Turf algal/sediment (TAS) mats: a chronic stressor on scleractinian corals in Akumal, México

Turf algal/sediment (TAS) mats are currently prevalent on reefs near Akumal, México. In contrast to turf algae that lack bound sediments, TAS mats outcompete scleractinians by employing one or more of three encroachment methods identified during this investigation. In 2000, all colonies of Diploria strigosa and most Montastraea faveolata were 100% surrounded by well-developed TAS mats, as were about two thirds of the Siderastrea siderea. Given that these species are locally important reef framework-builders and flourished here in recent decades, encroachment by TAS mats is probably a recent phenomenon. Results indicate that D. strigosa is the most vulnerable of the three, losing, on average, 70 ± 13.2 cm2 area of tissue/coral/year. Three of the 42 studied colonies suffered complete mortality and another one-third regressed below their estimated

puberty size. M. faveolata is similarly vulnerable but survives longer due to its far larger mean colony sizes. S. siderea, however, not only loses tissues at a significantly lower rate, but also seems able to outcompete TAS mats with low sediment loads. Sixty-one species of algae and cyanobacteria (more than two thirds of which were Rhodophyta) were identified in TAS mats that recruited to settling plates and rubble. Manipulations of sediment load in recruited TAS mats indicate that it can affect mat composition at the genus level. Entrapped sediment weight/cm2 is positively related to the average turf algal filament height and averages 14.4 times the algal dry weight/cm2 in which it is embedded.
The prevalence of TAS mats and their encroachment over scleractinians is likely related to high sediment loading and low rates of herbivory (due to heavy, historical fishing efforts, relatively high territorial pomacentrid populations, and the continued absence of the key Caribbean herbivore, Diadema antillarum). The current decline in D. strigosa and M. faveolata is unsustainable over the long-term with serious impending consequences for local habitat heterogeneity and diversity. Wherever present in abundance, TAS mats should be included as a chronic stressor in future assessments of reef health, and programs to enhance herbivore populations, especially of D. antillarum which can effectively remove these mats (pers. obs.), should be developed.