Samuel J. Purkis

The Emerging Role of Lidar Remote Sensing in Coastal Research and Resource Management

Abstract Knowledge of coastal elevation is an essential requirement for resource management and scientific research. Recognizing the vast potential of lidar remote sensing in coastal studies, this Special Issue includes a collection of articles intended to represent the state-of-the-art for lidar investigations of nearshore submerged and emergent ecosystems, coastal morphodynamics, and hazards due to sea-level rise and severe storms. Some current applications for lidar remote sensing described in this Special Issue include bluegreen wavelength lidar used for submarine coastal benthic environments such as coral reef ecosystems, airborne lidar used for shoreline mapping and coastal change detection, and temporal waveform-resolving lidar used for vegetation mapping.

Present limits to heat-adaptability in corals and population-level responses to climate extremes.

Climate change scenarios suggest an increase in tropical ocean temperature by 1–3°C by 2099, potentially killing many coral reefs. But Arabian/Persian Gulf corals already exist in this future thermal environment predicted for most tropical reefs and survived severe bleaching in 2010, one of the hottest years on record. Exposure to 33–35°C was on average twice as long as in non-bleaching years. Gulf corals bleached after exposure to temperatures above 34°C for a total of 8 weeks of which 3 weeks were above 35°C. This is more heat than any other corals can survive, providing an insight into the present limits of holobiont adaptation. We show that average temperatures as well as heat-waves in the Gulf have been increasing, that coral population levels will fluctuate strongly, and reef-building capability will be compromised. This, in combination with ocean acidification and significant local threats posed by rampant coastal development puts even these most heat-adapted corals at risk. WWF considers the Gulf ecoregion as “critically endangered”. We argue here that Gulf corals should be considered for assisted migration to the tropical Indo-Pacific. This would have the double benefit of avoiding local extinction of the worlds most heat-adapted holobionts while at the same time introducing their genetic information to populations naïve to such extremes, potentially assisting their survival. Thus, the heat-adaptation acquired by Gulf corals over 6 k, could benefit tropical Indo-Pacific corals who have <100 y until they will experience a similarly harsh climate. Population models suggest that the heat-adapted corals could become dominant on tropical reefs within ∼20 years.

The paradox of tropical karst morphology in the coral reefs of the arid Middle East

Despite differences in reef growth between the Arabian Gulf and the Red Sea, a common distinctive pattern of polygonal sills surrounding ponded depressions consistently occurs in shallow water. Viewed from a satellite, these seafloors are reticulated and maze like. Despite little current rainfall, this patterning is best explained by karst dissolution of limestone during periods of lower sea level. This is a paradox since such fine-scale karstification is confined to areas with considerably more precipitation than currently observed in Arabia. We resolve this apparent contradiction by developing a Pleistocene–Holocene chronology of sea level and climate for the Red Sea and Arabian Gulf, and through the use of pattern analysis and computer simulation, reveal the mechanism of formation for these structures. We demonstrate that this patterning can be taken as a Quaternary signature of paleohumidity in the now hyperarid Red Sea and Arabian Gulf.

Spatial analysis of the invasion of lionfish in the western Atlantic and Caribbean

Pterois volitans and Pterois miles, two sub-species of lionfish, have become the first non-native, invasive marine fish established along the United States Atlantic coast and Caribbean. The route and timing of the invasion is poorly understood, however historical sightings and captures have been robustly documented since their introduction. Herein we analyze these records based on spatial location, dates of arrival, and prevailing physical factors at the capture sights. Using a cellular automata model, we examine the relationship between depth, salinity, temperature, and current, finding the latter as the most influential parameter for transport of lionfish to new areas. The model output is a synthetic validated reproduction of the lionfish invasion, upon which predictive simulations in other locations can be based. This predictive model is simple, highly adaptable, relies entirely on publicly available data, and is applicable to other species.

