Fluxes of water, sediments, and biogeochemical compounds in salt marshes

Tidal oscillations systematically flood salt marshes, transporting water, sediments, organic matter, and biogeochemical elements such as silica. Here we present a review of recent studies on these fluxes and their effects on both ecosystem functioning and morphological evolution of salt marshes. We reexamine a simplified model for the computation of water fluxes in salt marshes that captures the asymmetry in discharge between flood and ebb. We discuss the role of storm conditions on sediment fluxes both in tidal channels and on the marsh platform. We present recent methods and field instruments for the measurement of fluxes of organic matter. These methods will provide long-term data sets with fine temporal resolution that will help scientists to close the carbon budget in salt marshes. Finally, the main processes controlling fluxes of biogenic and dissolved silica in salt marshes are explained, with particular emphasis on the uptake by marsh macrophytes and diatoms

Stable-isotope (H, O, and Si) evidence for seasonal variations in hydrology and Si cycling from modern waters in the Nile Basin:implications for interpreting the Quaternary record

Seasonal variations in hydrology and Si cycling in the Nile Basin were investigated using stable-isotope (H, O, and Si) compositions and dissolved Si (DSi) concentrations of surface waters, as a basis for interpreting lacustrine diatom sequences. delta O-18 ranged from -4.7 to +8.0 parts per thousand in the wet season and +0.6 to +8.8 parts per thousand. in the dry season (through 2009-2011). Higher delta O-18 values during the dry season reflected increased evapotranspiration and open water evaporation under conditions of lower humidity. Progressive downstream enrichment in the heavy isotope O-18 also occurred in response to cumulative evaporative losses from open water bodies and swamps. delta Si-30 values of DSi ranged from +0.48 to +3.45 parts per thousand. during the wet season and +1.54 to +4.66 parts per thousand during the dry season, increasing the previously reported global upper limit for delta Si-30 values in natural waters by 1 parts per thousand. Si-isotope fractionation was most intense during the thy season when demand for DSi by aquatic ecosystems exceeded supply. Progressive downstream enrichment in the heavy isotope Si-30, coupled with decreasing DSi concentrations, represented cumulative Si uptake by diatoms, macrophytes and other Si-accumulating aquatic organisms. The pronounced seasonal variations in DSi concentrations and Si-isotope compositions in the River Nile suggest that its DSi flux to the ocean may have varied significantly on a glacial/interglacial time scale, with important consequences for the marine Si budget and consequently the global C cycle. Anthropogenic impacts were evident in both the water- and Si-isotope datasets, especially during the dry season and along the Main Nile, where water management is most intensive. (C) 2013 Elsevier Ltd. All rights reserved