How Might Future Climate Change Affect

Lake Temperature, Mixing, Algae,

and Small Invertebrates?

John T. Lehman

University of Michigan

15 June 2001

What types of Ecological Knowledge do we need to understand and predict the effects of climate change?

What types of Ecological Surprises might occur?

How can we obtain an environmental insurance policy against detrimental effects and surprises?

Climate Change and the Great Lakes

•Air Temperature

•Humidity

•Winds

•Precipitation

•Cloud Cover

•UV Radiation

What are the climate forces that are likely to change?

•Lake surface temperature

•Evaporation rates

•Surface currents and mixing depths

•Duration of thermal stratification

•Ice cover

What physical features of the lakes are sensitive to these forces?

General Types of Ecological Knowledge

•Individuals and Single Populations

•Population Interactions

•Communities

•Materials and Energy

• Uniqueness of individuals (sexual, asexual reproduction)

• Life history and demography

• Behavior

• Genetics and evolution

• Sex ratios

• Dispersion and distribution

Individuals and Single Populations

• Predator-Prey (and defense mechanisms)

• Parasitism

• Symbiosis: mutualism, commensalism

• Competition (exploitation, interference)

• Indirect Effects

• Fundamental and Realized Niche

Population Interactions

• Species Composition, Richness, Diversity

• Indicator Species

• Trophic Structure

• Keystone Species

• Stability and Resilience

• Invadability

• Substitutability

• Ecological Succession

Communities

• Primary Production

• Secondary Production

• Bioenergetics

• Biogeochemistry

• Biological Magnification or Concentration

Materials and Energy

Do we possess complete catalogs of ecological knowledge about the species now present in the Great Lakes?

Managing in the Face of Uncertainty

Absolutely not!

Does the knowledge we do possess permit us to make any predictions at all?

Certainly.

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Erie Huron Michigan Ontario Superior

Duration of thermal stratification (d)

Projected duration of thermal stratification under Canadian Climate Centre climate scenario.

Projected maximum temperature of the mixed layer under Hadley Centre climate scenario.

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Erie Huron Michigan Ontario SuperiorMax temperature of mixed layer (C)

Projected average temperature of the mixed layer under Canadian Climate Centre climate scenario.

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Erie Huron Michigan Ontario SuperiorMean temperature of mixed layer (C)

Projected average temperature of the lake bottom at average lake depth under Hadley Centre climate scenario.

Projected minimum mixing depth under Canadian Climate Centre climate scenario.

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Erie Huron Michigan Ontario Superior

Minimum daily mixing depth (m)

Existing theory about algal ecology teaches us that mixing pattern is a Master Variable for algal population growth and community composition.

Given extended thermal stratification and elevated temperatures, what are the:

• effects on Diatoms?

• effects on colonial Greens and Bluegreens?

Ecological Effects on the Algae

Existing ecological knowledge points to an impending change from fast-growing, opportunistic, rapid-sinking species to slower growing, stress-tolerant, loss-minimizing species.

Rapid-sinking diatom species are presently key to the transfer of energy-rich food from the water column to the sediments, and to the benthic food web.

Ecological Effects on the Algae

Most of the Great Lakes are presently optically shallow, in the sense that much light reaches below the mixed layer and permits the growth of metalimnetic and hypolimnetic algal populations.

Climate factors alone will not change this condition. However, if nutrient loading from watersheds and airsheds increase, the optical state could change.

Ecological Effects on the Algae

Metabolism by invertebrates varies with temperature. Temperature affects

• Growth rate

• Egg and embryo development

• Generation time

• Feeding, respiration, and excretion

But temperature also raises the metabolism and activity of most lake predators on the invertebrates, as well.

Ecological Effects on Lake Invertebrates

Most of the Great Lakes presently harbor a group of cold stenothermic invertebrate species which cannot tolerate warm temperatures.

Deep, cold water habitat for these species will not disappear. However, the oxygen content of that habitat could become compromised by the end of the thermal stratification period, particularly if nutrient loading were to increase.

Ecological Effects on Lake Invertebrates

Increased foraging and predation by planktivorous fish will result in changes of invertebrate species toward small bodied forms.

The vertical range of zebra mussels may expand, but other benthic invertebrates may suffer from diminished inputs of high quality diatoms as food, and from potential decreases in oxygen.

Ecological Effects on Lake Invertebrates

Complex interactions among many species increase the likelihood of indirect effects, some of which will lead to ecological surprises.

• Species invasions

• New or invigorated parasites

• Changes in breeding windows

• Altered distribution patterns

Ecological Surprises

Ecological Surprises

Further surprises await us regarding the transformations and fates of environmental toxins. Some toxins, such as Mercury or PCBs, become biomagnified up a food chain. Their chemistry, transport, and effects are tied to biology.

• Increase rates at which dissolved elemental mercury is oxidized to ionic mercury

• Increase rates of methylation of ionic mercury

• Increase accumulation of methyl mercury in food chains leading to large fish

Ecological Surprises

For example, elevated UV radiation and elevated temperature could

Science relies on the testimony of evidence rather than the fervor of belief. Observation and theory are the antidotes to ignorance, fear, and doubt.

Thoughtful measurements and analyses can provide an early warning system for changes within the Great Lakes ecosystem.

How does the rational Public protect itself from disaster and surprise?

Regional assessment of Great Lakes response to environmental changes must become a regular activity. This assessment has uncovered many gaps in ecological knowledge that must be filled.

Research and reporting collaborations among federal agencies, academic scientists, and interested citizens require public support.

How does the rational Public protect itself from disaster and surprise?

Special thanks to Art Brooks and the Workshop sponsors. GCM-projected climate data for the Great Lakes were supplied by NOAA-GLERL.