Geomorphology and benthic cover of mesophotic coral ecosystems of the upper insular slope of southwest Puerto Rico

• Published in: Coral reefs 2010-06, Vol.29 (2), p.347-360
• Main Authors: Sherman, C, Nemeth, M, Ruíz, H, Bejarano, I, Appeldoorn, R, Pagán, F, Schärer, M, Weil, E
• Format: Article
• Language: English
• Subjects: Accretion; Animal and plant ecology; Animal, plant and microbial ecology; Biological and medical sciences; Biomedical and Life Sciences; Cnidaria. Ctenaria; Coral reefs; Deglaciation; Ecosystems; Freshwater & Marine Ecology; Fundamental and applied biological sciences. Psychology; Geomorphology; Insular slope; Insular slopes; Invertebrates; Life Sciences; Marine; Meltwater; Mesophotic coral ecosystem; Oceanography; Relict reef; Sea level; Sea water ecosystems; Sediment transport; Synecology; Terraces; Water depth
• ISSN: 0722-4028; 1432-0975
• DOI: 10.1007/s00338-010-0607-4

The upper insular slope of southwest Puerto Rico is defined as extending from the shelf break at ~20 m water depth down to a depth of ~160 m where there is a pronounced change in geomorphic character and the basal slope begins. The upper slope is divided into two geomorphic zones separated by a pronounced break in slope gradient at ~90 m water depth. Descending from the shelf break, these are Zone I (20-90 m) and Zone II (90-160 m). As orientation of the shelf margin changes, geomorphology of Zone I shows systematic variations consistent with changes in exposure to prevailing waves. Within Zone I, exposed southeast-facing slopes have a gentler gradient and lower relief than more sheltered southwest-facing slopes, which are steep and irregular. Mesophotic coral ecosystems (MCEs) are largely restricted to Zone I and concentrated on topographic highs removed from the influence of active downslope sediment transport. Accordingly, MCEs are more abundant, extensive and diverse on southwest-facing slopes where irregular topography funnels downslope sediment transport into steep narrow grooves. MCEs are more sporadic and widely spaced on southeast-facing slopes where topographic highs are more widely spaced and downslope sediment transport is spread over open, low-relief slopes inhibiting coral recruitment and growth. Relict features formed during preexisting sea levels lower than present include deep buttresses at ~45-65 m water depth and a prominent terrace at ~80 m. Based on correlations with existing reef accretion and sea-level records, it is proposed that the 80-m terrace formed during the last deglaciation ~14-15 ka and subsequently drowned during a period of rapid sea-level rise associated with meltwater pulse 1A at ~14 ka and deep buttresses at ~45-65 m formed between ~11.5 and 13.5 ka and then drowned during a period of rapid sea-level rise associated with meltwater pulse 1B at ~11.3 ka.