Deep connections: How do changes in deep sea faunal communities relate to surface physics and biology?

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Predicting the impact and effects of changes on life in the oceans is of growing concern to marine scientists. Warming of the oceans and associated changes in the chemistry of sea water are expected to cause major shifts in abundance, diversity, and distribution of marine organisms, with possible consequences for natural capital and the capacity of the ocean to absorb carbon dioxide and other waste materials produced by humans.

The deep-sea is the largest ecosystem on earth, covering ~60% of the surface. It is home to the greatest biodiversity on the planet, with >220,000 marine species described and ~750,000 yet to be discovered. Understanding the role of biotic and abiotic parameters in driving deep-sea biodiversity is critical if we are to comprehend and predict how the ecosystem responds to change¹. Primary productivity (PP), and the flux of particulate organic material (POM) to the seafloor, play a significant role in structuring deep-sea faunal communities². Currently deep-sea environments face multiple stressors e.g., changes in food quantity and quality, temperature. Long-term time-series from deep-sea stations are rare, yet crucial to disentangle background “noise” of short-term natural fluctuations from long-term natural trends that drive changes in benthic faunal communities. Presently it is unclear how future changes in PP may affect deep-sea community structure. Predicting the response of deep-sea communities to climate-driven shifts in PP is imperative given the importance of this environment to global bioabundance, biodiversity and ecosystem function. This project will use samples collected from the mid-1970s through to 2016 (no samples collected post 2016) to examine what changes have occurred in the benthic faunal community at species and functional diversity levels.

Overarching aim: to quantify the extent of change in a deep-sea benthic community over a period spanning four decades – the longest benthic bathyal time series available globally, and to determine whether observed changes are linked with shifts in biotic (e.g., PP) and abiotic (e.g., temperature) variables.

The work proposed here is the first of its kind at these depths and is important given known changes in productivity and water temperature and the impact these may have on the benthic ecosystem.

The degree of change in bathyal continental margin ecosystems over multi-decadal timescales is poorly understood. Time-series data from bathyal (~200–4000m) sampling stations experiencing varying POM flux are sparse³ and contrasting theories exist regarding the relationships between deep-sea diversity and POM flux. Globally, at latitudes between 30-50°, the “diversity-energy theory” supports a positive linear relationship, with some fauna reaching maximum species richness in regions of high export flux. However, regionally it has been proposed that species diversity peaks at intermediate levels of food input. The relationship between regional surface productivity and deep-sea benthic communities remains unclear at a bioabundance level as well as at a species/functional level. Changes in megafaunal species density appear to be driven by inter-annual variation in POM flux to the seafloor at abyssal (>4000m) depths⁴; which is intimately linked to variations in surface PP and ultimately to large-scale climatic variation. However, these links have yet to be observed at bathyal depths and is a major knowledge gap. Compared to lower latitudes, our study site in the Rockall Trough (57.3°N), is predicted to have relatively low diversity for comparably high surface productivity.

The Extended Ellett Line (EEL) hydrographic section is one of the few places where long-term records of full ocean-depth temperature and salinity occur at the same temporal-spatial scale of benthic community sampling. The numerous water masses flowing through the UK’s northern deep-water territory⁵ are known to have a dramatic effect on benthic communities in this region, specifically the changes in water temperature. Bett et al.⁶ indicated links between the distribution of biological communities in the deep Rockall Trough and the occurrence of Labrador Sea Water. This water mass exhibits long-term trends in its characteristics thought to result from intermittent supply from its source⁷. This variation may link to North Atlantic-scale climatic conditions and act as an additional vector for temporal changes in deep-sea biological communities. Data collected from the EEL hydrographic time-series will provide great insight into temporal variation in the surrounding deep-sea environment.

Good Environmental Status (GES) is not being reached for many benthic marine habitats, particularly those in the deep-sea. The datasets generated by this research will allow for GES to be assessed of the soft sediment deep-water environment in the Rockall Trough region of the NE Atlantic. It is therefore vital that temporal variability of deep-water sedimentary habitats is understood and that the following scientific issues are addressed: (a) the impact of changes in physical conditions (primarily temperature) on quantity and nutritional quality of surface PP, (b) the impact of variations in quantity/quality/timing of food supply to the benthos on macrofaunal ecology and biodiversity, (c) how future climate-driven changes in physical forcing conditions will impact on GES of deep-sea benthic macrofaunal communities in the Rockall Trough region.

The start date of this project is: 2 October 2023

The 3½ year studentships cover:

• Tuition fees each year at Home (UK) rate. For International students, there may be funding available to cover the full international tuition fee and this will be discussed at interview.
• A maintenance grant each of around £15,000 per annum (for full-time study)
• Funding for research training
• Part-time study is an option, with a minimum of 50% of full-time effort being required.

Applicants should normally have, or be studying for:

• A postgraduate Master’s degree from a degree-awarding body recognised by the UK government, or equivalent, or
• A first or upper-second-class honours degree from a degree awarding body recognised by the UK government, or equivalent, or
• Other qualifications or experience that affords sufficient evidence of an applicant’s ability to work at the academic level associated with doctoral study.

We are looking for a student that is highly motivated, preferably has some faunal identification experience or is keen to learn the process of faunal identification. Interest in oceanography and primary productivity will be important. The student should also be willing to spend some time with the different partners

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