Stalking the critters of the... deep

[STWA]

A new 'dead zone' has been found in Lake Erie, which struggled back from contamination before.

Zebra and quagga mussels from the Caspian Sea could be to blame for a new "dead zone" recently found in central Lake Erie, an area where dissolved oxygen levels are low enough to endanger fish and other water life.

University of Waterloo biologist Ralph Smith suspects the mussels also contribute to unwanted algae growth and cloudy water along the beach-lined shore of the eastern basin.

He's among a number of Canadian and American researchers trying this summer to understand how the two mussel species and other invaders such as the round goby fish are affecting the lake's chemistry and biology.

Erie was threatened once before by low oxygen, the result of decaying algae that grew in overabundance because laundry detergents and sewage were pumping in huge quantities of the plant fertilizer, phosphorus.

Scientists spent years figuring out the processes at work, creating a model that allowed them to predict how the lake would react to changes, and convincing society to control phosphorus.

The lake's health improved, the water became clearer and beaches were cleaner, but invasive species now appear to be causing such dramatic effects that the model may no longer apply.

Dime-sized zebra mussels were first found in Lake St. Clair, near Windsor, in 1990. They have since spread throughout the Great Lakes and beyond.

They established massive colonies on the bottom of eastern Erie, but Smith says they've since been almost entirely replaced by their quagga cousins, for reasons not yet understood.

The filter-feeding mussels are so numerous they've vacuumed up enough tiny plants and animals to make the water much clearer in many places, but at a price to other species that depend on the same food.

In addition to clogging intake pipes at power plants, factories and municipal drinking water systems, it now looks as if they could be changing the lake enough to encourage algae growth and make the water murky along the shores where people play.

The murkiness results, Smith suspects, from small particles sucked up by mussels in the shallow, nearshore areas that stay there and don't wash back out to deeper water. In addition, their fecal waste acts as fertilizer for algae that's becoming a nuisance on beaches.

He and other scientists are also intrigued by the bottom-feeding gobies, fish that grow to about 17 centimetres in the lakes. He says these fish "have arrived in the east end of the lake in fantastic numbers.

"They will eat small mussels (including zebra and quagga) for sure, and bass will eat the gobies, so life for smallmouth bass looks to be good for awhile."

But no one really knows the long-term effects of what's happening.

Before the mussels, "we didn't have any organism that would attach to the bottom and filter large quantities of water. The freshwater temperate zone never had that.

"So the mussels perform a completely new type of ecological function that will persist unless the gobies can harm them enough to reduce their impact."

Eric McGuinness

At first, it seemed no other creatures were preying on the mussels in the Great Lakes, but some duck species are feeding on them and they're turning up in the guts of perch, walleye, sheepshead, carp, white bass -- almost all fish except smelt and rainbow trout.

But Smith says researchers don't yet know how many mussels the fish are eating, whether they like the shellfish, whether they offer good nutrition to the fish or how fish are affecting the mussel population.

"Sheepshead probably eat a lot, and gobies more, the little ones. They have the potential to bring the mussels under some degree of control."

In Polish lakes and other places in Europe to which zebras and quaggas have spread, populations reportedly rise and fall unpredictably. Storms can scour them off shorelines, and they may be subject to diseases, but little is really known.

Smith says: "What are gobies going to do? I'd love to know."

By Eric McGuinness

 

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