High Resolution Ground Verification, Cluster Analysis and Optical Model of Reef Substrate Coverage on Landsat TM Imagery (Red Sea, Egypt)

A combination of high-resolution ground verification, cluster analysis using Landsat Thematic Mapper (TM) data, and optical modelling, was applied to Red Sea reef substrate. Ground verification, in an area of 3 by 20 pixels (90 by 600 m) with one metre scale resolution, identified the presence of 30 different bottom types that were later reduced to twelve dominant bottom types. A combination of bispectral plots and principal component analysis using spectral bands 1, 2 and 3 confirmed the presence of nine bottom types. The identified clusters were separated and used as a training set to classify substrate. Optical modelling, using literature radiance values and coverage of the original twelve dominant bottom types and a simple model for atmospheric and water column absorption, revealed a difference of up to 60 W m -2 between predicted substrate radiance and the satellite sensor values in the reef top area. Considering the simple atmospheric correction model, the lack of in situ radiance measurements and the uncertainties with respect to possible changes in bottom type distribution since the acquisition of the 14 year old image, the results show the potential use of satellite imagery for reef research in both biological and geological analysis through very precise and semi-quantitative ground verification, including in situ reflectance measurements.

Coral reef remote sensing : a guide for mapping, monitoring and management

Section I Visible and Infrared 1 Visible and Infrared Overview 1.1 Introduction 1.2 Physical and Technical Principles 1.3 Image Processing 1.4 Future Directions Suggested Reading References 2 Photography Applications 2.1 Introduction 2.2 Photography of Coral Reefs 2.3 Photography Analysis and Classification Techniques 2.4 Example Applications of Photography 2.5 Conclusions and Future Directions Suggested Reading References 3 Multispectral Applications 3.1 Introduction 3.2 Multispectral Analysis and Classification 3.3 Example Applications 3.4 Conclusions and Future Directions Suggested Reading References 4 Hyperspectral Applications 4.1 Introduction 4.2 Hyperspectral Planning and Preprocessing 4.3 Hyperspectral Algorithms 4.4 Conclusions Suggested Reading References Section II LiDAR 5 LiDAR Overview 5.1 Introduction 5.2 Physical Principles 5.3 Image Products and Environmental Variables 5.4 Processing and Validation Requirements Suggested Reading References 6 LiDAR Applications 6.1 Introduction 6.2 Example LiDAR Applications 6.3 Future Directions in LiDAR 6.4 Conclusion Suggested Reading References 7 Integrated LiDAR and Hyperspectral 7.1 Introduction 7.2 LiDAR/Hyperspectral Processing 7.3 Applications of LiDAR/Hyperspectral Fusion 7.4 Summary and Discussion Suggested Reading References Section III Acoustic 8 Acoustic Methods Overview 8.1 Introduction 8.2 Physical and Technical Principles 8.3 Applications of Acoustics 8.4 Conclusion Suggested Reading References 9 Acoustic Applications 9.1 Introduction 9.2 Applications 9.3 State of the Science and Future Directions Suggested Reading References 10 Deep Acoustic Applications 10.1 Introduction 10.2 History of Mapping Cold-Water Coral Habitats 10.3 Cold-Water Coral Mapping Example 10.4 Conclusions and Recommendations Suggested Reading References Section IV Thermal and Radar 11 Thermal and Radar Overview 11.1 Introduction 11.2 Thermal Overview 11.3 Radar Overview 11.4 Conclusion Suggested Reading References 12 Thermal Applications 12.1 Introduction 12.2 Thermal Data Products and Analysis 12.3 Example Thermal Applications 12.4 Future Directions Suggested ReadingReferences 13 Radar Applications 13.1 Introduction 13.2 HF Ocean Radar 13.3 VHF High Resolution Radar 13.4 Synthetic Aperture Radar 13.5 Scatterometers 13.6 X-band Wave Radars 13.7 Conclusions and Future Directions Suggested Reading References Section V Effective Use of Remote Sensing in Science and Management 14 Validation 14.1 Introduction 14.2 Sampling Design and Accuracy Measures 14.3 Validation Literature Review 14.4 Conclusions and Recommendations Suggested Reading References 15 Science and Management 15.1 Introduction 15.2 Research and Management Needs 15.3 Example Applications 15.4 Conclusions and Recommendations Suggested Reading References Index

Satellite imaging coral reef resilience at regional scale. A case-study from Saudi Arabia

We propose a framework for spatially estimating a proxy for coral reef resilience using remote sensing. Data spanning large areas of coral reef habitat were obtained using the commercial QuickBird satellite, and freely available imagery (NASA, Google Earth). Principles of coral reef ecology, field observation, and remote observations, were combined to devise mapped indices. These capture important and accessible components of coral reef resilience. Indices are divided between factors known to stress corals, and factors incorporating properties of the reef landscape that resist stress or promote coral growth. The first-basis for a remote sensed resilience index (RSRI), an estimate of expected reef resilience, is proposed. Developed for the Red Sea, the framework of our analysis is flexible and with minimal adaptation, could be extended to other reef regions. We aim to stimulate discussion as to use of remote sensing to do more than simply deliver habitat maps of coral reefs.

