The Importance of Biodiversity in Deep-Sea Chemosynthetic Ecosystems: An Assessment of Ecosystem Functioning and Services

Abstract

As demand for deep-sea resources increases, understanding the non-extractive use values and ecosystem services associated with the deep sea is crucial. A broader understanding of the benefits provided by deep-sea ecosystems will demonstrate why humanity should have a vested interest in the health of the deep sea, and encourage decision makers to enact sustainable environmental management plans. This dissertation contributes to the literature connecting deep-sea ecosystem structures, functions and services, with a focus on hydrothermal vents and methane seeps. Part 1 uses a repetitive survey technique to explore the views of deep-sea stakeholders on the importance of rare-species research and the ecological importance of rare species. Overall, participants called for research that addresses deep-sea biodiversity as a whole (i.e., both rare and abundant species) but opposing opinions existed regarding the ecological importance of rare species. Opinion ranged from the acknowledgment that rare species may play important roles now or in the future to the viewpoint that, by definition, rare species play minor roles in the ecosystem. The opposing opinions likely correspond to a lack of knowledge concerning the role of rare deep-sea species in ecosystem functioning. Part 2 uses functional trait analysis to assess the contribution of rare and common species to hydrothermal vent food-web dynamics. Some rare and common species were found to possess individually unique trait combinations and to contribute to hydrothermal vent functional diversity. In addition, trait combinations were found to vary among vent sites within the same biogeographic region, highlighting the ecological heterogeneity of hydrothermal vent systems. Part 2 suggests that both rare and common species should be incorporated into environmental management planning and that, given the uncertainty regarding species distributions and population connectivity, ensuring the persistence of functionally distinct rare species may require all active hydrothermal vents to be protected from deep-sea mining impacts. Part 3 uses phototransects to characterize the epibenthic megafauna at seeps on the US Atlantic margin. Epibenthic megafauna were found to follow depth-related patterns in taxon-abundance, suggesting a faunal boundary exists between shallow (~400m) and deep (~1500m) seeps. Part 3 also uses stable isotope analysis, plankton samples and observational data to explore the importance of seeps to the life history of the commercially valuable deep-sea red crab (Chaceon quinquedens). Chemosynthetically derived carbon was found to contribute up to ~50% of nutrition to individual crabs, although the proportion was variable among individuals and seeps. At Chincoteague seep, dense aggregations (~15 individuals 10m-2) of red crab were observed, alongside fifteen mating pairs, three ovigerous females and numerous zoeal larvae; suggesting some seeps may act as a reproductive hotspot as well as food resource for red crab populations. Part 4 highlights a cultural connection to the deep sea, whereby the Atlantic seabed is considered the final resting place for ~1.8 million enslaved Africans who died during slaving voyages from Africa to the Americas. Part 4 asks that the International Seabed Authority, ahead of mineral extraction from hydrothermal vent deposits on the Mid-Atlantic Ridge, to consider ways to commemorate the Middle Passage in the Area Beyond National Jurisdiction and recognize that cultural artifacts relating to the transatlantic slave trade may be found during mining-related activities.