Coral Reef Substrates Research Articles

Novel in situ observations of asexual reproduction in the carpet sea anemone, Stichodactylamertensii (Stichodactylidae, Actiniaria).

Merten's carpet sea anemone, Stichodactylamertensii Brandt, 1835, is the largest known sea anemone species in the world, regularly exceeding one meter in oral disc diameter. A tropical species from the Indo-Pacific, S.mertensii drapes prominently over coral reef substrates and is a common host to numerous species of clownfishes and other symbionts throughout its range, which extends from the Red Sea through the Central Pacific Ocean. Long thought to reproduce via sexual reproduction only, recent genetic evidence suggests it may rarely reproduce asexually as well, although this process had never been confirmed through direct observation and the mechanism was yet to be described. Here, we directly observed and documented in situ asexual fragmentation via budding, in real time, by a Red Sea S.mertensii in a turbid inshore reef environment. While asexual reproduction is not unusual in sea anemones as a group, it is typically expected to be uncommon for large-bodied species. Herein, we describe S.mertensii fragmentation, provide high resolution images of the event from the Saudi Arabian coastline at multiple time points, and confirm asexual reproduction for this species.

Sea Urchen (Echinoidea) Diversity in the Coastal Area at Mawasangka District, Central Buton Regency

Sea urchins are marine invertebrates that are grouped in the Echinoidea class and Echinodermataphylum. Mawasangka is a district that has a large coastal area with a fairly good seagrass and coral reef ecosystem that plays an important role in the life cycle process of marine organisms, for instant as a habitat and food source for sea urchins. However, until now, information about the diversity of sea urchins in the coastal areas in Mawasangka District has not been reported. Thus, this research is very important. This research began with a location survey, field observation, and sampling. Supporting data such as temperature, pH, brightness, depth and salinity were also measured. The results showed that the number of species in 3 research locations in the coastal area of Mawasangka District was 5 species, consisting of 3 species from the genus Diadema, namely Diadema cytosum, Diadema antillarium, and Diadema savignyi, and 2 species from the Echinotrix genus, namely Echinothrix calamaris and Echinothrix diadema. The highest number of species diversity was found on the Pasi Kobungi beach. This happened because the habitat and environmental conditions at Pasi Kobungi Beach were very compatible for 5 species found. The species with the greatest abundance was sea urchins from the species of Diadema cytosum. It was found out that the coral reef substrate and seagrass in three research locations were important habitats for the growth of Diadema setosum and were easy to adapt to the environment. Due to the abundance of sea urchins from the Diadema cytosum species as well as many benefits of it for the environment, it can be concluded that the condition of the coastal area of Mawasangka District is still classified as healthy and well preserved.

High plasticity of nitrogen fixation and denitrification of common coral reef substrates in response to nitrate availability

Nitrogen cycling in coral reefs may be affected by nutrient availability, but knowledge about concentration-dependent thresholds that modulate dinitrogen fixation and denitrification is missing. We determined the effects of different nitrate concentrations (ambient, 1, 5, 10 μM nitrate addition) on both processes under two light scenarios (i.e., light and dark) using a combined acetylene assay for two common benthic reef substrates, i.e., turf algae and coral rubble. For both substrates, dinitrogen fixation rates peaked at 5 μM nitrate addition in light, whereas denitrification was highest at 10 μM nitrate addition in the dark. At 10 μm nitrate addition in the dark, a near-complete collapse of dinitrogen fixation concurrent with a 76-fold increase in denitrification observed for coral rubble, suggesting potential threshold responses linked to the nutritional state of the community. We conclude that dynamic nitrogen cycling activity may help stabilise nitrogen availability in microbial communities associated with coral reef substrates.

