Mark D. Graham

Resilience of Epilithic Algal Assemblages in Atmospherically and Experimentally Acidified Boreal Lakes

Abstract Algal assemblages can be highly responsive to environmental changes in recovering acidified lakes. We compared epilithic algal assemblages in boreal lakes during chemical recovery from atmospheric (Killarney Park, Ontario) and experimental (Lake 302S, Experimental Lakes Area, Ontario) acidification to assess the impact of spatial and temporal scale of severe acidification on taxonomic resilience (i.e. recovery rate). Resilience was measured as the distance traveled by lakes in ordination space during pH recovery based on canonical correspondence analysis. Resilience was relatively negligible in the Killarney lakes, suggesting that eight years of experimental acidification in Lake 302S had less impact on biological recovery than did decades of regional acidification. Increases in dissolved organic carbon, dissolved inorganic carbon, and calcium best explained temporal variance of epilithic species abundances in the recovering acidified lakes. In Lake 302S, contrasting trajectories of taxonomic resilience and resistance, i.e. displacement from reference conditions following a perturbation, indicated that ecological factors affecting epilithon differed at corresponding pH levels during recovery and acidification. Our findings reveal that modeling of ecosystem recovery from severe acidification must account for the spatial and temporal scale of the perturbation, and biological delay responses that result in differences between recovery and acidification trajectories.

Respiration of juvenile Arctic cod (Boreogadus saida): effects of acclimation, temperature, and food intake

Oxygen consumption (VO2) of juvenile Arctic cod (Boreogadus saida) was investigated at low tempera tures (six temperatures; range -0.5 to 2.7°C). Small (mean wt. 6–8 g) and large (mean wt. 14 g) fish were acclimated, or adjusted to a constant temperature (0.4°C), for 5 months and then tested for metabolic cold adaptation (elevated metabolic rates in polar fishes). Short-term (2 weeks) acclimated fish showed elevated VO2 similar to previously established values for polar fishes, but there was no such evidence after longterm acclimation. Long-term acclimation caused VO2 values to drop significantly (from 86.0 to 46.5 mg O2·kg−1·h−1, at 0.4°C), which showed that metabolic cold adaptation was a phenomenon caused by insufficien: acclimation time for fish in respiration experiments. We also measured the effects of temperature and feeding on VO2. A temperature increase of 2.3°C resulted in relatively large increases in VO2 for both longand short-term acclimated fish (Q10 = 6.7 and 7.1, respectively), which suggests that metabolic processes are strongly influenced by temperature when it is close to zero. Feeding individuals to satiation caused significant increases in VO2 above pre-fed values (34–60% within 1–2 days after feeding). Respiration budgets of starved and fed Arctic cod at ambient temperatures in Resolute Bay N.W.T., Canada, were used to model annual respiration costs and potential weight loss. Low respiration costs for Arctic cod at ambient temperatures result in high growth efficiency during periods of feeding and low weight loss during periods of starvation.

Glacially mediated impacts of climate warming on alpine lakes of the Canadian Rocky Mountains

Climate warming is having a more pronounced impact ontemperature change in alpine and polar regions than on aglobal scale (Beniston 2006, Lemke et al. 2007). For theCordillera region of North America, climate models predictthat warming will increase with elevation, reaching its maxi-mum at mid-northern latitudes (Bradley et al. 2004). Den-drochronological models have already shown that summertemperatures in the Canadian Rockies were anomalouslywarm during the 20