Temporal Redox Fluctuations in Sediments: Pulsed Phosphorus Loadings in Freshwater Marsh Systems?

English Abstract:

Phosphorus concentrations in many surficial wetland sediments have increased substantially over the last century due to excessive anthropogenic loading of phosphorus to surface water. Wetland sediments are also often subject to rapidly oscillating redox conditions, driven by bioturbation or fluctuating dissolved oxygen concentrations in surface water. We used in situ mesocosms, alongside bioreactor sediment suspension experiments, to evaluate the effect of short temporal redox fluctuations on sedimentary phosphorus speciation and mobility in Cootes Paradise, a hypereutrophic coastal marsh which drains into Hamilton Harbour.

Results from bioreactor experiments show that repetitive re-oxidation of surficial wetland sediment (0-15cm) results in rapid degradation of organic phosphorus species (P_o) in autochthonous algal material to orthophosphate. Therefore, there is little accumulation of P_o in sediments (P_o = 13% of total sediment P). Despite enhanced extracellular phosphatase activities under oxic conditions (e.g. phosphomonoesterase activity of 2.4 mmol hr^-1 kg^-1), maximum phosphorus mobilization occurs during reducing conditions due to a strong metal oxide control. Reductive dissolution of iron oxides under anoxic conditions results in the reversible redistribution of iron oxide bound P to other solid phase pools and the aqueous phase.

Preliminary in situ mesocosms suggest that diurnal cycling of dissolved oxygen concentrations, controlled by photosynthesis and respiration, does not exert a strong influence on internal phosphorus loading to surface water. This is due to excessive nitrate concentrations (~200 µM) in surface water which inhibit iron reduction at the sediment water interface during short periods of water column anoxia and prevent pulsed phosphorus release.