Unexpected role of communities colonizing dead coral substrate in the calcification of coral reefs

AbstractGlobal and local anthropogenic stressors such as climate change, acidification, overfishing, and pollution are expected to shift the benthic community composition of coral reefs from dominance by calcifying organisms to dominance by non‐calcifying algae. These changes could reduce the ability of coral reef ecosystems to maintain positive net calcium carbonate accretion. However, relationships between community composition and calcification rates remain unclear. We performed field experiments to quantify the metabolic rates of the two most dominant coral reef substrate types, live coral and dead coral substrate colonized by a mixed algal assemblage, using a novel underwater respirometer. Our results revealed that calcification rates in the daytime were similar for the live coral and dead coral substrate communities. However, in the dark, while live corals continued to calcify at slower rates, the dead coral substrate communities exhibited carbonate dissolution. Daytime net photosynthesis of the dead coral substrate communities was up to five times as much as for live corals, which we hypothesize may have created favorable conditions for the precipitation of carbonate minerals. We conclude that: (1) calcification from dead coral substrate communities can contribute to coral reef community calcification during the day, and (2) dead coral substrate communities can also contribute to carbonate mineral dissolution at night, decreasing ecosystem calcification over a diel cycle. This provides evidence that reefs could shift from slow, long‐term accretion of calcium carbonate to a state where large daily cycling of calcium carbonate occurs, but with little or no long‐term accumulation of the carbonate minerals needed to sustain the reef against erosional forces.

Simultaneous Measurements of Dinitrogen Fixation and Denitrification Associated With Coral Reef Substrates: Advantages and Limitations of a Combined Acetylene Assay

Nitrogen (N) cycling in coral reefs is of key importance for these oligotrophic ecosystems, but knowledge about its pathways is limited. While dinitrogen (N¬2) fixation is comparably well studied, the counteracting denitrification pathway is under-investigated, mainly because of expensive and relatively complex experimental techniques currently available. Here, we combined two established acetylene-based assays to one single setup to determine N2-fixation and denitrification performed by microbes associated with coral reef substrates/organisms simultaneously. Accumulating target gases (ethylene for N2-fixation, nitrous oxide for denitrification) were measured in gaseous headspace samples via gas chromatography. We measured N2-fixation and denitrification rates of two Red Sea coral reef substrates (filamentous turf algae, coral rubble), and demonstrated, for the first time, the co-occurrence of both N-cycling processes in both substrates. N2-fixation rates were up to eight times higher during light compared to dark, whereas denitrification rates during dark incubations were stimulated for turf algae and suppressed for coral rubble compared to light incubations. Our results highlight the importance of both substrates in fixing N, but their role in relieving N is potentially divergent. Absolute N2-fixation rates of the present study correspond with rates reported previously, even though likely underestimated due to an initial lag phase. Denitrification is also presumably underestimated due to incomplete nitrous oxide inhibition and/or substrate limitation. Besides these inherent limitations, we show that a relative comparison of N2-fixation and denitrification activity between functional groups is possible. Thus, our approach facilitates cost-efficient sample processing in studies interested in comparing relative rates of N2-fixation and denitrification.

Changes of Coral Reefs Condition in the Core Zones of Kapoposang Island MPA, Makassar Straits

Coral reefs in Kapoposang Islands have been degraded for years due to environmental pressures and human activities. After designated as marine protected area (MPA), coral reefs conditions (e.g. living coral coverage - both hard and soft corals) were monitored regularly since 2015 by several agencies. This study aims to evaluate the effectiveness of MPA management in Kapoposang by analyzing changes of coral reef coverage in its two core zones. The survey was conducted in July 2019 using Line Intercept Transect (LIT), video transect, and satellite imagery analysis, then compared to data from the previous assessment. Results of recent survey, i.e.: living coral reefs coverage ranged from 46.73% - 73.08%; 21 genera found with 10 dominant genera, which Porites sp., is a resistant coral genus and the most dominant genus in Kapoposang MPA; the highest coral recruitment was found in Suranti island at a depth of 10 meters, while the lowest is in Papandangan island; several genera in the form of recruitment (e.g. Acropora sp., Goniopora sp., Fungia sp., Favona sp., and Porites sp.); percentage of macroalgae is 2.95% of total coral reef substrate cover; the most densely macroalgae coverage is found in Suranti, while the lowest is in Kapoposang island.

