Investigating the impact of in situ soil organic matter degradation through porewater spectroscopic analyses on marsh edge erosion

Surface water temperature impacts on coastal wetland denitrification: Implications for river reconnection

Louisiana, located in the southeast United States, is home to 40% of the continental US's coastal wetlands yet accounts for 80% of the nation's coastal wetland loss. This loss is generally attributed to decreased sediment supply, hydrologic alteration from levees, channelization, subsidence, sea-level rise, and wave and tidal induced marsh edge erosion. The Mid-Barataria Sediment Diversion is a US $1.3 billion coastal restoration project that will divert up to 2100 m³ s^-1 of sediment-laden Mississippi River water directly into Barataria Basin. The influx of colder, nutrient-rich, springtime river water could negatively impact water quality of the receiving basin. We quantified the effects of colder, surface water temperature on the nitrate (NO₃^-) reduction rate in vegetated marsh and open water bay sediments. Colder water limited NO₃^- removal processes averaging 17.1 mg N m^-2 d^-1 in the range of 5-14 °C, before increasing almost 3-fold in the 20 °C treatments at 50.6 mg N m^-2 d^-1. Low N removal rates, especially near the project inflow where temperatures will be coldest will favor transport of NO₃^- further into Barataria Basin where eutrophic conditions could become expressed. These results will inform coastal managers around the world of the potential ecosystem response to coastal restoration aimed at river reconnection where colder waters enter warmer, shallow basins.

Peripheral freshwater deltaic wetlands are hotspots of methane flux in the coastal zone

Methane (CH₄) emissions are low in the coastal zone due to a higher redox poise, related to sulfate reduction. However, river deltas are a potential source of CH₄ flux in coastal zones globally, due to fresh condition and high primary production. The goal of this study was to seasonally measure CH₄flux at three different geomorphic settings (newly forming island, river channel bottom and established freshwater marsh) within the Wax Lake Delta, Louisiana, USA. CH₄ flux rates were 386 ± 327 mg C m^-2 d^-1 in March and 2859 ± 1286 mg C m^-2 d^-1 in June at the freshwater marsh site. At the island site, CH₄ flux was significantly smaller at 7.94 ± 3.57 mg C m^-2 d^-1 in March and 215 ± 153 mg C m^-2 d^-1 in June while at adjacent river channel bottom site, CH₄ flux was lowest at 2.49 ± 3.38 mg C m^-2 d^-1 in March and 19.5 ± 1.12 mg C m^-2 d^-1 in June at the air-water interface. CH₄ emission rates show significant spatial heterogeneity with rates up to two orders of magnitude greater at the marsh site at the periphery of the delta, related to greater soil total C. Therefore regions within the active delta do not provide a significant source of methane, due to a lack of soil C, despite freshwater conditions. However, marshes at the periphery within the halo of fresh water, populated with established plant communities can be significant hotspots of CH₄ emissions, despite their location within the coastal zone.

The role of short-term and long-term water level and wave variability in coastal carbon budgets

We investigated soil organic carbon dynamics at three freshwater coastal sites in the Laurentian Great Lakes using a simple carbon budget box model. Long-term carbon budgets (1939–2018) were developed using aerial photography and then compared to short-term carbon export (2018–2019) developed using drone data. This study puts forth a refined coastal carbon budget model that advances previous model iterations by: (1) examining spatial variability in carbon budgets, (2) including a temporally dynamic carbon inventory term, and (3) updating the erosional term. Half of the initial carbon stock of the combined sites was lost in the 80-year study period, which is severely imbalanced with the age of those coastal habitats (400–2000 cal years BP). Major periods of carbon loss corresponded to periods of elevated water level. Short-term loss of carbon during 2018–2019 corresponded to northeasterly extreme wave events during a period of above-average water level.

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A coastal carbon budget model was refined to account for spatiotemporal heterogeneity

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Half the soil organic carbon stored at the study sites was exported in 80 years

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Carbon loss occurs during decadal periods of water level rise

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High wave events and/or elevated water level cause carbon loss