A new technique pioneered by scientists from the Plymouth marine laboratory in the UK is uncovering the secrets of life in our oceans.
The Mesopelagic zone is the area between 100 and 1000m deep in the oceans, and comprises one of the largest ecosystems on Earth. It was thought that the primary source of nutrients for this ecosystem was a ‘rain’ of sinking organic carbon and materials from the upper layers of the ocean. However, marine scientists realised that this source was not enough to fuel the enormous biomass of the Mesopelagic ecosystem.
Using a combination of satellite images of ocean colour and in-situ floats, researchers discovered a process of seasonal mixing that circulates organic matter from surface waters into the deeper realms of the Mesopelagic.
Spring storms and wind mix surface waters with organic carbon and carry particles too small to sink and dissolved carbon deeper into the ocean. In summer a mixed layer sits at the surface, trapping the deeply mixed carbon inside the Mesopelagic region. It was found that at high latitudes this process supplies an average of 23% of the flux of sinking food sources, although it can be greater than 100% in some instances. The research team estimates that globally this ‘seasonal mixed-layer pump’ moves around 300 million tonnes of carbon into the Mesopelagic zone each year.
“Most methods for measuring carbon transport into the deep ocean have concentrated on the particles that sink at relatively fast rates, but have not measured how neutrally buoyant or slowly sinking organic particles are redistributed through the water column” said lead researcher Giorgio Dall’olmo. “Current global estimates of carbon export in the ocean are missing the contribution of the seasonal mixed-layer pump”.
While it was previously known that variations in the mixed surface layer could provide organic carbon to the Mesopelagic, this is the first effort to estimate the total amount of organic carbon supplied in this way. This provides quantification of a major additional flux of organic carbon to the Mesopelagic that was previously unaccounted for.
More: Satellites help understand what fuels the twilight zone, European Space Agency