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III.12 Freshwater Carbon and Biogeochemical Cycles Darren Bade OUTLINE 1. Freshwater ecosystems 2. Reduction–oxidation reactions 3. Metal cycling: Fe and Hg 4. Phosphorus cycling 5. Nitrogen cycling 6. Sulfur cycling 7. Carbon cycling 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 in- fluences 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. GLOSSARY 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 1. FRESHWATER ECOSYSTEMS 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. Lakes provide an ideal example to consider element cycling for several reasons. Because lakes often have easily defined ecosystem boundaries and fairly long water residence times, studying the internal cycling of elements becomes more tractable. Also, lakes contain many habitats that are similar to habitats in other freshwater ecosystems. A brief description of these habitats is warranted (figure 1). Two important zones, the photic and aphotic zones, are present in many lakes with sufficient depth. The photic zone represents the surface water of a lake where light is sufficient for photosynthesis to occur. The aphotic zone is defined as the volume of water where photosynthetically available radiation is less than 1% of that present at the surface. Because light limits photosynthesis in the aphotic zone, respiratory processes usually dominate. Processes dominant in the aphotic zone of a lake are also likely to occur in groundwater. In streams, the hyporheic zone, an area below the streambed that contains the interaction between surface and groundwater, has similar processes as the aphotic zone. Although relatively unstudied, lakes, like streams, also contain an area where lake and groundwater interact. Lake habitats can be further divided into littoral and pelagic zones. The littoral zone is the region of the lake, near shore, where there is significant interaction between bottom substrates and the photic zone. Surface water in streams and wetlands is often subject to similar light and temperature regimes as the littoral zone of a lake. Because rooted plants can occupy the littoral zone of a lake, the littoral zone provides many similarities to wetlands, and many times there is little distinction between...


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