Lake Michigan Warming: Climate Threats
You don’t expect to see 75- or even 80-degree water in the Great Lakes in early July or, in most years,
anytime. But an exceptionally hot weather pattern has pushed water temperatures in most of the lakes to
the highest levels on record so early in the summer. Over lakes Erie and Ontario, the water is the warmest
it has been since records began being kept, and could warm more in the coming weeks.
The abnormally warm waters, consistent with climate-change trends in recent decades, could compromise
water quality and harm marine life in some areas.
These water temperatures over the Lakes are some 6 to 11 degrees warmer than normal.
Lake Michigan’s average water temperature reached 75.1 degrees on July 8, nearly 11 degrees above
normal, and the warmest mark on record so early in the year. The water temperature in July has only
been this warm one other time, at the end of the month in 1999.
Lake Michigan’s average water temperature on July 8, 2020 was 11 degrees above normal, and the warmest
mark on record so early in a year.
National Oceanic and Atmospheric Administration (NOAA) aircraft have already photographed 'blue-green
algae' or 'cyanobacteria' over western waters of Lake Erie in recent days. The foul-smelling
algal blooms can harm fish and make people who are exposed to the water sick. In 2014, cyanobacteria
from Lake Erie entered the water supply in Toledo, and residents were ordered not to drink or touch the
The jump-start to the algal bloom due to the warm water temperatures means it will be around for several
weeks longer than normal. NOAA stated that this was the second earliest that they have seen algae since 2002
Many fish do not do well in water that is too warm, so they get ‘squeezed’ into a smaller and smaller area
between surface water that is too warm, and bottom water that doesn’t have enough oxygen
The warming is expected to reduce the amount of time cold water fish spend in the southern basin of
Lake Michigan, where the chance of catching these species is already limited because it’s shallower
and more tepid than the rest of the lake.
Much of the lake is so deep and cold in open water that most fish can’t survive there, but warming
will likely open up more habitat for the majority of fish. Whether they will be able to find sufficient
food in those new waters is unclear.
This could become a significant problem for some game fish, like trout and salmon, that depend
on cold, oxygen-rich waters. However, milder water temperatures are expected to expand the
range of warm water fish like bass, which are confined to southern Lake Michigan.
To withstand warmer temperatures, cold water fish, like chinook salmon, will expend more energy
and require more food.
Invasive mussel species
have decimated the abundance of 'plankton', small organisms that serve as base of the
food chain and the staple of many small fish diets.
Invasive mussels have
also contributed to the decline of the chinook salmon’s primary prey, a small fish known as the
'alewife', whose population has crashed in the past several decades.
Warming temperatures has already increased bacteria levels in the Lake Michigan, as the water warms
earlier in the spring and warming contributes to vertical mixing that changes lake ecosystems. Sewer
overflows, the dumping of ship ballast water, and nutrient runoff from agriculture and industry all
contribute to growth of bacteria and several invasive species in the lakes.
It’s become common in recent years for beaches in Chicago and Michigan to close or be under swim
advisories because of bacterial contamination. Beach closures are expected to increase as heavy
precipitation exacerbates issues associated with runoff and pushes up bacterial counts as well as
algal blooms and E. coli alerts