Nocturnal variation of the zooplankton community in coral reef substrates at Isla del Coco National Park (Eastern Tropical Pacific)

Introduction: Zooplankton is a major link in coral reef food webs and a source of nutrients for corals. Demersal zooplankton in coral reefs makes diel migrations at night to avoid predation by fish and corals, increasing abundance and biomass at night. Objectives: To study the composition and variation of the demersal and pelagic zooplankton community at night and to increase the taxonomic knowledge of the benthic copepod fauna over different substrates of a coral reef. Methods: Traps made of plastic, with a 100 μm mesh size collector cup attached, were placed for 12 hours between 5-10 m depth over four substrates: (1) sand, (2) small and (3) big colonies of Porites lobata, and (4) dead and alive corals. Sampling started at dusk (1800 h), and the collector cups were collected and replaced at 2100 and 0000, and the last samples removed at down (0600 h). Results: The highest abundance of zooplankton was capture over P. lobata colonies between 0000 to 0600 h. Copepods (adults, copepodites, and nauplii) dominated the community structure and were more abundant during all the time intervals and substrates. Pelagic copepods were more abundant than the benthic, however, not more diverse. Among the benthic copepods, 20 new records of harpacticoids and calanoids were registered for Isla del Coco and Costa Rican waters. Appendicularians and decapod larvae were also abundant. Changes in coral reef community structure were related with time and not with the kind of substrate, although some taxa were found only in a specific time interval or substrate. Conclusions: Community structure and abundance of demersal and pelagic coral reef zooplankton in Chatham Bay were dominated by copepods, larvaceans and decapod larvae, similar to other coral reefs around the world. The highest abundance peak of organisms in the water column between 0000 to 0600 h could be related to diel migration.

Diversity of gastropods epifauna based on substrate in littoral zone in Mesjid Raya, District of Aceh Besar, Indonesia

The gastropod is one class of the Mollusca phylum using the abdomen as the leg. Generally gastropods are crawling on the bottom of the waters and submerged in the mud and are often called epifauna and infauna gastropods. Research on diversity of epifauna gastropods in the littoral zone has been done in Mesjid Raya, Aceh Besar District. This research aims to determine the diversity of gastropod species based on the substrate types (coral reef, muddy and sandy). The exploratory method by using transect line was used in this study. In each substrate, a line transect was drawn for five transects with a length of 50 m. The transects were started from the tidal line. The distance between transects were 50 m. Each transect was made in 10 sampling plots measuring lm × lm, and the distance between plots was 5m. The gastropods were collected directly and without damaging the substrates. The epifauna gastropods obtained was put into a sample bottle and preserved using 70% alcohol. The results showed that 23 families and 54 species of epifauna gastropods were identified. The number of species of gstropods was found in the sandy and muddy substrate more than in the sandy and corral reef substrate, but the highest diversity index was found in the sandy substrate, meanwhile the lowest diversity was found in the coral reef substrate. The highest species density was obtained on Rhinoclavis aspera.

Three new Licmophora species (Bacillariophyta: Fragilariophyceae) from Guam, two with an axial wave in the valve

