Vertical distribution and population structure of copepods along the northern Mid-Atlantic Ridge

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The Mid-Atlantic Ridge (MAR) between 40°N (Azores) and 63°N (Iceland) is the largest topographic feature in the North Atlantic Ocean. Despite generally limited surface production, there is evidence that the mid-ocean ridges are ecologically important for higher trophic levels relative to the surrounding open ocean. Vertical migrations of zooplankton are one of the primary mechanisms for the vertical transfer of carbon from surface waters to the deeper waters and sediments. The complicated topography of the MAR influences local and regional circulation patterns, which in turn are likely to affect the distribution of the zooplankton fauna. The crest of the MAR rises to 1000 m, thus intersecting the meso- and bathypelagic layers. In this paper we explore the vertical distribution and population structure of selected copepod species on the northern MAR, with the goal of better understanding the nature of the interactions between zooplankton and a mid-ocean ridge system. Zooplankton were sampled on the ridge from Iceland to the Azores (~60-41°N, 25-35°W) in June 2004. Depth stratified sampling revealed information on vertical distributions from surface down to 2500 m. The Subpolar Front (SPF) is the major biogeographic boundary in the studied area. Species with a wide vertical range also had a wide geographical distribution, occurring both north and south of the SPF. Several species were observed to change their vertical distributions along the transect, becoming deeper on the southern stations. Factors influencing vertical distributions are evaluated and relationships between zooplankton, water masses, and ridge topography are discussed.