Scott C. Neubauer

Moving Beyond Global Warming Potentials to Quantify the Climatic Role of Ecosystems

For decades, ecosystem scientists have used global warming potentials (GWPs) to compare the radiative forcing of various greenhouse gases to determine if ecosystems have a net warming or cooling effect on climate. On a conceptual basis, the continued use of GWPs by the ecological community may be untenable because the use of GWPs requires the implicit assumption that greenhouse gas emissions occur as a single pulse; this assumption is rarely justified in ecosystem studies. We present two alternate metrics—the sustained-flux global warming potential (SGWP, for gas emissions) and the sustained-flux global cooling potential (SGCP, for gas uptake)—for use when gas fluxes persist over time. The SGWP is generally larger than the GWP (by up to ~40%) for both methane and nitrous oxide emissions, creating situations where the GWP and SGWP metrics could provide opposing interpretations about the climatic role of an ecosystem. Further, there is an asymmetry in methane and nitrous oxide dynamics between persistent emission and uptake situations, producing very different values for the SGWP vs. SGCP and leading to the conclusion that ecosystems that take up these gases are very effective at reducing radiative forcing. Although the new metrics are more realistic than the GWP for ecosystem fluxes, we further argue that even these metrics may be insufficient in the context of trying to understand the lifetime climatic role of an ecosystem. A dynamic modeling approach that has the flexibility to account for temporally variable rates of greenhouse gas exchange, and is not limited by a fixed time frame, may be more informative than the SGWP, SGCP, or GWP. Ultimately, we hope this article will stimulate discussion within the ecosystem science community about the most appropriate way(s) of assessing the role of ecosystems as regulators of global climate.

Effects of sea level induced disturbances on high salt marsh metabolism

Salt marshes, which provide a transition between the marine and terrestrial environments around much of the temperature world, will be the first ecosystem to feel the effects of an increased rate of sea level rise. This study examined the metabolic responses of a high salt marsh to increased inundation and wrack deposition associated with sea level rise. We measured changes in ecosystem and soil photosynthesis and respiration by analyzing carbon dioxide fluxes in the light and dark. Data from seasonal flux measurements were combined with continuously measured light and temperature data to develop a model that estimated annual production and respiration. Results suggested that increased inundation will reduce respiration rates to a greater extent than production, yielding a moderate net loss of organic carbon from the high marsh. The model also predicted a substantial loss of organic carbon from wrack-affected areas. This decreased organic carbon input may play an important role in the ability of the marsh to maintain elevation relative to sea level rise.

Nitrogen cycling and ecosystem exchanges in a Virginia tidal freshwater marsh

Tidal freshwater marshes are diverse habitats that differ both within and between marshes in terms of plant community composition, sediment type, marsh elevation, and nutrient status. Because our knowledge of the nitrogen (N) biogeochemistry of tidal freshwater systems is limited, it is difficult to assess how these marshes will respond to long-term progressive nutrient loading due to watershed development and urbanization. We present a process-based mass balance model of N cycling in Sweet Hall marsh, a pristine (i.e., low nutrient)Peltandra virginica-Pontederia cordata dominated tidal freshwater marsh in the York River estuary, Virginia. The model, which was based on a combination of field and literature data, revealed that N cycling in the system was largely conservative. The mineralization of organic N to NH4+ provided almost twice as much inorganic N as was needed to support marsh macrophyte and benthic microalgal primary production. Efficient utilization of porewater NH4+ by nitrifiers and other microbes resulted in low rates of tidal NH4+ export from the marsh and little accumulation of NH4+ in marsh porewaters. Inputs of N from the estuary and atmosphere were not critical in supporting marsh primary production, and served to balance N losses due to denitrification and burial. A comparison of these results with the literature suggests that the relative importance of tidal freshwater marsh N cycling processes, including plant productivity, organic matter mineralization, microbial immobilization, and coupled nitrification-denitrification, are largely independent of small changes in water column N loading. Although very high (millimolar) concentrations of dissolved inorganic N can affect processes including denitrification and plant productivity, the factors that cause the switch from efficient N recycling to a more open N cycle have not yet been identified.

