The sight of a painting-perfect loon on a lake this weekend reminded me that I covered the annual loon count on this blog three years ago. In that post, I discussed how Bird Studies Canada had turned the national summer pastime of watching loons into an exercise in “citizen science” — the involvement of non-scientists in scientific endeavours.

Now the Canadian Lakes Loon Survey has yielded important scientific evidence.

The common loon, with its characteristic silhouette and unearthly like-no-other-animal calls, stands out in another way. It is an “indicator” or “sentinel” species, defined as an organism that is sensitive to the environment and hence can provide an indication of the health of that environment. A canary in a coal mine, a distant-early-warning system.

In a study published this year in Avian Conservation and Ecology, authors Tozer, Falconer and Badzinski used the data from citizen observations to model the reproductive success of common loons in Canada. Loons are sensitive to changes in pH (acidity) and to methylmercury, making them an important indicator species for changes in these variables. Furthermore, methylmercury in combination with higher temperatures and acidity has a synergistic effect — more than the sum of its separate effects — on loon breeding success.

The analysis shows that loon reproduction has been going down at a worrying rate since 1992. Reproduction is more successful in western Canada than eastern. These findings are correlated with changes in acidification of lakes and methylmercury levels, but the authors caution that there may be unknown factors in addition. A correlation like this one is suggestive, but cannot prove cause-and-effect.

According to the model these authors developed, at a certain pH level in a lake, loons no longer reproduce enough to replace themselves. That is, the population no longer increases but starts to decline. In biology, this is called the “source-sink” threshold.

In lakes with a pH of 6.0, this threshold may have already been breached in 2001, according to the calculations. Other lakes with a current pH of 8.0 probably won’t hit this point until about 2034. Overall, the authors figure that the population is still increasing, but marginally, and if the declining trend continues, the total population will soon start to decline.

This is a different picture from the one painted by another study, the breeding bird survey, which showed increasing numbers over the same period. By contrast, while this new study indicates overall increase, it shows lower success rates, decelerating into the negative zone in some areas.

It’s an impressive achievement for citizen science, which has proven its worth, providing more data than possible in a limited scientific survey.

It’s also a red flag on environmental mercury and continuing acid rain. These problems have not gone away — far from it. The loon’s call is a warning not only about one of our iconic fauna but also about the future of our delicate lake ecosystems.