Freshwater Carbon and
|3.||Metal cycling: Fe and Hg|
Freshwater lakes provide an ideal example for considering the carbon cycle and other biogeochemical cycles. A range of redox conditions exists in lakes that allows observation of numerous chemical and biochemical processes. The processes are not limited to freshwater lakes, and similar examples can be found in marine and terrestrial systems. The cycles of carbon and other elements are closely linked. Production of organic carbon depends on cycling of nutrients such as nitrogen and phosphorus. Respiration of organic carbon alters the redox condition, which in turn influences the cycling of nutrients. Many other elements can be influenced by redox conditions (e.g., sulfur, iron, and mercury). The elements can have indirect effects on carbon cycling or can be deleterious to organisms present in the ecosystem.
airshed. A region sharing a common flow of air
biogeochemistry. The scientific study of the physical, chemical, geological, and biological processes and reactions that govern the cycles of matter and energy in the natural environment
ecosystem. A natural unit consisting of all plants, animals and microorganisms (biotic factors) in an area functioning together with all of the nonliving physical and chemical (abiotic) factors of the environment.
lake. A body of water of considerable size surrounded entirely by land
micronutrient. A chemical element necessary in relatively small quantities for organism growth
nutrient. A chemical element necessary for organism growth
watershed. The area of land where all of the water that is under it or drains off of it goes into the same place
Streams, rivers, lakes, and wetlands constitute some of the most obvious natural freshwater ecosystems. Additionally, groundwater and intermittent pools can be considered in this context. Artificial ecosystems, such as small impoundments, large reservoirs, and engineered wetlands, also are labeled as freshwater ecosystems.
There are many physical, chemical, and biological differences among these varied ecosystems. Hydrology offers one brief indication of the differences. Water residence time, the average time a molecule of water spends in an ecosystem, varies from minutes to years to millennia in streams, lakes, and ancient groundwater. Related to the water residence time is the movement of water. At one end of the spectrum are streams, which have strong unidirectional flow governed by gravity. At the other end of the spectrum, lakes typically show little directional flow.
There is one key commonality that links all the freshwater ecosystems. Indeed, this commonality also links marine and terrestrial counterparts. The cycling of elements in all ecosystems is mainly controlled by aqueous chemical reactions, including those reactions mediated by organisms. In that vein, the cycles examined in this chapter are considered mainly in the context of freshwater lakes.