Physical oceanography

Networking in nature

Source: Schwarz, C., F. van Rees, D. Xie, M. G. Kleinhans, and B. van Maanen. 2022. Salt marshes create more extensive channel networks than mangroves. Nature Communications 13:1–9. https://doi.org/10.1038/s41467-022-29654-1

Mangroves, marshes, and more

The plants that line our coasts are, above all, survivors. The grasses and trees that make up salt marshes, mangrove forests, and other ecosystems experience constantly changing conditions and bear the brunt of brutal storms. Nevertheless, they are well-suited to the task. Salt marsh grasses form elaborate root networks that trap sediment and reduces water flow. Mangrove roots act similarly, protecting and stabilizing the shoreline.

At the end of the day, however, these two systems are not identical. This is of growing concern for two reasons: 1) Mangroves, typically found in tropical regions, are expanding their range as climate change progresses, crowding into the salt marsh range, while 2) overall sea level rise and storm intensities are also accelerating, increasing strain on both species. If we want to know how our coastlines may change in the future, we must understand how each species serves its shoreline stabilizing service.

Coastal channeling

A crucial aspect of both marshes and mangroves is that neither is a completely consistent region. Both feature meandering channels that divide the vegetated area while providing a route for nutrients and sediment to move inland.

A team of researchers from Belgium, the Netherlands, the United Kingdom, and the United States noticed through satellite images that these channels were not all created equal. They hypothesized that the differences in reproduction strategies between plant types influences how water moves within the ecosystems.

Salt marsh grasses reproduce by flowers and seeds but can also spread through clonal expansion. When spreading clonally, a structure called the “rhizome” that extends underground with the root network sprouts a new plant. The new sprout is both connected to and genetically identical to the original. This creates patches of vegetated area, as the clones typically survive better than seeds.

Image: Floating mangrove propagules in search of their forever home. Photograph from Brian Gatwicke, Flickr.

Mangrove seeds, however, stay attached to the parent plant until they are seedlings (also called propagules). At this point, they fall into the water and float to a new destination to settle and grow. This is a less continuous process than the grasses’ cloning, but also creates a more consistent landscape.

These original differences in settlement could remain as an “imprint” on the landscape, as vegetated areas enhance erosion elsewhere, creating the channels described above.

Following the flow

To test this, the research team used two approaches. The first was a study of existing salt marshes and mangrove forests, employing satellite imagery to map out the extent of channels in areas all over the world. These maps were then used to calculate a set of metrics describing the amount of channeling, distance between unchanneled areas, and the geometry of the channel networks. The second was a lab experiment, in which they used a flume to imitate the flow of water over patchy and consistent vegetation.

The satellite images painted a striking picture. The salt marshes surveyed had, on average, more channels per area than the mangrove forests. Likewise, the distance that a drop of water would have to travel to the nearest channel was shorter in the salt marshes than the mangroves. However, the overall geometry of the channels created by the two systems was the same, meaning that the branching patterns were similar.

The flume experiments also supported the researchers’ original hypothesis; the water flow dug out more channels when vegetation was patchy than when it was a large, consistent block. When vegetation was patchy, the channels also extended further along the flume.

Salt marshes, sediments, and sea level rise

Based on their two experiments, the researchers concluded that salt marshes have more extensive and efficient channel networks than mangrove forests, likely due to differences in plant reproduction. While other factors influence channel creation in coastal communities, such as sediment type and wave activity, this allows us to better understand how the existing plant community may shape future wetlands.

The more extensive channels of the marshes allow sediment to reach further inland, building up the coastline and possibly allowing these areas to keep pace with sea level rise. However, these same properties allow storm surge waves to reach further inland, limiting the protection provided to onshore areas. The researchers plan to continue studying this topic, with the goal of determining what other factors may contribute to channel formation and their implications for future resilience.

 

Header image: Aerial view of the Plum Island Long Term Ecological Research site salt marsh in Ipswich, MA. Taken from Google Earth

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