The ocean plays a major role in regulating the amount of carbon dioxide in the atmosphere. Carbon dioxide can be dissolved in seawater and carried to depth by ocean currents (the solubility carbon pump) but it can also be carried there having entered the marine food web (the biological carbon pump). The total growth of the tiny phytoplankton of the ocean each year rivals that of all land plants. Some of this new organic material sinks directly to depth. Other organisms also consume these phytoplankton (and each other) creating many pathways by which previously atmospheric carbon is sequestered deep in the ocean.
However, there remains considerable uncertainty over the relative importance of the different routes, or fluxes, of carbon from the ocean surface to the abyss transported as organic material. One of the aims of SUMMER is to reduce this uncertainty. To do so it will focus on measuring the magnitude of three key fluxes that form part of the biological carbon pump.
The gravitational flux involves the sinking material. This is not just dead phytoplankton but also includes faecal pellets from organisms such as fish and crustaceans, sometimes all coalescing into aggregates called marine snow. The DOC flux arises from the release of dissolved organic carbon (DOC) by marine organisms which is then transported to depth through ocean mixing and turbulence. The active flux is due to the regular daily migration of many fish and zooplankton between the surface and depths of 1000m or more, described as the largest animal migration on earth. The reason for the migration, with the organisms rising at dusk and descending at dawn, is thought to be a balance between avoiding being seen by predators while still needing to visit the surface in search of food.
Although each of the three fluxes has been measured individually, we still lack a clear picture of the relative magnitudes of the three and how this might vary with the environment. To tackle this, within Work Package 4 of SUMMER we plan to carry out simultaneous observations of the three fluxes in contrasting locations. In particular we want to determine the importance of the active flux due to migrating mesopelagic fish. If significant, then the service they are doing us by transferring atmospheric carbon to the deep ocean needs to be balanced with any short term gain obtained from fishing them for food or other products which may diminish their ability to pump carbon.
As with many areas of life and science, the Covid-19 pandemic is making this difficult. Scientific cruises are usually multi-national affairs and current travel restrictions are making previously simple tasks such as getting scientists to a research vessel safely much more challenging. Nevertheless, our first cruise is due to leave Barcelona this autumn, looping clockwise round Spain and Portugal to finish at Vigo in Spain. En route, our team will hopefully deploy equipment ranging from high-tech optical sensors to good old-fashioned nets, allowing us to quantify the three fluxes and begin assessing the importance of fish in pumping carbon at last.
Written by: Adrian P. Martin, United Kingdom Research and Innovation/The National Oceanography Centre (NOC)
Photo: The biological carbon pump (Turner 2015). https://www.sciencedirect.com/science/article/abs/pii/S0079661114001281