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Tidal Wetland Fluxes at Browns Island
 Tidal marshes are depositional
environments characterized by emergent vegetation, fine sediment substrates,
and dendritic channel networks. They are common in deltaic settings and in
sheltered coastal areas where fine sediments accumulate to a level where
wetland plants can colonize. Due to the biological value of these systems, many
regulatory agencies seek to restore lost tidal marshes and protect existing
areas from drainage or development.
Ecological concerns regarding habitat and contaminants highlight the need for
understanding marsh accretion processes. The import/export of suspended
sediment plays a large role in the formation and maintenance of tidal wetlands,
as well as in the overall sediment budget within an ecosystem (Cahoon et al.
1996; Yang 1998). A decrease in sediment supply can halt the accretion
necessary for wetland survival. In consideration of ongoing rise in sea level,
fluxes of material to and from tidal marshes may control their sustainability
(Pont et al. 2002; Temmerman et al. 2004) and therefore must be quantified. The
presence of sediment-associated contaminants increases the relevance of marsh
accretion, especially for studies of contaminant uptake and transformation
within wetlands (e.g. Marvin-DiPasquale and Agee 2003). Ultimately the
quantification of sediment fluxes will assist in relevant management decisions.
Previous work has established the variability of fluxes within tidal creek
cross sections (Pillay et al. 1992), as well as the need for continuous, long
term data (Suk et al. 1999).
An established wetland was chosen to serve as a model for estimating mature
marsh fluxes, representing the end point of restoration efforts, as well as
natural marsh conditions. Browns Island is a Scirpus (tule) marsh located at
the confluence of the Delta and Suisun Bay, California. This wetland was chosen
based on accessibility, age, and measurement feasibility. We measured water and
sediment fluxes through two channels on Browns Island, at the landward end of
San Francisco Bay, to determine the factors that control sediment fluxes on and
off the island. In-situ instrumentation was deployed between October 10 and
November 13, 2003. Acoustic Doppler current profilers (ADCPs) and the index
velocity method were employed to calculate water fluxes. Suspended-sediment
concentrations (SSC) were determined with optical sensors and cross-sectional
water sampling. All procedures were analyzed for their contribution to total
error in the flux measurement. The inability to close the water balance and
determination of constituent concentration were identified as the main sources
of error; total error was 27 percent for net sediment flux. The water budget
for the island was computed with an unaccounted input of 0.20 cubic meters per
second (22 percent of mean inflow), after considering channel flow, change in
water storage, evapotranspiration, and precipitation. The net imbalance may be
a combination of groundwater seepage, overland flow, and flow through minor
channels. Change of island water storage, caused by local variations in water
surface elevation, dominated the tidally averaged water flux. These variations
were mainly caused by wind and barometric pressure change, which alter regional
water levels throughout the Delta. Peak instantaneous ebb flow was 35 percent
greater than peak flood flow, indicating an ebb-dominant system, though
dominance varied with the spring/neap cycle. SSC were controlled by wind-wave
resuspension adjacent to the island and local tidal currents that mobilized
sediment from the channel bed. During neap tides sediment was imported onto the
island but during spring tides sediment was exported because the main channel
became ebb-dominant. Over the 34-day monitoring period 14,000 kg of suspended
sediment were imported through the two channels. The water imbalance may affect
the sediment balance if the unmeasured water transport pathways are capable of
transporting large amounts of sediment. We estimate a maximum of 2,800 kg of
sediment may have been exported through unmeasured pathways, giving a minimum
net import of 11,200 kg. Sediment flux measurements provide insight on tidal to
fortnightly marsh sedimentation processes, especially in complex systems where
sedimentation is spatially and temporally variable.
Publications
Bergamaschi, B.A., Downing, B.D., Wheeler, G.A., Schoellhamer,
D.H., Ganju, N., Fram, M.S., Erickson, D.E., Kendall, C., Bemis, B.E.,
Stepanauskas, R., Hollibaugh, J.T., and Fujii, R., 2003, Quantifying the
contributions of tidal wetlands to dissolved organic material in the San
Francisco Estuary, California, USA. Proceedings of the 17th Biennial Conference
of the Estuarine Research Federation, Seattle, Washington, September 14-18,
2003, p. 13.
Link to Abstract
Downing, B.D., Boss, E., Bergamaschi, B.A., Fleck, J.A.,
Lionberger, M.A., Ganju, N.K., Schoellhamer, D.H., and Fujii, R., in review,
Quantifying fluxes and characterizing compositional changes of dissolved
organic matter in aquatic systems in situ using combined acoustic and optical
measurements. Limnology and Oceanography: Methods.
Downing, B., Wheeler, G., Emerson, S., Ganju, N., and
Bergamaschi, B., 2003, Continuous, real-time optical measurement of DOC fluxes
in a tidal wetland. Proceedings of the 6th biennial State-of-the-Estuary
Conference, Oakland, California, October 21-23, 2003, p. 73.
Fujii. R., Bergamaschi, B.A., Ganju, N.K., Fleck, J.A.,
Burow-Fogg, K.R., Schoellhamer, D., Deverel, S.J., 2003, Preliminary Assessment
of DOC and THM Precursor Loads from a Freshwater Restored Wetland, an
Agricultural Field, and a Tidal Wetland in the Sacramento-San Joaquin River
Delta. Proceedings of the 2003 CALFED Science Conference, Sacramento,
California, January 14-16, 2003, p. 206.
Link to
Abstract
Ganju, N.K., Schoellhamer, D.H., and Bergamaschi, B.A, 2005,
Suspended sediment fluxes in a tidal wetland: measurement, controlling factors,
and error analysis. Estuaries 28(6), 812-822.
PDF File
Lionberger, M.A., Ganju, N.K., Schoellhamer, D.H., Downing,
B.D., Bergamaschi, B.A., and Wheeler, G.A., Wetland fluxes of dissolved organic
carbon and sediment at Browns Island, California: balancing the water
budget. Proceedings of the 2004 CALFED Science Conference.
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