Spatial and temporal abundance patterns of oceanic chaetognaths in the western North Atlantic—II. Vertical distributions and migrations

Abstract

Vertically stratified zooplankton tows were collected with the MOCNESS in the upper 1000 m of the western North Atlantic during the period from August 1975 to November 1977. Vertical distributions of 18 abundant chaetognath species were studied from 18 slope water (SW), 18 northern Sargasso Sea (NSS) and two Gulf Stream (GS) tows. Krohnitta pacifica, Pterosagitta draco, Sagitta bipunctata, S. enflata, S. helenae, S. hexaptera, S. minima, S. serratodentata and S. tasmanica were classified as epipelagic species, Eukrohnia hamata, Krohnitta subtilis, Sagitta decipiens, S. lyra, S. maxima and S. planctonis were classified as mesopelagic species, and Eukrohnia bathypelagica, E. fowleri and Sagitta macrocephala were classified as bathypelagic species. Nine species were significantly shallower in the SW relative to the NSS, while K. subtilis and S. lyra were significantly shallower in the NSS. Few seasonal differences were detected in either the SW or NSS for any of these species, and vertical distributions in the two GS tows are similar to those found in the SW and NSS. No diel vertical migrations were detected for any of these species, but ontogenetic vertical migrations were detected for several of the meso- and bathypelagic chaetognaths. That many species occur at shallower depths in the SW is consistent with the hypothesis that they follow isotherms in selecting the depths at which they live. E. hamata is the only species living at approximately the same temperature in the SW and NSS, however; the remaining species live at higher temperatures in the NSS. Other factors, such as pressure and light, may be important in determining the species' vertical distributions. S. lyra occupies greater depths in the SW relative to the NSS because its SW population consists mostly of larger, deep-living individuals, while its NSS population is dominated by younger, shallow-living individuals. It is possible that many of the regional differences in vertical distribution exhibited by the other species are due also to regional differences in population stage structure coupled with ontogenetic migrations.