|Loons feed heavily on Great Lakes fish en route to their |
summer habitat further north. Credit: EPA
Wednesday, November 27, 2013
Bird die-offs in the Great Lakes: a side effect of ecosystem chaos
Early this year, loons on the north shores of Lake Michigan suffered a massive die-off , with thousands of the migratory birds washing ashore along Michigan's Upper Peninsula. The birds died of botulism E poisoning, a toxin produced by Clostridium bacteria. These die-offs have been recorded in the Great Lakes dating back to the 1960s, but appear to be increasing in frequency and intensity.
The current explanation for the die-offs is that the birds are increasingly falling victim to the ongoing disruption of the Great Lakes ecosystem. Invasive quagga mussels, which are filter feeders, are stripping the water column to such an extent that sunlight is penetrating unusually deeply. The availability of sunlight in deeper waters is thought, in turn, to provoke algal blooms. Of the several kinds of resident algae, bird die-offs seem to match most closely with blooms of Cladophora glomerulata . These algal mats deplete oxygen from the water, promoting formation of botulinum toxin by Clostridium, which is then ingested with the algae or along with the quagga mussels, by yet another invasive species, the round goby. And the botulinum toxin becomes concentrated as it moves up the food chain, felling migratory birds and larger fish. However, although this is probably the major route by which migratory birds become ensured, Clostridium is present in several niches within the Great Lakes and may end up in the food chain through other routes.