Circulation Processes in Lakes
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CARBON CYCLE IN LAKES Amy Myrbo Department of Earth Sciences, LacCore/Limnological Research Center, University of Minnesota, Minneapolis, MN, USA
Introduction The lacustrine carbon cycle involves the interactions of gaseous, dissolved, and solid forms of C. Inputs include carbon derived from the atmosphere, surface water, groundwater, regolith, and bedrock. Transformations within the lake system include biotic and abiotic processes occurring in both the presence and absence of oxygen. Losses occur to the atmosphere, to streams and groundwater, and to the sediment underlying the water (Figure 1). Many lacustrine processes have familiar counterparts in marine systems. Lakes are not small oceans however; important aspects of the carbon cycle such as the relationship between dissolved gases and the atmosphere, and the role of organisms in carbonate mineral formation, differ strikingly in lakes as compared with the ocean. Lakes are strongly influenced by processes occurring in their catchments, because of the high ratio of shoreline to open water, while the oceans are affected only locally by continental processes. Large lakes such as the Laurentian and East African Great Lakes behave more like oceans than like small lakes; however, their water chemistries differ significantly from typical ocean chemistry and from one another. Lakes process a tremendous amount of carbon, given their small collective areal extent, sequestering organic and inorganic carbon in their sediments as well as emitting large quantities of carbon dioxide and methane to the atmosphere. Density stratification of lakes plays a major role in carbon cycling. The differentiation of the cold, dark,
stagnant hypolimnion from the warmer mixed epilimnion, part or all of which may be within the photic zone, promotes different biological and chemical processes in the two layers. Photosynthesis, wind mixing, and atmospheric exchange lead to higher pH and dissolved oxygen saturation (or supersaturation) in the epilimnion, while the hypolimnion, isolated from the atmosphere and receiving sinking organic matter, may evolve toward anoxia, carbon dioxide supersaturation, and low pH due to respiration. Lake water chemistry is highly variable, ranging from almost pure rainwater in lakes with small catchments on crystalline bedrock, to several times the salinity of seawater, and from low to high pH (~3–11). The ionic composition of lake water is a reflection of the surrounding geology and is further affected by climatic factors such as precipitation-evaporation balance. Major ions in surface water may include virtually any combination of K+, Na+, Mg2+, Ca2+, HCO3, CO32, SO42, Cl; under reducing conditions in anoxic bottom waters, dissolved Fe2+ and Mn2+ may also be major. Lakes with sufficient alkalinity may precipitate carbonate minerals, similar to the formation of limestone in the oceans; these are sometimes termed “hardwater” lakes, and in these lakes carbonate geochemistry represents an important part of the C cycle.
C reservoirs and fluxes in lakes Lakes
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