Day/night vertical distribution of euphausiids in the eastern tropical Pacific |
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Authors: | D Sameoto L Guglielmo M K Lewis |
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Institution: | (1) Department of Fisheries and Oceans, Marine Ecological Laboratory, Bedford Institute of Oceanography, P.O. Box 1006, B2Y 4A2 Dartmouth, Nova Scotia, Canada;(2) Dipartmento di Biologia Animale ed Ecologia Marina, Universita di Messina, Via dei Verdi, 75, I-98100 Messina, Sicily, Italy |
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Abstract: | Between March 23 and April 4, 1981, samples were taken in the eastern tropical Pacific. The day/night vertical distribution
of euphausiid species and biomass are described and contrasted in detail on two eastern tropical Pacific stations, the DOME
station, in a region of continuous upwelling and the BIOSTAT station, in a nonupwelling area. The effects of various biological
parameters, such as temperature, salinity and oxygen concentrations on the distributions of the species are examined. The
numbers of euphausiids m-2 on both stations were highest during the day, indicating that avoidance of the sampler was not a problem. During the day
the largest concentration of adult euphausiids was between 300 and 350 m whereas the juveniles were concentrated between 170
and 80 m on both stations. Very few individuals were found within the oxygen minimum layer, but low concentrations of some
species were found below the oxygen minimum down to 1 000 m. At night the euphausiid concentration migrated upward into the
mixed layer (20 to 30 m) at BIOSTAT and to the base of the mixed layer at the DOME. Significant differences in the night depths
of the species were found on both stations. The oxygen minimum layer appeared to act as a barrier to the vertical distribution
of all species. Only two species were found in water with an oxygen concentration of <0.1 ml O2 l-1. Twentyone species of euphausiids were found on the two stations but the adult population was dominated by only two or three
species on both stations. The reproductive state of the species suggested that some species reproduced earlier on the DOME
than on BIOSTAT. Analysis of the depth distribution by cluster analysis showed that the most abundant species occupied different
depths during the night and day at BIOSTAT but the two most abundant species were concentrated at the same depth at the DOME
station although portions of each species population occupied different pelagic zones. |
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