By WENLEY FERGUSON, MARCI COLE EKBERG and MEG KERR/special to ecoRI News
Salt marshes are tidal areas that contain plants tolerant of salt water. Rhode Island salt marshes are found along the shores of salt ponds, Narragansett Bay, estuarine rivers — such as the Narrow River estuary — and small embayments, such as Allin’s Cove in Barrington. Salt marshes provide nursery grounds and foraging habitat for hundreds of species of fish, shellfish, birds and mammals.
In addition to their habitat value, salt marshes filter out pollutants before they reach coastal waters, and provide a buffer to adjacent developed coastal communities during storms and flooding.
In the past 300 years, more than 50 percent of Narragansett Bay’s salt marshes have been destroyed, and most of the remaining salt marshes are damaged by development throughout the watershed.
Downtown Providence was once a large tidal estuary known as the Great Salt Cove. This marsh was completely filled to form the uplands and the urban landscape we see today. In fact, throughout the Narragansett Bay estuary many marshes have been partially filled. Partial filling impacts the marsh by altering the tidal exchange of water and impacting the vegetation communities that rely on twice-daily flooding. Often the result of such changes in elevation and flooding is the invasion by non-native species such as phragmites australis (common reed). Phragmites are tolerant of these conditions of limited tidal flushing and lower salinity, and can rapidly overtake partially filled marshes.
Development that favors phragmites growth includes the construction of dikes, roads and rail crossings across salt marshes. This limits the natural flow of salt water and reduces salinity. Once tidal flooding is reduced, phragmites, which are tolerant of reduced salinity, invade rapidly. Phragmites out-compete native salt-marsh vegetation, and reduce local biodiversity.
Mosquito ditching also has impacted many marshes in the Narragansett Bay watershed. Mosquito ditches are straight, narrow channels that were dug to drain the upper reaches of salt marshes. Historically, it was believed that ditching marshes would control populations of mosquitoes that breed there. But it is now known that ditching drains salt-marsh pools, which support populations of mosquito-eating fish, such as mummichogs, leading to increases in mosquitoes.
These fish are an important prey item for wading birds such as herons and egrets, as well as larger, predatory fish species. Mosquito ditching alters natural patterns of groundwater drainage, which alters plant community composition and nutrient cycling.
Polluted runoff from adjacent uplands can degrade salt marshes. Runoff from roads and other paved surfaces, and nutrient-rich runoff from fertilized lawns, agricultural areas and septic systems can degrade marshes by encouraging growth of phragmites and other invasive species. Forested areas that buffer salt marshes have diminished as population growth in coastal areas increases.
Monitoring and restoration
For more than a decade, Save The Bay and local, state and federal partners have coordinated salt-marsh restoration projects throughout the Narragansett Bay watershed. Projects have improved water flow through tidal restrictions, removed historic fill, treated stormwater flow to limit sediment deposition and pollution into the marsh, and planted buffers to protect marshes from upland runoff.
In some cases, salt-marsh plants are planted after the marsh elevation has been restored. Once high and low marsh plants are able to flourish, both plant and animal diversity in the marsh increases.
The natural assemblage of plants in a salt marsh is adapted to the wetness of the area and the salinity of the surrounding water.
Climate change with its associated sea-level rise will negatively affect salt marshes. When sea-level rise occurs gradually, plant communities have sufficient time to adapt to shifting conditions by increasing their elevation through the accretion of sediment and organic material and retreating landward. In this scenario, low marsh gradually replaces high marsh, and the entire marsh migrates inland. If sea-level rise occurs too quickly, salt marshes can’t build up elevation fast enough, and can drown in place. In addition, salt-marsh retreat is often blocked by human-built structures such as seawalls or houses or by natural terrain like bluffs or bedrock outcroppings.
Rhode Island has seen an increased rate of sea-level rise from 2.6 millimeters per year between 1931 to 2009, to 3.6 millimeters per year between 1990 and 2009. The Narragansett Bay Research Reserve calculated (pdf) a marsh accretion rate for Nag marsh of 2.9 millimeters a year. If this accretion rate is representative of other Rhode Island marshes, then marsh accretion rates aren’t keeping up with sea-level rise.
Additionally, between 2010 and 2012, observed tidal heights during the growing season exceeded predicted heights — both high and low tides — by about 9 centimeters on average during the growing season, according to National Oceanic and Atmospheric Administration (NOAA) tide data. This phenomenon is attributed to El Nino–related changes to atmospheric pressure over the Gulf of Mexico and eastern Canada as well as to winds over the Northeast Atlantic.
Rapid sea-level rise and increased tidal heights may lead to erosion of the low-marsh margin, as well as increased inundation and possible loss of the high marsh. In many local marshes, it appears that instead of a slow shift in the vegetation community from high marsh to low marsh, higher water levels are flooding the high marsh. The water remains trapped on the high marsh, which then loses vegetation and creates open water areas that subside to an elevation that doesn’t support typical salt-marsh vegetation. High temperatures and high salinities in the high marsh during droughts can also cause die-off of high-marsh vegetation. Most marshes have additional impacts including manmade ditches or tidal restrictions, barriers to inland migration, increased nutrient inputs that can degrade below-ground biomass, and increased herbivory pressure by grazers such as Canada geese and the purple marsh crab.
In recent years, Save The Bay ecologists and other experts have observed that the regions’ salt marshes may be showing initial signs of response to the effects of rapid sea-level rise and increased inundation. Most of these observations have been anecdotal and haven’t been supported by quantitative field data. To better understand the changes, Save The Bay has developed a three-tiered rapid assessment to collect data to quantify the extent of the shift from vegetated salt marsh to ponded water on the marsh surface. Tier 1 of the approach is a landscape scale, GIS analysis. Tier 2 is a field-based rapid assessment. Tier 3 is a set of detailed, less rapid, research-based assessments.
In 2012, Save The Bay began conducting Tier 2 of the approach by reviewing numerous salt marsh rapid assessments, and testing out different assessment techniques in the field. Protocol included a “belt” transect from upper marsh to creek or water’s edge. Along the transect the width of each dominant plant community was measured. In dominant marsh zones, the bearing capacity of the soil was measured, which indicates the extent to which the organic matter is decomposed and how susceptible the soil is to erosion.
Preliminary results show that short-form Spartina alterniflora, once thought to be a minor component of New England salt marshes, covers 50 percent to 80 percent of the marsh.
Save The Bay has identified and begun to implement adaptive management at some marshes, including Hog Island, Barrington Beach salt marsh and Rocky Hill marsh on the Potowomut River. The goal of each of these projects is to reduce the amount of water impounded on the marsh surface to allow the marsh surface to revegetate and to prevent further marsh subsidence.
Each marsh has different factors causing water to be impounded on the former high marsh, such as migrating barrier beaches. Yet a consistent factor affecting all of these marshes and others throughout the region is the increased rate of sea level-rise.
The goal this year is to implement all three assessment tiers and to continue adaptive management planning at other marshes, including the Narrow River, Hundred Acre Cove and Round Marsh in Jamestown.
Wenley Ferguson is the director of habitat restoration and Marci Cole Ekberg is a coastal ecologist at Save The Bay. Meg Kerr is the watershed program manager at the Narragansett Bay Estuary Program. This article originally was published in the Spring 2013 Narragansett Bay Journal.