CASI Mapping of the Farasan Islands (Saudi Arabia): Ecosystem Processes in an Atypical Red Sea Setting

The Red Sea is typified by expansive fringing-reefs affixed to a narrow and steep coastal shelf. We present work from the South of the Red Sea where a rather different geomorphology exists. The little studied marine system of the Farasans consists of vast shallow sediment-rich banks rising from deep water, a morphology that profoundly affects the depositional environment. Satellite imagery and 3,000 km2 of airborne CASI imagery were acquired over this remote seascape. Hydrodynamic measurements atop the banks revealed low flushing rates and temperatures notably higher than encountered in adjacent deep water. These stressful factors appear to render the setting sub-optimal for corals. The hyperspectral CASI data coupled with exhaustive field survey allowed mapping of benthic character across the Farasan archipelago at exceptionally fine spatial resolution. In addition, bathymetry was retrieved from a spectral derivation of the CASI data, trained using numerous acoustic soundings made during the field campaign. The investigation reveals an abundance of fine-grained sediment sheets on-lapping topographic highs dominated by algal meadows. Despite the unfavorable environmental conditions, expansive areas of live coral were also found. These were concentrated either within shallow micro-atoll fields in the bank interior, or in deeper habitats within large but diffuse ‘coral fields’ interspersed with high algal cover. Though the region does not lack suitable substrate and is locally rich in coral, little evidence for frame-building was observed. The mapping revealed that the location of coral-dominated habitat was not random, but rather clustered in the northern (upstream) sector of the archipelago. Corals were not found to favor a particular depth regime, but the axis of orientation for these areas was consistent between banks. The study highlights an unusual coral setting in the Red Sea and successfully proves the existence of a vibrant but atypical ecosystem