Redox dynamics of a tidally-influenced wetland on the San Jacinto River
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abstract
The stage height, dissolved oxygen, and redox potential of the surface water and sediments of an intertidal Texas wetland were evaluated using continuous monitoring techniques. Surface water oxygenation was most drastically affected by photosynthesis, which served to elevate daytime oxygen concentrations compared to those measured at night. Colder temperatures affected dissolved oxygen concentrations by increasing solubility and decreasing heterotrophic consumption. Waters of the San Jacinto River were generally more oxygenated than those in the wetland, and incoming tides typically elevated dissolved oxygen levels unless the tide was in direct conflict with the daily photosynthetic cycle. The redox potentials in the sediments could not be correlated with a daily cycle as was found in the surface waters. Redox potentials in the intertidal zone fluctuated between -150 mV and 300 mV throughout normal cycles of periodic inundation and exhibited decreasing potentials with depth. Sediments further from the waterline had denser vegetation and exhibited higher redox potentials under similar hydrodynamic loading. During extremely dry periods, redox potentials varied from 300 mV to 600 mV, and deeper sediments were often more oxygenated than sediments closer to the sediment-air interface, due to the higher rates of metabolism in the upper stratum. Rates of oxidation and reduction in the sediments were lognormal and evenly distributed about the steady state, with maximum rates of 300 mV min -1 during intermittent periods of inundation. The results show that biological carbon oxidation in sediments can entirely deplete oxygen from surface waters overlying the mud flats, that vegetation serves to oxygenate the rhizosphere, and although redox potential changes can be large in the dynamic intertidal zone, large changes are infrequent.
