Distribution and physiology of zooplankton in an oxygen minimum layer Richard Alan William Hoos
Date
1970
Authors
Hoos, Richard Alan William
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Abstract
Saanich Inlet, a British Columbia fjord, is deep, stratified, and has a permanent oxygen minimum layer below the sill depth of 80 meters. Zooplankton are exceptionally plentiful down to and below sill depth and several species inhabit the oxygen deficient zone.
The relationship between the vertical distribution of zooplankton and dissolved oxygen concentrations in Saanich Inlet was examined and related to results of experiments of their low oxygen tolerance levels, respiration rates, and respiratory blood pigment concentrations. Similar analyses were conducted in Bute Inlet, a control environment without an oxygen deficient zone.
All zooplankton was collected and quantified from predetermined depths utilizing the Brown-McGowan opening and closing net (Brown & McGowan, 1966) with a depth-time recorder. The survival of the species was measured at an oxygen concentration of 0.40 ml/1, the concentration at which the zooplankton aggregation occurred in Saanich Inlet. Laboratory experiments were run in oxygen tolerance equipment developed at the University of Victoria. Survival levels were determined in the field by lowering specimens in cylinders which were in communication with the environment. The Winkler method was used to determine respiration rates, while starch gel electrophoresis and spectrophotometric analyses were utilized for respiratory blood pigment assays.
The vertical distributions of the Saanich Inlet zooplankton reflect the trends observed in the oxygen tolerance tests. In general, the longer the organisms survived the tests, the deeper they occur in Saanich Inlet, and the more time they spend in the minimum layer.
Orchomenella obtusa and Calanus plumchrus could survive at oxygen levels of 0.4 ml / 1 for at least 24 hours. Their main populations inhabited the oxygen minimum continually.
Euphausia pacifica survived for the shortest period of time (1-2 hrs) of the species tested. Furthermore, this species was always situated above the oxygen deficient layer of Saanich Inlet. The Bute Inlet E. pacifica survived for similar short periods of time, suggesting that the oxygen minimum in Saanich Inlet was not influencing survival time. No significant respiration rate difference was found between the Bute and Saanich Inlet euphausiids, further supporting the hypothesis that E. pacifica has not adapted to the low oxygen environment.
Metridia lucens from both inlets survived for identical periods of time, again suggesting that no adaptation has occurred in this species.
Euchaeta japonica and Cyphocaris challengeri had similar vertical distributions and oxygen tolerance levels in Saanich Inlet. The Bute Inlet organisms survived for much shorter periods of time, indicating that a physiological adaptation has possibly taken place, however, the results of the respiration experiments for these animals proved to be contradictory. E. japonica had a considerably higher rate in Bute than in Saanich, but the reverse was observed f or C. challengeri.
Parathemisto pacifica had a significantly higher respiration rate in Bute Inlet than in Saanich Inlet, but these results could not be related to oxygen tolerance values as Bute Inlet data was lacking.