Down in the mud at the bottom of Burnham Harbor and Montrose Harbor and Calumet Harbor and all the other shallow sheltered waters along our lakefront lives a bright red worm, an animal so interesting they named it twice: Tubifex tubifex. It lives in a tube it fashions out of the bottom mud. The tube straddles the water-mud interface, extending a few centimeters down into the mud and a few centimeters above it.

It is not alone down there. It has an equally bright colored relative named Limnodrilus hoffmeisteri that is probably even more common in the mud of our harbors.

Both worms live upside down. Their bright red tails extend out of the tops of their tubes. The tails wave and wiggle almost constantly. If you were scuba diving in one of our harbors, you might find the entire bottom practically coated with these tiny bright red writhing tails. Recorded population densities for Tubifex range up to 8,000 worms per square meter.

Of course, if you are scuba diving someplace where the bottom is coated with Tubifex and Limnodrilus, you should seriously consider getting back to dry land as soon as possible. Both of these worms are what ecologists call indicator species, species whose presence tells us something significant about the environment. And what these worms tell us is that there is a whole lot of sewage in the water. The common name for both species is sludge worm.

When sewage or other kinds of organic wastes are dumped in water, all sorts of organisms get involved in breaking them down into their constituent elements. Bacteria are the most important of these decomposers. Decomposers, like the rest of us, need oxygen. The heavier the load of sewage, the more oxygen they need.

In a shallow harbor full of clean water, oxygen saturation can reach 100 percent; that is, the water is holding all the dissolved oxygen it can. Add sewage, and the decomposers begin to multiply. Oxygen levels start to go down. As they drop, animals sensitive to oxygen deprivation start to vanish from the local fauna. Limnodrilus and Tubifex populations go up. Tubifex can live in water with saturation levels of oxygen as low as 10 percent. In laboratory experiments one-third of test populations have survived up to 48 days with no oxygen at all. Limnodrilus is not quite as tolerant of low oxygen levels, so in the most heavily polluted situations Tubifex may be the only living thing present bigger than a bacterium.

Indiana Harbor, generally regarded as the most polluted place in the Great Lakes, is paradise for Tubifex. In that environment it has no predators and no competitors.

The bright red color of sludge worms is a result of their strategy for surviving in low oxygen levels. The color is created by a chemical very like the hemoglobin in our own blood. Like hemoglobin, it is an oxygen carrier, and these worms have so much of it that they can both collect oxygen from nearly stagnant waters and store oxygen in their bodies for future use.

Sludge worms belong to a family of annelids called the oligochaetes. The name means “few chaetae.” The chaetae in question are hairlike bristles that extend from the sides of the bodies of these worms. Earthworms are oligochaetes, and Tubifex and Limnodrilus feed just as the earthworms do, ingesting dirt, digesting all the digestible material in it, and excreting the rest. If you wade in water with a muddy bottom and feel the mud squishing between your toes, what you are squishing is mainly worm do-do, the castings left by oligochaetes.

In 1974 and ’75 a graduate student at Northeastern Illinois University named David M. Heffernan collected samples of bottom mud from 19 locations in southwestern Lake Michigan. Ten of his locations were near shore in Burnham Harbor, Calumet Harbor, and near Navy Pier. The other nine were offshore, ranging from a spot one-half kilometer from Navy Pier to a location 40 kilometers out. He found the pollution-tolerant oligochaetes at 13 of his 19 locations, and overall the worms made up 39 percent of the total animals collected. Our protected near-shore waters–harbors and waters inside breakwaters–are polluted.

But oligochaetes are not the only bright red bottom dwellers in our harbors. We also have bloodworms. Bloodworms are not worms at all. They are the larval stage of a family of nonbiting midges called chironomids. Chironomid larvae burrow into the bottom. They eat mostly detritus, the bits of dead plant material that constantly rain down on the lake bottom.

Some chironomids spin nets of fine filaments across the entrances of their burrows. They wait a few minutes to allow the net to collect whatever is floating by, and then they eat the whole business, both the collected particles and the net. And then they spin another net.

Chironomid species vary in their tolerance for low oxygen levels, so we can get an indication of pollution levels by identifying the species of bloodworms that live in it.

Our harbors are also home to large numbers of clams. If you dig up some bottom mud, you will almost invariably find clam shells in it. The shells endure long after the animals in them have died, so shells will be much more abundant than living clams. Snails of various species are also common in shallow water.

Offshore, where the water gets cleaner, Heffernan found a different sort of fauna living on or near the bottom. The dominant animals here were tiny crustaceans. There were amphipods, also called side-swimmers or scuds. And there were aquatic sow bugs, relatives of creatures that live under rocks and in damp leaf litter on land.

Pontoporeia is the most typical scud genus in the Great Lakes. It lives only in cold, clean waters, so its presence is an indication that away from shore Lake Michigan is still in pretty good shape. Pontos, as limnologists call them, are vertical migrants. They spend the day on the bottom and then rise into the water at night. They never get to the surface, but they do get high enough to feed on plankton.

Another clean-water indicator in Heffernan’s collection was Mysis relicta, commonly called an opossum shrimp. This is another vertical migrant that spends the day on the bottom and the night in the water. In winter opossum shrimp may rise all the way to the surface. In summer they stay in the deeper, colder water.

Heffernan’s results reveal the great variety in communities of benthic (bottom-dwelling) animals in Lake Michigan. To learn something about how all these communities sort out, I talked to Dr. Jerry Kaster of the Center for Great Lakes Studies in Milwaukee. According to Dr. Kaster, the benthic communities of the lake can be classified by the nature of the bottom and by the amount of pollution present.

Mud bottoms feature oligochaetes, clams, and chironomids. Bottoms of hardpan clay have lots of snails, some pontos, some chironomids, and usually leeches. Sand bottoms feature chironomids almost exclusively. The abrasiveness of the sand seems to eliminate almost everything else. A fourth kind of bottom, which Kaster calls cobble reef, is paved with rocks ranging in size from Ping-Pong balls to VWs. Cobble reefs have the richest benthic fauna in the lake, including freshwater sponges, hydras, and bryozoans. Bryozoans (the name means moss animals) are colonial creatures that live attached to rocks and feed by filtering edible matter out of the water.

Cobble reefs are the cleanest benthic environment in the lake. The exposed rocks on the bottom show that waves and currents are preventing runoff from accumulating. Mud bottoms include the most polluted waters in the lakes. However, there are unpolluted mud bottoms far from shore where wind and currents combine to allow particles to settle. Oligochaetes are common in such places, but the genus is likely to be Stylodrilus rather than Tubifex or Limnodrilus.

The benthic communities of the Great Lakes didn’t get much attention until quite recently. In the past decade we have learned that persistent toxics such as PCBs, DDT, and polyaromatic hydrocarbons collect in the bottom mud. This was once thought to be a very good place for them, but now we know it isn’t.

Oligochaetes and chironomids take in toxics from the mud as they feed. Present estimates are that a given molecule might be recycled 100 times through living organisms before it gets buried deep enough to be out of reach. Bottom-feeding fish take up the toxics when they eat a bloodworm or an oligochaete. Bottom-feeding fish include obligate bottom feeders like bullheads and carp as well as species like perch and lake trout that feed on the bottom sometimes and higher up other times.

Scuds feed on the bottom during the day and carry their load of toxics up at night, providing food for small trout and salmon. And when a trout that has been eating toxic food all its life finally dies, its body washes ashore and gets eaten by a bald eagle. Toxic loadings in bald eagles that live near the lakes are much higher than in inland birds, and as a result their reproductive success is much lower. Things on the bottom can rise up to haunt us.