Mid-ocean mystery explained

Mid-ocean mystery explained

 

PML's remote sensing scientists provide clues to explain apparent richness of the Mid-Atlantic Ridge.

Amongst the most biologically productive areas of the ocean are the coastal shallow seas where a combination of high light levels and rapid recycling of nutrients from the seafloor, often enriched from land-based sources, provides ideal conditions for plankton growth - the basis of most ocean food chains supporting higher animals. In the open ocean, where nutrients are restricted, much of this primary production is transported down where it supports sea life through the water column to the very deep, abyssal, seafloor.

Yet, with increasing distance from land and greater depth the quantity of benthic life on the deep ocean seabed dwindles to less than one percent of that in coastal areas. However in some areas, such as where seamounts occur, or along the Mid Atlantic Ridge, theInfluence of MAR on overlying water column: Seasonal oceanic front frequency map- % of time a strong front was observed at each location, Spring (Mar-May), from merged microwave/infrared SST data 2007-2009. Blue arrows-inferred paths of branches of North Atlantic Current (NAC) delineating sub-polar front.

Frontal features related to bathymetry: B–Rockall Bank, C-Iceland-Greenland branch of NAC, H-deeper hollow in seabed, M-recurring meander, N-near to MAR, PAP-Porcupine Abyssal Plain, PSB-Porcupine Seabight, R-Reykjanes Ridge shallower water has the potential to raise the quantities of sea life, so bucking the accepted trend. Indeed such areas are attractive to a wide range of marine life and have become the focus of fisheries which exploit enriched fish assemblages, and the accepted view has been that these raised areas are ‘hotspots’ of productivity and biodiversity.

So is the Mid-Atlantic Ridge more productive than the deeper seas that surround it? Knowing the answer to this is crucial to an understanding of how oceans work, and facing increasing pressures, how they can be managed. At PML remote sensing scientists and experts in ocean productivity have played key roles in understanding what exactly is going on far out in the Atlantic Ocean. In a remarkable example of scientific collaboration, the UK ECOMAR project operated within an international team of 37 scientists in 16 research institutes from 8 countries to bring a wide range of expertise to bear on trying to answer the question.

PML’s contribution provided a unique insight into how surface currents and fronts are influenced by the Mid-Atlantic Ridge, several thousand metres beneath the surface, while others from the Remote Sensing Group at PML provided crucial data on plankton productivity in this area which showed that the Mid-Atlantic Ridge was no more productive than the seas that surround it. Dr Peter Miller added: “Our research on ocean fronts at the Mid-Atlantic Ridge has also generated unexpected outcomes, for example in helping to explain seabird and cetacean foraging zones and to define marine protected areas.”

Dr Gavin Tilstone, also from PML, showed from his research how important the sub-polar front is to the total productivity of the North Atlantic and how this area enhances potential food available to the deep sea. The international team, led by Professor Monty Priede, Director of Aberdeen University’s Ocean Lab, concluded that while the Mid-Atlantic Ridge appears to increase productivity in the area it is really only replacing and concentrating what would be available in the water column if the ridge was not there.

This has important implications for the way we might take advantage of the apparent riches of the Mid-Atlantic Ridge. With marine life, such as corals, crabs, starfish, sea cucumbers, crabs and fish crowded on the slopes of the Mid-Atlantic Ridge, rather than spread through the water column, caution is necessary when it comes to fishing – the Mid-Atlantic Ridge is no richer than its surroundings and so any exploitation will require careful management.

 

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