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Evaluating gas exchange rates in salt marsh tidal creeks using in situ noble gas concentrations

Summary

Request for funding:

I am requesting $1040 to cover the expense of analyzing two seawater samples for noble gas concentrations at the WHOI Isotope Geochemistry facility ($520 each). These samples are part of a larger campaign to better constrain ecosystem level metabolic fluxes in salt marsh tidal creeks, the focus of my thesis. The gas exchange rate is a major control on fluxes of metabolically active dissolved gases. By directly evaluating gas exchange rates from changes in dissolved noble gas concentrations, I avoid many of the attendant difficulties with indirect parameterizations and artificial tracer releases. 

While important to my thesis, this work is unfunded and noble gas analysis has not been budgeted into the grant supporting my thesis. In addition to the two samples that I am requesting COI funding for, I may obtain funding to run 2-4 samples from MBL researchers who wish to simultaneously evaluate denitrification rates using dissolved nitrogen to argon ratios and argon concentrations from our samples. However, this source of support has not been finalized.

Description of research:

Salt marshes are widely distributed, highly productive, and likely represent an important sink for atmospheric carbon dioxide. Tidal creeks mediate exchange of nutrients and terminal metabolic products between estuarine water and the grass dominated salt marsh platform, including metabolically active dissolved gases such as oxygen and carbon dioxide. Quantifying tidal creek metabolic fluxes such as gross primary production, net ecosystem metabolism, and respiration requires accurate estimation of the gas exchange rate. 

However gas exchange is difficult to evaluate in salt marsh tidal creeks using traditional parameterizations based on wind speed or current flow (Raymond and Cole 2001). This is because of complicated channel geometry (eg sinuous, limited fetch), surfactant films, submerged vegetation (which alter energy dissipation and mixing regimes), and sheltering grass canopies (Carini et al. 1996, Neph 1999). Deliberate tracer release studies are sensitive to in situ conditions in larger tidal estuaries for a limited period before dispersal of the tracers (Carini et al. 1996, Ho et al. 2011). However the rapid tidal flushing of salt marsh tidal creek systems constrains the potential study period when using introduced tracers. In contrast, noble gas concentrations are both sensitive to in situ conditions and reset to near equilibrium with the atmosphere by incoming estuarine water with each flooding tide.

As part of my thesis work, in summer and fall of 2012 I measured the effect of nutrient loading in fertilized and unamended salt marsh tidal creeks (part of the TIDE experiment at the Plum Island Estuary-LTER, Massachusetts) by quantifying two metabolic fluxes: gross oxygen production, an indicator of total photosynthesis, and net oxygen production, which is related to the balance of autotrophic and heterotrophic processes (Fig. 1). These community level metabolic rates are derived using in situ dissolved triple oxygen isotope ratios and an oxygen mass balance; the dominant uncertainty in these calculations is the magnitude of the gas exchange rate.

A subset of noble gas samples was collected in tidal creeks at the study site in summer and fall of 2012.  These samples were analyzed without dedicated funding. Concentrations of He, Ne, Ar, Kr, and Xe were input to their respective advection-diffusion equations, with the horizontal concentration gradient assumed to be negligible within a short stretch of tidal creek. This system of equations was optimized for best fit gas exchange parameters including sea-air gas transfer and bubble processes (Stanley 2009). Calculation results indicated that gas exchange in this system may be very high relative to a nearby tidally influenced river and larger estuarine systems (Table 1; Carini et al. 1996, Ho et al. 2011, Raymond and Cole 2001). 

However, samples were only collected at a single location at each tidal creek for logistical and monetary reasons. In order to confirm the accuracy of these rates, and of the dependent ecosystem fluxes, net advective transport must be evaluated. I am collecting 16 noble gas samples along transects in each study creek to evaluate the along-creek concentration gradient at two representative tidal stages (4 samples per timepoint per creek). 

Analyzing these samples will contribute to completing and publishing one of my thesis chapters on salt marsh tidal creek metabolic fluxes. If confirmed or improved with the proposed measurements, these gas exchange estimates will also inform estimates of other biogeochemical fluxes in the same system, such as denitrification rates being evaluated from dissolved nitrogen to argon ratios by Anne Giblin at the Marine Biological Laboratory. Noble gas derived estimates may help better bound gas exchange rates in similar tidal creek systems found globally, and provide another means to evaluate air-water transfer in systems that are challenging for parameterization and tracer release methods.

Table 1: Gas transfer rates corresponding to higher and lower tidal stages in the creek from this study (shaded blue) and other studies. Other parameterizations use average windspeed and water depth at the tidal creeks in this study. Water-air gas transfer rate (s-1)

Average depth 1.7m

0.7m

Measurement

Location

-1.60E-04

-5.80E-05

Noble gases

Fertilized tidal creek, Plum Island Estuary1

-6.30E-05

-1.20E-05

Noble gases

Unamended tidal creek, Plum Island Estuary1

-3.30E-06

-7.90E-06

3He/SF6 dual tracer release

Parker River, Plum Island Estuary2

-3.10E-06

-7.50E-06

3He/SF6 dual tracer release

Hudson River3

-7.70E-07

-1.90E-06

Purposeful and natural tracers

Compilation of rivers and estuaries4

-1.80E-06

-4.50E-06

Bottom induced turbulence

Compilation of streams4

 Carini, S., N. Weston, C. Hopkinson, J. Tucker, A. Giblin, and J. Vallino. Gas exchange rates in the Parker River Estuary, Massachusetts. Biological Bulletin 191: p. 333-334

Ho, D.T., P. Schlosser, and P.M. Orton. 2011. On factors controlling air-water gas exchange in a large tidal river. Estuaries and    Coasts 34: p.1103-1116, doi: 10.1007/s12237-011-9396-4

Nepf, H.M. 1999. Drag, turbulence, and diffusion in flow through emergent vegetation. Water Resources Research 35: p.479-489, doi: 10.1029/1998WR900069

Raymond, P.A., and J.J. Cole. Gas exchange in rivers and estuaries: Choosing a gas transfer velocity. Estuaries and Coasts 24: p.312-317, doi: 10.2307/1352954

Stanley, R.H.R., W.J. Jenkins, D.E. Lott, and S.C. Doney. Noble gas constraints on air-sea gas exchange and bubble fluxes. Journal of Geophysical Research Oceans 114: C11020, doi: 10.1029/2009JC005396

Last updated: December 3, 2013