BackgroundDiatoms in genus Licmophora are found as epiphytes on marine shores and are members of fouling communities. They can be abundant on natural coral reef substrates such as filamentous algae and as fringes on Halimeda. Biodiversity of benthic tropical diatoms is generally poorly known. Several new species of Licmophora have been reported from Guam but many are still undescribed.MethodsSamples of seaweeds with evident diatom coatings were collected during scuba dives, examined live, preserved in formalin, and processed with nitric acid cleaning following standard practice. Cleaned samples were examined in LM and SEM.ResultsWe describe two species unusual in having an axial wave in the valve, and a further long, straight species. L. repanda n. sp. and L. undulata n. sp. were of similar lengths (ca. 200 μm long) and both had axial waves but L. undulata was more slender with an inflated basal pole, had colonies on mucilage pads vs. long, single-stranded mucilage stalks. In addition, L. undulata had one apical rimoportula vs. two, elongated areolae along the midrib vs. uniform areolae, and the valvocopular advalvar striae ended halfway along, whereas in L. repanda they continued to the end. L. joymaciae n. sp. grew on long, single-stranded mucilage stalks with valves 300–400 μm long, with two apical rimoportulae, and basal pole not inflated. As in L. undulata its valvocopular advalvar striae ended halfway along the band. Stria densities were different at apex vs. base in L. joymaciae but not in the two wavy species. In all species, the 4th pleura had an apical cap and a narrow band with a single row of pores but it was larger in L. joymaciae. The other three girdle bands did not present any major differences among the three species but are documented for future comparisons with other species.ConclusionsAlthough the morphometric differences are small among the new species and between them and similar known species, the combinations of character states clearly shows that they are different.

Distribution Of Brittle Star (Ophiuroidea) on Rancabuaya Coastal Areas, Garut, West Java

This study aims to determine the distribution of brittle star (Ophiuroidea) in the waters of Rancabuaya coasts, Garut, West Java. The study was conducted in January-April 2016. The method used purposive sampling technique. The results showed that there were eight species (Gymnolophus obscura, Ophiomastix venosa, Clarkcoma canaliculata, Ophiocoma alexandri, Ophiocoma echinata, Ophiocoma valenciae, Ophiocoma erinaceus and Ophiocoma scolopendrina) into 4 genera and 2 families. Rancabuaya coastal has habitat characteristics with three types of substrate (rock, rocky and coral reef substrate) and the water conditions based on the substrate types (DO, pH, temperature, salinity) are in normal condition and suitable for the life of the Ophiurodea. The species of Ophiocoma scolopendrine has the widest distribution compared to other species because the substrate is in accordance with habitat its species. The results of this study can be used as baseline data for future coastal waters management.

Habitat associations of fish-parasitic gnathiid isopods in a shallow reef system in the central Philippines

Gnathiid isopods are among the most common ectoparasitic organisms infesting marine fishes in benthic marine environments, and are best known in coral reef ecosystems. These micropredators are ecologically important, as they can impact their hosts directly through blood loss and tissue damage, and indirectly by transmitting blood parasites. Their abundance, and thus expected impact on hosts, varies greatly over multiple spatial scales. Most studies on gnathiids in coral reef systems have been conducted in the Caribbean and on the Great Barrier Reef, Australia, with few studies conducted in the heart of marine biodiversity, the “Coral Triangle”. This is the first field study of gnathiid ecology in the Philippines. The primary goals of this study were to characterize substrate associations and diel activity patterns of gnathiids in shallow reef areas. Gnathiid larvae were found to infest at least 28 species of hosts and were found in both seagrass and reef habitats. Within reef habitat, there was a significant difference in gnathiid abundance among coral reef substrates. Gnathiid larvae were most abundant in coral rubble substrates and least abundant on live hard coral. This is in spite of host abundance (fish) being greater near live coral. More and larger-sized gnathiids were collected from dusk to dawn compared to daytime, and gnathiids were able to find hosts using non-visual sensory cues. Our results are, thus, consistent with previous studies in the Caribbean.

Volcanic ash supports a diverse bacterial community in a marine mesocosm.

