Differential effects of chronic and acute simulated seawater intrusion on tidal freshwater marsh carbon cycling

Abstract

Tidal freshwater ecosystems experience acute seawater intrusion associated with periodic droughts, but are expected to become chronically salinized as sea level rises. Here we report the results from an experimental manipulation in a tidal freshwater Zizaniopsis miliacea marsh on the Altamaha River, GA where diluted seawater was added to replicate marsh plots on either a press (constant) or pulse (2 months per year) basis. We measured changes in porewater chemistry (SO 4 2− , Cl − , organic C, inorganic nitrogen and phosphorus), ecosystem CO 2 and CH 4 exchange, and microbial extracellular enzyme activity. We found that press (chronic) seawater additions increased porewater chloride and sulfate almost immediately, and ammonium and phosphate after 2-4 months. Chronic increases in salinity also decreased net ecosystem exchange, resulting in reduced CO 2 and CH 4 emissions from press plots. Our pulse treatment, designed to mimic natural salinity incursion in the Altamaha River (September and October), temporarily increased porewater ammonium concentrations but had few lasting effects on porewater chemistry or ecosystem carbon balance. Our findings suggest that long-term, chronic saltwater intrusion will lead to reduced C fixation and the potential for increased nutrient (N, P) export while acute pulses of saltwater will have temporary effects.

Salinity pulses interact with seasonal dry‐down to increase ecosystem carbon loss in marshes of the Florida Everglades

“…, Herbert et al. ). While the effect on ecosystem C cycling of saltwater pulses into tidal marshes has been studied (e.g., Chambers et al.…”

“…Very few salinity manipulations have been conducted in the field (Neubauer , Herbert et al. ), but since scale often matters to interpretation of results, in situ manipulations are desirable for deducing mechanisms of change inferred from smaller‐scale/benchtop experiments or larger‐scale descriptive studies.…”

“…Saltwater intrusion can lead to changes in soil biogeochemical cycling that affects ecosystem C dynamics (Weston et al 2011, Herbert et al 2015. Many studies have documented changes in C cycling along coastal marsh salinity gradients, often with contrasting results as to whether increasing salinity enhances or depresses C storage (Neubauer 2013, Weston et al 2014, Wilson et al 2015, Herbert et al 2018. While the effect on ecosystem C cycling of saltwater pulses into tidal marshes has been studied (e.g., Chambers et al 2013, Neubauer 2013, Weston et al 2014, the biogeochemical and ecosystem C responses of non-tidal, nearshore, fresh-and brackish-water marshes to saltwater pulses are still not well known.…”

Phosphorus alleviation of salinity stress: effects of saltwater intrusion on an Everglades freshwater peat marsh

“…Saltwater can be a stressor for freshwater wetland vegetation and has been associated with decreased species richness (Sharpe and Baldwin , Neubauer ), gross ecosystem productivity (GEP; Neubauer , Herbert et al. , Wilson et al. ), and primary production (Delaune et al.…”

“…Saltwater intrusion is defined here as the intrusion of marine-origin water composed of many salt-forming ions (i.e., Cl À , NO 3 À -N, SO 4 2À , PO 4 3À ), also referred to as marine water intrusion. Saltwater can be a stressor for freshwater wetland vegetation and has been associated with decreased species richness (Sharpe andBaldwin 2012, Neubauer 2013), gross ecosystem productivity (GEP; Neubauer 2013, Herbert et al 2018, Wilson et al 2018a, and primary production (Delaune et al 1987, Spalding and Hester 2007, Herbert et al 2015. In non-tidal, non-riverine, peatdominated wetlands that do not receive an allochthonous sediment supply, a decline in productivity of freshwater plants can be coupled with a decrease in soil organic matter inputs, which can result in marsh instability (Delaune et al 1994).…”