NOAA’s National Centers for Coastal Ocean Science (NCCOS) approved an Event Response Award of $25,000 for Louisiana State University to evaluate the impact of a historic drought on coastal wetland porewater properties.
Surface water salinity tends to increase during droughts (or lack of precipitation), as dry conditions lead to water evaporation and concentrate existing salts in rivers, lakes, and soil. But what exactly happens to the salinity of pore water, the water found in the spaces between sediment particles where the plant roots grow? The chemical composition of porewater is heavily influenced by the surrounding sediment, and doesn’t necessarily respond to drought in the same way as surface water.
The Palmer Drought Severity Index for Coastal Louisiana for the past 125 years shows fluctuating periods of very wet as well as very dry conditions (Figure 1). In 2023, coastal Louisiana experienced its second driest (lowest) index reading in the past 100 years. The year 2000 holds the record for the lowest index in the past 100 years, which resulted in the die off, or “browning,” of more than 100,000 acres of tidal marsh vegetation. The region also experienced significant losses to coastal forests. It was unclear what specifically caused the marsh browning event because there was no surface or porewater salinity or hydrologic data collected in the affected areas before or during the drought event.
The Louisiana Coastal Protection and Restoration Authority now maintains a network of almost 400 coastal monitoring stations called the Coastwide Reference Monitoring System (CRMS), which monitor surface water level and surface salinity. Data from the CRMS has indicated that severe drought can induce increased surface salinities across the fresh-brackish-salt marsh continuum. But how does the surface salinity affect the porewater salinity?
Understanding the chemical triggers that allow severe droughts to result in marsh vegetation death will improve coastal managers’ ability to prepare for and mitigate these events. Because much of the stress to the marsh vegetation is due to salinity in the rhizosphere (root zone), it is important to obtain real time measurements in the soil pores using porewater sensors, to assess how surface salinity is impacting the root zone.
This funding supported the establishment of a network of real time porewater sensors that can assess the marsh condition to varying salinity events, including hurricanes, drought, or even high and low Mississippi River stage. The research team collected and processed the soil porewater throughout the 2023 drought, installed the in-situ conductivity and moisture content sensors to track changes continuously, and collected the drought cores to conduct a reflooding experiment with fresh water in the laboratory to examine over what time scales the porewater salinity might return to standard conditions. Notably, the team has leveraged this work to expand their sensor network to better capture future events.
The Effects of Sea Level Rise Event Response Program augments current or prior research to help offset costs of immediate mobilization of response and/or assessment efforts associated with events that are difficult to plan as part of a scientific study.
This work is authorized by the NOAA Authorization Act of 1992, Pub. L. 102-567 (Oct. 29, 1992); sec. 201(c), which directs appropriation for the NCCOS Competitive Research Program to augment and integrate existing NOAA programs, with a specific focus on improving predictions of coastal hazards and protecting human life and property.