The spatial and temporal abundance patterns of chaetognaths in the western North Atlantic Ocean

Abstract

Zooplankton samples were collected with the MOCNESS (Multiple Opening/ Closing Net and Environmental Sensing System) on six cruises in the western North Atlantic Ocean during the period from August 1975 to November 1977 as part of the study of the biology, physics, and chemistry of Gulf Stream cold core rings. The physical, chemical and biological properties of the Slope Water to the North of the Gulf Stream differ substantially from the properties of the Northern Sargasso Sea to the south. The purpose of this thesis was to identify the horizontal, vertical and seasonal abundance patterns of chaetognaths in this region and to relate these patterns to environmental conditions. Twenty one chaetognath species were identified, of which eighteen were abundant enough for analysis of the spatial and temporal abundance patterns. The most important sources of sampling error in this study were mesh selection and avoidance, which bias the abundance estimates, and patchiness and subsampling, which add variability to the abundance estimates. The 0.333 mm mesh of the MOCNESS does not appear to sample individuals of any of the species less than 6 mm in length very well. Avoidance appears to be a problem only for the larger individuals (>20 mm) of the larger species. Variability due to small scale patchiness was less for this study than for many previous studies, and this may have been due either to the location of the tows in presumably less variable open ocean regions or to the increased volume of water filtered relative to previous studies. Subsampling with the Folsom plankton sample splitter introduced more variability than predicted by the binomial distribution, and for some species the subsampling variability was greater than that due to patchiness. In spite of these sources of variability, significant spatial and temporal abundance patterns were detectable for most of the species in this region. Vertical distributions of chaetognaths in this region were similar to those reported for these species from other regions. Nine species were classified as epipelagic, four were classified as mesopelagic, and five species were classified as bathypelagic. Diel vertical migrations were not detected for any of the species, but due to the depth intervals sampled, migrations of less than 100 m were not detectable. Ontogenetic migrations were detected for the seven meso- and bathypelagic species for which these migrations were examined. The epi- and bathypelagic species were shallower in the Slope Water than in the Northern Sargasso Sea, suggesting that they were orienting their vertical distributions to temperature. The mesopelagic species showed little difference in vertical distribution between the two hydrographic regions. For Sagitta lyra, the only species for which size frequency data was collected from all the tows, the vertical distribution as measured by median population depth was found to be significantly correlated with average individual size. This suggests that observed differences in vertical distribution for the meso- and bathypelagic species at different times and places in the oceans may be due to changes in the size structure of the population coupled with ontogenetic migrations rather than to responses to differing environmental conditions. It may be possible to correlate the vertical distributions of size or sexual maturity classes with environmental variables such as temperature and light. The differences in hydrography between the Slope Water and the Northern Sargasso Sea were also reflected in the species abundances. Seven species were significantly more abundant in the Slope Water, nine were significantly more abundant in the Northern Sargasso Sea, and two species showed no significant differences between the two regions. Recurrent group analysis did not delineate these groupings, and it is suggested that quantitative methods employing abundance estimates be used to detect associations among species. Ordination analysis (e.g. factor analysis and correspondence analysis) was found to delineate the species groupings adequately. The hydrographic differences in abundance for these chaetognaths were most likely related to differences in food abundance and temperature structure of the water column between the Slope Water and Northern Sargasso Sea. Seasonal abundance differences were not detected for the seven Slope Water species, yet seasonal abundance differences are expected in such temperate regions. Most of the Northern Sargasso Sea species were most abundant in the spring, and this was presumably related to the late winter-early spring peak in primary productivity and zooplankton biomass in this region. The spatial and temporal abundance patterns of chaetognaths in Gulf Stream cold core rings were also studied. The patterns were similar to predictions made on the basis of their abundance differences between the Slope Water and Northern Sargasso Sea, but a few species showed anamolous patterns. Further studies of the chaetognath fauna of both warm and cold Core rings should provide invaluable insight into the processes regulating the spatial and temporal abundance patterns of these organisms.