Although sulfur is an essential element for marine primary production and critical for climate processes, little is known about the oceanic pool of nonvolatile dissolved organic sulfur (DOS). We ...present a basin-scale distribution of solid-phase extractable DOS in the East Atlantic Ocean and the Atlantic sector of the Southern Ocean. Although molar DOS versus dissolved organic nitrogen (DON) ratios of 0.11 ± 0.024 in Atlantic surface water resembled phytoplankton stoichiometry (sulfur/nitrogen ~ 0.08), increasing dissolved organic carbon (DOC) versus DOS ratios and decreasing methionine-S yield demonstrated selective DOS removal and active involvement in marine biogeochemical cycles. Based on stoichiometric estimates, the minimum global inventory of marine DOS is 6.7 petagrams of sulfur, exceeding all other marine organic sulfur reservoirs by an order of magnitude.
Here we explore strategies of resource utilization and allocation of algal versus terrestrially derived carbon (C) by lake bacterioplankton. We quantified the consumption of terrestrial and algal ...dissolved organic carbon, and the subsequent allocation of these pools to bacterial growth and respiration, based on the δ(13)C isotopic signatures of bacterial biomass and respiratory carbon dioxide (CO2). Our results confirm that bacterial communities preferentially remove algal C from the terrestrially dominated organic C pool of lakes, but contrary to current assumptions, selectively allocate this autochthonous substrate to respiration, whereas terrestrial C was preferentially allocated to biosynthesis. The results provide further evidence of a mechanism whereby inputs of labile, algal-derived organic C may stimulate the incorporation of a more recalcitrant, terrestrial C pool. This mechanism resulted in a counterintuitive pattern of high and relatively constant levels of allochthony (~76%) in bacterial biomass across lakes that otherwise differ greatly in productivity and external inputs.
Northern rivers and lakes process large quantities of organic and inorganic carbon from the surrounding terrestrial ecosystems. These external carbon inputs fuel widespread CO₂ supersaturation in ...continental waters, and the resulting CO₂ emissions from lakes and rivers are now recognized as a globally significant loss of terrestrial production to the atmosphere. Whereas the magnitude of emissions has received much attention, the pathways of C delivery and processing that generate these emissions are still not well-understood. CO₂ outgassing in aquatic systems has been unequivocally linked to microbial degradation and respiration of terrestrial organic carbon (OC) r but the nature (i.e., age and source) of this OC respired in surface waters is largely unknown. We present direct radiocarbon measurements of OC respired by bacteria in freshwater aquatic systems, specifically temperate lakes and streams in Quebec. Terrestrial OC fuels much of the respiration in these systems, and our results show that a significant fraction of the respired terrestrial OC is old (in the range of 1,000-3,000 y B.P.). Because the bulk OC pools in these lakes is relatively young, our results also suggest selective removal of an old but highly bioreactive terrestrial OC pool and its conversion to CO₂ by bacteria. The respiration of ancient ¹⁴C-depleted terrestrial C in northern lakes and rivers provides a biological link between contemporary aquatic carbon biogeochemistry and paleo-conditions in the watershed, and it implies the aquatic-mediated return to the atmosphere of C putatively considered permanently stored, thus challenging current models of long-term C storage in terrestrial reservoirs.
Net ecosystem production (NEP) and the overall organic carbon budget for the estuaries along the East Coast of the United States are estimated. We focus on the open estuarine waters, excluding the ...fringing wetlands. We developed empirical models relating NEP to loading ratios of dissolved inorganic nitrogen to total organic carbon, and carbon burial in the sediment to estuarine water residence time and total nitrogen input across the landward boundary. Output from a data‐constrained water quality model was used to estimate inputs of total nitrogen and organic carbon to the estuaries across the landward boundary, including fluvial and tidal‐wetland sources. Organic carbon export from the estuaries to the continental shelf was computed by difference, assuming steady state. Uncertainties in the budget were estimated by allowing uncertainties in the supporting model relations. Collectively, U.S. East Coast estuaries are net heterotrophic, with the area‐integrated NEP of −1.5 (−2.8, −1.0) Tg C yr−1 (best estimate and 95% confidence interval) and area‐normalized NEP of −3.2 (−6.1, −2.3) mol C m−2 yr−1. East Coast estuaries serve as a source of organic carbon to the shelf, exporting 3.4 (2.0, 4.3) Tg C yr−1 or 7.6 (4.4, 9.5) mol C m−2 yr−1. Organic carbon inputs from fluvial and tidal‐wetland sources for the region are estimated at 5.4 (4.6, 6.5) Tg C yr−1 or 12 (10, 14) mol C m−2 yr−1 and carbon burial in the open estuarine waters at 0.50 (0.33, 0.78) Tg C yr−1 or 1.1 (0.73, 1.7) mol C m−2 yr−1. Our results highlight the importance of estuarine systems in the overall coastal budget of organic carbon, suggesting that in the aggregate, U.S. East Coast estuaries assimilate (via respiration and burial) ~40% of organic carbon inputs from fluvial and tidal‐wetland sources and allow ~60% to be exported to the shelf.
