Date of Award
2024
Degree Type
Dissertation
Degree Name
Doctor of Philosophy in Biological and Environmental Sciences
Department
Biological Sciences
First Advisor
Andrew Davies
Abstract
Historically, the deep ocean has been considered biologically sparse and food-limited, made cryptic by its inaccessibility and therefore separate from conversations of human-ecosystem relationships and conservation priorities. However, this perception belies the vibrant interconnected reef communities in the deep sea, particularly those built by cold-water corals. Corals are known as some of the most significant ecosystem engineers of the ocean. By providing complex structural habitat, and modifying water and particle movement, deep-sea corals support levels biodiversity and biomass that rival those of their shallow-water counterparts and far exceed surrounding deep-sea areas. Baseline knowledge of these habitats, primarily formed by the deep-sea scleractinian Desmophyllum pertusum, has advanced in recent years due to advances in technology and the growing recognition of the importance of these habitats to global ecosystem function. These slow-growing reefs are also highly vulnerable to accelerating anthropogenic threats, including global climate-induced changes in ocean cycling and chemistry, impacts from fisheries, mineral & fuel extraction, and offshore development. Despite this, not enough is understood about the dynamic environmental drivers that support the establishment, maintenance and function of cold-water coral reefs, and the potential avenues to resilience against future changes. To that end, the work presented here sought to fill key scientific gaps and develop a deeper understanding of the physical-ecological relationships within cold-water coral reefs across multiple scales.
Chapter 2 investigated how a regional oceanographic feature, the Gulf Stream, drives environmental conditions in the vicinity of a recently discovered reef and determines how temporal variability and stochastic patterns in these conditions may drive coral distribution. This site existed in a region of high thermal variability, with temperature spikes up to 10.8 °C occurring over a matter of hours, that would be expected to confer significant metabolic and physiological stress to corals. However, the Gulf Stream’s offshore meandering and incursion over the deep continental plateau temporarily enhanced the supply of particulate organic matter to the reefs in the area. Vertical mixing within the Gulf Stream’s frontal eddies and upwelling of a deep, nutrient rich undercurrent both stimulated enhanced primary productivity in the region and mitigated the risk of prolonged thermal stress. This mechanism highlighted the limitations of our understanding of corals’ ecological niche; temporal relationships between key environmental parameters like temperature and food supply have largely been omitted from conversations of environmental tolerances and habitat suitability studies. This concept led to the work that is presented in Chapter 3, in which I formally defined this environmental mechanism to explain the mitigating factors allowing corals to persist in areas of thermal stress.
In Chapter 3, using long-term high-resolution time series of temperature, currents, and particulate supply, I tested for the presence of this mitigating mechanism at 18 separate cold-water coral locations with disparate hydrodynamic regimes habitats across the Atlantic Ocean. This synthesis found periods of elevated temperature had a positive relationship with food supply at the majority of sites, and that this mechanism was stronger for deeper, more food-limited communities. Findings from this chapter strongly imply that cold-water habitats could be resilient to environmental change due to the coupling between stress and ameliorating factors, but that these relationships themselves may be vulnerable to future climatic shifts.
Lastly, the regional and global perspectives from Chapters 2 and 3 were paired with a fine-scale investigation of the interactions between coral structure and benthic currents in Chapter 4. This study used quantitative 3D modeling techniques based on scans of real D. pertusum skeletons and a particle image velocimeter (PIV) to show non-linear effects of coral structural complexity on downstream flow patterns within an experimental flume. This was paired with measurements from acoustic doppler velocimeter (ADV) observations of larger composite coral structures to determine whether relationships between structural complexity and flow moderation were scalable. It was found that complex gradients in velocity, turbulence, and directional stress were generated within 10 cm of the coral fragments, and that factors like length, rugosity, and packing (density of surface area) had strong relationships to turbulence. These results provide important context for ecological processes that are controlled via flow modification, including resuspension of benthic sediments into the boundary layer, particle availability for downstream suspension feeders, and protected habitat for coral associates.
From a single colony to a reef, and even up to a global scale, cold-water corals interact with their environment in complex ways that produce a dynamic mosaic of physical and ecological processes that together make up the cold-water coral niche. The body of work presented in this thesis, explored individual components of the niche interactions, but largely focused on relationships between hydrodynamic processes and the physical environment that controls the distribution of corals. It is clear that the coral niche exists within a system of positive and negative feedback loops between stress inducing factors and those that ameliorate them. The findings from this thesis allows for a broader view of the processes that must be accounted for when predicting coral distribution and the capacity of these habitat forming species to survive under future environmental change.
Recommended Citation
Carrick, Jane V., "PHYSICAL-ENVIRONMENTAL DRIVERS AND HYDRODYNAMIC MECHANISMS THAT DEFINE THE NICHE OF REEF-BUILDING COLDWATER CORALS" (2024). Open Access Dissertations. Paper 1714.
https://digitalcommons.uri.edu/oa_diss/1714
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