Diversity of the Deep

Previous work has characterized diversity gradients in terrestrial and shallow-water system. Are these previously described diversity gradients also applicable to hard-substrate features in the deep sea?

Above: 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).

Investigation into the distribution of life on our planet and the co-existence of different species has fascinated naturalists for centuries. Early work in the 19th and 20th centuries began characterising patterns of where life is found, focusing on terrestrial flora and fauna. This was followed in more recent times by the characterisation of diversity gradients of shallow water marine ecosystems. However, the deep sea, that’s waters with depths of 200 m and below, remains understudied in this field, largely due to the extreme technological, logistical, and financial challenges associated with studying such a remote area.

Cover image article: (Free to read online for two years.)
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 https://doi.org/10.1111/jbi.14355.

In the 1960s and 70s, several studies investigated the latitudinal and bathymetric (depth) diversity gradients of various groups of deep-sea taxa. Results were extremely variable, with some groups displaying increases in diversity polewards and with depth, others a decline in diversity polewards and with depth, and some simply displayed no significant diversity gradients. This said, when considering all the studies, ‘traditional’ diversity gradients in the deep sea may be described as a bathymetric gradient that sees diversity increase from the surface to the mid-bathyal depths between 1,000 and 3,000 m, after which it decreases; and a latitudinal gradient that sees highest diversity in the temperate latitudes of each hemisphere, with lowest diversity levels at the equator and poles. Whilst these broad generalisations are useful to describe the distribution of life in the deep sea, there is one crucial flaw – most of the samples used to characterise these gradients come from soft-sediment ecosystems on continental shelves and slopes. Therefore the key question is, are previously described diversity gradients applicable to hard-substrate features in the deep sea?

Seamounts and other features provide hard substrate in an otherwise soft-substrate deep sea, and thanks to their complex hydrodynamic and productivity regimes, they often host diverse, important, and protected ecosystems. These may include cold-water coral reefs stretching for kilometres and hosting commercially important fish species, or sponge aggregations home to undiscovered novel molecules that may significantly benefit humankind. Whilst some seamounts are reasonably well-studied, others remain entirely unsampled. In order to support effective science-based decision making and ensure their sustainable management, we need a broad-scale understanding of how diversity is distributed across seamounts and other hard-substrate features, as drivers of the ‘traditional’ gradients may not apply to these unique features.

Example gradients reported for terrestrial and shallow-water marine systems: do they also characterize the fauna of hard substrate habitats in the deep sea?

In this study, we compiled image datasets from nine different seamounts and oceanic islands, collected during four research cruises to the UK Overseas Territory of Saint Helena, Ascension and Tristan da Cunha in the South Atlantic. These features span 32 degrees of latitude, with data points spanning 700 m along the flanks and therefore this study represents the most comprehensive, broad-scale analysis of diversity gradients of hard-substrate ecosystems in the deep sea. We used regression modelling approaches to investigate the presence and trends in both α- and β-diversity.

We found that the ‘traditional’ latitudinal gradient (parabolic per hemisphere) was detectable across these features in the South Atlantic, a key driver of which was the increased productivity seen in temperate regions thanks to nutrient-rich frontal zones. But, when it came to bathymetric diversity gradients, we found no reliable relationship between depth and α-diversity (i.e. no peak in diversity at any point). However, having decided to investigate bathymetric β-diversity, we discovered a significant gradient in the form of turnover with depth, with this also being most pronounced (i.e. most rapid change) in temperate latitudes. This effectively means that although the number of species at each depth is not significantly changing, the identities of those species are, and this is likely one of the reasons that seamounts have previously been described as ‘oases of biodiversity’.

The application of ecological ‘rules’ from one type of ecosystem to another is something that should not be undertaken lightly. We evidence this by demonstrating that although the ‘traditional’ latitudinal diversity gradient appears to hold true for hard-substrate ecosystems, the bathymetric gradient does not. Should management decisions be made when an incorrect assumption has been made, it opens the door to potentially devastating damage to many of these fragile ecosystems. Additionally, the difference in the α- and β-diversity gradients with depth demonstrate the importance of considering multiple metrics when investigating diversity.

Written by:
Amelia Bridges
School of Biological and Marine Sciences, University of Plymouth, Plymouth, UK

Additional information:
Twitter: http://www.twitter.com/Amelia_Bridges
Webpage: http://www.plymouth.ac.uk/staff/amelia-bridges
ResearchGate: http://www.researchgate.net/profile/Amelia-Bridges
LinkedIn: http://www.linkedin.com/in/amelia-bridges-694391108/

Further reading:
Rex, M.A., Etter, R.J. and Stuart, C.T., 1997. Large-scale patterns of species diversity in the deep-sea benthos. In: Marine biodiversity.
McClain, C.R., Lundsten, L., Barry, J. and DeVogelaere, A., 2010. Assemblage structure, but not diversity or density, change with depth on a northeast Pacific seamount. Marine Ecology, 31, pp.14-25.

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