A 3D perspective on sediment accumulation in algal turfs: Implications of coral reef flattening

Abstract

Globally, coral reefs are being transformed by a suite of stressors, the foremost being climate change. Increasingly, it is expected that these reconfigured reef systems will emerge with lower‐complexity and will be dominated by algal turfs. Understanding this new operating space is vital if we are to maintain the services, such as fishable biomass production, that reefs provide. However, the functionality of these systems appears to depend on the nature of the algal turfs themselves, which is in‐turn, intimately linked to the sediments they contain. As reefs are losing complexity, we need to understand if, and to what extent, algal turf condition and complex reef structure are connected. To address this issue we took advantage of recent developments in 3D structure‐from‐motion technology to examine how complexity metrics (elevation and surface angle) related to the nature of algal turfs on a heavily climate‐impacted coral reef. This represents a novel application of this technology in the context of coral reef ecosystems. We found that as both elevation and surface angle decreased, the nutritional value of the epilithic algal matrix also decreased while sediment accumulation increased. Furthermore, we showed that elevated surfaces were characterized by far shorter algal turfs, and are potentially herbivory hotspots, offering fertile grounds for further exploration of herbivory dynamics at sub‐metre spatial scales. Synthesis. This study yields new insights into the operating‐space of future reefs, and suggests that as reefs flatten, sediment accumulation is likely to increase even if sediment inputs remain unchanged, altering algal turfs fundamentally. Maintaining key services provided by climate‐transformed, low‐complexity algal turf‐dominated reefs of the future, will depend on managing the complex interactions between herbivory, sediments, algal turfs and reef structural complexity.

Habitat zonation on coral reefs: Structural complexity, nutritional resources and herbivorous fish distributions

“…To date, coral reef ecologists have employed a vast array of metrics, across different scales, to characterize complexity (e.g. [16,29,56]). As the broad range of metrics all measure different facets of complexity, one may expect that they will differ markedly from each other.…”

“…For example, the nature of algal turf resources can vary dramatically among habitats in terms of their productivity, sediment loading and detritus levels [10,[97][98][99]. All such factors can affect herbivorous fish feeding activity [55,[100][101][102] and are also likely to be closely intertwined with habitat complexity [29,103]. Furthermore, hydrodynamic activity such as wave energy [10,104] varies substantially among habitats and can structure fish assemblages.…”

“…However, habitat structure and nutritional resource availability are not necessarily mutually exclusive and can be intertwined, e.g. more complexity can mean more surface area for fishes to graze [28], along with higher-quality nutritional resources [29]. It is currently unclear how the different facets of complexity differ among distinct reefal zones, how they are inter-related, and what this might mean for the distribution of herbivorous fishes.…”

“…Accurately quantifying three-dimensional (3D) features in marine environments has proven to be a bottleneck for coral reef ecologists for decades. Issues can arise due to: a) differences in methodologies, metrics and scales used to measure complexity among systems (reviewed by [29]); and b) practical and logistical issues associated with identifying, then quantifying, the functional components of complexity on reefs [24]. However, advances in computer vision and camera technologies have allowed underwater researchers to utilise closerange photogrammetry techniques.…”

“…However, the application of this technology is still in its infancy and to-date, most studies utilising 3D-methods have been methodological (e.g. [30][31][32]) rather than being used as a tool to address ecological hypothesis (but see [28,29,33,34]).…”