In Defense of Mud II: Lakes as Carbon Sinks

Abstract

In late 1969, Edward S. Deevey, Jr. delivered a talk to the National Water Commission, with the catchy title “In Defense of Mud.” The text of his speech was published the following year in the Bulletin of the Ecological Society of America (Deevey 1970). The year of publication coincided with the first celebration of Earth Day and was a time of growing realization that humans were transforming the planet, thereby threatening many species with extinction. In what may have appeared to be an odd argument for conserving non-marine aquatic ecosystems, Deevey proposed that lakes, wetlands, ponds, and estuaries deserved protection, not only because they are homes for iconic birds, mammals, and fish, but because the sediments that accumulate within them are habitats for the bacteria that reduce nitrate and sulfate, making them important players in key global biogeochemical cycles. Humans were emitting tremendous amounts of sulfate and nitrate into the atmosphere. At the same time, they were ditching and draining wetlands for agriculture and construction, thus endangering the anoxic, muddy realm in which nitrate- and sulfur-reducing microbes live. Although it is unlikely that anyone ever saw a “Save the Microbes” bumper sticker, Deevey's publication represented a novel and holistic perspective on the need to conserve continental aquatic ecosystems.

Fifty years later, Hale (2020) published a commentary, “In Praise of Mud,” also in the Bulletin of the Ecological Society of America. It expanded upon Deevey's work by providing a list of 38 reasons why humans and other animals should appreciate lake sediments, among them because they provide myriad ecosystem services, are used in the manufacture of construction and beauty products, and are rich archives of past climate and environmental information. Hale mentioned that we should also be grateful for a particular service provided by lake mud, i.e., that it “sequesters carbon, preventing its return as carbon dioxide to the atmosphere.” We agree with his claim and will focus here on that important ecosystem function. We address the issue because there exists confusion about the role lakes play in global carbon (C) cycling, particularly with respect to their being sources or sinks of C relative to the atmosphere. Given the recent rise in the concentration of CO₂ in the atmosphere and consequent global climate change, it is crucial that we understand the processes associated with C as it moves through inland aquatic ecosystems.

For this discussion, we use the generally accepted definition of a “sink” as it applies to any element. Broadly, an element sink can be thought of as an area of the landscape or a volume of the biosphere where the mass of an element increases over time, i.e., where inputs of the element exceed outputs. We define a C sink as a pool or compartment in the carbon biogeochemical cycle where C-containing material accumulates and is stored for a protracted period. The C sink absorbs more carbon from the atmosphere than it releases to the atmosphere. A C source, on the other hand, supplies more carbon to the atmosphere than it sequesters. We note that some engineering strategies designed to reduce C concentration in the atmosphere involve the capture of gaseous CO₂ and its subsequent storage, in solid form, in a subterranean sink. The percent of C stored (inputs minus outputs) relative to total C outputs has no bearing on the sink versus source determination, and it follows from our definition that the term “net sink” is redundant.

We contend that almost all lakes behave as C sinks on timescales lasting millennia, and in some cases, much longer. Lake basins continue to act as C sinks as long as they accumulate C-bearing sediment, in the form of organic matter (OM), typically ~45–50% organic C (OC) by mass, or carbonate (e.g., as CaCO₃, 12% C_inorg by mass), as snail, bivalve, or ostracod shells, or as photosynthetically or chemically induced precipitates. Inorganic C storage is particularly notable in lakes of karst terrains (Martin 2017), where carbonates can account for >90% of the sediment mass in shallow-water areas of waterbodies (Hodell et al. 2007).

We felt compelled to address this topic, given the number of studies that purport to show that lakes are “sources” of C to the atmosphere. We do not dispute the fact that in many lakes, perhaps most, more gaseous CO₂ is transferred across the air–water interface from the water column to the atmosphere than in the opposite direction. Nonetheless, if any C that enters a lake is “permanently” stored on the lake bottom, in any form, we argue that the basin functions as a C sink. That is, the lake is a C sink if there is a growing amount of C being accumulated and stored below the area circumscribed by the lake shoreline.

