MEASUREMENT OF PHOSPHORUS FLUX AT THE SEDIMENT WATER INTERFACE IN MISSISQUOI BAY, LAKE CHAMPLAIN

Eutrophication, due to internal and external loading of nutrients, is becoming a problem world wide as cyanobacterial blooms dominate the fresh water ecosystem. Iron oxides in the sediment at the lake bottom can sorb phosphorus, a potentially limiting nutrient for cyanobacteria. Development of anoxic conditions at the sediment-water interface (SWI) can cause dissolution of iron oxide particles and the release of sorbed P from the sediment into the water column. In order to quantify internal loading properly, determination of P flux under different redox conditions is critical. In this study mesocosms composed of sediment cores gathered from a bloom prone bay are manipulated by controlling headspace O2 levels to provoke movement of the redox front (the boundary between oxic and anoxic conditions) into and out of the sediment. The location of the redox front is found using microelectrodes to determine the concentration and location of O₂, Mn and Fe(II) within the sediment and water column. Migration of the redox front into the sediment allows for phosphorus sorption and lower SRP and TP values in the water column, while migration of the redox front into the water column causes desorption of phosphorus complexes and SRP and TP values are high in the water column. This study shows that changing redox conditions dominantly controls the P flux intensity and direction at the SWI. A more dynamic environment, characterized by more frequent migration of the redox front across the SWI, can also affect iron oxide particle size, that in turn affects sorption capacity.