Cladoceran body size as a metric of environmental change in New Brunswick lakes

Abstract

Cladocerans are a widespread order of zooplankton frequently used as bioindicators of aquatic conditions and environmental change. Shifts in cladoceran body size, at both the taxon and assemblage level, can occur as a response to environmental stressors. A standardized body size metric was developed for cladoceran taxa commonly observed in lakes of eastern Canada to provide an assessment method for understanding body size responses to environmental change. The metric was then applied to dated sediment records of five New Brunswick lakes that experienced severe DDT contamination in the mid-20th century. Body sizes of Bosmina sp. and Daphnia sp. remains were also measured from two lakes to determine whether size trends based on the metric occurred at a taxon level. A decline in body sizes in two lakes initially associated with historical DDT inputs has remained to present, and suggests that past and contemporary stressors, such as climate change, may structure both assemblage composition and body sizes of cladocerans. The shift towards smaller body sizes during the DDT impact period was reflected at both the assemblage (via the size metric) and taxon levels. The agreement in the metric and measurements of cladoceran remains from the sediment record suggests that the metric accurately tracks trends in average cladoceran body size through time. Overall, the body size metric provides a tool to further assess the impact of environmental stressors on cladocerans and determine potential implications for aquatic food webs.