The Biology of the Surface Zone Zooplankton of a Boreo-Arctic Atlantic Ocean Area

Abstract

Zooplankton samples were taken from the surface zone waters of the central Labrador Sea at International Ocean Station "B" (56° − 30′N., 51° − 00′W.) at weekly intervals throughout one calendar year for the purpose of determining the composition of the zooplankton, the seasonal progression, fluctuations and pertinent physical factors. An attempt was made to correlate the physical and biological factors.Ships of the United States Coast Guard and the Royal Canadian Navy were used as collecting units, and these were provided with nets, samplers and other equipment with which to carry out the shipboard phase of the investigation. Mid-day, 20 minute oblique hauls from 150-0 metres were made with the Clarke-Bumpus plankton sampler using No. 2 and No. 12 mesh silk nets, along with bathythermograph and Secchi disc lowerings. Later in the day, but at least one hour after dark, an open half-metre net 20 minute surface tow was made for additional qualitative data. Bioluminescence records also were obtained at this time.All available physical and biological data pertaining to the area under investigation were examined to provide a basis for comparison.Synopses of the species found in the material are presented individually, including their seasonal occurrence, probable breeding seasons, numerical and economic importance, and values as indicator forms. In all, 61 species in seven phyla, 10 classes and 17 orders are represented.The zooplankton population of the surface zone in the central Labrador Sea is characterized by relatively few species, but at the same time by large numbers of individuals. The plankton consists mainly of Calanus finmarchicus, Thysanoessa longicaudata, Oithona atlantica, Scolecithricella minor, Oikopleura labradoriensis and Eukrohnia hamata. The remainder of the species assume comparatively minor importance in the economy of the region.The main augmentation season was found to occur generally in late summer, although mid-winter breeding was noted for a few animals. The zooplankton of this region was found to be distinctive, being consistently oceanic, but neither true arctic nor true boreal. Definite biological evidence was obtained that indicated the influx of boreal-subtropical surface waters into the area in winter.Extinction coefficients determined from white Secchi disc readings are graphed, as are comparisons between black and white disc readings. It was concluded that the great variations of transparency were caused by fluctuations in the amount and kinds of phytoplankton and detritus in the water, and that the zooplankton density had little effect on water clarity.No direct correlation was noted between the occurrence of bioluminescence and individuin species or planktonic groups, but it was found that the greatest bioluminescence was observed al the summer and fall.