Lake Superior Drops to Third
Using a Sechii disk
I grew up fishing in sluggish rivers, backwaters and ponds here in Northwest Indiana so I never fished in crystal clear water. Around here the water was brown, except in the middle of the summer when it cleared up up a bit because there were so many weeds the wind and carp couldn’t roil the water, stirring up the bottom sediments. Then, the algae in the water made the water green. It still wasn’t crystal clear.
Perhaps that was a part of the allure of the Great Lakes when I started fishing there. The water in Lake Michigan was clear - at least to me. Later, when I got the chance to fish Lake Superior I learned what “clear” really meant when it came to water clarity.
Scientists use a devise called a Secchi Disk to measure water clarity. Invented by a guy named Secchi in 1865, it’s still in every fish biologist’s tool chest. It’s a flat, 8 or 12-inch disk, divided into quarters like you would slice a pie, with alternate pieces of the pie painted either black or white. A cord is fastened in the middle of the disk. Lower the disk into the water on the cord and see how far down you can still see it.
Really muddy, turbid water may have a Secchi reading (the depth it’s still visible) of only a foot or two. Actually, a foot is quite muddy, two feet isn’t all that uncommon in slow moving rivers after a rain event. A really clear freshwater lake can produce a Secchi reading of around 30 feet. Lake Superior has had Secchi readings up to 80 feet.
Personally, my introduction to checking out the ultra clear water in Lake Superior was a much more fun experience. I was fishing in Lake Superior near Isle Royale National Park on a bright, sunny day. The waves were calm and I just happened to be reeling up a deepwater lake trout on a reel with a line-counter meter on it. The sun was perfect, shining high and over my shoulder and the fish was being cranked up from almost straight below.
As the fish struggled at the end of the line I was able to first spot the flashes of sunbeams reflecting off the fish’s flanks with 45 feet of line out and by the time I’d reeled in 10 more feet, I could easily see the entire fish. I don’t know what the Secchi reading would have been but I’m confident I never saw a fish that deep under my boat previously. The water was the clearest I’d ever fished. At the time, Lake Superior was the clearest of all the Great Lakes. Why not? It’s deep, cold, much of the lake bottom is rock, indeed much of the lake shore is rock and the number of large, slow moving rivers flowing into Lake Superior are few as well - especially when compared to the other Great Lakes.
I was surprised, then, when news crossed my desk that Lake Superior no longer has the clearest water of the five Great Lakes. In fact, it’s dropped to number three behind Lake Huron and Lake Michigan. Lakes Erie and Ontario trail all three.
“This is a change of significant historical and economic importance,” according to scientists from Michigan Technological University, the University of Michigan, University of California Los Angeles and Colorado State University who conducted the research.
What accounts for the dramatic shift? Did Secchi readings in Lake Superior decrease or did the lower lakes get clearer?
The second premise is what happened. Lake Huron and Lake Michigan has become more clear in recent years and that has major ecological implications. The study, published in the Journal of Great Lakes Research, identified probable causes for this shift.
One cause is a reduction in phosphorus entering lakes Huron and Michigan, largely from agricultural run-off of fertilizers and poor waste treatment from shoreline cities. Phosphorus is “algae fertilizer” with a direct relationship between the amount of phosphorus in a lake and the amount of algae that can grow in it. Less phosphorus means less algae means clearer water.
More significant is the proliferation of the invasive quagga mussels in Lakes Huron and Michigan which feed by filtering plankton from the water. Their sheer numbers - an estimated 10 trillion blanket the bottom in each lake - and their ability to strain out the nearly microscopic plant and animal life from the water and have doubled water clarity in the past decade.
Clear water may look nice and seem a good thing, but the lack of plankton floating in the water means less food for creatures at the bottom of the food web. In the big-things-eat-little-things world of the Great Lakes, clearer water means less food in the food chain, including food for the game fish we all like to catch.