Spatial patterns, rates and mechanisms of saltmarsh cycles (Westerschelde, The Netherlands)

Saltmarsh development was studied in a spatial context, in order to understand the mechanisms involved in saltmarsh cycles. A 30-y time-series of very high resolution false colour aerial photographs was studied for eight saltmarsh sites in the Westerschelde estuary (southwest Netherlands). Detailed maps of vegetation cover were produced for each year, based on a supervised classification, and changes in vegetation cover were analysed in a geographical information system. The results of the vegetation change maps and complementary series of topographic surveys have shown evidence of saltmarsh cycles, but without spatial synchrony in these cycles for the Westerschelde. For each saltmarsh, a different status of net erosion or accretion was found. Within each saltmarsh, there were both areas with vegetation loss and with vegetation expansion in each period. Most saltmarshes showed a simultaneous expansion of Spartina anglica by tussock growth, and a lateral retreat of the saltmarsh plateau through undercutting of a cliff at the edge of this plateau, resulting in saltmarsh rejuvenation. The rate of lateral retreat of the saltmarsh plateau decreased with the establishment of pioneer vegetation in front of the cliff (probably through dissipation of wave and tidal energy caused by the friction associated with the pioneer vegetation), and was generally not related to the frequency of high magnitude tides and high magnitude onshore winds. For the saltmarsh site that showed severe lateral erosion, a relationship with the mudflat topography was found, with lateral retreat of the saltmarsh plateau being compensated by an increase in mudflat elevation, eventually slowing down the erosion rate, and facilitating the subsequent expansion of pioneer vegetation in front of the cliff. The study demonstrates the significance of intrinsic processes in saltmarsh development, and the necessity to consider the local feedback mechanisms between plant growth, morphology and hydrodynamics of both the saltmarsh and the mudflat, when assessing the status of saltmarshes. It also shows the importance of assessing the changes in saltmarsh area in a spatial context, rather than looking at changes in total area of saltmarsh vegetation.