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

Ketil Koop-Jakobsen,Martin S Gutbrod

doi:10.3389/fenvs.2019.00137

Ketil Koop-Jakobsen, Martin S Gutbrod

Open Access

https://doi.org/10.3389/fenvs.2019.00137

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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 O2, pH and CO2 in shallow salt marsh tidal ponds in the summer time. High benthic photosynthetic activity, fueled by CO2 from the sediment, resulted in steep vertical O2 gradients at the sediment-water interface, increasing from anoxia to extremely supersaturated peak concentrations up to 886±139 µmol L-1 (391% atmospheric O2 saturation) over a short distance of 6mm. These characteristic peaks developed even at low light conditions down to 150 µmol m-2 sec-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. CO2 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 O2 and pH, which was restricted to the time of the flood. As the flooding water receded, the preceding O2 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.

Highlights

Tidal ponds, referred to as tide pools or potholes, are naturally occurring in salt marshes, creating a mosaic of permanently inundated ponds with bare sediment on the otherwise vegetated salt marsh platform (Harshberger, 1916; Adamowicz and Roman, 2005)
The tidal ponds had high O2 dynamics ranging from anoxic conditions inside the sediment to extreme supersaturation
A thick layer of benthic microalgae was present on the sediment surface (Figure 1C) resulting in a marked peak in O2 and steep O2 gradients during daylight

Summary

Introduction

Referred to as tide pools or potholes, are naturally occurring in salt marshes, creating a mosaic of permanently inundated ponds with bare sediment on the otherwise vegetated salt marsh platform (Harshberger, 1916; Adamowicz and Roman, 2005). Salt marsh tidal ponds constitute a unique habitat characterized by rapid fluctuations in O2, temperature and salinity, which can reach extreme levels over a day-night cycle (Smith and Able, 2003). There is high benthic photosynthetic O2 production during the day, and a high sediment O2 demand caused by microbial respiration and reoxidation of reduced compounds, which leads to marked daily O2 fluctuations (Ruber et al, 1981)

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