Abstract
In mid-winter 2018, an unprecedented sediment deposition event occurred throughout portions of the Great Marsh in Massachusetts. Evaluation of this event in distinct marsh areas spanning three towns (Essex, Ipswich, and Newbury) revealed deposition covering 29.2 hectares with an average thickness of 30.1±2.1 mm measured shortly after deposition. While sediment deposition helps marshes survive sea level rise by building elevation, effects of such a large-scale deposition on New England marshes are unknown. This natural event provided an opportunity to study effects of large-scale sediment addition on plant cover and soil chemistry, with implications for marsh resilience. Sediment thickness did not differ significantly between winter and summer, indicating sediment is not eroding or compacting. The deposited sediment at each site had similar characteristics to that of the adjacent mudflat (e.g., texture, bivalve shells), suggesting that deposited materials resulted from ice rafting from adjacent flats, a natural phenomenon noted by other authors. Vegetative cover was significantly lower in plots with rafted sediment (75.6±2.3%) than sediment-free controls (93.1±1.6%) after one growing season. When sorted by sediment thickness categories, the low thickness level (1–19 mm) had significantly greater percent cover than medium (20–39 mm) and high (40–90 mm) categories. Given that sediment accretion in the Great Marsh was found to average 2.7 mm per year, the sediment thickness documented herein represents ~11 years of sediment accretion with only a 25% reduction in plant cover, suggesting this natural sediment event will likely increase long-term marsh resilience to sea level rise.
Highlights
Salt marshes, widely recognized for their valuable suite of ecosystem services, are both dependent upon tidal flooding and vulnerable to increased rates of sea level rise [3, 4]
A total of 29.2 ha of naturally deposited sediment was calculated from the orthomosaics and resulting supervised classifications (Fig 3). This total is comprised of 1.7 ha at Essex Bay (EB), 3.2 ha at Jeffrey’s Neck (JN), and 24.3 ha at Plum Island (PI) (Table 1)
Thickness appeared to differ between sites in both winter and summer (Fig 4a), with sediment at JN being almost double the thickness of EB in both cases
Summary
Widely recognized for their valuable suite of ecosystem services (flood protection, habitat provision, carbon storage, etc. [1, 2]), are both dependent upon tidal flooding and vulnerable to increased rates of sea level rise [3, 4]. Widely recognized for their valuable suite of ecosystem services [1, 2]), are both dependent upon tidal flooding and vulnerable to increased rates of sea level rise [3, 4]. Effects of a natural sediment deposition event on salt marsh plant cover collection and analysis, decision to publish, or preparation of the manuscript
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