Shallow Salt Marsh Tidal Ponds–An Environment With Extreme Oxygen Dynamics

Abstract

In marshes, tidal ponds are increasing in number and areal coverage. Getting a better understanding of their unique biogeochemistry is a prerequisite for foreseeing their future role in salt marsh ecosystems. Using in situ microprofiling, this study investigated the spatiotemporal dynamics of O 2 , pH, and CO 2 in shallow salt marsh tidal ponds in the summer time. High benthic photosynthetic activity, fueled by CO 2 from the sediment, resulted in steep vertical O 2 gradients at the sediment-water interface, increasing from anoxia to extremely supersaturated peak concentrations up to 886 ± 139 µmol L −1 (391% atmospheric O 2 saturation) over a short distance of 6 mm. These characteristic peaks developed even at low light conditions down to 150 µmol photons m −2 s −1 photosynthetically active radiation (PAR). The oxygen gradients were restricted to the layer of benthic microalgae on the sediment surface and did not extend into the water column, which was well-mixed throughout the day showing no vertical variation. The benthic photosynthesis and respiration controlled the oxygen concentration in the water column, creating net supersaturated conditions during the day and hypoxic conditions at night. The tidal ponds were generally well-buffered showing only attenuated pH fluctuations ranging from 6.2 to 7.3, and no persistent gradients built up, despite the high photosynthetic activity at the sediment water interface. CO 2 accumulated in the sediment and was present in the water column during the morning hours, but depleted in the afternoon due to the high photosynthetic uptake. Tidal ponds also experienced event-driven changes in their biogeochemistry. Sea foam developed on the water surface during the day and accumulated on one side of the pond blocking light penetration and lowering oxygen concentrations under the foam. Inundation at high tide caused a short-lived temporal variation in O 2 and pH, which was restricted to the time of the flood. As the flooding water receded, the preceding O 2 and pH conditions were immediately restored. Altogether shallow tidal ponds comprise a marsh habitat with distinctive spatiotemporal oxygen dynamics driven by benthic photosynthesis and respiration, which differ from the surrounding vegetated marsh, and could drive changes in salt marsh biogeochemistry in response to increased pond coverage.

Optical Sensing and Imaging of pH Values: Spectroscopies, Materials, and Applications

“…Such fiber microsensors can also be used for profiling, for example to analyze pH gradients in saltmarsh ponds at sediment-water interface. 1017 …”

Modeling the spatial dynamics of marsh ponds in New England salt marshes

“…On the contrary, many marsh platforms are highly heterogeneous because of the presence of pondssemicircular depressions permanently inundated even at low tide. This heterogeneity is particularly relevant for the Mid-Atlantic and New England coast of the USA, where ponds are ubiquitous features (Adamowicz and Roman, 2005;Harshberger, 1909;Koop-Jakobsen and Gutbrod, 2019;Redfield, 1972;Schepers et al, 2017). Thus, considering the presence of ponds is necessary to accurately predict the landscape-level evolution and persistence of these salt marshes under climate change.…”

Microbial dark carbon fixation fueled by nitrate enrichment

“…The use of multiple electron acceptors by the collection of chemoautotrophic MAGs 396 in each of the functional groups could provide an important adaptation to inhabiting the 397 top 25 cm of marsh sediment. Salt marsh sediments contain steep gradients in oxygen 398 (37,38) and nitrate (19,39), and a sulfate to sulfide transition zone (40), all of which are 399 influenced by diurnal tidal inundation that provides a re-supply of fresh organic matter 400 and nutrients (41). The biogeochemistry of the sediment is also influenced by the 401 release of oxygen and carbon from plant roots (42,43) and animal burrows, providing 402 additional structure and permeability to sediment that enhances chemoautotrophic 403 growth (44).…”