There are several possible reasons why some investigators have concluded that lakes are not C sinks: (1) they misunderstand the definition of the term “sink” or adopt a modified definition, and/or (2) they use inappropriate measures to determine if a lake is accumulating C. For example (a) they look at partial pressure of CO₂ in surface waters, or pH and alkalinity values, simply to evaluate whether dissolved CO₂ in the water column will tend to evolve to the atmosphere, (b) they directly measure gas exchange across the air-water interface, or (c) they look at the ratio of CO₂ uptake during lacustrine primary production (photosynthesis) to carbon dioxide gas produced by community respiration in the water column (P/R). In such studies, accruing C-containing sediments are typically ignored. Lastly, investigators may decide that some arbitrary proportion of C fixed in the lake must be sequestered in the sediments to rightfully qualify the waterbody as a C sink. This requirement resulted in the contradictory claim that subtropical stormwater ponds are net sources of C to the atmosphere at the same time that they continue to accumulate ever greater amounts of C through time (Goeckner et al. 2022). In contrast, a study of temperate artificial ponds determined that those waterbodies displayed high OC sequestration rates, and suggested that global OC burial in inland waters is probably underestimated (Holgerson et al. 2024).

We are not the first to address the confusion about the role of lakes as “sources vs. sinks” in the global C cycle. Dean and Gorham (1998) mentioned that some studies had suggested lakes were sources of CO₂ in the atmosphere (Cole et al. 1994, Molot and Dillon 1997), but they then went on to make the case that sedimentation in inland waterbodies stored substantial C and that lakes and wetlands were thus C sinks. The authors estimated that lakes, reservoirs, and peatlands around the world accumulate OC at a rate of ~300 Tg/year, whereas OC accumulation in the oceans amounts to ~100 Tg/year. By their reckoning, lakes account for ~42 Tg/year (14%) of the total OC stored annually in inland lentic aquatic ecosystems.

Much of the recent interest in C cycling in lakes is related to how climate and human activities influence C storage in (Anderson et al. 2020) or loss from lakes (St. Louis et al. 2000, Kosten et al. 2010). It has been noted that the C cycle, as it applies to lakes, both regulates and is regulated by climate change (Tranvik et al. 2009, Yasarer 2015). Carbon enters lakes in gaseous form across the air-water interface, as dissolved inorganic C (DIC) in rainfall and other input waters, and as dissolved or particulate organic C (DOC/POC) from the watershed and airshed. Carbon that is fixed via photosynthesis or bio-precipitated within the lake ultimately came from outside the waterbody (i.e., from the atmosphere). It is indisputable that large amounts of gaseous C are lost from lakes to the atmosphere, in the form of CO₂ and methane (CH₄). Some lakes are even thought to be heterotrophic (Bachmann et al. 2000), i.e., they display higher rates of water column respiration (CO₂ generation) than primary production (CO₂ consumption). We note that even such heterotrophic lakes can function as C sinks, as long as they continue to accumulate carbonaceous sediment. Indeed, most lakes probably evolve almost all the C that enters their water column back to the atmosphere, whether the C in the water column is present as DIC, allochthonous DOC/POC, or C fixed within the lake by algae and macrophytes, the latter representing POC that was fixed by photosynthesis, using lake DIC that ultimately came from the atmosphere. It is important, however, to keep in mind that the C in the lake, whether allochthonous DIC, DOC, POC, or fixed in situ via photosynthesis (autochthonous), ultimately came from the atmosphere. As some have suggested, it might be more appropriate to view lakes as “conduits” through which substantial C passes on its way back to the atmosphere (Tranvik et al. 2009, Yasarer 2015).

The case for lakes being C sinks can benefit from drawing an analogy to household economies (Fig. 1A, B). Although an imperfect comparison, it may be helpful to think of the money that enters and leaves household accounts as analogous to C flow into and out of lakes. Just as lakes receive C from multiple external sources, household income can come from wages, interest, investments, etc. Some hobbyists may sell artisanal wares like ceramics and jewelry that they fabricate at home, which can be thought of as an autochthonous (in-house) revenue source. As is the case for C loss from lakes, a large proportion of household income leaves home accounts to cover living expenses, i.e., necessities like rent or mortgage payments, groceries, clothing, furniture, utilities, car payments, etc., as well as discretionary expenditures such as vacations and entertainment. A wise money manager will, however, arrange for some portion of their income to be earmarked for use later in life. That money can be put into savings or retirement accounts, stocks and bonds, etc., all of which, one hopes, will continue to grow in value over time, i.e., accrue more money. Depending on monthly household income and financial obligations, smaller or larger sums of money can be earmarked for savings. Money deposited in savings is temporarily (decades?) removed from circulation in the economy, just as C in lake-bottom sediments is prevented from cycling back into the atmosphere, often on timescales lasting thousands of years.

