Some lakes are known for their stunning clarity, while others are as green as grass, from bright green to dull olive. Lake Tahoe has long been extolled for its deep blue color due to the clearness of its water, although over time it has diminished, and the pressures of development, sedimentation, fire, and more, mean trouble for its future. Most lakes are not so clear, and feature at least seasonal colors as summer’s warmth fuels the growth of phytoplankton, and their uncountable numbers cloud up the waters in shades of green. But a green lake, though perhaps less scenic, isn’t necessarily a bad thing. All that microscopic algae feeds microscopic animals, which are fed upon by somewhat bigger animals, and eventually by fish, whose food chain also ranges up from the small-sized to the large. You can’t have a big old salmon without a healthy population of phytoplankton to get it all going.
Lake Michigan has long had a range of clarity through its yearly cycle. Wintertime brings freezing temperatures, during which phytoplankton growth slows, leading to good clarity. As the year warms, growth ramps up and the phytoplankton provide the initial serving at the lake’s cafeteria, where everybody eventually gets a meal. At the apex, that includes non-gilled lakeside residents, who hook a fish meal out of the greener waters. This system in the lake has worked rather well through the ages, even given the various pollution problems that have impacted its water quality. In fact, in the 1980s, as much as 10 million pounds of king salmon was caught in Lake Michigan annually. Things aren’t quite so fecund these days.
Late in the 1980s, about the time record salmon hauls were coming in, an outsider came to the Great Lakes. The zebra mussel, Dreissena polymorpha, is a small, striped mollusk which came from the Caspian and Black Seas of western Asia. International shipping carried it across the wide ocean in ballast water, invisibly depositing the microscopic larvae into North American waters. Arriving in the early 2000s was the zebra’s close relation, the quagga mussel (D. bugensis). Each has the habit of massive reproduction—100,000 eggs is a common occurrence, but some females can produce a million eggs in a season. They grow in such profusion that there can be 20,000 of them in a square meter. They are filter-feeders, and each one can filter about a quart of water a day. Given their massive numbers—we’re talking trillions here—and efficient processing, the population in Lake Michigan could filter the entire volume of that massive lake in four to six days.
Needless to say, these little invaders are chowing down mind-boggling amounts of phytoplankton. Hence, the waters of Lake Michigan, and Lake Huron as well, are now a lovely shade of blue all summer, charming the tourists and providing easy viewing of the lakes’ many shipwrecks. But those perks come at a huge cost. There is no food left for the zooplankton, and the effect of their depletion moves on up the line from the small shrimp-like Diporiea to the big fish. The Diporiea population is already crashing, and they are a food source not only for the small-sized fish, but for the big ones as their lives are getting started. The salmon catch is now less than a third of its 1980s highpoint. Scientists studying this unfolding drama call it unprecedented in the life of the Great Lakes. They say the effect will alter the entire ecology of these lakes. Outside of the food-chain nightmare, the unstoppable production of mussels is causing endless headaches for owners of boats, docks, intake pipes, and any structure that presents a surface to the water, because the kajillions of veligers (as the larvae are called) will attach to whatever they can in their search for a permanent home. Even as they die and break off of their moorings, clumps of their shells wash up on beaches and present sharp edges to bare feet, and a not-so-pleasant smell as they decompose.
What is to be done? Quagga mussels can grow to depths of 400 feet, and unlike the zebras, which attach themselves only to hard structures, quaggas can grow on soft surfaces like the sandy lake bottom. Trying to physically remove them is not feasible. Powerful chemicals that might poison them would no doubt wreak havoc on all the rest of the biota. These invasive mussels do serve as food themselves for some fish, but the mussels bioaccumulate toxins in their tissues, so feasting on them is not a healthy practice for the fish. The best chance for controlling mussel numbers may be their own voracious habits. As they strip the waters of its algae, they exhaust their own food supply. The population, then, has a maximum sustainable level, and whether anything else in the lake system can manage to get itself fed while the mussels are at those maximum numbers remains to be seen.
Recent years, however, have seen the development of a possible wonder product made from nothing more complicated than the dead and desiccated bodies of Pseudomonas, a naturally-occurring soil bacteria. Naturally occurring in North America, that is, and it seems the western Asian mussels get a rather severe tummyache when they ingest Pseudomonas. A fatal one, in fact. So far, the product, called Zequanox, has only been approved for use in enclosed areas, such as the intake pipes of water processing plants. Zequanox seems to have no effect on any of the other animals it has been tested on, perhaps because it is virtually everywhere in the soil here and the locals have evolved with it. Still, before open-water testing is done, the EPA wants to be sure it won’t cause some new, unforeseen problem. Of course, it is expensive, and applying it effectively so that it doesn’t just wash away with the currents could be tricky. It will not eradicate the invasive mussels from the Great Lakes, nor drive them all out of any of the far-flung other areas to which they have spread. But at least there may now be a fighting chance to keep these many aquatic habitats in a more balanced state.
Want to have a better look at the Great Lakes while you ponder the path of invading mussels? This National Geographic map of the region also features copious notes on interesting locations for the curious tourist. Available here from Maps.com:
caption: The clear waters of Lake Michigan….maybe too clear?
source: Flickr: Delta Whiskey (CC by ND 2.0.)
caption: Zebra mussels—tiny, stripy trouble.
source: https://www.fws.gov/lodi/aquatic_invasive_species/quagga_and_zebra_mussels.htm: US Fish and Wildlife Service (Public domain)
caption: They’ll coat any surface they can find.
source: Flickr: NOAA (CC by SA 2.0)
caption: March of the invading mussel army. Lookout California, the enemy has breached the gates.
source: https://nas.er.usgs.gov/UserImages/current_zm_quag_map.pdf: USGS (Public domain)
caption: Threat-risk map shows where the mussels may go next. Low calcium levels in waters seem to limit their success.
source: Flickr: Paul Ringold, Sue Pierson, et al (CC by SA 2.0)