Reducing lake pollution drives away ducks

Cleaning up a polluted lake ought to be a good thing for the birds that spend winters there, right? Apparently not — at least in the case of one large lake in Northern Ireland. Researchers have found that reducing nutrient run-off from farms may have caused the number of bugs and crustaceans in the lake to plummet, forcing migratory ducks to look elsewhere for food.

At more than 380 square kilometers, Lough Neagh is the British Isles’ biggest lake. It has also been a major wintering spot for pochard, tufted, and goldeneye ducks. But from 2000 to 2009, the number of diving ducks on the lake during the winter dropped by 63 percent.

The team wondered if the ducks weren’t finding enough food, in the form of invertebrates such as midges. To find out, they surveyed the lake’s invertebrates in 2010 and compared the numbers to those in 1997 and 1998. During that time, the density of bottom-dwelling macroinvertebrates dropped from 15,300 per square meter to about 5,000. That decrease in food supply might have driven ducks elsewhere, the authors say in Freshwater Biology.

The lake was plagued with nutrient pollution in the 1970s, but clean-up efforts may have since changed nutrient levels. “Historically the lake was heavily affected by organic pollution as a result of nutrients from agricultural run-off. This artificially boosted its productivity,” said co-author Irena Tomankova of Queen’s University Belfast, UK in a press release. “Now that conservation schemes are beginning to have an effect and reduce levels of pollution we are seeing increasing water quality and the unexpected consequence is fewer invertebrates and as a result less duck food.” Roberta Kwok | 14 November 2013

Source: Tomankova, I. et al. 2013. Chlorophyll-a concentrations and macroinvertebrate declines coincide with the collapse of overwintering diving duck populations in a large eutrophic lake. Freshwater Biology doi: 10.1111/fwb.12261.

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