Circulation and Reaction Hotspots in an Intertidal Salt Marsh: a Modeling Study

Abstract

Intertidal salt marshes are highly productive, dynamic ecosystems at the interface between the land and the ocean that can play a significant role in reducing nutrient loading to the coastal ocean. To assess the spatio‐temporal patterns in salt marsh biogeochemistry, a reactive transport model describing tidally‐driven flow as well as solute dynamics across a marsh cross‐section was developed. Porewater residence times were computed to identify zones of rapid fluid exchange. Model simulations suggest the presence of circulation hotspots at the creek bank and the upland‐marsh transition zone, whose intensity varies over a tidal cycle. The location and magnitude of these regions of rapid fluid exchange depend on the tidal amplitude, and on the presence or absence of terrestrial groundwater input from the upland. The introduction of oxygenated creek water to the marsh subsurface also promotes biogeochemical reactions and hence may be important for regulating the marsh’s filter function. Reaction hotspots are located at the interface between chemically distinct water bodies such as upland‐derived groundwater and the intruding tidal creek water. As a result, these hotspots develop at the fringes of circulation hotspots, but are not identical to the locations of highest infiltration. The relative importance of reaction hotspots varies substantially with tidal amplitude and their presence has important implications for the placement of monitoring wells in field studies.