Linking deep seabed structure to biodiversity: an exploration of seamounts and deeper reefs in the South and Western Indian Ocean
Keywords: geomorphology
seamounts
seabed mapping
mesophotic coral ecosystems
habitat mapping
Indian Ocean
seascape ecology
marine ecology

Environmental heterogeneity, understood as spatial or temporal variability in environmental conditions, influences biodiversity and ecosystem processes over multiple scales, including at the deep seabed. However, as a result of the inaccessibility of the ocean beyond conventional SCUBA depth (> 30 m), key knowledge gaps remain on the biotic and abiotic patterns that influence the occurrence and distribution of seabed-associated taxa, habitats and resulting ecological processes and ecosystem services. Although seabed and habitat mapping documents seabed environmental and habitat heterogeneity, very little research explicitly quantifies it to draw conclusions on its ecological consequences. To study the character and effect of environmental heterogeneity on biodiversity and marine ecological processes in the deep sea, this thesis explored the potential of combining existing seabed and habitat mapping practices with the theoretical framework and analytical techniques from land- and seascape ecology.
This thesis first reviews seascape ecology and its potential to study the ecological implications of environmental heterogeneity at the deep seabed, and identifies theoretical focal areas for the application of tools and concepts from seascape ecology deeper than 30 m (Chapter 1: Introduction). The objectives of this thesis, based on these focal areas, can be divided in three main themes: 1) characterising spatial heterogeneity using spatial pattern metrics; 2) assessing the ecological relevance of spatial heterogeneity quantified using spatial pattern metrics; and 3) applying this knowledge to inform environmental management. Objectives are addressed through a set of case studies that provide the opportunity to explore the multi-scale relationship between seabed structure and ecology in habitats at seamounts (km-m scale, Chapter 2 and 3) and reefs found between 30 m-250 m on atoll slopes (m-cm scale, Chapter 4 and 5) in theWestern Indian Ocean. Case studies test specific ecological hypotheses using spatial pattern metrics quantifying seascape composition, configuration and terrain structure, which function as predictors for the occurrence and distribution of benthic assemblages and demersal fish.
Chapter 2 combines habitat mapping and spatial pattern metrics from seascape ecology to quantitatively test for and compare differences in seascape composition and configuration between five seamounts on the Southwest Indian Ridge (SWIR). Results quantitatively demonstrate that seamounts are highly variable in morphology, even when part of the same geological feature. As heterogeneity in the relative proportion and spatial relationships of habitats may influence ecological functioning, habitat mappers and marine managers focusing on representational protection of seamounts could benefit from such spatially-explicit approaches to quantify seabed heterogeneity.
Chapter 3 examines the influence of multi-scale seabed spatial heterogeneity on 15 commercially important fish families at three SWIR seamounts, focusing on patch affinity, patch complexity, patch size and seascape aggregation. Although strongly driven by site and depth, demersal fish respond to unique combinations of seascape composition, configuration and terrain structure depending on their family. Further, seascape composition and configuration (i.e. habitat size, shape and structural connectivity) had higher predictive power than terrain derivatives commonly used in developing proxies for deepwater fish biodiversity. These outcomes indicate the importance of incorporating spatial pattern metrics when identifying environmental predictors of fish distributions and suitable habitat in deep-sea environments.
Chapter 4 tests whether multi-scale geomorphology can act as a reliable spatial proxy for deeper reef assemblage (30 m-250 m) distribution. It found that assemblage occurrence and distribution is determined by a combination of environmental parameters, explained by the functional characteristics of each assemblage. Depth and structural complexity were main predictors, and broad scale predictors (25 m) proved more informative than finer scale predictors (2 m). Findings addressed geographical gaps in our knowledge of the distribution of deeper reef habitats and generated insights into ecological relationships. Complex geomorphological structures, including terraces and paleoshorelines, supported particularly high densities of mesophotic benthic assemblages and could be considered priority habitats for management.
Chapter 5 investigates the effect of fine-scale (cm-m) environmental heterogeneity on fish associated with mesophotic reefs (30 m-120 m). Spatial pattern metrics quantifying benthic composition, configuration and terrain structure were extracted from transect terrain models and orthomosaics produced with Structure-from-Motion (SfM) photogrammetry. In addition to known drivers (depth and geographic location), results show a combination of fine-scale seascape metrics of terrain structure, patch composition and patch configuration best explains mesophotic fish assemblage structure. Overall, sites with steep slopes and high terrain complexity hosted highest fish abundance and biomass.
Across case studies, spatial pattern metrics allowed quantification and comparison of seascape structure and functioned as reliable predictors for the occurrence and distribution of benthic assemblages and demersal fish at deeper reefs and seamounts in the Western Indian Ocean. Overall, spatial pattern metrics facilitated a better understanding of biodiversity-environment relationships in heterogeneous environments, in some cases functioning as the main explanatory variable. This thesis therefore recommends their further application in deep sea ecology, monitoring and ecosystembased management and conservation, whilst accounting for the biological phenomenon under consideration and scale- and context dependency in survey and analysis.