Will sea level rise harm tidal marshlands?

Article: Delgado, P., P. Hensel, and A. Baldwin. “Understanding the Impacts of Climate Change: an Analysis of Inundation, Marsh Elevation, and Plant Communities in a Tidal Freshwater Marsh.” Estuaries and Coasts 41, no. 1 (2018): 25-35. DOI: https://doi.org/10.1007/s12237-017-0342-y

Change is coming, but what will it mean?
Figure 1: The Jug Bay Wetlands Sanctuary, featuring a tidal freshwater wetland. Credit: Wikimedia Commons

A habitat type commonly found in the upper reaches of estuaries (water bodies where fresh river waters mix with salty ocean water) is tidal freshwater wetlands. These habitats, which experience tides but have very low salinity, are home to a diverse array of plant species, especially in higher elevation marsh zones where many salt intolerant plant species can be found. However, climate change pressures, such as sea level rise, make the fate and vulnerability of these unique habitats highly uncertain.

Sea level rise will almost certainly translate to an increased amount of time the marsh is flooded. In a tidal freshwater wetland, whether you are underwater or dry during high tide is largely controlled by elevation, meaning that even a slight change in water level could have an impact. Changes in flooding patterns could change which plants can exist in this habitat –  and a decrease to that plant diversity could mean changes to all the benefits wetlands provide, including protection from storm surge, nursery space for many critters, and filtration of harmful nutrients.

The Chesapeake Bay, like many tidal systems around the Mid-Atlantic, is experiencing rapid sea level rise (2.9-5.8 mm/yr). This change in water level, and potentially salt levels, will likely mean a change in how often, and for how long, tidal freshwater wetlands are flooded. This study by Patricia Delgado and colleagues sought to better understand how even small changes in the local water patterns could affect the fate of these diverse habitats.

Utilizing monitoring data to answer multi-year questions
Figure 2: This image shows the distribution of the tidal freshwater plants species found at Jug Bay, where the plants on the x-axis (horizontal) are arranged as those being found from the greatest to lowest elevation range. Note how the majority of the plants have their minimum elevation range located above Mean Sea Level. Source: Delgado and Baldwin 2018, Figure 2. Permission to use this image was obtained from RightsLink.

This study took place at a tidal freshwater marsh located in Jug Bay (Fig. 1), which is part of the Chesapeake Bay National Estuarine Research Reserve in Maryland. As a Research Reserve, Jug Bay has multiple years of monitoring data including plant surveys, water level data, and a vertical control network (think GPS) to measure elevation. All these data allowed the researchers to study how long Jug Bay’s plant communities were flooded in the early growing season in addition to the diversity and distribution of plants growing at different elevations.

The early growing season was defined here as the time period between Mid-March and Mid-May and the peak growing season was defined as occurring between the end of June and the middle of July. Water depths and plant coverages for more than 5 years, in addition to an intense elevation survey, were used to better understand the two main questions of this study:

  • Is the distribution of plants in the middle to upper portions of a tidal freshwater marsh related to differences in elevation?
  • Is there a negative relationship between the amount of plant coverage and amount of time tidal freshwater wetlands plants are flooded during the early growing season?

A final, and important thing to note was that the analysis was not a cause and effect since it uses field-based data. In other words, the researchers couldn’t purposefully flood the marsh to see what would happen. Instead, they used the data to detect patterns between flooding and plant coverage.

What did they find?

The majority of plant species (62%) at Jug Bay where found between the middle and upper elevations of the marsh (Fig. 2). This infers that the middle and upper parts of the marsh were the most diverse in plant species.

The data also showed that the amount of time the marsh was flooded was negatively related to the plant coverage. It was found that when the marsh was flooded for longer early in the growing season, there was a sharp decrease in the plant coverage later in the growing season (Fig. 3).

Figure 3: This plot shows the relationship between peak plant coverage and the amount of time the marsh was flooded in the early growing season. The dashed vertical line is the threshold denoting when the amount of time flooded corresponded to a drastic decline in plant species. Source: Delgado and Baldwin 2018, Figure 5. Permission to use this image was obtained from RightsLink.
What might this mean for a tidal freshwater marsh in the face of climate change?

This study found that elevation and early season flooding play important roles in plant species coverage at Jug Bay’s tidal freshwater wetlands. For example, only ~12% of the plant species found were present below the mean sea level, indicating that the middle and upper elevations have the most plant diversity. However, this elevation range is small, so any water level or elevation changes could greatly affect the system. For example, 65% of the plant species were exclusively found in the upper elevations of Jug Bay, thus if the tidal marsh cannot keep pace with sea level rise, those plant species could be vulnerable to loss, making this habitat less diverse. Any change in plant diversity could have rippling affects up the ecosystem by removing things such as shelter and food sources and could also affect the resiliency (or ability to ‘bounce back’ from a disturbance) of the marsh.

The researchers hypothesize that one reason early flooding could affect plant distribution is that flooding may affect the seed bank and germination process. Many of these tidal freshwater plants begin their seed germination in that early window between March and May, so increased flooding may affect the ability of seedlings to emerge and grow.

Early flooding is just one potential manifestation of climate change that could make tidal freshwater systems, such as Jug Bay, vulnerable to habitat loss and decreased diversity. This study showed that even small changes in the elevation and/or flood period could greatly impact the abundance and presence of certain plant species, particularly those found in higher elevations.

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Kari St. Laurent

Kari St. Laurent

I earned a Ph.D. in Oceanography from the University of Rhode Island Graduate School of Oceanography in 2014. My research focused on the sources and fluxes of black carbon in the Subtropical Atlantic. After, I was a postdoctoral scholar at the University of Maryland Center for Environmental Science studying extreme climate change. I am currently the Research Coordinator for the Delaware National Estuarine Research Reserve in the Delaware Department of Natural Resources and Environmental Control.

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