Long-term oxygen conditions assessed using chironomid assemblages in brook trout lakes from Nova Scotia, Canada

Abstract

We examined the larval remains of chironomids in 12 stocked brook trout lakes from Nova Scotia using 2 paleolimnological approaches (i.e., top–bottom approach and stratigraphic analyses). Limited monitoring data have indicated that almost all of the survey lakes contained poor coldwater-fish habitat; therefore, concerns have been raised that oxygen conditions have deteriorated and suitable habitat for brook trout may have declined. Our goal was to evaluate shifts in water quality, with a focus on understanding trends in hypolimnetic oxygen concentrations. No strong directional changes in the relative abundances of chironomids were recognized using top–bottom (12 lakes) and downcore stratigraphic (4 lakes) analyses. Chironomid assemblage compositions have not differed significantly between modern and preindustrial time periods (ANOSIM: R = −0.06, P = 0.94). Changes (if any) in oxygen conditions over the last ∼2 centuries likely were subtle and not of sufficient duration and (or) magnitude to strongly influence chironomid assemblage compositions through time. Detrended correspondence analysis of downcore assemblages indicated that taxa turnover was minimal (≤1 SD) at 3 of 4 lakes. We conclude that oxygen conditions (which our proxy data indicate were likely naturally low) have not deteriorated significantly since preindustrial times, and no overall directional trends are yet obvious in the paleolimnological record. Combining the often complimentary information gained from paleolimnological, monitoring, and modeling analyses, as we do here, can aid in the management of aquatic ecosystems and highlight the relative importance of regional trends in water quality and its influence on aquatic organisms such as recreationally significant salmonids.