ARE PONDS RESPONSIBLE FOR PERMANENT MARSH LOSS?

Ponds are water-filled, un-vegetated rounded depressions, 0.1-1 m deep and 1-100 m large, commonly present on marsh platforms. Ponds play an important ecological and biochemical role, but it is not clear whether they also contribute to permanent marsh loss. A simple model for pond vertical and planform dynamics is implemented to study how ponds affect the long term morphological evolution of tidal marshes. Isolated ponds form, deepen and enlarge likely because of biochemical processes that oxidize existing organic matter and prevent further organic matter accumulation. Ponds eventually connect to the channel network and thus can re-vegetate and accumulate sediment. If pond size at the time of drainage is smaller than the critical size for runaway expansion by wave erosion, ponds quickly fill up and recover the original platform elevation. In this case a dynamic cycle establishes: ponds constantly form, expand and recover without causing permanent marsh loss. This scenario seems to occur in marshes of the US Atlantic coast, which are characterized by large tidal ranges and moderate sea level rise rates. On the contrary, connected ponds in the marshes of the Mississippi Delta seem to expand independently on their size, suggesting the presence of an edge erosion mechanism not related to waves. In this case connected ponds might still fill up if the inorganic deposition rate is larger than the rate of relative sea level rise, which seems to occur on marshes adjacent to a large riverine sediment supply. If ponds cannot fill up they will eventually enter the runaway expansion by wave erosion. The model suggests that pond-driven marsh loss is not dictated by the accretion on the vegetated platform, but rather by inorganic sediment accretion in the absence of vegetation and by wave-unrelated processes that erode the pond edge. Further research should determine whether the latter mechanism is caused by physical or biochemical processes.