Physical and functional responses to experimental marsh surface elevation manipulation in Coos Bay's South Slough

Dike material was used as fill to construct high, mid, and low intertidal elevations in a subsided marsh located in the South Slough National Estuarine Research Reserve, Oregon. Marsh surface elevation change (including fill consolidation and compression of the original marsh soils), vertical accretion, tidal channel development, emergent vegetation colonization, and fish use were monitored over 3 years. Significant marsh surface elevation loss was detected at all elevations, with fill consolidation accounting for 70% of the loss at the highest elevation. Vertical accretion averaged 0.19 cm/yr in the sparsely vegetated Kunz Marsh compared with 0.70 cm/yr at the densely vegetated reference sites. Tidal channel development was influenced as much by marsh surface gradient as by marsh surface elevation. Vegetation colonization was directly correlated with elevation, whereas density and species richness of fish was inversely correlated with elevation. Manipulating the marsh surface to mid-marsh elevations favors rapid vegetation colonization and facilitates vertical accretion-dominated tidal channel development. Low marsh elevations result in initially slower developing vegetation colonization and channel development but are more beneficial to fish during the early stages of marsh recovery. High marsh elevations appear to sacrifice tidal channel development and associated fish access for rapid vegetation colonization.