Seascape ecology of remote Pacific coral reefs

Abstract

Spatial patterns in ecology can reveal much about the processes driving them but can also be scale dependent. Observational scales in the marine realm can be constrained by traditional approaches, leading to a limited understanding of broader seascape patterns and processes. For the first time, this thesis characterised the spatial structure of coral reef benthic communities, along with their key physical drivers over multiple scales and geographies. Our first study used 6022 digital images, from sub-surface towed-diver surveys spanning ~140 linear km of reef, around five uninhabited tropical Pacific islands, to yield a spatially expansive and thematically resolved dataset. Benthic functional groups (e.g. hard coral and macroalgae) and coral morphologies (e.g. branching and plating) displayed natural spatial clustering across the seascape, with individual groups dominating coastlines over km-scales. Interestingly, at each island, the scales at which variables became spatially random were similar between benthic communities and two key physical drivers of benthic community structure on uninhabited coral reefs, wave energy and temperature. The second study in this thesis then looked at these spatial scaling patterns over time; before and after multiple bleaching events at a single remote Pacific Island. Despite little change in overall coral cover leading up to a mass bleaching event (coral cover increased by 4% following a minor bleaching event), the spatial structure of the island’s hard coral had become more random across multiple scales. The mass bleaching event also disrupted the ability to predict distributions of the functional groups with known environmental drivers of the island’s benthic seascape. The impact of bleaching events on community structure at smaller spatial scales (~10 m2) was then investigated. This entailed the analysis of co-occurrence patterns within benthic communities around the remote Howland and Baker islands. In our baseline year, prior to bleaching, 43% of pairwise combinations were significant, 26 of which were positive and 23 negative. Significant co-occurrences were found in all survey years. But the organisms with more numerous positive associations changed from involving CCA and hard corals (e.g. Porites, Favites, Pocillopora and Pavona) to turf algae and more stress-tolerant genera after repeat bleaching events. Finally, this thesis discusses how novel technologies coupled with theoretical framing and analytical approaches provided by the field of landscape ecology could progress coral reef monitoring and science. The aim of this thesis explores the randomness in coral reef benthic community structure at remote islands. This thesis reveals the clustered nature of spatial patterns of organisms within coral reef benthic communities across spatial scales, contributing vital baseline information during a time when coral reefs may be changing irreversibly. This thesis also demonstrates that bleaching events can affect and alter coral reef benthic communities across scales, regardless of the degree of protection or remoteness of the reef. It therefore contributes knowledge to how coral reefs may change spatially and at different scales with climate change but also suggests avenues to better understand and monitor these changes into the future.

Date of Award	26 Oct 2023
Original language	English
Awarding Institution	Bangor University
Supervisor	Gareth Williams (Supervisor) & John Healey (Supervisor)