positively skewed. Differences in size-structure of coral populations were attributed to faster growth and lower rates of partial mortality at inshore habitats. While the underlying causes for the favorable condition of inshore coral communities are not yet known, we hypothesize that the ability of corals to shift their trophic mode under adverse environmental conditions may be partly responsible for the observed patterns, as shown in other reef systems. This study, based on data collected from a uniform reef habitat type and coral species with diverse life-history and stress-response patterns from a heavily exploited reef system, showed that proximity to potential sources of stressors may not always prove an adequate proxy for assigning potential risks to reef health, and that hypothesized patterns of coral cover, population size-structure, growth, and mortality are not always directly related to water quality gradients.