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Matt Benson '12 helps assess live coral cover

Matt hard at work in Belize, summer 2011

Traditionally it has been assumed that since shallow water corals grow faster than deeper corals, overall reef accretion rates should follow a depth-related patterns of decreasing net carbonate production with depth (Schlager, 1981; Boscher & Schlager, 1992).These assumptions are at odds with recent studies suggesting no clear depth-related pattern in Caribbean reef building with time (Gischler, et al. 2008; Hubbard, 2009). It is well documented that coral reefs are in declining health across the Caribbean and wider Atlantic region. Understanding carbonate production and reef accretions paradigms may help with understanding the current potential shift of reefs from net carbonate production towards net carbonate destruction.

This study attempted to investigate the relationship between individual coral growth, percent live coral cover, and overall carbonate production with depth from 2005 to 2011 in St. John. Additionally, this study also develops and explores the potential for future related research off Ambergris Caye, Belize through an exploratory survey of the area. In this study, linear-extension rates for over 70 Montastrea annularis coral samples were estimated by analyzing x-radiographs of specimens collected off of St. John. Percent live coral, macro algae, turf algae, sand, sponge, dead coral, gorgonian, and pavement cover data were collected along 11 transects using video surveys of randomly placed 10 m transects that have been repeated at least annually since 2005. These transects span a depth range of 3-20 m. Bulk density and linear extension data from individual samples combined with percent live coral cover were used to calculate carbonate production per 10m transect per year at these sites. In Belize, subsurface photographic and video surveys of line transect and meter square quadrants along five A. cervicornis-dominated patch reefs were collected. These Belizean transects were located in a previously undocumented potential refuge for Acropora cervicornis, an endangered species of coral. From data collected in Belize, 61 quadrat photos were digitized scaled, and all live tissue was traced. Each scaled image was analyzed for percent live coral tissue and number of live branch tips.

Off St. John, live coral cover shows a significant decline after 2005 but some recovery after 2009. Data suggest nothing in particular (algae, sediment, or pavement) replaced the declining coral cover in a spatial or temporal pattern. On shallow reefs coral cover declined by a greater percent and recovered faster than on deep reefs. The difference suggests that for the deeper reefs water depth potentially buffered or lessened the effects of the environmental stress that spurred the original decline. Despite faster growing shallow water corals declining more rapidly than higher density deeper corals with less variable coral cover changes there appears to be little impact on net reef accretion over the project period. Off Belize, the average percent of live coral tissue per quadrant was calculated at 35%. This level of percent cover is consistent with percent cover figures from St. John prior to the 2005 heat event. This research project is attempting to better the understanding of Caribbean reef depth, time, and space related patterns, and is also the first attempt to assess the health of an endangered species of coral at a previously undocumented site off the coast of Belize.