Testing biodiversity theory using species richness of reef-building corals across a depth gradient

Photosynthetic usable energy explains vertical patterns of biodiversity in zooxanthellate corals

The biodiversity in coral reef ecosystems is distributed heterogeneously across spatial and temporal scales, being commonly influenced by biogeographic factors, habitat area and disturbance frequency. A potential association between gradients of usable energy and biodiversity patterns has received little empirical support in these ecosystems. Here, we analyzed the productivity and biodiversity variation over depth gradients in symbiotic coral communities, whose members rely on the energy translocated by photosynthetic algal symbionts (zooxanthellae). Using a mechanistic model we explored the association between the depth-dependent variation in photosynthetic usable energy to corals and gradients of species diversity, comparing reefs with contrasting water clarity and biodiversity patterns across global hotspots of marine biodiversity. The productivity-biodiversity model explained between 64 and 95% of the depth-related variation in coral species richness, indicating that much of the variation in species richness with depth is driven by changes in the fractional contribution of photosynthetically fixed energy by the zooxanthellae. These results suggest a fundamental role of solar energy availability and photosynthetic production in explaining global-scale patterns of coral biodiversity and community structure along depth gradients. Accordingly, the maintenance of water optical quality in coral reefs is fundamental to protect coral biodiversity and prevent reef degradation.

Factors Limiting the Range Extension of Corals into High-Latitude Reef Regions

Reef-building corals show a marked decrease in total species richness from the tropics to high latitude regions. Several hypotheses have been proposed to account for this pattern in the context of abiotic and biotic factors, including temperature thresholds, light limitation, aragonite saturation, nutrient or sediment loads, larval dispersal constraints, competition with macro-algae or other invertebrates, and availability of suitable settlement cues or micro-algal symbionts. Surprisingly, there is a paucity of data supporting several of these hypotheses. Given the immense pressures faced by corals in the Anthropocene, it is critical to understand the factors limiting their distribution in order to predict potential range expansions and the role that high latitude reefs can play as refuges from climate change. This review examines these factors and outlines critical research areas to address knowledge gaps in our understanding of light/temperature interactions, coral-Symbiodiniaceae associations, settlement cues, and competition in high latitude reefs.

Algal turf productivity on coral reefs: A meta-analysis

Algal turfs are an abundant and highly productive component of coral reef ecosystems. However, our understanding of the drivers that shape algal turf productivity across studies and among reefs is limited. Based on published studies we considered how different factors may shape turf productivity and turnover rates. Of the factors considered, depth was the primary driver of turf productivity rates, while turnover was predominantly related to turf biomass. We also highlight shortcomings in the available data collected on turf productivity to-date; most data were collected prior to global coral bleaching events, within a limited geographic range, and were largely from experimental substrata. Despite the fact turfs are a widespread benthic covering on most coral reefs, and one of the major sources of benthic productivity, our understanding of their productivity is constrained by both a paucity of data and methodological limitations. We offer a potential way forward to address these challenges.

Drivers of coral bleaching in a Marine Protected Area of the Southern Gulf of Mexico during the 2015 event

Here we report the bleached coral cover and its drivers observed at Alacranes Reef in 2015. Our results show that 2015 was an unprecedented heat stress event. However, we observed low coral bleaching, with the most substantial impact on sites with a 10-20% of coral cover with bleaching. Depth was the most relevant variable related to coral bleaching and the bleaching severity index, with deeper reefs being most affected. Further, our results show that reefs with higher structural complexity based on species composition were among the most affected. We identified that accumulated heat stress and thermal variation in the last 28 days were relevant drivers of coral bleaching. This work highlights the importance of multidimensional frameworks in assessing the spatial variation of coral bleaching, demonstrating the importance of structural habitat variables such as depth in high heat stress events and at a reef scale.