Bleaching responses and bleaching threshold temperatures were developed from coral monitoring sites. An unprecedented data set of benthic temperature records and coral bleaching responses collected by the TCRMP were used to develop bleaching threshold temperatures for monitoring sites dominated by star corals (genus Orbicella) and on one lower mesophotic site dominated by scroll coral (Agaricia undata). The resulting publication challenges the notion that mesophotic reefs will be refuges from a warming ocean without additional factors besides cooler temperatures with depth (Smith et al. in press). Previously, a more moderate bleaching and mortality response of USVI MCE reefs during seawater warming in 2005 and 2010 was thought support the idea of the Deep Reef Refugia Hypothesis (Glynn 1996; Riegl and Piller 2003). This hypothesis states that the cooler temperatures at deeper depth suggest that these corals are buffered from the impacts of mass coral bleaching. However, recent work has shown that MCE corals are acclimatized to the moderate temperatures at depth in the USVI. Because of this, temperatures above the MCE local maximum can cause bleaching, even if the same temperatures would not be stressful at shallow depths (Smith et al. in press). MCE bleaching and mortality was demonstrated during the 2005 northeastern Caribbean seawater warming event and MCE bleaching occurred in 2012 without any shallow water bleaching.
Of more practical importance to management of reefs in the USVI was the creation of bleaching threshold temperatures for many TCRMP coral monitoring sites and one site in the Virgin Islands National Park, St. John. These bleaching threshold temperatures can be used to more accurately assess the response of coral communities over years with potential thermal stress.
Reef bioerosion, the breakdown of calcium carbonate reef structure and sediment by organisms, is a key process that determines the structure of reefs and their persistence with time. Constructional forces, such as coral and other organisms that form limestone skeletons, balance bioerosion and determine reef structural complexity and diversity. These factors are critical to coral reef ecosystem services, such as the provision of habitat to fish and invertebrates. The main biological component of modern coral reefs consists of a thin living veneer draped upon a biologically-produced non-living foundation. The non-living structural components of a coral reef consist of its framework, rubble, cement, and sediment. Although monitoring the health of the living coral cover is critical to short-term ecological management of the natural resource, long-term projections of coral structural sustainability require an understanding of how the non-living components of the reef are constructed, modified, and eroded. In some cases, when bioerosion outpaces the ability of reefs to add structure, the complexity of reefs can degrade and the coral ecosystem suffers.
Caribbean reefs have experienced significant declines in architectural complexity in the past four decades (Alvarez-Filip et al., 2009; Bozec et al., 2014). Despite the importance of bioerosion, little is known regarding this process in mesophotic coral reef habitats. Therefore, in conjunction with the TCRMP data sets, long-term and short-term bioerosional trends were evaluated in U.S. Virgin Islands mesophotic reefs and shallow water counterparts using a variety of techniques. TCRMP reef sites evaluated included (from deepest to shallowest) the Grammanik Bank, College Shoal, Seahorse Cottage Shoal, and Black Point. Two additional mesophotic sites with very different composition were also evaluated in the Hind Bank Marine Conservation District. Cylinders of shallow water star coral (Orbicella spp.) skeletons were deployed for up to three years to evaluate natural processes of bioerosion on dead coral substrate, reef sediments were analyzed to understand if they reflect reef composition, and coral skeletons were analyzed to understand rates of growth of star corals in different mesophotic environments.
Bioerosion rates were different among TCMRP mesophotic sites sampled (Weinstein 2014; Weinstein et al. 2014). Most mesophotic reefs showed low rates of bioerosion relative to shallow reefs. Parrotfish grazing was found to be the dominant initial bioerosion method for shallow and upper mesophotic zone coral reefs in the northern USVI. A reduction in parrotfish grazing was partially responsible for the low substrate bioerosion rates of mesophotic reef communities in the middle to lower depths of the mesophotic reef zone. Sediment grain composition and bulk geochemistry were found to reflect the distribution and abundance of coral and macroalgae communities in mesophotic and shallow reefs (Weinstein et al. 2015). Sediment analyses also indicate that hydrodynamic forces do not transport a significant amount of potentially harmful terrigenous material to USVI mesophotic reefs, as was found previously from the Hind Bank FSA site (Smith et al. 2008). Also, sediment does not tend to be transported from areas outside of where they were formed on the reef. Coral growth rates of star corals were significantly slower in mesophotic depths than at shallow reefs, even at sites with very in mesophotic high coral cover (Weinstein et al. submitted). This suggests that fast coral growth is not needed to form extensive and vibrant mesophotic coral reefs, possibly because of the limited amount of bioerosion. However, slow growth may indicate the sensitivity of these reefs to loss of growth, such as from coral mortality, or any processes that increases bioerosion, such as nutrient pollution.
These data have been used to construct a model of reef carbonate budget that can show if the systems is growing or declining. Overall, results indicate that all mesophotic reefs analyzed have net positive production of carbonate, unlike shallow sites, which showed declines. Differences between the amounts of positive production also indicates that the habitats have different growth potential over long time scales and may partially explain the high variability and complexity of geomorphology at the different sites. This work is in preparation for publication.