Marine biology from space

Who: Olav R. Godø, Annette Samuelsen, Gavin J. Macaulay, Ruben Patel, Solfrid Sætre Hjøllo, John Horne, Stein Kaartvedt and Johnny A. Johannessen
What: Mesoscale eddies are oases for higher trophic marine life.
Where: Plos One

Large oceanic eddies or vortices make rich oceanic oasis for phytoplankton and small zooplankton. Using satellite images and advanced echo sounding techniques, the biomass in the eddy can be mapped in detail. They reveal that fish higher up in the food chain also take advantage of the plentiful and easily available food along the eddy’s edges.

The ocean may seem like a never-ending, dark, monotonous surface, but the ocean waters are transported, structured and mixed by the ocean currents and their interaction with the sea floor. A large number of phenomena occur in the ocean, one of these are so-called mesoscale eddies (mesoscale: of medium horizontal range - from ten to 100 km). In Norwegian waters they may be formed when the northernmost extension of the Gulf Stream changes direction as it hits the continental shelf. The sudden change in direction, leads to instabilities and vortices, similar to a backwater in a river. The eddies have a diameter of up to 100 kilometres and can reach more than 1000 meters down. They may develop and intensity before they gradually loose their energy and can last from a few days to several months.

The figure shows the surface distribution of phytoplankton in the Faroe-Shetland channel in 2008, the phytoplankton concentration differ between the centre and the periphery of the eddy and thus reveal their location.

The vortices contribute to the pumping of nutrients from the deep, dark waters into the well-lit surface waters and stimulate the phytoplankton growth. They are apparently important for the biomass production in Norwegian waters, but we know little about how they influence the production of plankton and fish. Up till recently eddies were surveyed using traditional oceanographic methods, measuring the temperature and salinity and collecting plankton and fish using nets and trawls. This approach is very time-consuming and gives only a partial picture of the eddy and the marine life connected to it.

By combining satellite-based technology and sensitive acoustic measurements (echo sounder) it is now possible to obtain a 3 dimensional image of the eddies. In addition we can obtain new quantitative information on how the eddies influence the biological production in the ocean. A mesoscale eddy makes an imprint on the ocean surface. The impression is so strong that it is visible in satellite images. When the scientists discover an eddy in the area that is being surveyed, a cruise-track can be planned based on the location and extent of the eddy. Sensitive acoustic measurements reveal the distribution of biomass at different depth in the eddies. These measurements make it possible to visualize the physical and biological properties of the eddy. In this way the scientists can “see” the eddy, its shape and the amount of marine life connected to it. Analysis of the date have revealed that the higher trophic levels (mostly fish) are concentrated and structured along the edge of the eddy. For example, a large abundance of blue whiting (identified by trawl) was found along the periphery of an eddy. This species, that normally forage at 300-500 meters, was found feasting on large concentrations on krill found close to the surface. The survey showed that there was a close relationship between the distribution of biomass and the eddy’s physical structure.

We suppose that mesoscale eddies function as local oasis in the nutrient-poor open ocean and that larger fish can swim between such eddies in the search for food. The availability of concentrated prey is particularly important for the survival of fish larvae that can only swim a few centimetres. Mesoscale eddies may play a crucial part in the survival of larvae because they provide locations with large concentrations of food, although at this point the link between eddies and larvae is only speculation.

By studying the distribution of biomass in the mesoscale eddies we can improve out understanding of how larvae of commercial fish species can find sufficient concentrations of food in the first months after the hatching. In this way we can get a better foundation for early calculating of year class strength. The survival of larva is the key to understanding the recruitment mechanisms in our most important fish species. This knowledge is central to the long-term projections of stocks and quotas.