As Earth’s climate warms, coastal wetlands face a new challenge in the form of sea level rise, which can drown vegetation and drive erosion. Wetlands and their channels thrive on a steady input of mineral sediment that builds up slowly over time to counter sea level rise, but declining sediment supply and wave-induced erosion threaten this balance.
Previous modeling shows that suspended sediment concentration (SSC) and accretion are proportional to the maximum sea level rise a marsh can withstand, but the relationship may be more nuanced. Here Ganju et al. analyzed SSC and accretion rates at neighboring wetlands along the eastern shore of the Chesapeake Bay to determine how variability in sediment transport influences wetland vulnerability.
The researchers measured accretion, turbidity, tidal velocity, and SSC from the Blackwater River and Transquaking River marshes, where there are low river flows and low watershed sediment supply. Over the past century, sea level rise, shoreline erosion, subsidence, and invasive rodents have driven disintegration in the Blackwater River wetlands, while the Transquaking River area has stayed relatively stable.
The results of the current study stand in stark contrast to conventional wisdom. The researchers found that SSC and accretion rates were actually higher in the deteriorating Blackwater River wetland complex due to marsh erosion, which suggests they are not strong indicators of marsh stability. Instead, the team found that the difference in SSC between ebb tide and flood tide had a greater influence over sediment transport direction—and therefore which wetlands are more stable. The ratio of organic to inorganic SSC served as a better indicator of relative marsh degradation.
Ultimately, the scientists stress the importance of assessing wetland vulnerability as the result of complex, interrelated physical and biological processes. Understanding how sea level rise and storms will impact wetlands requires further long-term, comprehensive research into these interactions. (Geophysical Research Letters, doi:10.1002/2015GL065980, 2015)
—Lily Strelich, Freelance Writer
Citation: Strelich, L. (2016), How sediment transport sways wetland stability, Eos, 97, doi:10.1029/2016EO046199. Published on 17 February 2016.