Assessing short-term sediment accretion rates and hydrological influences on a microtidal estuarine wetland: Mustang Island, TX

As sea level rises there has been a growing concern whether salt marsh wetlands can withstand an accelerated rise in sea level by vertically accreting. Sediment accretion is a natural process that changes the elevation of the marsh surface relative to sea level. For a wetland to persist in the long-term, its accretion rate must at least match the rate of relative sea level rise. This study describes sedimentation rates in the estuarine wetlands located on Mustang Island, TX, a sandy barrier island. Sedimentation rates were measured bi-weekly from June 2014 to July 2015 using sediment plates and erosion pins, and over periods of 2.4 to 3.3 years (2012- 2014/2015) using horizon marker techniques. Water level loggers were used to assess hydrological controls on bi-weekly sedimentation patterns. Shallow cores (~15 cm) were collected from the horizon marker plots in August 2014 and July 2015. Vertical accretion rates were compared across different timescales including decadal rates determined using 137Cs from a previous study on Mustang Island, TX.

Results indicated sediment accretion across the study area was not significantly influenced by hydrological patterns, with the exception of low marsh environments near tidal creeks (r2=0.52, p < 0.1). The most important factor in determining sediment deposition on sediment plates located near the main tidal creek was the number of flooding events, suggesting that deposition increases as frequency of flooding events increases. The total accumulation deposited on plates was dominated by inorganic sediments, suggesting there is a limit of detrital organic matter contribution for this area.

Average vertical accretion using horizon markers was 8.15 ± 5.21 mm yr-1 in upland environments; 4.51 ± 5.21 mm yr-1 in high marsh environments; 3.36 ± 3.57 mm yr-1 in high flat environments; 11.92 ± 9.73 mm yr-1 in low marsh environments; and 1.88 ± 2.54 mm yr-1 in low flat environments. There was a significant difference in vertical accretion rates between both horizon markers and erosion pins, which provide annual-scale accretion rates, when compared to 137Cs, which provide decadal-scale accretion rates (p < 0.1). On average annual vertical accretion rates were 2.8 times higher than decadal rates. Differences between annual and decadal accretion rates are mostly attributed to shallow sediment compaction within the top 3 cm of the wetland surface. Variation in wetland vertical accretion rates increased significantly going from decadal (± 0.41 mm) to annual (± 2.87 mm) to annualized biweekly rates (± 9.60 mm).

Annual-scale accretion rates measured using horizon markers in low marsh and upland environments appear to be keeping up with relative sea level rise (RSLR), which is 5.27 ± 0.48 mm yr-1 as measured since the 1950’s at a nearby tide gauge. However horizon marker vertical accretion rates in tidal flats and high marsh environments are not sufficient to overcome sea level rise. Vertical accretion rates were positively correlated with organic and inorganic accretion for all horizon markers (p < 0.1); however, the relative contribution of organic matter decreases as inorganic matter increases. Our findings anticipate environmental shifts in habitats with accretion rates below RSLR. Furthermore, vertical accretion was dominated by inorganic matter, making the wetlands reliant on constant wind and episodic storms to transport sediment to the area. Importantly, these data suggest that storm-induced sedimentation acts to stabilize coastal wetlands and helps certain environments cope with RSLR, but is not sufficient to prevent shifts in the relative composition of the wetland.