Level Loggers Assess Salt Marsh Responses to Sea Level Rise
The Narragansett Bay National Estuarine Research Reserve (NBNERR) in Rhode Island is an estuarine protected area located on four islands within the heart of Narragansett Bay. The approximately 4,400 acre-reserve spans both land and water environments and includes salt marshes, eelgrass beds, rocky intertidal zones, pine barrens, deciduous forest, and coastal meadows.
The NBNERR’s research arm conducts research and monitoring of coastal and estuarine systems as part of a larger goal to track changes in the Reserve and within the Narragansett Bay estuary. These efforts help forecast environmental trends and serve to protect Narragansett Bay by fostering science-based ecosystem management.
A recent study conducted by the NBNERR focused on the assessment of marsh vegetation responses to sea level rise during the current period of extreme water level increases. Although stresses on marsh systems resulting from sea level rise are well understood, most studies in southern New England on these impacts pre-date the more recent cycle of rapidly accelerating sea level rise coupled with episodic events of extreme water level surges.
To investigate this, the NBNERR leveraged data from two salt marsh monitoring and assessment programs in Rhode Island that were designed to evaluate marsh responses to sea level rise. Data from these programs were combined to document temporal and spatial patterns in marsh vegetation during the current period of extreme water level increases.
Of the programs, one evaluated variations over the long term in two National Estuarine Research Reserve Sentinel Site salt marshes, while the other examined changes occurring across 40 marshes during a single season.
“The data from these two different, yet complimentary, programs really went hand-in-hand to build a stronger story,” says Kenneth Raposa, Ph.D., research coordinator at NBNERR.
Results from the NBNERR’s research revealed the ongoing decline of a high salt meadow foundation species, called Spartina patens, which is being replaced mostly by Spartina alterniflora, a low marsh grass species that thrives in lower elevations and is adapted to withstanding more tidal inundation.
“These findings were supported with additional, ancillary data such as elevations and water levels that were obtained through monitoring, which helped form a more accurate and comprehensive understanding of the processes that are occurring,” Raposa says. “From this, we concluded that changes in salt marsh ecosystem dynamics are tightly linked to higher water levels in Narragansett Bay, which, in turn, are associated with sea level rise.”
In running the study’s monitoring programs—which covered dozens of marsh plot locations—Raposa and his research colleagues used HOBO water level data loggers offered by Massachusetts-based Onset (onsetcomp.com). The loggers provide accurate water level and temperature monitoring and feature robust designs for deployment in saltwater environments such as brackish wetlands and tidal areas.
Specifically, the research team used the level loggers to track and record short-term fluctuations in marsh water levels. This information was coupled with elevation data—after installing the data loggers in marshes, survey equipment was utilized to determine elevations of both the marsh surface and the data loggers. By linking these data sets together and performing simple calculations, the researchers established the extent of marsh inundation by tidal waters over different periods of time.
“We deployed the loggers in marsh creeks by simply fastening them to PVC pipes driven into the sediment,” Raposa says. “The loggers’ depths varied, but they were essentially positioned about 25 cm from the bottom of the creek, in areas with a tidal range of about one meter. Data were collected for about a month with measurements taken every 10 minutes.”
Raposa, who switched to the HOBO data loggers roughly five years ago, had previously used another manufacturer’s data logger. “Compared to the other product, the HOBO data loggers are much more user-friendly for programming and deployment,” he says. “They also offer greater durability and are easier to maintain. But even with all these advantages, the HOBO data loggers are still more affordable. We are continuing to use them in new projects with great success.”
Importantly, the HOBO data loggers delivered accurate and reliable data to Raposa and his team, helping bolster their conclusions that Spartina patens losses in southern New England marshes can accelerate during periods of extreme high water levels.
“Unless adaptive management actions are taken, we predict that marshes throughout Rhode Island will continue to lose salt meadow habitat along with the ecosystem services and wildlife support functions that are dependent on these valuable areas,” Raposa says.