Key Points
Organic carbon budget of U.S. East Coast estuaries is estimated
In the aggregate, estuaries are net heterotrophic
In the aggregate, estuaries export organic carbon to the shelf
Food webs in aquatic systems can be supported both by carbon from recent local primary productivity and by carbon subsidies, such as material from terrestrial ecosystems, or past in situ primary ...productivity. The importance of these subsidies to respiration and biomass production remains a topic of debate. While some studies have reported that terrigenous organic carbon supports disproportionately high zooplankton production, others have suggested that phytoplankton preferentially support zooplankton production in aquatic ecosystems. Here we apply natural abundance radiocarbon (Δ14C) and stable isotope (δ13C, δ15N) analyses to show that zooplankton in Lake Superior selectively incorporate recently fixed, locally produced (autochthonous) organic carbon even though other carbon sources are readily available. Estimates from Bayesian isotopic modeling based on Δ14C and δ13C values show that the average lake-wide median contributions of recent in-lake primary production and terrestrial, sedimentary, and bacterial organic carbon to the bulk POM in Lake Superior were 58%, 5%, 33%, and 3%, respectively. However, isotopic modeling estimates also show that recent in situ production contributed a disproportionately large amount (median, 91%) of the carbon in mesozooplankton biomass in Lake Superior. Although terrigenous organic carbon and old organic carbon from resuspended sediments were significant portions (median, 38%) of the available basal food resources, these contributed only a small amount to mesozooplankton biomass. Comparison of zooplankton food sources based on their radiocarbon composition showed that terrigenous organic carbon was relatively more important in rivers and small lakes, and the proportion of terrestrially derived material used by zooplankton correlated with the hydrologic residence time and the ratio of basin area to water surface area.
It has often been hypothesized that the dissolved organic carbon (DOC) pool of algal origin in lakes is more bioavailable than its terrestrial counterpart, but this hypothesis has seldom been ...directly tested. Here we test this hypothesis by tracking the production and isotopic signature of bacterial respiratory CO2 in 2 week lake water incubations and use the resulting data to reconstruct and model the bacterial consumption dynamics of algal and terrestrial DOC. The proportion of algal DOC respired decreased systematically over time in all experiments, suggesting a rapid consumption and depletion of this substrate. Our results further show that the algal DOC pool was used in proportions and at rates twice and 10 times as high as the terrestrial DOC pool, respectively. On the other hand, the absolute amount of labile terrestrial DOC was on average four times higher than labile algal DOC, accounting for almost the entire long‐term residual C metabolism, but also contributing to short‐term bacterial C consumption. The absolute amount of labile algal DOC increased with chlorophyll a concentrations, whereas total phosphorus appeared to enhance the amount of terrestrial DOC that bacteria could consume, suggesting that the degradation of these pools is not solely governed by their respective chemical properties, but also by interactions with nutrients. Our study shows that there is a highly reactive pool of terrestrial DOC that is processed in parallel to algal DOC, and because of interactions with nutrients, terrestrial DOC likely supports high levels of bacterial metabolism and CO2 production even in more productive lakes.