Shallow‐water coral reef ecosystems, particularly those already impaired by anthropogenic pressures, may be highly sensitive to disturbances from natural catastrophic events, such as volcanic eruptions. Explosive volcanic eruptions expel large quantities of silicate ash particles into the atmosphere, which can disperse across millions of square kilometres and deposit into coral reef ecosystems. Following heavy ash deposition, mass mortality of reef biota is expected, but little is known about the recovery of post‐burial reef ecosystems. Reef regeneration depends partly upon the capacity of the ash deposit to be colonised by waterborne bacterial communities and may be influenced to an unknown extent by the physiochemical properties of the ash substrate itself. To determine the potential for volcanic ash to support pioneer bacterial colonisation, we exposed five well‐characterised volcanic and coral reef substrates to a marine aquarium under low light conditions for 3 months: volcanic ash, synthetic volcanic glass, carbonate reef sand, calcite sand and quartz sand. Multivariate statistical analysis of Automated Ribosomal Intergenic Spacer Analysis (ARISA) fingerprinting data demonstrates clear segregation of volcanic substrates from the quartz and coral reef substrates over 3 months of bacterial colonisation. Overall bacterial diversity showed shared and substrate‐specific bacterial communities; however, the volcanic ash substrate supported the most diverse bacterial community. These data suggest a significant influence of substrate properties (composition, granulometry and colour) on bacterial settlement. Our findings provide first insights into physicochemical controls on pioneer bacterial colonisation of volcanic ash and highlight the potential for volcanic ash deposits to support bacterial diversity in the aftermath of reef burial, on timescales that could permit cascading effects on larval settlement.

Bidirectional reflectance of coral growth-forms

ABSTRACTIn situ spectral reflectance measurements of substrates in a coral reef are often obtained by viewing a substrate at nadir. However, it is likely that off-nadir oblique viewing would show different spectral characteristics for most coral reef substrates and provide valuable information on structural properties. To understand the relationship between substrate structure and spectral response, this study examined the bidirectional reflectance distribution function (BRDF) of various growth-forms of hard corals and algae, as well as rock, rubble, and sand. BRDF measurements were collected on Heron Reef, the Great Barrier Reef, Australia, during the spring (October to November) of 2010, using a visible and near-infrared (VNIR) spectroradiometer attached to a goniometer. The measurements were made in the same five view zenith angles as the PROBA-1 Compact High Resolution Imaging Spectrometer (CHRIS) satellite (+55°, +36°, 0°, −36°, −55°) in the solar principal plane (SPP). A correction algorithm was used to remove both water column and water surface effects. Uncorrected measurements for sand covered with benthic microalgae appeared to have BRDF effects, but when corrected showed an essentially diffuse spectral response. Corrected measurements for branching corals showed BRDF effects dependent on branch spacing, branch length, and colour. The results indicate that spectral reflectance does vary with view angle for complex substrates such as caespitose corals, macroalgae, and irregular beach-rock and to a lesser extent for digitate corals and rippled sand and that the morphology of the coral and the shadowing between branches determines total spectral response. It is concluded that BRDF information can provide additional discriminating features for some coral reef substrates, particularly in the wavelength range of 550–650 nm.

Regional status assessment of stony corals in the US Virgin Islands.

States may protect coral reefs using biological water quality standards outlined by the Clean Water Act. This requires biological assessments with indicators sensitive to human disturbance and regional, probability-based survey designs. Stony coral condition was characterized on a regional scale for the first time in the nearshore waters of the US Virgin Islands (USVI). Coral composition, abundance, size, and health were assessed at 66 stations in the St. Croix region in fall 2007 and at 63 stations in the St. Thomas and St. John region in winter 2009. Indicators were chosen for their sensitivity to human disturbance. Both surveys were probability-based (random) designs with station locations preselected from areas covered by hardbottom and coral reef substrate. Taxa richness was as high as 21 species but more than half the area of both regions exhibited taxa richness of <10 species in the 25m(2) transect area. Coral density was as high as 5 colonies m(-2) but more than half the area of both regions had <2 colonies m(-2). Both regions showed similar dominant species based on frequency of occurrence and relative abundance. Because of large colony sizes, Montastrea annularis provided more total surface area and live surface area than more abundant species. The surveys establish baseline regional conditions and provide a foundation for long-term regional monitoring envisioned by the USVI Department of Planning and Natural Resources. The probabilistic sampling design assures the data can be used in Clean Water Act reporting.

