Great Lakes dead zone a mystery

Satellite photo of Lake Erie, showing "whiting" event from calcium carbonate in water (photo: NASA Earth Observatory)

In the wake of a report in Science two weeks ago that concluded that the number of dead zones — areas of low oxygen that choke off life — in the ocean are doubling every 10 years, renewed attention may be focused on a major dead zone in one of the Great Lakes that continues to be a mystery.

Dead zones do not happen only in the oceans; they happen in freshwater lakes of sufficient depth as well. Lake Erie has had a large dead zone for decades, one that covers almost the entire central basin of the lake (Sandusky, Ohio, divides the western basin from the central; Erie, Penn., divides the eastern basin from the central basin; everything in between is the central basin, the bulk of the lake’s area and volume), but for a time it was getting better.

In the 1970s, Lake Erie was called a dead lake, but in fact it was an extremely productive lake due to enormous amounts of agricultural and industrial pollution that put huge volumes of nutrients into the lake. Major steps were taken to reduce the nutrient load in the lake and clean things up, and for a long time it was working. The EPA takes constant readings of the oxygen levels in the water at points all around the lake and for more than a decade and a half the level of anoxia (oxygen depletion) had steadily decreased. The plans put in place were clearly working.

In the early 90s, however, that trend began to change; levels of anoxia began to fluctuate significantly season to season and year to year. A new round of scientific studies was started to determine the extent of the problem as well as possible causes and solutions. Those studies continue today. Before we look at what those studies have said, however, we need to understand how dead zones occur.

Dr. Jim Lubner, education coordinator for the University of Wisconsin Sea Grant Institute, explained to Michigan Messenger how dead zones operate in the Great Lakes:

In the summertime, the Great Lakes stratify — two distinct bodies of water, lying one over the top of the other, and a discontinuity layer between them where temperature drops rapidly. In the springtime, the lake is relatively the same temperature from top to bottom due to mixing. As the temperatures warm up, the lake warms up, but it warms from the surface. So what you end up with after a period of time, and it varies from body of water to body of water, is a layer of water that can go 20 or 30 meters deep that is warm and underneath it you have a layer of significantly colder water with a sharp drop in temperature between them called a thermocline. That thermocline layer effectively separates the upper lake from the lower lake. There is little mixing between those two distinct layers. The sunlight and the nutrients in the upper body of water is where all the productivity occurs — algae growing, animals circulating, etc. Eventually, the algae and animals die and they settle down into the lower, colder body of water and get decomposed by the bacteria. That decomposition uses up the oxygen and results in dead zones.

This process is particularly bad in Lake Erie because that lake is just barely deep enough for stratification to occur, meaning the bottom layer (scientists call this the hypolimnion; the upper layer is the epilimnion) is much smaller, has less oxygen in it to begin with and is thus more rapidly depleted of that oxygen. For decades, this process was spurred on by those heavy nutrient loads from external pollution; the elevated amount of nutrients encourages tremendous growth in the higher strata, but when that life dies and sinks to the bottom and decomposes, it results in the rapid depletion of oxygen in the lower strata.

The problem facing scientists now is figuring out why the dead zone in Lake Erie may be worsening again despite the progress in reducing nutrient inputs into the lake. The input of nutrients into the lake is divided into two sources: point sources and nonpoint sources. Point sources are things like wastewater treatment plants, where the amount of nutrients released into the water can be measured; we know that those levels have been reduced significantly since the 1970s. Nonpoint sources are things like agricultural runoff, which can’t be measured in the same manner because fertilizers are spread on the ground and the amount of runoff produced varies by any number of factors. But it seems almost certain that the amount of nonpoint source nutrient pollution has also decreased significantly due to tighter regulations on fertilizer use on farms.

But we also know that anoxia can occur even in the absence of human-induced nutrient loads. Geologists have identified periods of anoxia in the sediments of bodies of water all over the world, periods that took place long before there was anything like agricultural or industrial waste. Clearly, there are natural inputs to this problem as well. Are those natural inputs at work in Lake Erie? If so, are they being worsened by human activities that could be altered to fix the problem?

Scientists have proposed at least two credible explanations.

The first is the invasive species hypothesis. We all know about the problem of zebra mussels in the Great Lakes, but they’ve now been joined by quagga mussels, a closely related species that can live in more varied environments than the zebra mussels and are therefore spreading more widely than their cousins. Those mussels live on the bottom of the lake and tend to pull nutrients down into the lake floor sediments with them, making the water more anoxic. There is no doubt among scientists that these invasive species have changed the nutrient cycles in the lakes, but how exactly that relates to dead zones is still not entirely known.

The second hypothesis is that global warming may be fueling the problem. Warmer water stays warm for longer periods of time, which exacerbates the stratification problem. If you lengthen the period of stratification by increasing the temperatures, you isolate the bottom for a longer period of time, which means more possibility of using up all the oxygen at the bottom.

Hunter Carrick, an aquatic ecologist at Penn State University who has collaborated on several studies of the Lake Erie dead zone, cautions against drawing unjustified conclusions at this point. He isn’t even entirely convinced that the dead zone has gotten worse because the data show greater variability rather than a clear trend:

I don’t think we have all the data to make an evaluation of whether the dead zone has gotten larger in recent years. The temporal change in hypoxia is not clearly defined yet, so making assumptions about whether it’s worse or better puts the cart before the horse. We need more work to iron out the patterns. The pattern from the 80s to the 90s had a discernible linear pattern in a reduction of anoxic conditions. Since then it has been less clear with more variation from year to year.

Carrick also says that it’s entirely possible that nutrient loading is still a problem. “Certainly the loads to Lake Erie have come down markedly,” he told the Michigan Messenger, “but we don’t know if there are new and unaccounted for sources that might be affecting the lake in some way.”

He also suggests that both proposed explanations noted above may be valid: “The exotic introduction of invasive species is a reasonable hypothesis. The impact of global warming is a reasonable hypothesis. We need more research to confirm or deny those hypotheses.”