The household economy analogy may be imperfect for several reasons, perhaps most obviously because human life spans and the life spans of lakes on the landscape are so different. After leaving their jobs, retirees may begin to draw down their savings accounts, which they rely on when they no longer receive salary, rendering those accounts a money source to the economy rather than a sink. To further analogize, we can envision circumstances in which accumulated carbonaceous lake sediments cease to accrue and then become sources of C to the atmosphere: (1) shallow (ephemeral) lakes may dry completely, exposing their accumulated organic and carbonate deposits to sub-aerial conditions and oxidation/dissolution, (2) accumulated peat deposits may be mined and combusted for fuel or used as soil amendments, (3) wetland soils may be ditched and drained for cropping and animal grazing, and (4) lake sediments may be dredged to enhance navigation or improve water quality. But these examples are exceptions to the general rule. Almost all lakes around the world, most of which formed since the last deglaciation, continue to hold water and accumulate C-bearing sediments.

Perhaps one way to address the “source/sink conundrum” is to pose the following hypothetical question: If a lake were to sequester in its sediments all the C that entered its water column, i.e., if it were 100% efficient at storing incoming C on the lake bottom, could we all agree that the lake behaves as a C sink? We suspect that most or all individuals would respond in the affirmative. We can now pose the same question for a lake that is 75% efficient at storing C in its sediments, i.e., it permanently buries three-quarters of all the incoming C. Again, most respondents would probably agree that such a lake is a C sink. If we reduce the efficiency to 50%, half of all incoming C is preserved on the lake bottom, and half makes its way back to the atmosphere. Here we might refer to our household economy analogy and ask, who would not be happy to be able to put half of their annual income into a savings account? Let's now entertain the same question for lakes that sequester in their sediment just a few percent of their incoming C (analogous to the mean percent of disposable personal income that US residents put into savings, ~4%). This is probably the case for most lakes, and we contend that they are nevertheless C sinks; places where C is stored on timescales of years to many millennia. We also argue that the assignment of an arbitrary criterion, in terms of percent of incoming C stored, cannot be used to determine if a lake is a C source or sink.

We began this discussion by presenting our definition of a C sink to avoid semantic ambiguity. It is our contention that the “source/sink debate” as applied to inland waterbodies, diverts attention from interesting questions about the role of lakes in C cycling (Cole et al. 2007). Fortunately, many who study continental aquatic ecosystems, especially paleolimnologists, i.e., scientists who focus on the sediment compartment where C is stored, are addressing those questions. To list just a few such questions: (1) how do C accumulation rates differ spatially within lakes? (Lin et al. 2022), (2) how do C accumulation rates differ among lakes, with respect to trophic state, temperature, and dominant primary producer community (macrophytes versus phytoplankton)? (Kosten et al. 2010, Balmer and Downing 2011, Alcocer et al. 2014, Heathcote et al. 2015, Reed et al. 2018, Waters et al. 2019, Anderson et al. 2020), (3) how has C sequestration in lakes changed throughout the Holocene? (Kenney et al. 2016), (4) how does C storage on lake bottoms change in response to land clearance? (Heathcote and Downing 2012, Dietz et al. 2015), (5) what are the principal sources (allochthonous/autochthonous) of C in lake sediments? (Meyers and Ishiwatari 1993), (6) what factors affect the efficiency of C sequestration [storage/input] in lakes? (Tranvik et al. 2009), (7) what is the magnitude of CH₄ evolution from lakes? (Johnson et al. 2022), (8) what is the importance of lake and reservoir C storage in the global C biogeochemical cycle (Mendonça et al. 2017)? and how do terrestrial and aquatic biogeochemical processes in karst terrains influence C flux to and from the atmosphere (Chen et al. 2023)? As we continue to further address those questions, it will be helpful to keep in mind that lake sediments are C sinks and that continental water bodies play an important role in sequestering C and keeping it out of the atmosphere.