Spatial variation in mechanical properties of coral reef substrate and implications for coral colony integrity

The physical structure of coral reefs plays a critical role as a barrier to storm waves and tsunamis and as a habitat for living reef-building and reef-associated organisms. However, the mechanical properties of reef substrate (i.e. the non-living benthos) are largely unknown, despite the fact that substrate properties may ultimately determine where organisms can persist. We used a geo-mechanical technique to measure substrate material density and strength over a reef hydrodynamic gradient. Contrary to expectation, we found a weak relationship between substrate strength and wave-induced water flow: flow rates decline sharply at the reef crest, whereas substrate properties are relatively constant over much of the reef before declining by almost an order of magnitude at the reef back. These gradients generate a novel hump-shaped pattern in resistance to mechanical disturbances for live corals, where colonies closer to the back reef are prone to dislodgement because of poorly cemented substrate. Our results help explain an intermediate zone of higher taxonomic and morphological diversity bounded by lower diversity exposed reef crest and unstable reef back zones.

DEVELOPMENT OF A MODEL TO CORRECT MULTI-VIEW ANGLE ABOVE WATER MEASUREMENTS FOR THE ANALYSIS OF THE BIDIRECTIONAL REFLECTANCE OF CORAL AND OTHER REEF SUBSTRATES

Abstract. Spectral reflectance characteristics of substrates in a coral reef environment are often measured in the field by viewing a substrate at nadir. However, viewing a substrate from multiple angles would likely result in different spectral characteristics for most coral reef substrates and provide valuable information on structural properties. To understand the relationship between the morphology of a substrate and its spectral response it is necessary to correct the observed above-water radiance for the effects of atmosphere and water attenuation, at a number of view and azimuth angles. In this way the actual surface reflectance can be determined. This research examines the air-water surface interaction for two hypothetical atmospheric conditions (clear Rayleigh scattering and totally cloudcovered) and the global irradiance reaching the benthic surface. It accounts for both water scattering and absorption, with simplifications for shallow water conditions, as well as the additive effect of background reflectance being reflected at the water-air surface at angles greater than the critical refraction angle (~48°). A model was developed to correct measured above-water radiance along the refracted view angle for its decrease due to path attenuation and the "n squared law of radiance" and the additive surface reflectance. This allows bidirectional benthic surface reflectance and nadir-normalised reflectance to be determined. These theoretical models were adapted to incorporate above-water measures relative to a standard, diffuse, white reference panel. The derived spectral signatures of a number of coral and non-coral benthic surfaces compared well with other published results, and the signatures and nadir normalised reflectance of the corals and other benthic surface classes indicate good class separation.

Spectral reflectance of coral reef benthos and substrate assemblages on Heron Reef, Australia

Studies investigating the spectral reflectance of coral reef benthos and substrates have focused on the measurement of pure endmembers, where the entire field of view (FOV) of a spectrometer is focused on a single benthos or substrate type. At the spatial scales of the current satellite sensors, the heterogeneity of coral reefs even at a sub-metre scale means that many individual image pixels will be made up of a mixture of benthos and substrate types. If pure endmember spectra are used as training data for image classification, there is a spatial discrepancy, because many pixels will have a mixed endmember spectral reflectance signature. This study investigated the spectral reflectance of coral reef benthos and substrates at a spatial scale directly linked to the pixel size of high spatial resolution imaging systems, by incorporating multiple benthos and substrate types into the spectrometer FOV in situ. A total of 334 spectral reflectance signatures were measured of 19 assemblages of the coral reef benthos and substrate types. The spectra were analysed for separability using first derivative values, and a discrimination decision tree was designed to identify the assemblages. Using the decision tree, it was possible to identify 15 assemblages with a mean overall classification accuracy of 62.6%.

