When depth is no refuge: cumulative thermal stress increases with depth in Bocas del Toro, Panama

Abstract

Coral reefs are increasingly affected by hightemperature stress events and associated bleaching. Monitoring and predicting these events have largely utilized sea surface temperature data, due to the convenience of using large-scale remotely sensed satellite measurements. However, coral bleaching has been observed to vary in severity throughout the water column, and variations in coral thermal stress across depths have not yet been well investigated. In this study, in situ water temperature data from 1999 to 2011 from three depths were used to calculate thermal stress on a coral reef in Bahia Almirante, Bocas del Toro, Panama, which was compared to satellite surface temperature data and thermal stress calculations for the same area and time period from the National Oceanic and Atmospheric Administration Coral Reef Watch Satellite Bleaching Alert system. The results show similar total cumulative annual thermal stress for both the surface and depth-stratified data, but with a striking difference in the distribution of that stress among the depth strata during different high-temperature events, with the greatest thermal stress unusually recorded at the deepest measured depth during the most severe bleaching event in 2005. Temperature records indicate that a strong density-driven temperature inversion may have formed in this location in that year, contributing to the persistence and intensity of bleaching disturbance at depth. These results indicate that depth may not provide a stress refuge from high water temperature events in some situations, and in this case, the water properties at depth appear to have contributed to greater coral bleaching at depth compared to near-surface locations. This case study demonstrates the importance of incorporating depth-stratified temperature monitoring and small-scale oceanographic and hydrologic data for understanding and predicting local reef responses to elevated water temperature events.

Caribbean massive corals not recovering from repeated thermal stress events during 2005–2013

“…Furthermore, the relatively small growth rates we report here, and the large variance in growth across colonies, point out the difficulty in determining true growth rates for massive‐type corals, which must be measured over many years, i.e., longer than the 7 years of this study. Given inherent methodological variance in measuring planar area from photographs (Neal et al., ) and the absence in the record of some colonies in some years due to tag shedding or missed photographs, the narrowly positive rates we report may not be maximum individual growth rates for these corals, but could alternatively indicate net maintenance of tissue (i.e., a flat growth rate) which may persist for many years. In either case, these even or positive rates for the recovery period do provide a good comparison to the marked losses of the initial and response periods.…”

“…Colony photographs were analyzed for projected planar area (in units of cm 2 ) and pigmentation condition of live coral tissue, expressed as a percent of the total live area (with any patches of dead exposed skeleton surrounded by live tissue removed from the live area analysis), following Neal et al. (). All photographs included a color and size reference in the image and were analyzed with a custom MATLAB‐based image segmentation tool (http://vision.ucsd.edu/content/coral-colony-segmentation-and-area-measurement-tools), resulting in planar area measurements of live coral tissue, dead area on the colony, and bleached and partially bleached tissue (as subsets of the live area measurement) (Neal et al., ).…”

“…(). All photographs included a color and size reference in the image and were analyzed with a custom MATLAB‐based image segmentation tool (http://vision.ucsd.edu/content/coral-colony-segmentation-and-area-measurement-tools), resulting in planar area measurements of live coral tissue, dead area on the colony, and bleached and partially bleached tissue (as subsets of the live area measurement) (Neal et al., ). Bleached tissue was defined as tissue exhibiting nearly completely white areas, and partially bleached tissue as that with residual pigmentation remaining in the bleached areas (also referred to as paling).…”

Tropical dead zones and mass mortalities on coral reefs

“…S1); however, we detected no influence of temperature or salinity variation on patterns of coral cover and richness among sites when considering their effects separately or their potential interactions with dissolved oxygen (P > 0.12 all analyses). Nevertheless, overall elevated temperatures in the region during the hypoxic period may have increased background stress levels and the susceptibility of the coral community to hypoxia (17,18), as is known to occur in other taxa (19). Lowered pH, which can co-occur with hypoxic conditions (20), is unlikely to have played a primary role in the mortality event we observed because coral survivorship is relatively unaffected by acidified conditions (21,22), and coral reefs commonly tolerate wide fluctuations in pH (23).…”

Bleaching events regulate shifts from corals to excavating sponges in algae‐dominated reefs

“…This is attributed to competition by algae, which impact the growth and recruitment of excavating sponges (Chaves‐Fonnegra & Zea, ; López‐Victoria et al., ). Although the data showed that sponges decreased from 2007 to 2010, this trend changed again, toward an increase in 2011, after the 2010 coral mortality event in Florida and other areas in the Caribbean (Neal et al., ). While many corals and some species of sponge also died during the 2010 cold event (Lirman et al., ), we did not notice major mortality of C. delitrix in the Fort Lauderdale population (Chaves‐Fonnegra, Maldonado, Blackwelder, & Lopez, ).…”