HOW GOOD IS THE “LIMESTONE-ON-A-STICK METHOD” AT CHARACTERIZING PATHWAYS AND RATES OF BIOEROSION?

Whether or not the “Anthropocene” represents the “greatest extinction in earth history”, modern-ancient comparisons require valid estimates of the rate at which communities change. Recent reef decline provides an opportunity to examine shifts in community structure in response to dramatic environmental change. However, while coral loss has been quantified, reliable rates of post-mortem alteration are lacking. Samples from natural substrates provide a sense of the major bioeroders and their relative importance. However, because we do not know when corals died, reliable rates of processes after their demise are illusive. The solution has typically involved carbonate blocks of known volume placed near the reef to track bioerosion over time. However, studies to verify that these artificial substrates behave like recently killed corals are lacking.

Forty transects (d = 3-40m) have been monitored in Virgin Islands National Park since late 2005 – initially on a bi-monthly basis to track rapid changes following severe disease and bleaching, and annually thereafter. The video records were used to determine the time of death for individual corals, and 90 of these were recovered across the depth range represented by the transects. These samples provide the first quantitative record of post-mortem alteration for natural substrates that have not been manipulated in any way and have a known time of death.

While analyses are ongoing, several trends have emerged. Grazing, which has been characterized as the dominant process of substrate removal, has been less intense than expected over the 5-year post-mortem history of the sampled corals. Bioerosion along upward facing surfaces has been surprisingly light and is much lower than reported values from artificial substrates. Both macro- and micro-erosion are more pronounced on the sides of dead colonies, especially on cryptic surfaces where access by grazers is limited. Based on a nominal linear-extension rate of 8-10 mm/yr, those “protected” surfaces are less than 20 years old and total bioerosion rates of whole colonies are again below those based on experiments using artificial substrates. This study raises potential concerns over estimates of bioerosion based on manipulated experimental surfaces and may cast doubt on models of sclerobiont succession using those studies.