The spatial and temporal abundance patterns of chaetognaths in the western North Atlantic Ocean
Citable URI
https://hdl.handle.net/1912/2759Location
Western North Atlantic OceanDOI
10.1575/1912/2759Keyword
Chaetognatha; Chain (Ship : 1958-) Cruise CH125; Knorr (Ship : 1970-) Cruise KN53; Knorr (Ship : 1970-) Cruise KN62; Knorr (Ship : 1970-) Cruise KN65; Knorr (Ship : 1970-) Cruise KN71; Endeavor (Ship: 1976-) Cruise EN11Abstract
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.
Description
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution August 1982
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Suggested Citation
Thesis: Cheney, Jerry, "The spatial and temporal abundance patterns of chaetognaths in the western North Atlantic Ocean", 1982-08, DOI:10.1575/1912/2759, https://hdl.handle.net/1912/2759Related items
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