Status and trends of breeding waterbirds in boreal Ontario lakes (1983-2010)

Background/Question/Methods Regulated controls on the emissions of acidifying pollutants are anticipated to lead to the chemical, and subsequent biological, recovery of acid stressed aquatic ecosystems. We analysed temporal trends, and their associations with lake water chemistry, for breeding waterbird (waterfowl, common loon, gulls, herons) pairs and young using observations collected between 1983 and 2010 on 775 lakes in the acid sensitive Boreal Ecozone in Ontario, Canada.  Lakes were surveyed by helicopter on a rotating schedule over the 28 year period, with a visit in May to estimate the number of breeding pairs and a visit in July to estimate the number and age of young. Data from a lake water monitoring program characterized the chemical and physical attributes of surveyed lakes and provided estimates of temporal change in chemical conditions. Surveyed lakes tended to be small, moderately acidic, dilute, and oligotrophic.

Results/Conclusions Over 28 years, counts of indicated pairs per survey increased significantly for Hooded Merganser (0.86%/yr), Bufflehead (5.66%), Common Goldeneye (1.07%), Common Merganser (2.10%), Ring-necked Duck (1.95%), Mallard (3.53%), Wood Duck (2.66%), Canada Goose (16.09%), Common Loon (0.68%), Great-blue Heron (6.56%) and Sandhill Crane (16.91%). The number of young observed during brood surveys declined for Hooded Merganser (1.67%/yr), Common Goldeneye (1.93%), Common Merganser (3.35%) and American Black Duck (4.15%) but increased for Bufflehead (13.01%). Overall increasing trends in pair counts suggests improving population status in the surveyed regions but declining breeding productivity may signal cause for ongoing concern. Although sulphate concentrations declined in a large proportion of lakes, improvements in lake acidity status were less common, possibly due in part to widespread and large declines in conductivity and base cations. We detected limited but variable evidence of correlation between waterbird and water chemistry changes.  Our study suggests that drivers other than local lake chemical change are influencing observed increases in pair counts but that lake-level responses to acidification stress may be important to waterbird breeding productivity.