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Title

Remote Sensing the Diversity, Distribution and Resilience of Coral Reef Environments

Author

Gwilym Rowlands, Nova Southeastern University

Defense Date

8-8-2013

Document Type

Dissertation - NSU Access Only

Degree Name

Ph.D. Oceanography/Marine Biology

Department

Oceanographic Center

First Advisor

Samuel J. Purkis

Second Advisor

Andrew W. Bruckner

Third Advisor

Richard E. Dodge

Fourth Advisor

James A. Goodman

Fifth Advisor

Bernhard M. Riegl

Abstract

Chapter 2:

This chapter introduces the five study sites (Ras Al‐Qasabah; Al Wajh; Yanbu; Farasan Banks; and Western Farasan Islands) along with the fieldwork and detailed benthic mapping and bathymetry mapping conducted in the Saudi Arabian Red Sea. In the Western Farasan Islands two candidate mapping technologies were compared. Firstly, the QuickBird multispectral satellite sensor and secondly the CASI‐550 airborne hyperspectral sensor. In processing the CASI imagery, it was necessary to customize processing to correct for an unusual across‐track artifact caused by lens condensation. On the basis of cost, logistical constraints, spectral reliability, and project needs, multispectral imagery was found to be the most appropriate technology for regional‐scale mapping. Over 20,000 sq. km of high quality QuickBird imagery were amassed across the five study sites. This represents approximately half the shallow water (<20 m) environment of the Saudi Arabian Red Sea. The work presented in this chapter provides a blueprint for processing such large image data sets. Maps with a minimum mapping unit (MMU) of 7.5 sq. m, and a thematic resolution of fifteen habitat classes were produced at an overall accuracy (Tau statistic) of 70%. The five study sites were found to differ substantially in terms of the type, quantity and spatial arrangement of habitats present. The study illustrates the power of remote sensing for delivering regional‐scale audits of coral reef environments.

Chapter 3:

Coral reefs and their associated accumulations of carbonate sediment adopt particularly complex planform geometries atop the coastal shelf of the Saudi Arabian Red Sea. By assembling 95,000 sq. km of remote sensing data into a GIS, this study aims to relate the morphology of these shallow‐water depositional environments to processes that sculpt the coastal zone. A typology is developed that sorts carbonate systems into end‐members on the basis of their morphology and relationship to the coastline. The resulting GIS was interrogated for spatial patterns in the distribution and abundance of the end‐members. While several depositional morphologies are present throughout the length of the Saudi Arabian Red Sea, the occurrence of others is restricted to narrow regions of latitude. Such differences in distribution can be explained in process‐terms by the rift tectonics of the Red Sea basin, spatial variability in the presence of sub‐seafloor evaporites, and the input of siliciclastic detritus onto the coastal shelf via wadis. This chapter provides a foundation for understanding the morphological diversity of shallow‐water carbonate systems in both the modern ocean and rock record.

Chapter 4:

In this chapter a framework is proposed for spatially estimating a proxy for coral reef resilience using remote sensing. Data spanning over 20,000 sq. km 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 indices 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 the analysis is flexible and with minimal adaptation, could be extended to other reef regions. The chapter illustrates how remote sensing can be used to deliver more than simply habitat maps of coral reefs.

Chapter 5:

In this chapter, a fundamental measure of coral reef health, coral cover, is assessed in relation to two physical parameters, water depth and wave height. Light availability declines rapidly with depth, which influences the photosynthetic productivity of coral. Where waves break, they produce a severe increase in marine turbulence, and generate currents that may extend beyond the surf zone. The study is focused on the Farasan Banks where some 4000 sq. km of reef habitat are spread across 12,000 sq. km of the Saudi Arabian coastal shelf. The size of the system creates logistical challenge for standard field‐based monitoring methodologies, such as SCUBA surveys. Here, rapid video assessments were employed to deliver measures of coral health across eight percentage cover classes at 472 locations. Whilst water depth can be reliably derived from satellite, assessing wave height is problematic since the parameter is both spatially and temporally variable. Using daily, satellite derived meteorology, a spatially explicit wave model was developed spanning the nine year period from 1999 to 2008. For the majority of the

video sites in the Farasan Banks, coral cover was found to be <11%. This statistic hides the counter trend, however that there are robust patterns in higher coral abundance that can be characterized by water depth and wave height. In the inshore, wave height had little bearing on coral cover, instead video sites with a high coral cover were found with a greater probability in shallow (<9m, reef environments. In the offshore, wave exposure exercises stronger control on coral cover than in the inshore, such that video sites with a coral cover greater than 50% were exclusively found in areas where significant wave height exceeds 2 m. The water depth at which the highest coral cover occurs is also deeper offshore than inshore. Once quantified, the conservative behavior of coral cover with respect to water depth and hydrodynamic exposure offers relevant insight to the management of coral reef environments at regional extent.

Chapter 6:

Carbonate sequence stratigraphy is founded on the principle that changes in relative sea level are recorded in the rock record by the accumulation of sediment with relative‐water‐depth dependent attributes. While at the scale of a shelf‐to‐basin transect, facies clearly stratify by water depth, the relationship blurs for depths <40 m, the most vigorous zone of carbonate production, where the intrinsic processes of storm and wave reworking influence the seabed through submarine erosion and sediment redistribution. Remote sensing imagery, field observations, and hydrodynamic models for two reef‐rimmed shore‐attached carbonate platforms in the Red Sea show neither water depth nor energy regime to be reliable indicators of facies type when considered in isolation. Considered simultaneously, however, the predictive power of the two variables rises significantly. The results demonstrate it to be an oversimplification to assume a direct link between palaeo‐water depth and depositional lithofacies diversity, while highlighting the importance of hydrodynamics in directing the accumulation of carbonate sediments in the shallow photic zone. While the size distributions of facies in the two focus areas, Al Wajh and Ras Al‐Qasabah, follow power laws, no direct relationship between the lateral continuity of the facies belts and water depth or wave height are reported. The work is relevant for the interpretation of meter‐scale subtidal carbonate cycles throughout the geologic record.

Comments

Additional keywords:

Habitat Mapping, Thematic map, Facies, Satellite, QuickBird, CASI, MODIS, QuikSCAT, ASTER, Evaporite, Doline, Tower, Platform, Digital Elevation Model, DEM, SST, Water depth, Wave model, Wave height, Wave exposure, Fishing Pressure

NSUWorks Citation

Gwilym Rowlands. 2013. Remote Sensing the Diversity, Distribution and Resilience of Coral Reef Environments. Doctoral dissertation. Nova Southeastern University. Retrieved from NSUWorks, Oceanographic Center. (75)
https://nsuworks.nova.edu/occ_stuetd/75.