Horizontal and vertical copepod distribution and abundance on the Mid-Atlantic Ridge in June 2004

Abstract

Abstract Copepod composition, abundance and distribution were studied in June 2004 on the Mid-Atlantic Ridge, along a transect extending from Iceland (60°N) to the Azores (41°N). The samples were depth-stratified from the surface down to a maximum depth of 2500 m. The transect covered relatively warm and relatively saline Modified North Atlantic Water (MNAW) on the Reykjanes Ridge in the northern part, colder and less-saline Sub-Arctic Intermediate Water (SAIW) further south to the Charlie-Gibbs Fracture Zone (CGFZ), and warm high-salinity North Atlantic Central Water (NACW) in the southern part. In total, 68 copepod genera and 117 species were identified. Calanoid copepods dominated the generic richness with 57 genera. There was a clear increase in the number of genera southwards, from 30 in the northern region to 57 in the south. The total copepod number was variable between stations, ranging from 45,000 to 178,000 m−2; however, the variability did not show any clear north–south trend, and the lowest abundance was observed close to the Sub-Polar Front, located near the CGFZ. Cluster analysis divided the transect into three main regions as based on copepod community structure: one in the northern part (containing MNAW and SAIW) where Calanus finmarchicus and Pareuchaea tonsa characterized the region; one in the Frontal Region (SAIW/NACW) where C. hyperboreus was more abundant than in the other two regions and one southern region (containing NACW), characterized by several species, e.g., Calanus helgolandicus, Mecynocera clausi and Pleuromamma gracialis. Vertically, the copepods mainly occurred in the upper part of the water column. In total, 53% of the individuals occurred above 100 m and 75% above 500 m depth. Below 1000 m depth, only low numbers were recorded (∼10% of the total copepod abundance). There were large differences in vertical distribution between genera/species, with M. clausi, Acartia spp. and Pseudocalanus spp. showing the shallowest distribution and Oncaea spp., Pleuromamma spp. and Microcalanus spp. the deepest distribution. Depth distribution, however, was also affected by hydrographic differences between stations. In the deepest samples, there was a tendency for a higher total abundance close to the bottom than at greater distances from bottom.