ECR feature: Amelia Bridges on deep-sea ecological gradients

Amelia Bridges is a postdoc at the University of Plymouth in the UK. She is a marine biologist interested in deep-sea ecology. Here, Amelia shares her recent work on seamount benthic community gradients.

Dr Amelia Bridges presenting her research at the National Marine Aquarium.

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Institute. University of Plymouth

Academic life stage. Postdoc

Major research themes. Benthic ecology, marine spatial planning, fundamental ecology of the deep sea, habitat mapping.

Current study system. The deep sea represents an immensely vast area of our planet, and yet comparatively little is known about ecosystems within it. Throughout my career I’ve been lucky enough to work on a number of deep-sea ecosystems including the Pheronema carpenteri sponge aggregations of the North Atlantic, and the cold-water coral reefs and gardens in the South Atlantic. What I find so interesting and engaging is that we’re still learning about the fundamental ecology of these ecosystems. For example, what functional roles do they have? How do these fit in with the wider biosphere, global cycles and ecosystem services?

Recent JBI paper. Bridges, A. E., Barnes, D. K., Bell, J. B., Ross, R. E. & Howell, K. L. (2022). Depth and latitudinal gradients of diversity in seamount benthic communities. Journal of Biogeography, 49(5), 904-915

The RRS James Clark Ross coming into Jamestown, St Helena (Credit: Nils Piechaud).

Motivation behind this paper. Shifts in diversity over environmental gradients represent one of the most fundamental ecological fields of study, with research dating back centuries. Investigation of diversity gradients in the deep sea began in the 1900s, but samples were only available from soft-substrate ecosystems due to the technologically challenging nature of data collection. Just like in terrestrial and shallow marine ecosystems, we know that different substrate types support different communities in the deep sea. Additionally, certain features that provide hard substrates, such as seamounts, are also exposed to different hydrodynamic regimes that alter key environmental drivers of diversity such as food availability. Here, we wanted to investigate whether diversity gradients described for the deep sea, but hypothesized from soft-substrate data, applied to hard-substrate ecosystems also. Understanding diversity gradients of communities living on hard-substrate is important as seamounts and oceanic islands often serve key ecological roles such as stepping stones for dispersal and providing nursery habitat and refugia.

Key methodologies. We used regression modelling to investigate the relationship between α- and β-diversity, depth and latitude, and other ecologically/biologically relevant parameters correlated with them. Whilst some previous studies have characterised these relationships over individual or small chains of seamounts in close proximity, our data comes from nine different features across a 32-degree latitudinal range, and therefore represents the most broadscale study of such relationships. These data were collected aboard large, ocean-going research vessels equipped with high-resolution camera equipment and sensors collecting environmental data. This approach has allowed us to provide insight on gradients and drivers that are relevant at broad ocean-basin-scales as opposed to regionally, finer-scale relationships. Additionally, the investigation of α- and β-diversity within a single dataset is not often undertaken, but our results show that differences in the two indices are important to consider, particularly through the lens of sustainable management.

Some example seabed images from the cruises around St Helena, Ascension and Tristan da Cunha (Credit: British Antarctic Survey/Centre for Environment, Fisheries and Aquaculture Science).

Unexpected challenges. As many who work in deep-sea science will attest, characterising the diversity of ecosystems from imagery can be particularly challenging. Not only is the initial annotation process manual and slow, but the taxonomic resolution achieved is often below what could be reached if physical specimens were examined using traditional taxonomic approaches like dissection. However, when working with fragile ecosystems like cold-water coral gardens, there is an important balance to strike between non-invasive scientific sampling and taxonomic resolution. This is why we used an Operational Taxonomic Unit (OTU) approach in our study, given that we can rarely identify an individual to species level. Still, we can say that this group of individuals is very similar morphologically. Therefore, an OTU is treated as a species (or morphospecies) in diversity analyses.

Major results. Whilst patterns in the data we collected aligned with previously described parabolic latitudinal diversity gradients per hemisphere in the deep sea, likely driven by productivity regimes, we did not detect shifts in α-diversity gradient caused by depth that are commonly reported from soft-substrate ecosystems. This demonstrates that ecological ‘rules’ based on data from one ecosystem may not be transferable to other ecosystems, particularly where key characteristics such as substrate type are different. Although the number of ‘species’ (OTUs) didn’t change with depth, upon further data exploration, we found that the ‘species’ present across the depth range did vary significantly, aligning with a small number of studies that have hypothesised the high species richness of seamount features in the deep sea derives from a turnover, or change in, species as you progress down the slope into deeper water (β-diversity gradient). The difference in α- and β-diversity gradients observed here shows the importance of considering both metrics when characterising seamounts and determining sustainable management strategies in the future.

Sunrise at sea near Tristan da Cunha (Credit: Nils Piechaud).

Next steps for this research. The next steps would be to conduct similar studies on other seamounts/oceanic island features within the South Atlantic to determine if the identification of key drivers holds true, but also in other ocean basins for the same reason. The global south is extremely understudied compared to the northern hemisphere, and as we have shown, caution needs to be taken when applying ecological ‘rules’ to less studied ecosystems/areas. Through equitable scientific exploration of the global south, I hope we will further understand the fundamental ecology of deep-sea ecosystems, their distribution, and how we can best ensure their sustainable management going forward.

If you could study any organism on Earth, what would it be? I know I’m biased, but I really do love deep-sea ecosystems! I think the fact that we still have so much to learn about them is the most exciting aspect – the only comparison I can think of would be turning the clock back and being a terrestrial ecologist centuries ago. Also, from a management perspective, we still have the chance to ensure robust environmental regulations are in place in the deep sea before mass exploitation happens.

Anything else to add? Although I wasn’t present, during one of the cruises the fibreoptic cable connecting the camera system to the ship snapped (possibly due to a shark bite!) leaving the camera system totally unconnected on the seafloor, hundreds of metres below the surface. Luckily, thanks to the clever work of Captain and crew, it was retrieved… on the first attempt!

Published by jbiogeography

Contributing to the growth and societal relevance of the discipline of biogeography through dissemination of biogeographical research.

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