S. Leigh McCallister

Evidence for the respiration of ancient terrestrial organic C in northern temperate lakes and streams

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), 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 Québec. 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 and organic carbon balance of U.S. East Coast estuaries: A synthesis approach

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.

Selective consumption and metabolic allocation of terrestrial and algal carbon determine allochthony in lake bacteria.

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 δ13C 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.

Dissolved organic sulfur in the ocean: Biogeochemistry of a petagram inventory

Inventory of an essential marine element Sulfur is necessary for marine primary production and has a large impact on climate processes. Because it is difficult to detect accurately, the amount of dissolved organic sulfur in the ocean is poorly defined. Ksionzek et al. measured dissolved organic sulfur in the Atlantic to estimate its distribution and infer its quantity in the worlds oceans (see the Perspective by Levine). The findings suggest that dissolved organic sulfur exceeds all other forms of organic sulfur by a factor of 10. Science, this issue p. 456; see also p. 418 The dissolved fraction of marine organic sulfur is more abundant than all other forms by a factor of 10. 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.

Response to Comment on “Dissolved organic sulfur in the ocean: Biogeochemistry of a petagram inventory”

Dittmar et al 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.Dittmar et al. 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.