Small-scale zooplankton community variability in a northern British Columbia fjord system

Abstract

We have used multivariate classification, ordination, and discriminant function analyses to describe the small-scale spatial pattern of zooplankton community composition in a northern British Columbia fjord system. Finely ground tailings from a molybdenum mine were discharged into one branch of the fjord (Alice Arm) during a 17-month span which included our two sampling periods (August 1981 and June 1982). Our study had two major objectives. The first was to learn whether or not the zooplankton community was strongly altered in the vicinity of the tailings discharge, and the second was to characterize the intensity and dominant spatial grain scale of ‘normal’ community variability in the remainder of the deep inlet system. Zooplankton were collected by water column integrating oblique net hauls. The statistical analyses examined relative and absolute changes in the contribution of individual species to the local zooplankton community. Within each time period, the range of spatial variability was small in comparison both to the between-time period (probably seasonal) differences within the inlet, and to the range of spatial variability observed by identical analysis methods in open continental shelf waters to the south. However, despite this small overall community variance, we found a reasonably consistent regional partitioning of the inlet; the samples most similar in composition tended to be those from the same or adjoining locations. Sites near the turbid heads of the branching inlet (including those near the tailings discharge) had higher concentrations of Euphausia pacifica and other large non-copepod zooplankton, and relatively low concentrations of the copepod Neocalanus plumchrus. The magnitude of the between region-differences was small and did not appear to be increasing with time; we therefore conclude that the impact of tailings discharge on zooplankton community structure was relatively minor. The distance over which neighbouring samples show significant compositional correlation is smaller in the inlet than in open coastal waters previously studied; we interpret this as due largely to the absence of meso-scale horizontal turbulent eddies within the inlet.