Modelling the linkage between coral assemblage structure and pattern of environmental forcing

Influence of upwelling on coral reef benthic communities: a systematic review and meta-analysis

Highly competitive coral reef benthic communities are acutely sensitive to changes in environmental parameters such as temperature and nutrient concentrations. Physical oceanographic processes that induce upwelling therefore act as drivers of community structure on tropical reefs. How upwelling impacts coral communities, however, is not fully understood; upwelling may provide a natural buffer against climate impacts and could potentially enhance the efficacy of spatial management and reef conservation efforts. This study employed a systematic review to assess existing literature linking upwelling with reef community structure, and a meta-analysis to quantify upwelling impact on the percentage cover of coral reef benthic groups. We show that upwelling has context-dependant effects on the cover of hard coral and fleshy macroalgae, with effect size and direction varying with depth, region and remoteness. Fleshy macroalgae were found to increase by 110% on inhabited reefs yet decrease by 56% around one well-studied remote island in response to upwelling. Hard coral cover was not significantly impacted by upwelling on inhabited reefs but increased by 150% when direct local human pressures were absent. By synthesizing existing evidence, this review facilitates adaptive and nuanced reef management which considers the influence of upwelling on reef assemblages.

Stochastic diffusion characterizes early colony formation in Mediterranean coral Corallium rubrum

The colony formation in Mediterranean coral Corallium rubrum is initiated by a larva that metamorphoses into the first polyp of the emerging colony approximately two weeks after settlement. The primary polyp then sets up a slow process that eventually, at least after several years, gives rise to a tree-like rigid colony structure on which other polyps flourish. For a mature colony, this axial skeleton provides support for new polyps. However, the first emergence of the characteristic axial skeleton takes two to four years from the larva stage. The early colony morphology, instead, is shaped exclusively by the polyps' abundant deposition of sclerites, a magnesian calcite biomineral that has a different granularity from the distinctive red-coloured skeleton. With the appearance of the first polyp, a growing sclerite heap in a mesoglea layer provides a base for the emerging colony. In this paper, to elucidate the mechanical processes of early skeleton development in C. rubrum colonies, we present a computational model whereby the mesoglea layer provides a diffusion medium for the sclerites that the polyps deposit. We show that our stochastic model with three parameters captures the dynamic variability observed in measurements on living colonies. Our simulation results provide evidence for a diffusion process whereby the interplay between polyp budding and sclerite deposition are the main determinants of structure in early colony formation. Our model demonstrates that the frequency of budding events in an early colony can be described as a function of the available mesoglea surface whereas the number of polyps on the colony plays a secondary role in determining this frequency. We show that these model predictions are confirmed by direct observations on the colonies in our sample. Moreover, our results indicate that diffusion is a prevalent mechanism of colony development also at later stages of a colony's life span.

Post-settlement demographics of reef building corals suggest prolonged recruitment bottlenecks

For many organisms, early life stages experience significantly higher rates of mortality relative to adults. However, tracking early life stage individuals through time in natural settings is difficult, limiting our understanding of the duration of these ‘mortality bottlenecks’, and the time required for survivorship to match that of adults. Here, we track a cohort of juvenile corals (1–5 cm maximum diameter) from 12 taxa at a remote atoll in the Central Pacific from 2013 to 2017 and describe patterns of annual survivorship. Of the 537 juveniles initially detected, 219 (41%) were alive 4 years later, 163 (30%) died via complete loss of live tissue from the skeleton, and the remaining 155 (29%) died via dislodgement. The differing mortality patterns suggest that habitat characteristics, as well as species-specific features, may influence early life stage survival. Across most taxa, survival fit a logistic model, reaching > 90% annual survival within 4 years. These data suggest that mortality bottlenecks characteristic of ‘recruitment’ extend up to 5 years after individuals can be visually detected. Ultimately, replenishment of adult coral populations via sexual reproduction is needed to maintain both coral cover and genetic diversity. This study provides key insights into the dynamics and time scales that characterize these critical early life stages.