A light in the dark: using cabled seafloor observatories to study abundance and behaviour of seafloor megafauna in response to environmental change

Command, Rylan J. (2022) A light in the dark: using cabled seafloor observatories to study abundance and behaviour of seafloor megafauna in response to environmental change. Masters thesis, Memorial University of Newfoundland.

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Abstract

Surface primary productivity forms the base of most marine food webs and contributes significantly to global carbon cycling, providing a key link from surface to seafloor. High seasonal primary productivity along temperate latitude coastlines provides crucial nutrients for seafloor communities, driving spatiotemporal patterns in abundance, behaviour, biodiversity, and distribution of benthic megafauna. Many factors, including ocean warming, deoxygenation, and increasing frequency and duration of marine heatwaves (MHW), may alter the dynamics governing primary production, threatening benthic organisms that depend on the seasonal input of phytodetritus for food. The focus of this thesis is to make use of two seafloor observatories, NEPTUNE near Vancouver Island, British Columbia, and a new platform in Conception Bay, NL, to examine variability in abundance, behaviour, and composition of seafloor communities in response to environmental change across temporal scales. First, the response of the deep-sea pink urchin Strongylocentrotus fragilis to a recent MHW on the continental margin off the coast of Vancouver Island was investigated using a combination of benthic trawls (2004-2018) and seafloor observatory data (2013-2020). Sea urchin density declined during the MHW, likely in response to reduced kelp subsidies from coastal waters. Next, the new Holyrood Underwater Observatory in Conception Bay was used to study benthic community response to the spring phytoplankton bloom. High-frequency variability in seafloor environmental dynamics was documented during the winter-spring transition, and the unexpected emergence of >200 sea cucumbers (Psolus sp.) coinciding with the arrival of phytodetritus at the seafloor was observed. These data will provide a baseline against which to monitor changes in phenology as climate change progresses. This work comes at a critical point in ocean observing as we approach global climate tipping points. Now more than ever, it is essential to document the current state of marine communities to understand and predict community responses to changing ocean conditions, and to sustainably manage ocean resources.

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