The abstract:
Reproductive success of Common Loons (Gavia immer) is a powerful indicator of aquatic ecosystem health, especially in relation to mercury and acid precipitation. We examined relationships between Common Loon reproductive success and longitude, year, lake area, and pH across southern Canada using data collected from 1992 to 2010 by participants in Bird Studies Canada’s Canadian Lakes Loon Survey. Our goal was to indirectly describe the health of lakes in southern Canada with respect to mercury and acid precipitation. The overall model-predicted number of six-week-old young per pair per year was 0.59 (95% confidence limits: 0.56–0.62). Six-week-old young per pair per year decreased by 0.19 from west-to-east (−127° to −52° longitude), decreased by 0.14 between 1992 and 2010, increased by 0.22 as lake area increased from 10 to 3000 ha, and increased by 0.43 as acidity decreased from pH 5 to 9. The relationships were likely linked to acid- and temperature-mediated exposure to methylmercury and/or acid-induced reductions in forage fish. The temporal decrease was unexpectedly steeper in southwestern than in southeastern Canada. Projections suggested that reproductive success across southern Canada may not drop below the demographic source-sink threshold until ~2016 (range: 2009-2029). Reproductive success on pH 6.0 lakes, however, may have passed below the source-sink threshold as early as ~2001 (1995-2009), whereas reproductive success on pH 8.0 lakes may not pass below the threshold until ~2034 (2019-2062). There were ~0.1 more six-week-old young per pair per year on 2500 ha lakes than on 20 ha lakes. Reproductive success crossed below the source-sink threshold on 20 ha lakes at pH 6.4 (5.8–7.1) and on 2500 ha lakes at pH 5.5 (4.1–6.6). Our results show that citizen science is powerful for monitoring ecosystem health and indirectly support further action to abate emissions of mercury and the harmful components of acid precipitation throughout North America and globally.
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