Three water masses occur within a radius of 120 nautical miles off Cape Hatteras, North Carolina. Several species of shell-bearing pteropods characterize these water masses: subarctic Limacina retroversa in the slope water, subtropical Limacina inflata in the Sargasso
Sea, and tropical Limacina trochiformis and Creseis virgula conica in the Gulf Stream. The pattern of vertical distribution of pteropod species is compatible with the seasonal structure of the water. In summer, a thin, warm surface layer bearing tropical species of pteropods
overlies thick, cold, slope water carrying subarctic species in the region of 35°30′N to 37°10′N. In winter, the slope water surfaces at about 36°N. The sharp gradient exhibited by the vertical slope of isotherms acts as a temperature barrier separating subarctic from
tropical species of pteropods. Pteropod shells are found in sediments beneath the Gulf Stream at depths of about 400-2150 m. Creseis virgula conica is the dominant species in the sediments.
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