By Juanita Asapokhai
Salt marsh meadows, also known as tidal marshes, grow from the long-term accumulation of plants such as Spartina alterniflora and sediment close to the shore. This growth is dependent on maintaining a balance between the periods of wetness and dryness caused by high tide and low tide that allows the plants and sediments to amass enough material to form the marsh. The ability of salt marsh to withstand fluctuations in sea level is essential to their survival. As global warming causes sea levels to rise, this balance is in jeopardy, and salt marsh plants like the Spartina alterniflora, also known as smooth cordgrass, will be forced to adapt, or die out. The worst outcomes of rising temperatures are still decades away, but, thanks to this research study funded by the Long Island Sound Study (see main story), researchers are seeking to get a better understanding of long-term impacts.
The role of the salt marsh in Long Island Sound’s ecosystem is multifaceted, with a long environmental history as a habitat for various species, and a historical and contemporary asset in the lives of Long Island Sound communities.
A visit to a beach or shoreline in the Sound with a thriving salt marsh will quickly showcase sprawling patches of Spartina alterniflora stretching up out of the water, not too far away from sediment and sand in the low marsh. A long, slender grass, Spartina alterniflora, also known as smooth cordgrass, is the foundation species of salt marshes in the Sound. The grass provides the base material that, over time, integrates biomass —plants and their petals, stems, roots, and other organic materials —with sediment to form the mass of salt marsh. In comparison with other “high” or upland marsh plants, Spartina alterniflora is particularly tolerant of the flooding brought by high tide, as the surface of the marsh and part of the plant is underwater. The optimal conditions for the growth of salt marsh occur within the intertidal zone. The intertidal zone refers to the cycle between wetness (as well as exposure to saltwater) and dryness directed by the tides that enables salt marsh to pace sea level rises by growing tall enough to avoid being submerged beneath the water too frequently.
The underground system of roots and material of plants like Spartina alterniflora enable salt marshes to function as effective nutrient sinks. Absorbed by the marsh plants through photosynthesis and by their roots, nutrients like carbon, nitrogen, and phosphorus accumulate in the below ground biomass for a long period of time, removing them from the Sound and the atmosphere. This is a particularly valuable feature of salt marshes in the case of carbon, a greenhouse gas.
Salt marshes play host to a variety of coastal birds. Ospreys and their nests are easily spotted close to these estuary habitats. Marshes are also a breeding ground for willets, a once-endangered species that, thanks to conservation efforts started in the 1970s, has since seen a significant boost in its numbers in the past few decades. The great blue heron, whose uniquely webbed feet allow them to navigate the muddy terrain of salt marshes with ease, lives on salt marshes throughout the year. Salt marshes support the broader food chain of the Long Island Sound by providing a habitat for fish like the mummichog and striped killifish that serve as food for species that dwell offshore. The juvenile offspring of horseshoe crabs and American eels reside within marshes until they have matured, at which point they move offshore. The biodiversity preserved and sustained by salt marshes makes them an essential fixture in the ecology of the Long Island Sound.
For communities of Long Island Sound, salt marshes have had several important roles throughout history. Precolonial indigenous cultures utilized salt marshes for fishing and farming for hay. The salt marshes present across the Sound today are a fraction of preexisting marshes, a large percentage of which have been eliminated for coastal development in the form of roads and residences. As homes continue to populate the coast and shorelines, the presence of salt marshes is needed now more than ever to act as a buffer for flooding and storm damage. When high-energy waves push water onto the shore, salt marsh vegetation such as Spartina alterniflora reduces the energy of the waves by providing friction, slowing the waves’ momentum and force on the shore and the homes beyond it. The below ground biomass of salt marshes helps to retain the position of the coastline and resist erosion due to storm waves.
The ecological conditions needed for this “vertical” growth are in jeopardy: as rising sea levels push salt marshes farther up the shore, where they run into developments on the Sound, like roads and train tracks. Further, as sea levels rise, the plant’s high tolerance for a wet environment has led to the creep of the species into higher marsh areas, where flooding is less frequent, and that serve as habitats for species, like the endangered salt marsh sparrow. Salt marsh conservation and restoration efforts are important steps to mitigating the projected effects of rising sea levels on the salt marsh ecosystem of the Long Island Sound and the various species that call marshes home.
The ability of the salt marsh meadows to withstand fluctuations in sea level is essential to their survival. The worst outcomes of rising temperatures are still decades away, but, research projects such as a 2022-fund project through the Long Island Sound Study Research Grant Program are providing decision-makers with information on how get a better understanding of long-term impacts and plan for the future.
Juanita Asapokhai was a Communications Intern for the Long Island Sound Study in summer 2023. She attends Tufts University and will be graduating with degrees in Community Health and Sociology in the spring of 2024. Dr. Sarah Crosby, Director of Conservation and Policy at The Maritime Aquarium at Norwalk, provided information about salt marshes for this article.