Herbert, Ellen R., et al. “Differential Effects of Chronic and Acute Simulated Seawater Intrusion on Tidal Freshwater Marsh Carbon Cycling.” Biogeochemistry, vol. 138, no. 2, 24 Mar. 2018, pp. 137–154., doi:10.1007/s10533-018-0436-z.
The reality is that coastal wetlands are disappearing right before our eyes, and the majority of these losses are freshwater ecosystems. According to US EPA, more than 80,000 acres/year were lost on average during 2004-2009 and the rate is increasing! One major cause of wetland loss is saltwater intrusion – when saltwater is introduced into freshwater – which is known to be caused by human activities locally and climate change globally.
Of the different types of coastal wetlands, let’s focus on tidal freshwater coastal wetlands (Figure 2). While these wetlands are adapted to low levels of salinity from the tides, additional seawater can disrupt many interconnected processes within the ecosystem. Soil microbes drive the carbon cycle and the production of gases like CO2 and CH4. Some microbes breathe oxygen and emit CO2 similarly to us, but others respire sulfate and convert it to other sulfur-containing compounds like hydrogen sulfide. Plants have a lead role in ecosystem productivity through photosynthesis and are responsible for the uptake of nutrients like nitrogen and phosphorus during growth. Since the health of the wetland is dependent on these processes, it is important to understand how saltwater intrusion can be expected to affect them.
Dr. Herbert and colleagues recognized that there are two distinct ways that climate change is expected to influence saltwater intrusion into tidal freshwater coastal wetlands:
- Periodic or “pulse” basis: Climate change is predicted to intensify climatic weather events, such as droughts and large storms. Saltwater can move with less resistance into areas suffering from drought, and storms (like hurricanes) can cause storm surges transporting large amounts of saltwater from the ocean to land.
- Long-term or “press” basis: Climate change is also resulting in rising sea levels, with an expected increase of 0.38 to 2m in the next 100 years. As sea levels increase, saltwater reaches further inland, above and below ground, and changes the ecosystem dynamics irreversibly.
In this study, the authors focused on these two types of saltwater intrusion to see what changes occur in water chemistry, microbial activity, ecosystem productivity, and greenhouse gas production. By observing these factors, the researchers can better understand what’s happening in the ecosystem in response to the treatments.
The Big Experiment
This study occurred from 2013-2016 and was part of the “Seawater Addition Long-Term Experiment” (SALTEx) in coastal Georgia. The study area was a tidal freshwater coastal marsh on the Altamaha River in which the dominant vegetation is the giant cutgrass (Zizaniopsis miliacea; Figure 3). Thirty experimental plots of 2.5 m x 2.5 m were distributed over a 0.1-hectare area, separated by at least 3 m as a buffer.
The researchers outlined the plots with frames which contained the dosed water during treatment but otherwise let the plots exchange naturally. They assigned each plot to one of five treatments at random: control (with frame), control (no frame), freshwater, “press” salinity (brackish mixture), and “pulse” salinity (freshwater for 10 months, brackish for 2 months).
Throughout the study, the researchers looked at the following parameters:
- Water chemistry: They sampled porewater (the water between soil pores) and analyzed it for dissolved inorganic nitrogen and phosphorus species (DIN and DIP, respectively), chloride, sulfate, and dissolved organic carbon (DOC).
- Microbial extracellular enzyme activity: They took soil cores and analyzed for enzymes that indicate microbes’ ability to acquire carbon (part of the carbon cycle).
- CO2 and methane (CH4) exchange: They measured these gases to assess ecosystem productivity (the rate of CO2 uptake during daylight) and greenhouse gas production.
The Data We’ve Been Waiting For
Dr. Herbert and colleagues observed significant differences in water chemistry between the press and pulse treatments. In the press plots, chloride and sulfate increased almost immediately and DIN and DIP increased after 2-4 months, whereas changes in the pulse plots were either insignificant or short-lived. DOC dropped initially in both pulse and press plots, but the pulse plots had recovered a year later while the press plots remained low. Overall, the press plots showed lasting changes, whereas the pulse plots were able to return to control levels over time.
Microbial Extracellular Enzyme Activity:
Notably, the increase in sulfate did not correspond with increased activity of microbes. In the soil core samples, the researchers observed 53% lower carbon-acquiring enzyme activity in the press plots after a year of treatment. The pulse plots, on the other hand, did not respond significantly during the 2-month saltwater addition, however, five months afterward the carbon-acquiring enzyme activity had risen to 50% greater than the controls. These results show significant differences in enzyme activity between the press and pulse salinity methods, implying shifts in the microbial communities in response to the treatments.
CO2 and CH4 Exchange:
Three months after treatment began, the gross ecosystem productivity of the press plots was significantly lower than in all other treatments and fell to 30% below the plots that were not treated. Furthermore, the decrease in productivity was associated with loss of vegetation – including both the giant cutgrass itself and the understory species that normally surround it. The press plots also had reductions in greenhouse gas emissions. CH4 emissions were 72% lower than the control plots for the first year of the study. This was not enough to change the press plots from a CH4 source to a CH4 sink, but overall it lowered their net release from 5491 g CO2e/m2·year down to 1579 g CO2e/m2·year (where the units are based on CO2 equivalents). In conjunction with the loss in productivity, the CH4 emission reduction suggests that freshwater wetlands will have reduced carbon fixation in response to long-term increased salinity.
The Bottom Line
Dr. Herbert and colleagues witnessed significant changes in response to long-term saltwater intrusion, including altered water chemistry, decreased microbial activity, decreased ecosystem productivity, and loss of vegetation. The authors state that chronic increased salinity can have lasting effects on tidal freshwater coastal wetlands, but that periodic saltwater intrusion can be expected to have only temporary effects. These results imply that tidal freshwater coastal wetlands will be threatened by continued sea level rise, but that there is hope for recovery in the wake of strong storms.
The SALTEx project will continue for several more years to better define some of the trends observed in this research. The greater research group, GC LTER, is also monitoring a tidal freshwater forest and studying changes at the landscape scale. By learning more about the response of freshwater coastal wetlands to saltwater intrusion, researchers can help coastal communities adapt their management strategies to better protect these fragile areas.
Herbert, Ellen R., et al. “Differential Effects of Chronic and Acute Simulated Seawater Intrusion on Tidal Freshwater Marsh Carbon Cycling.” Biogeochemistry, vol. 138, no. 2, 24 Mar. 2018, pp. 137–154, doi:10.1007/s10533-018-0436-z.
Craft, Christopher B., et al. “Climate Change and the Fate of Coastal Wetlands.” (2016). Wetland Science and Practice, vol. 33, no. 3, Sep. 2016, pp. 70–73.
Mohlenbrock, Robert H. “Zizaniopsis miliacea (Michx.) Döll & Asch. giant cutgrass.” Southern wetland flora: Field office guide to plant species. South National Technical Center, Fort Worth. Hosted by the USDA-NRCS PLANTS Database / USDA SCS. 1991. https://plants.usda.gov/core/profile?symbol=ZIMI.
U.S. Environmental Protection Agency. “Coastal Wetlands.” 13 Jun. 2018, https://www.epa.gov/wetlands/coastal-wetlands.