SPATIAL AND TEMPORAL IMPACTS OF CRAB BURROWS ON GROUNDWATER-SURFACE WATER INTERACTIONS AND REDOX CONDITIONS IN A TIDAL WETLAND

Tidal wetlands are hydrologically complex ecosystems due to tidally induced water level oscillations, subsurface heterogeneity, and microtopography. Diurnal water level fluctuations in tidal creeks create hydraulic gradients that drive water into and out of creek banks. Spring-neap tidal oscillations result in alternating periods of consistent marsh platform inundation and prolonged drought. By altering the hydroperiod on a fortnightly cycle, the water table undergoes not only diurnal changes, but also spring-neap oscillations. However, these groundwater-surface water interactions are made more complex due to subsurface heterogeneity in the form of macropores. Macropores, excavated by the marsh fiddler crab, Uca pugnax, create preferential flow paths in the low-permeability marsh sediment where groundwater and surface water can readily exchange. The goal of this study was to determine how macropores spatially and temporally impact porewater flow and groundwater-surface water interactions, and, in turn, subsurface redox conditions in a mid-latitude salt marsh in Dover, Delaware. Through the use of seepage meters in the tidal creek, slug tests across the marsh platform, and crab burrow size and density assessments, our results suggest that macropores increase groundwater-surface water interactions and fluxes of water between the marsh and tidal channel from late spring to late fall, when macropores are present. Fluxes of water between the marsh platform and tidal creek decrease when fiddler crab burrows do not perforate the creek banks. Continuous reduction/oxidation potential data from in-situ sensors show that the redox potential near the creek varies tidally. However, the depth and extent of the redox oscillation increases due to tide stage and the presence of crab burrows. This result is confirmed by the creation of macropore casts, which show a complex subsurface network of burrows that reach 35 cm or greater in some areas. Results from this study show that macropores have a large impact on the biogeochemistry of the marsh platform, and could potentially impact the quantity of solute exchange between the marsh platform and tidal creek, thus impacting coastal chemical budgets.