Key Points
Algal and terrestrial C degraded in parallel but differentiallyHigh bacterial consumption of terrestrial organic C on short‐ and long‐termPhosphorus increases the degradation of terrestrial DOC in lakes
We explore the role of lakes in carbon cycling and global climate, examine the mechanisms influencing carbon pools and transformations in lakes, and discuss how the metabolism of carbon in the inland ...waters is likely to change in response to climate. Furthermore, we project changes as global climate change in the abundance and spatial distribution of lakes in the biosphere, and we revise the estimate for the global extent of carbon transformation in inland waters. This synthesis demonstrates that the global annual emissions of carbon dioxide from inland waters to the atmosphere are similar in magnitude to the carbon dioxide uptake by the oceans and that the global burial of organic carbon in inland water sediments exceeds organic carbon sequestration on the ocean floor. The role of inland waters in global carbon cycling and climate forcing may be changed by human activities, including construction of impoundments, which accumulate large amounts of carbon in sediments and emit large amounts of methane to the atmosphere. Methane emissions are also expected from lakes on melting permafrost. The synthesis presented here indicates that (1) inland waters constitute a significant component of the global carbon cycle, (2) their contribution to this cycle has significantly changed as a result of human activities, and (3) they will continue to change in response to future climate change causing decreased as well as increased abundance of lakes as well as increases in the number of aquatic impoundments.
This study applies radiocarbon and stable carbon isotopic distributions to investigate carbon sources and cycling within Lake Superior. We report the radiocarbon (Δ13C) and stable carbon isotope ...(δ13C) values and the carbon concentrations within dissolved organic carbon (DOC), particulate organic carbon (POC), and dissolved inorganic carbon (DIC) in the lake’s western basin water column. Samples were taken during spring mixing and late-summer thermal stratification over a 2-yr period (2007–2009). Distinct processes operating in the surface (photosynthesis) and deep waters (sediment resuspension and pore-water intrusion) control the relative contribution of modern and ancient DOC and POC in the water column. The terrigenous carbon input to the open lake POC varied from 13% ± 4% during late summer stratification to 9% ± 3% during spring mixing, with most of the terrestrial carbon being 14C-enriched (modern). The DIC reservoir cycles rapidly, with a bulk Δ13CDIC value that records atmospheric radiocarbon levels from 3 yr prior to sampling. The DOC pool recycles on a longer time scale than does the DIC, with a DOC residence time of ≤ 60 yr. The suspended POC was in most cases older than co-occurring DOC, most likely as a result of resuspension of lake sediments.
Annually, rivers and inland water systems deliver a significant amount of terrestrial organic matter (OM) to the adjacent coastal ocean in both particulate and dissolved forms; however, the metabolic ...and biogeochemical transformations of OM during its seaward transport remains one of the least understood components of the global carbon cycle. This transfer of terrestrial carbon to marine ecosystems is crucial in maintaining trophic dynamics in coastal areas and critical in global carbon cycling. Although coastal regions have been proposed as important sinks for exported terrestrial materials, most of the global carbon cycling data, have not included fjords in their budgets. Here we present distributional patterns on the quantity and quality of dissolved OM in Fiordland National Park, New Zealand. Specifically, we describe carbon dynamics under diverse environmental settings based on dissolved organic carbon (DOC) depth profiles, oxygen concentrations, optical properties (fluorescence) and stable carbon isotopes. We illustrate a distinct change in the character of DOC in deep waters compared to surface and mid-depth waters. Our results suggest that, both, microbial reworking of terrestrially derived plant detritus and subsequent desorption of DOC from its particulate counterpart (as verified in a desorption experiment) are the main sources of the humic-like enriched DOC in the deep basins of the studied fjords. While it has been suggested that short transit times and protection of OM by mineral sorption may ultimately result in significant terrestrial carbon burial and preservation in fjords, our data suggests the existence of an additional source of terrestrial OM in the form of DOC generated in deep, fjord water.
•Organic matter dynamics in fjords were studied using fluorescence and stable isotopes.•Distinct changes in DOM character with depth were observed.•Terrestrial DOM source in deep fjord waters suggested as POM desorption derived.
Dittmar
proposed that mixing alone can explain our observed decrease in marine dissolved organic sulfur with age. However, their simple model lacks an explanation for the origin of sulfur-depleted ...organic matter in the deep ocean and cannot adequately reproduce our observed stoichiometric changes. Using radiocarbon age also implicitly models the preferential cycling of sulfur that they are disputing.
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