Date Awarded


Document Type


Degree Name

Master of Science (M.Sc.)


Virginia Institute of Marine Science


The Mid-Atlantic Ridge (MAR) is the largest topographic feature in the Atlantic Ocean, yet little is known about the food web structure and trophic ecology of fishes inhabiting mesopelagic waters along the MAR. To better understand the food web structure and to compare the feeding ecology of abundant MAR fishes to those in offridge areas, MAR-ECO, a Mid-Atlantic Ridge ecosystem field project of the Census of Marine Life, sampled the pelagic fauna of the northern MAR (Iceland to the Azores) during June-July, 2004. Samples were collected with two midwater trawls outfitted with multiple opening and closing cod ends to sample discrete depths from the surface to >2300 m at predefined stations within four ridge sections. Fishes of the family Myctophidae were the most abundant fishes collected, with the myctophid Benthosema glaciale the numerically dominant species. The feeding ecology of three myctophid species, B. glaciale, Protomyctophum arcticum, and Hygophum hygomii, were quantified through dietary analysis. Each species was predominantly zooplanktivorous, consuming copepods, euphausiids, ostracods, and other crustacean zooplankton, with each myctophid species exhibiting unique dietary patterns. Different abiotic parameters influenced variability in the diet of B. glaciale and P. arcticum as determined by canonical correspondence analysis. Of the measured parameters, ridge section and time of day were significant explanatory variables in the diet of B. glaciale, while only depth was significant in the diet of P. arcticum. Daily consumption by B. glaciale and P. arcticum was highest at the Reykjanes Ridge and Charlie-Gibbs Fracture Zone, respectively; consumption by H. hygomii was only measured at the Azorean Zone. Daily consumption of all three species was less than 1% of dry body weight. Active transport of carbon out of the euphotic zone by diel vertically migrating MAR myctophids through respiration of CO2, excretion of dissolved organic carbon, and egestion of particulate organic carbon (POC) was also estimated, and compared to passive sinking of POC in the North Atlantic. Active carbon flux by the 0-200 m integrated migrant myctophid biomass, uncorrected for trawl efficiency, ranged from 0.01-0.4% of sinking POC flux at 150 m and 0.02-0.95% at 300 m. If myctophid biomass was corrected for low gear efficiency, flux increased to 0.1-1% and 0.3-3.5%, respectively, of sinking POC. Lower MAR myctophid biomass resulted in lower active carbon transport compared to myctophids in the Pacific and zooplankton in the North Atlantic estimated in previous studies, but myctophid active transport should still be considered in models of MAR carbon cycling. Due to the role of myctophids as both predators of numerous zooplankton taxa and as prey of higher taxa, information on the feeding ecology and daily consumption of these fishes is necessary to accurately assess their role in the MAR food web and the overall trophic structure of this hydrodynamically and topographically unique ecosystem.



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