The Statistics of Natural Shapes in Modern Coral Reef Landscapes

Spatial heterogeneity is a fundamental characteristic of modern and ancient depositional settings, and the scaling of many carbonate environments has been shown to follow power function distributions. The difficulty in obtaining information on the horizontal persistence of sedimentary lithotopes at the basin scale has, however, hampered evaluation of this fact over larger geographic areas. In recent years, large‐scale maps of reef facies derived from remotely sensed data have become widely available, allowing for an analysis of reef‐scale map products from 26 sites spread through four reef provinces, covering >7000 km2 of shallow‐water habitat in the U.S. territorial Pacific. For each site, facies maps were decomposed to polygons describing the perimeter of patches of differing sedimentologic/benthic character. A suite of geospatial metrics quantifying unit shape, fractal dimension, and frequency‐area relations was applied to investigate the intra‐ and intersite variability. The spatial architecture of these reef sites displays robust fractal properties over an extended range of scales with remarkable consistency between provinces. These results indicate the possibility of extrapolating information from large to small scales in various depositional environments.

Bucket structure in carbonate accumulations of the Maldive, Chagos and Laccadive archipelagos

Peritidal platforms rimmed by reefs, and raised reefs rimming deep lagoons, are characteristic morphologies of the tropical carbonate factory; their geometry contrasts sharply with the seaward-sloping shelves of siliciclastic margins. The structure has been compared to a bucket—stiff reef rims holding a pile of loose sediment. Remote-sensing data from the Maldive, Chagos and Laccadive archipelagos of the Indian Ocean show that ring reefs with bucket structure are the dominant depositional pattern from patch reefs of tens of meters to archipelagos of hundreds of kilometers in diameter, that is, over more than 4 orders of magnitude in linear size. Over 2.5 orders of magnitude, the bucket structures qualify as statistical fractals, exhibiting self-similar patterns and size distributions following power laws. However, most regional or genetic subsets of the data follow lognormal distributions and small subsets of lagoon reefs exhibit exponential distributions. Seismic data and boreholes in the Maldives indicate that the bucket has been a dominant depositional motif since the Oligocene. Ecological and hydrodynamic studies on modern reefs suggest that the bucket structure is a form of biotic self-organization: the edge position in a reef is favored over the center position because bottom shear is higher and the diffusive boundary layer between reef and water thinner. Thus, the reef edge has easier access to nutrients and is less likely to be buried by sediment. The bucket structure reflects these conditions. Karst processes have accentuated the surface relief of the buckets, particularly in the late Quaternary.

Monitored and modeled coral population dynamics and the refuge concept.

With large-scale impacts on coral reefs due to global climatic change projected to increase dramatically, and suitability of many areas for reef growth projected to decrease, the question arises whether particular settings might serve as refugia that can maintain higher coral populations than surrounding areas. We examine this hypothesis on a small, local scale in Honduras, western Caribbean. Dense coral thickets containing high numbers of the endangered coral Acropora cervicornis occur on offshore banks while being rare on the fringing reef on nearby Roatán. Geomorphological setting and community dynamics were evaluated and monitored from 1996 to 2005. A model of population dynamics was developed to test assumptions derived from monitoring. Coral cover on the fringing reef declined in 1998 from >30% to <20%, but the banks maintained areas of very dense coral cover (32% cover by A. cervicornis on the banks but <1% on the fringing reef). Bathymetry from satellite images showed the banks to be well-separated from the fringing reef, making asexual connectivity between banks and fringing reef impossible but protecting the banks from direct land-runoff during storms. Exposure to SE tradewinds also causes good flushing. Only four A. cervicornis recruits were recorded on the fringing reef over 6 years. Runoff associated with hurricanes caused greater mortality than did bleaching in 1998 and 2005 on the fringing reef, but not on the banks. Since 1870, our analysis suggests that corals on the banks may have been favored during 17 runoff events associated with tropical depressions and storms and potentially also during five bleaching events, but this is more uncertain. Our model suggests that under this disturbance regime, the banks will indeed maintain higher coral populations than the fringing reef and supports the assumption that offshore banks could serve as refugia with the capacity to subsidize depleted mainland populations.