Reorganization of aquatic communities from low-nutrient lakes in northwestern New Brunswick, Canada

Sparse monitoring of New Brunswick (Canada) lakes creates challenges for understanding mechanisms of deteriorating water quality, such as recent instances of increased cyanobacterial biomass in low-nutrient systems. To assess long-term environmental change experienced by low-nutrient, dimictic New Brunswick lakes we use sedimentary remains of algal pigments and Cladocera in dated cores from impact (prone to late-summer algal blooms, Lac Unique) and reference (no observed algal blooms, First and States) lakes. Overall, all three lakes now exhibit greater bosminid abundance and fewer daphniids, lower Cladocera richness, and smaller average cladoceran body size, although some lake-specific differences in assemblage response exist that cannot be related solely to top-down or bottom-up forces. Zooplankton trends are most pronounced at Lac Unique and First Lake, where algal production is generally greater. Across all three lakes, the only significant (IndVal: p < 0.05) cladoceran bioindicator of the post-1990 period is the pelagic bosminid group, whereas prior to 1990, Daphnia longispina-complex and several littoral taxa were significant (IndVal: p < 0.05) bioindicators. These temporal shifts suggest that the smaller-bodied bosminid group may now be favored over its larger-bodied pelagic competitor Daphnia sp. in these lakes, irrespective of heterogeneous long-term algal patterns inferred from stable sedimentary pigments. We suggest that the indirect effects of climate change, principally during the spring and summer quarters in New Brunswick, may be associated with marked shifts in limnological structure, as evidenced by changes in dominant zooplankton of the pelagic zone. However, further research is needed to rule out whether or not size-selective predation, and its potential interactions with climate change and other stressors, are key mechanisms favoring increased bosminids in low-nutrient, dimictic lakes.