Restoring a Coastal Wetland Under Altered Hydrology
A 400-hectare tidal marsh behind a 1950s earthen levee has subsided 40 cm below adjacent marsh elevations and is now dominated by cattail and reed canary grass. Managers propose two options. Option A: remove a 200 m levee section to restore full tidal exchange, which would reintroduce brackish water (salinity 8-15 ppt) and tidal range of 1.2 m. Option B: install a managed flap-gate culvert that allows controlled freshwater inflow and limited tidal exchange, keeping salinity under 3 ppt. Sediment cores show the marsh accretes about 2 mm per year under current conditions, while sea level is rising 4 mm per year locally. The endangered salt marsh harvest mouse uses high-elevation pickleweed habitat, which needs salinity above about 6 ppt to outcompete cattail.
1. Which option better addresses the vertical elevation deficit, and what does the accretion-versus-subsidence arithmetic imply?
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Option A is stronger on elevation. Current accretion (2 mm/yr) is below local sea-level rise (4 mm/yr), so the marsh loses about 2 mm/yr of relative elevation even before accounting for the 40 cm subsidence deficit. Restoring tidal exchange delivers sediment-laden tidal water and promotes organic accretion, which is the mechanism that can raise accretion above the local rate of rise. Option B's freshwater, low-energy regime likely keeps accretion near or below current levels and keeps the marsh on a drowning trajectory. The judgment call: elevation persistence favors A even though A has ecological tradeoffs.
2. Explain the biodiversity tradeoff between the two options using competitive exclusion and habitat requirements.
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Under Option B, low salinity (<3 ppt) favors cattail and reed canary grass, which competitively exclude the salt-tolerant pickleweed that the endangered harvest mouse requires. The marsh would persist structurally but lose the specialist habitat and its associated species. Option A raises salinity to 8-15 ppt, which suppresses cattail and allows pickleweed to reestablish, benefiting the harvest mouse, but full tidal exchange may stress or displace freshwater-dependent species currently using the site. The judgment call: A better serves the endangered specialist and overall native diversity, while B protects the current freshwater assemblage but locks in a low-diversity, invasive-dominated state. Managers must weigh irreplaceability of endangered habitat against transitional losses.