Measuring coral reef community metabolism using new benthic chamber technology

Accurate measurement of coral reef community metabolism is a necessity for process monitoring and in situ experimentation on coral reef health. Traditional methodologies used for these measurements are effective but limited by location and scale constraints. We present field trial results for a new benthic chamber system called the Submersible Habitat for Analyzing Reef Quality (SHARQ). This large, portable incubation system enables in situ measurement and experimentation on community-scale metabolism. Rates of photosynthesis, respiration, and calcification were measured using the SHARQ for a variety of coral reef substrate types on the reef flat of South Molokai, Hawaii, and in Biscayne National Park, Florida. Values for daily gross production, 24-h respiration, and net calcification ranged from 0.26 to 6.45 g O2 m−2 day−1, 1.96 to 8.10 g O2 m−2 24 h−1, and 0.02 to 2.0 g CaCO3 m−2 day−1, respectively, for all substrate types. Field trials indicate that the SHARQ incubation chamber is an effective tool for in situ isolation of a water mass over a variety of benthic substrate types for process monitoring, experimentation, and other applications.

Hyperspectral analysis of chlorophyll content and photosynthetic capacity of coral reef substrates

Few studies have assessed the biophysical properties controlling reflection and absorption of light in coral reef environments and their relationships with quantitative measures of reef health and productivity. The present article examines the relationship between spectral reflectance, photosynthetic capacity, and chlorophyll a from common coral reef substrates. Reflectance readings of several targets (massive corals Montipora sp., n = 49, and Porites sp., n = 80; macroalgae Chlorodesmis sp., n = 24; and sediment interspersed with benthic microalgae, n = 35) were obtained in situ on Heron Reef, southern Great Barrier Reef (23°27'S, 151°55'E). Measurements of photosynthetic capacity and chlorophyll content were acquired simultaneously. Linear correlations were examined between spectral reflectance at all wavelengths and both photosynthetic capacity and pigment content (Chl a). Reflectance plots for all targets exhibited an absorption feature centered at 675 nm, and spectral reflectance at this wavelength decreased with increasing Chl a levels. The strength of this correlation varied between features, being highest for Porites sp. and lowest for sediment, highlighting the complexities of coral reef environments and the difficulties associated with relating spectral reflectance to biophysical properties. Photosynthetic capacity did not exhibit statistically significant correlations to spectral reflectance or absorption at any wavelength. Our results demonstrate the capabilities and difficulties associated with field scale hyperspectral data for measuring select biophysical properties of coral reefs and the need for assessment of the capabilities of airborne and satellite imaging sensors for similar purposes.

Modeling spectral discrimination of Great Barrier Reef benthic communities by remote sensing instruments

Remote sensing can monitor coral reef health, provided the benthic substrates are spectrally resolvable through the water column and surface. We studied the separability of eight substrate types (live coral, dead coral, soft coral, sand, brown algae, green algae, red algae, cyanobacteria) and the influence of the overlying water. A spectral library of coral reef benthic communities was collected from the Great Barrier Reef. Hydrolight 4.1 was used to simulate remote sensing reflectances. One multispectral and two hyperspectral sensors were simulated: the Advanced Land Imager (ALI, space borne), Hyperion (space borne), and HyMap (airborne, at 1.5 km altitude). Spectral radiance differences above different substrates were calculated to estimate what substrates can be separated and to what depth of waters this can be done. The dominant features in reflectance spectra of coral reef substrates are in the wavelength range 550–700 nm. Distinguishing various substrates in this part of the spectrum is limited to water depths of 5–6 m due to attenuation of the water. Below 550 nm some substrates have spectral features that are detectable by hyperspectral instruments even in deeper waters. Broader band instruments (e.g., ALI, Landsat) can provide some information about the substrate type. Sensors with a broad bandwidth provide fewer possibilities for developing analytical remote sensing algorithms for resolving significant numbers of substrate types in waters with variable depth. Hyperspectral sensors increase our capability to detect narrow spectral features that can be used for resolving various benthic communities.