Mangrove encroachment of salt marsh in Western Port Bay, Victoria: The role of sedimentation, subsidence, and sea level rise

Surface elevation tables, feldspar marker horizons, and²¹⁰Pb analysis of core profiles were implemented at four sites in Western Port Bay, Victoria, Australia, to provide information on the role of sedimentation, subsidence or compaction, and enhanced sea-level rise in contributing to salt marsh decline. Photogrammetric surveys indicate that the rate of salt marsh decline that is attributable to mangrove encroachment is lower in Western Port Bay than in comparable sites in New South Wales. Differences in the rate of mangrove encroachment at Western Port Bay may be attributed to the inverse relationship found between the degree of mangrove encroachment and surface elevation increase. While sedimentation contributes to surface elevation changes, surface elevation is not solely explained by sedimentation; factors including autocompaction and changes in the water table also play a significant role in Western Port Bay. Historic sedimentation rates measured using²¹⁰Pb dating techniques corresponded to contemporary sedimentation rates determined from feldspar marker horizons. Core sediment profiles show no change in sedimentation rates at three sites. A fourth site (French Island) was the only site that exhibited high rates of sedimentation, which appears to be related to local land-use changes in the area. All sites maintained their elevation with respect to sea level over the study period. Historic sedimentation exceeded sea-level rise for the past 32 yr, but it is difficult to determine the extent to which belowground processes affect surface elevation, causing deviations between surface elevation and sedimentation over longer periods.