Surficial Sediment Transport and Deposition Processes in a Juncus roernerianus Marsh, West-Central Florida

Lynn A. Leonard, Albert C. Hine, Mark E. Luther

Abstract


Flow speed, water level, total suspended solid (TSS) concentration and sediment deposition were measured on the surface of a Juncus roemerianus marsh in west-central Florida in order to: (1) determine magnitude and direction of suspended material transport across the marsh surface; (2) identify the processes controlling this transport, and; (3) relate water column processes to net surficial deposition. Flow data, measured by hot-film anemometry speed sensors, indicate that (1) flow speed is inversely related to distance from the creek edge, and (2) the duration of slack-high water is limited to less than 30 minutes. TSS concentrations measured on the levee generally reflect those measured within the creek itself (15 to 20 mg/l). However, as distance from the creek increases, current speeds decrease and a corresponding decrease in TSS concentration is observed. Low current speeds in the overland flow promote deposition throughout the entire inundation event until the last parcel of water leaving the marsh surface has a minimum TSS concentration of approximately 6-8 mg/l. Rates of total deposition per tidal cycle (tc) were calculated from petri-dish sediment traps deployed over both fortnight and single tidal cycles. The results indicate that deposition rates are affected by proximity to the tidal creek and also by season. Levee deposition rates average 24 ± 9 g/m2/tc and exceed depositional rates measured 10 meters from the creek edge (9.5 ± 3 g/m2/tc). When divided into seasonal components, mean deposition rates are greater for both the levee (31 ± 17 g/m2/tc) and the inner (18 ± 4 /g/m2/tc) marsh sites during the summer than those measured in the winter (18 ± 3g/m2/tc and 6.7 ± 2 g/m2/tc, respectively). The trap data also indicate that during the winter, surficial deposition is significantly affected by storm activity. Surficial sediment fluxes derived from TSS concentration, water level, and flow speed data agree with deposition rates determined from trap data and suggest that direct settling accounts for almost all surficial deposition measured by the traps. Other potential sediment sources (e.g., fecal material and plant litter) appear to contribute little overall to marsh surface accretion on the time scales considered for this study. Despite the fact that the observed deposition rates are less than those reported for other marshes on the Gulf of Mexico and the southeast U.S. coastlines, vertical accretion rates (extrapolated from bulk density measurements of surficial sediments and deposition rates in the study area) suggest that the west-central Florida marshes are accreting at rates comparable to local rates of sea-level rise.


Keywords


Coastal marsh; sediment flux; wetland; marsh accretion; total suspended solid concentration

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