Inland Waters

14.2.1 Early Understandings

The study of carbon cycling in lakes, streams, and large rivers started in the early part of the last century with the development of the ecosystem concept as a functional unit by which scientists could define the physical, chemical, and biological structure of the world around them. This concept was adapted from terrestrial to aquatic systems through seminal work (Lindeman 1942) partitioning the movement of energy, and as a result carbon, across trophic levels in lakes. A second concept relevant to carbon cycling in inland waters is the tracing of elements through natural systems, which has a long history in geochemistry and had developed prior to the notion of ecology. The convergence of these two concepts that define the interactions among biological, physical, and chemical environments was permanently established by the need to 1) improve water quality from eutrophication of freshwaters by agricultural fertilizer inputs and 2) understand the impacts of acid rain through the exploration of elemental cycling in whole lakes (Johnson and Vallentyne 1971) and at the watershed scale (Likens 1977). Although carbon remained secondary to the tracing of nutrients and other chemical species, research clearly established that carbon from terrestrial systems provided energy to and influenced the structure of aquatic systems (Pace et al., 2004) and that the boundary between these two systems might not be so discrete. A rich field of ecosystem-based science subsequently developed that expanded dramatically into this century. In an attempt to synthesize carbon dynamics in freshwaters, a group through the National Center for Ecological Analysis and Synthesis produced a seminal paper that highlighted the magnitude of the flows of carbon through freshwaters at the global scale (Cole et al., 2007), laying the foundation for the research that supports this chapter.

14.2.2 First State of the Carbon Cycle Report

The First State of the Carbon Cycle Report (SOCCR1) identified rivers and lakes as a net sink of 25 Tg C per year into sediments across North America (CCSP 2007; Pacala et al., 2001; Stallard 1998). The total lateral transfer of carbon (including both DIC and DOC) to the ocean was estimated to be 35 Tg C per year (Pacala et al., 2001) and was considered highly uncertain. These estimates did not include Canada, Mexico, or the Great Lakes because of a lack of available data for each. It is important to note that all estimates for rivers were considered sinks or net transfers of carbon to the coastal environment, as well as storage of carbon in lake and reservoir sediments. Since 2007, the research community has widely accepted that inland aquatic ecosystems also function as an important interface for carbon exchange between terrestrial ecosystems and the atmosphere (Cole et al., 2007; Tranvik et al., 2009). Evidence summarized herein shows that, over short timescales, freshwaters function as sources of atmospheric CO₂. Also provided are improved estimates of burial in lakes and reservoirs and lateral transfer to the coast. The updated budget increases the total carbon fluxes from inland waters by a factor of two over those reported in SOCCR1 (see Table 14.1) and alters the previous perception of inland waters as a sink of atmospheric CO₂. These estimates of inland water fluxes, coupled with a better understanding of flow paths for carbon losses and export from wetland and coastal environments, provide evidence that the majority of terrestrially derived carbon moving through inland waters is released to the atmosphere as CO₂.