Stuart A. Sandin

Coral reef communities are often studied by tracking the percentage (or fraction) of the reef covered by coral through time. However, coral community dynamics result, in part, from underlying colony-level growth and mortality, which in turn depend on characteristics of individ- ual colonies, such as size, taxon, life history strategy, and morphology. Colonies are also subject to external disturbances that propel fission into smaller coral fragments and fusion where related fragments later fuse into contiguous colonies. To quantify how changes in coral growth through time depend on individual colony characteristics and colony fission and fusion processes, 4385 individual Caribbean coral colonies representing 4 dominant coral types (Madracis mirabilis, mounding coral species, Agaricia agaricites, and Millepora spp.) were tracked at 6 mo intervals for 4 yr. Despite overall stable percent coral cover, colonies belonging to different coral types experi- enced differential growth, shrinkage, mortality, fission, and fusion processes. All coral types dis- played size-dependent allometric growth patterns whereby relative, or proportional, growth in colony area decreased with increasing colony size. The largest changes in relative colony growth resulted from colony fission or fusion with other colonies, which occurred in 16.4% of all moni- tored colonies. Colony longevity, or survival, increased significantly with increasing colony size for all hard-coral groups that did not experience fission, fusion, or a combination of these pro- cesses. Our findings illustrate the usefulness of a size- and life-history-dependent approach to coral demography that elucidates the factors driving community dynamics of colonial organisms, which are not captured by traditional approaches based on benthic cover alone.

One mechanism giving fleshy algae a competitive advantage over corals during reef degradation is algal-induced and microbially-mediated hypoxia (typically less than 69.5 µmol oxygen L⁻¹). During hypoxic conditions oxygen availability becomes insufficient to sustain aerobic respiration in most metazoans. Algae are more tolerant of low oxygen conditions and may outcompete corals weakened by hypoxia. A key question on the ecological importance of this mechanism remains unanswered: How extensive are local hypoxic zones in highly turbulent aquatic environments, continuously flushed by currents and wave surge? To better understand the concert of biological, chemical, and physical factors that determine the abundance and distribution of oxygen in this environment, we combined 3D imagery, flow measurements, macro- and micro-organismal abundance estimates, and experimentally determined biogenic oxygen and carbon fluxes as input values for a 3D bio-physical model. The model was first developed and verified for controlled flume experiments containing coral and algal colonies in direct interaction. We then developed a three-dimensional numerical model of an existing coral reef plot off the coast of Curaçao where oxygen concentrations for comparison were collected in a small-scale grid using fiberoptic oxygen optodes. Oxygen distribution patterns given by the model were a good predictor for in situ concentrations and indicate widespread localized differences exceeding 50 µmol L^-1 over distances less than a decimeter. This suggests that small-scale hypoxic zones can persist for an extended period of time in the turbulent environment of a wave- and surge- exposed coral reef. This work highlights how the combination of three-dimensional imagery, biogenic fluxes, and fluid dynamic modeling can provide a powerful tool to illustrate and predict the distribution of analytes (e.g., oxygen or other bioactive substances) in a highly complex system.