Component greenhouse gas fluxes and radiative balance from two deltaic marshes in Louisiana: Pairing chamber techniques and eddy covariance

Coastal marshes take up atmospheric CO2 while emitting CO2, CH4, and N2O. This ability to sequester carbon (C) is much greater for wetlands on a per area basis than from most ecosystems, facilitating scientific, political, and economic interest in their value as greenhouse gas sinks. However, the greenhouse gas balance of Gulf of Mexico wetlands is particularly understudied. We describe the net ecosystem exchange (NEEc) of CO2 and CH4 using eddy covariance (EC) in comparison with fluxes of CO2, CH4, and N2O using chambers from brackish and freshwater marshes in Louisiana, USA. From EC, we found that 182 g C m−2 yr−1 was lost through NEEc from the brackish marsh. Of this, 11 g C m−2 yr−1 resulted from net CH4 emissions and the remaining 171 g C m−2 yr−1 resulted from net CO2 emissions. In contrast, −290 g C m2 yr−1 was taken up through NEEc by the freshwater marsh, with 47 g C m−2 yr−1 emitted as CH4 and −337 g C m−2 yr−1 taken up as CO2. From chambers, we discovered that neither site had large fluxes of N2O. Sustained-flux greenhouse gas accounting metrics indicated that both marshes had a positive (warming) radiative balance, with the brackish marsh having a substantially greater warming effect than the freshwater marsh. That net respiratory emissions of CO2 and CH4 as estimated through chamber techniques were 2–4 times different from emissions estimated through EC requires additional understanding of the artifacts created by different spatial and temporal sampling footprints between techniques.

Silliman, B. R., E. D. Grosholz, and M. D. Bertness (ed.) Human Impacts on Salt Marshes: A Global Perspective

“The state of our marshes is not good!” claim BrianSilliman, Edwin Grosholz, and Mark Bertness at the end oftheir edited book Human impacts on salt marshes: A globalperspective. As I sit here writing this review, I can look outmy window and see some of the expansive North Inlet saltmarsh and I am skeptical. The cordgrass is growing wellthis year, perhaps helped along by the freshwater dumpedfrom the regular summer thunderstorms that have playedhavoc with my field work. Water column nutrient concen-trations are again low, which is usual for this well-flushed,ocean-dominated system. My vertebrate-loving colleaguestell me that there are plenty of fish (and dolphins, andsharks) in the tidal creeks. Everything is as it should be. Butwait! Is that a small dieback area over there? And I think Isee some Phragmites at the edge of the marsh. Are thoseperiwinkles just hanging out or are they secretly plotting tomow down the entire marsh? Such is the paranoia that cangrip a fellow after reading through nearly 400 pagesdocumenting a suite of anthropogenic stressors that directlyor indirectly affect salt marshes.The basic objectives for the Human impacts on saltmarshes book are to: 1) summarize the causes andconsequences of anthropogenic impacts on salt marshes,both in North America and abroad, and 2) integrate thisinformation and provide it to coastal managers to allow themto combat a range of threats and promote cohesive marshconservationefforts.Thiseditedvolumecontains18chapters(plus introductory and concluding text by the editors) by atotal of 41 authors, almost a quarter of whom are locatedoutsidetheUnitedStates.Whilemuchofthebookfocusesonlessons learned from research in North American saltmarshes, the international authorship helps provide the“global perspective” mentioned in the book’s subtitle.The first part of the book covers Invasions in NorthAmerican Salt Marshes. The global invasion ofSpartinaspp., with an emphasis on invasions along the Pacificcoast of North America, is documented in Chapters 1and 2. Chapter 3 discusses several examples of invasiveanimals and their impacts on salt marshes, while the fourthchapter provides a long-term perspective on native vs.invasive Phragmites australis in eastern North Americanmarshes.The second partofthe bookdeals with Human InputsandConsumer Effects and includes chapters on goose over-grazing in Arctic marshes (Ch. 5), periwinkle grazing andfungal farming in southern U.S. marshes (Ch. 6), and ahypothesized alligator-nutria-plant trophic cascade inmarshes on the U.S. Gulf of Mexico coast (Ch. 7). Thesechapters provide compelling evidence that there are timesand situations when top-down control can limit primaryproduction and affect ecosystem structure and function. Ithink that Keddy and coauthors have it right in Ch. 7 whenthey suggest that salt marshes are controlled by a combina-tion of both top-down and bottom-up factors. There is justtoo much evidence for both modes of ecosystem regulationto believe that salt marsh food webs are exclusively top-down or exclusively bottom-up regulated.Land Use and Climate Change are covered in the thirdpart of the book. Two chapters use New England marshesas a case study on the effects of shoreline development(Ch. 8) and tidal restrictions and ditching (Ch. 9) onthe biological and physical properties of salt marshes.Chapter 10 is a wide-ranging chapter that covers marsh