Lakes as integrators

Paleolimnology is a subset of limnology (study of inland waters) focused on uncovering the past by examining the sediment record. The layers of lake sediments are arranged annually, much like tree rings, and age can be determined by counting (in some cases) or via isotopes (e.g., carbon dating). By examining sections microscopically for fossils of microorganisms and their eggs, paleolimnologists can infer the chemical and environmental requirements of certain organisms (using current data) and attribute those conditions to the past. A synthesis of multiple studies spanning more than 200 Northern Hemisphere lakes shows a distinct restructuring of the diatom community (a group of algae) since the 19th century due to increasing temperatures. Interestingly, changes were apparent in Arctic lakes up to 100 years before appearing in temperate lakes around 1970. By comparing the relative abundance of other microorganisms in the sediments, scientists concluded that an increase in sun energy to the lake was directly linked to the shift in diatom community composition, and not landscape changes or pollution in the watershed. Using these methods, paleolimnologists can look tens of thousands of years into the past. The greatest obstacle of these methods is separating the overwhelming effects of human activities in the watershed from the larger scale climate effects.

Lake as regulators

The two major greenhouse gases (CO2 and methane) can be produced within lakes, and carbon is a central element of both. Carbon enters lakes in either particulate or dissolved form and comes from vegetation and geologic sources (e.g., limestone). Once in the lake, carbon is either buried in the sediments, passed through the out-flowing stream or dam, incorporated into living material (plants, algae, bacteria), or out-gassed to the atmosphere as CO2 or methane.
Globally, lakes and reservoirs bury four times more carbon in their sediments than the oceans do. Because of their hydrology and their placement in the landscape near carbon-contributing human populations, reservoirs are particularly good at receiving and holding carbon. For example, small eutrophic (productive) reservoirs store carbon at a rate that is one to two orders of magnitude higher than natural lakes, based on surface area. While overall more carbon is stored in lakes and reservoirs than is emitted by them, this is not the case in every instance. New reservoirs contribute greenhouse gases to the atmosphere for roughly their first decade due to the decomposition of flooded terrestrial vegetation. Because of methane gas discharge from the hypolimnion, some hydroelectric reservoirs in the tropics may release more greenhouse gases than would be released by the fossil fuels that were offset by hydroelectricity.

Carbon enters lakes from from the atmosphere (as CO2) and from streams and runoff in both particulate and dissolved forms. Aerobic biological processes incorporate carbon into living organisms where it is either stored or respired (as CO2). In lake sediments, particulate carbon (including dead organisms) is decomposed by bacteria, producing CO2 and methane (CH4). Reservoirs that pull water from the hypolimnion release methane as well as particulate and dissolved carbon.

The Future

Rainfall in temperate regions is expected to decrease as the climate changes, meaning there will be less runoff flowing into Missouri’s lakes. Water use will increase as our water supplies are used to grow food and biofuels for an ever-expanding population. To address the water shortages humans will build more impoundments, and by 2050 the total global surface area of impoundments is estimated to increase 250%. At present, lakes and reservoirs bury an average of 0.6 Pg of carbon in their sediments each year, the equivalent of 1.3 million pounds TIMES one million (1 Pg = 1015 grams). That amount should increase significantly as the number of impoundments increases.
It seems certain that climate changes will have an effect on lakes, but those changes will vary from region to region or even lake to lake. Prolonged stratification will increase the amount of time deep sediments are without oxygen, thus increasing nutrient cycling from sediments. With less runoff entering lakes, residence times will increase, allowing suspended sediment particles to settle out. As a direct result of increased light penetration, higher nutrient concentrations, and higher temperature, Missouri lakes, particularly in the northern plains region, are likely to grow more algae in the summer than they do now. Community structures will change as differently adapted predator, prey, and forage species inhabit our lakes. The new reservoirs we build will store more carbon than they emit, but not for a number of years, and not enough to offset population growth. Ultimately, we will have to adapt to our new climate and our lakes’ new character.

The journal volume referenced can be found online:
http://aslo.org/lo/toc/vol_54/issue_6_part_2/index.html

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