APPLICATION OF HISTORICAL ACCUMULATION RATES TO SURFACEWATER MANGEMENT: MERRITT ISLAND NATIONAL WILDLIFE REFUGE

摘要

Previous research has demonstrated reconnection of Florida’s impoundedrnwetlands to the estuary benefits fisheries and emergent ecosystems. However, no effort hasrnyet been made to quantify the effects of managed surface water flow on patterns ofrnsedimentation. As sedimentation patterns strongly influence historical marsh evolution,rnincluding the distribution of open water and emergent landscapes, the enumeration ofrnmarsh sedimentation rates will provide a basis for reconstructing the physical processesrnresponsible for the observed historical landscape changes, as well as predicting possiblernlandscape changes that may accompany reconnection to the estuary. The goal of thisrninvestigation was to determine rates of historical marsh sediment accretion, organic matterrnaccumulation, and mineral sediment deposition at marsh locations subject to unmanagedrn(“native”) and managed hydrology.Impoundment D was subject to submergence as a result of surface waterrnmanagement that began in 1963. Sediment accumulation rates (2.8 mm/yr), calculatedrnfrom cesium activity profiles obtained from open areas of Impoundment D reflect surfacernerosion in response to wave-induced circulation across the impoundment during times ofrnsubmergence. This process lowers the substrate elevation, as has been quantified by recentrnhigh-resolution surveys. Two cesium peaks were identified in cores obtained along thernmargin of Impoundment D. The near-surface peak reflects recent deposition of cesiumladenrnsediment derived from the erosion of open marsh surface sediment duringrnsubmergence. Recent high-resolution surveys document the higher elevation, a featurernconsistent with the calculated historical accumulation rates (3.6 mm/yr) and presence ofrnmud-cracks.rnImpoundment K appears to be functioning as a natural wetland from arnsedimentological perspective as water management structures were removed more thanrnthree decades ago. The relatively thick, organic-rich surface layer has accumulated at ratesrn(3.3 to 4.3 mm/yr) approximating eustatic sea level rise in response to surface and subsurfacerndeposition of vegetative material derived in situ.rnThe effect of surface water management on marsh elevations has a significantrnmanagement implication; it suggests hydraulic reconnection to adjacent estuarine watersrnwill result in the submergence of Impoundment D open areas to water depths in excess ofrn~10 to 20 cm (msl). At these depths, most wetland plant taxa will not successfully recruit