Lake Michigan is teeming with life. But over time, its food web — from the smallest plankton to the largest fish —has been impacted.
Overfishing and waves of invasive species have taken their toll.
Now, climate change is flexing its muscles in the Lake Michigan ecosystem. On land we’ve felt and seen it during what’s been the warmest winter on record. If you want to put this past very warm winter in context, nobody does it better than Paul Roebber. The UWM distinguished atmospheric science professor says to prepare for more winters like this one.
“It’s been an incredibly warm winter globally and of course locally and we’ve been seeing it in terms of ice coverage on the Great Lakes obviously there’s been a declining amount of ice on the lakes for years now actually. The overall trend is clearly downward,” Roebber says.
We met at Bradford Beach on a recent clear and balmy morning; obviously, there was no ice in sight. “From my standpoint, it’s the impact on Great Lakes water levels that is most relevant and the reason for this weird aspect of freshwater,” Roebber says.
The Great Lakes stratify in the summer and winter, forming thermal layers, such as cooler at the bottom and warmer up top. Twice a year, the layers turnover or mix in the spring and again in the fall. It happens when the lake’s surface reaches 4 degrees Celsius — that’s 39.2 degrees Fahrenheit. Oxygen from the lake’s surface circulates to the bottom, and nutrients trapped deep in the lake are distributed throughout the lake. The phenomenon is vital to the lake’s ecosystem.
Roebber says during the spring cycle, "If there’s ice, that has to melt, and then it continues to warm up to 4 degrees. The overturning goes again, and from that point, you have the most stable water at the surface, and it can warm up."
But Roebber says when ice is absent, such as during this warmest winter on record, things change drastically.
"It accelerates that overturning process by several weeks and that means that layer of water gets a chance to warm up much, much more quickly. So the next summer, you’ll have much warmer surface water temperatures, and that has an impact on evaporation out of the lake, and that has an impact on lake levels," he says.
The Great Lakes have experienced swings in water levels, more recently dramatic swings — at times buffeting coastal areas, at others, impacting water intakes and marinas.
UWM School of Freshwater Sciences professor Harvey Bootsma is puzzling out how life beneath the surface stands up to the lakes lows and highs, and now as warm winters become pretty much the new normal.
Take plankton — it's the foundation of the lake's food web. Bootsma says plankton grows faster if the lake is warmer but also needs phosphorus found at the lake bottom to fuel its growth.
"By being warm, the lake remains in a stratified condition for longer throughout the year. There’s this warm surface layer and this cold bottom layer, and there’s restricted mixing between those two layers," Bootsma says.
Under those conditions, nutrients at the lake’s bottom couldn't reach the plankton up top. "They’re not getting the nutrients they need to grow," Bootsma says.
Scientists will also be watching how species like lake whitefish and lake trout fare under warming conditions. "They spawn in late fall, early winter and the eggs hatch in the spring," Bootsma says.
Normally, there’d be plenty of plankton for the wee ones to feed on. "But one possible scenario now is that as the lake warms up, the fish may still lay their eggs at the same time, but the temperature of the water affects the maturation rate of those eggs and the embryos in those eggs," Bootsma says.
If the eggs hatch sooner and if plankton is not plentiful," Those fish larvae are coming out and finding that there’s not that much food to eat so that could result in declines in some of the fish populations," Bootsma says.
He's one of a fleet of scientists around the Great Lakes trying to understand the workings of the lakes’ ecosystems and how they’re changing in order to come up with management strategies.
"There’s some simple questions we can answer, but when you look at the ecosystem as a whole, there are multiple factors you have to consider and multiple organisms and processes within the system that you have to consider when you’re figuring out how it’s going to respond," Bootsma says.
Bootsma says more than ever before, the scientific community will need to be nimble in anticipation what might come up next to challenge the Great Lakes.
