December 08, 2017 - Tracey T. SuttonBiogeography OBIS data

Illuminating the Twilight Zone - expert panel maps the world's mesopelagic zone

The mesopelagic, or “twilight” zone (open ocean waters between 200 – 1000 m depth), is the world’s second-largest cumulative ecosystem, trailing only the bathypelagic zone (waters > 1000 m depth). In this zone there is not enough sunlight to support photosynthesis (i.e., less than 1% of surface irradiance), but enough light that animals can detect the difference between night and day. The importance of deep-pelagic ecosystems in global ecosystem functioning, such as carbon cycling, is widely acknowledged, but poorly understood. To date less than 1% of this habitat has been sampled, hampering statistical approaches to map its inhabitants on a global scale. A recent paper provides a synthesis of what is known about the distribution of life in the Twilight Zone. Experts integrated available biological data with physical oceanographic spatial modelling to present a biogeographic classification of this massive ecosystem. Thirty-three global ecoregions were identified, of which 20 were truly oceanic, while 13 were ‘distant neritic.’ Each ecoregion harbors a characteristic combination of organisms, with ‘boundaries’ between ecoregions being more like gradients than sharp discontinuities. Each ecoregion is driven by a complex of driving factors, but some of the most important are phytoplankton production in the overlying waters, the presence or absence of oxygen minimum strata, upwelling, and water column stratification. Much work needs to be done to produce a truly dynamic mesopelagic biogeography – huge sections of the World Ocean are still unsampled, and seasonal sampling is rare in all but a few locations. As resource extraction from the deep increases, so too does the need for baseline information to assess human impacts. The proposed mesopelagic classification addresses a standing data gap in global ocean management and conservation efforts.

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