When I first lay eyes on the salt marsh on Bald Head Island, the swaths of Spartina grass (Spartina alterniflora) that flank the tidal creek are quiet and tranquil. They undulate ever so slightly with the breeze, a sleepy stretch of sandy gray.
Only a few moments into my conversation with Johnston Graduate Fellowship recipient Gabrielle Krueger, I learn that the salt marsh is a site for much more activity than appears at a glance. In addition to being in a state of constant flux with the daily surge and recession of tidal water, the salt marsh is home to many creatures who live beneath the protective shade of the marsh grasses: oysters (Crassostrea virginica), Atlantic ribbed mussels (Geukensia demissa), marsh fiddler crabs (Minuca pugnax or Uca pugnax), and marsh periwinkle snails (Littoraria irrorata), among others.
In July of 2025, Krueger conducted research on the salt marsh for two purposes: to assess overall marsh health and to study the mutualistic relationship between the Spartina and the mussels. To ensure that she was measuring faunal activity across the marsh, Krueger set up transects at multiple locations along the tidal creek system: long, straight measured lines between 40-50 meters in length from the creek further out to the interior of the marsh. Each transect provided a “slice” of the marsh for data collection at different distances from the creek bed.
Between the salt marsh and the organisms that live there exist mutualisms, symbiotic relationships between species. One of the measurements Krueger used to assess marsh health was the presence of fiddler crab burrows dug into the marsh sediment. Spartina provides shade and protection to organisms like crabs. It also benefits from their presence. Salt marshes can be subject to hypoxia, a condition in which oxygen levels decrease rapidly beneath the soil surface due to factors like constant tidal flooding and soil density.1 The burrows that crabs dig allow oxygen to permeate marsh sediment, enabling the Spartina’s roots to receive oxygen and absorb important nutrients.2
The mutualism between Spartina and Atlantic ribbed mussels, which Krueger is studying as part of her dissertation work at Georgia Institute of Technology, also enables both species to thrive. Mussels anchor themselves at the base of spartina and excrete waste that includes nitrogen, which acts as a fertilizer to spear spartina growth.3 During her fieldwork, Krueger measured and compared Spartina growth with mussel presence. She also studied the interactions between species at the microbial level, using porewater samples to analyze the impact of local microbes on the mutualism.
Salt marshes are crucial parts of the coastal ecosystem and are threatened by rising sea levels, stormwater pollution, and coastal development.4 In addition to providing a home for species like the fiddler crab and Atlantic ribbed mussels, marshes are also “sinks” for greenhouse gasses,5 preventing their release into the atmosphere. In a future where salt marshes shrink, our environment will stand to lose the benefits from the symbiotic relationship with the marsh: habitats for vulnerable wildlife and protection from coastal erosion and increasing global temperatures.
The complexity of mutualism between species on macro- and microbiological scales demonstrates just how much symbiotic activity occurs in the salt marsh environment. Through the data collected in Krueger’s health assessment and through their ongoing water quality monitoring, the Conservancy can not only understand the salt marsh’s current health but can use the data as a baseline of detailed information to compare against in future years. If negative impacts on the salt marsh become apparent, these assessments will assist in understanding those changes and will help preserve the salt marsh as a place where unique, important relationships between species occur.
