Impact of grazing management on silica export dynamics of Wadden Sea saltmarshes

During periods of silica limitation, its supply from tidal marshes is important for the stability of estuarine and coastal food webs. Saltmarshes are highly dynamic, grass dominated ecosystems: their large area, high salinity and location imply that they could efficiently contribute to the buffering of silica depletion events in the coastal zone. As grazing management potentially alters vegetation and sedimentation dynamics in saltmarshes, it could have an indirect impact on silica cycling in these systems. In two saltmarshes of the Wadden Sea coast, concentrations of dissolved and biogenic silica (DSi and BSi) were measured in eight creeks in four seasons under different management conditions. Export rates were calculated using simultaneous discharge measurements. Mean annual DSi concentration in the seepage water was 338 ± 112 μmol l−1. Ungrazed sites had significantly higher seepage water DSi concentrations than sites which were grazed by sheep. BSi concentrations were, in general, lower and more variable. DSi export rates from ungrazed sites (265 ± 155 μmol m−2 day−1) were twice as high as from grazed saltmarshes (126 ± 137 μmol m−2 day−1). DSi concentrations were among the highest values previously reported for saltmarshes and tidal freshwater marshes. Although differences in silica exports from grazed and ungrazed sites might be partly explained by silica uptake of benthic diatoms in the creeks, differences in hydrology appeared to be an overarching factor, controlling silica exports from Wadden Sea saltmarshes.