PORE WATER PHOSPHORUS AND IRON CONCENTRATIONS IN A SHALLOW, EUTROPHIC LAKE - INDICATIONS OF BACTERIAL REGULATION

Peak pore water SRP and iron(II) concentrations were found during summer in surface sediments in the shallow and eutrophic L. Finjasjon, Sweden, and the concentrations generally increased with water depth. The SRP variation in surface sediments (0-2 cm) was correlated with temperature (R2 = 0.82-0.95) and iron(II) showed a correlation with sedimentary carbon on all sites (R2 = 0.42-0.96). In addition, sedimentary Ch1a, bacterial abundances and production rates in surface sediments (0-2 cm) varied seasonally, with peaks during spring and fall sedimentation. Bacterial production rates were correlated with phosphorus and carbon in the sediment (R2 = 0.90-0.95 and R2 = 0.31-0.95, respectively), indicating a coupling with algal sedimentation. A general increase in sediment Ch1a and bacterial abundances towards sediments at greater water depth was found. Further, data from 1988-90 reveal that TP and TFe concentrations in the lake were significantly correlated during summer (R2 = 0.81 and 0.76, in the hypolimnion and epilimnion, respectively). The results indicate that the increase in pore water SRP and Fe(II) in surface sediments during summer is regulated by bacterial activity and the input of organic matter. In addition, spatial and temporal variations in pore water composition are mainly influenced by temperature and water depth and the significant correlation between TP and TFe in the water suggests a coupled release from the sediment. These findings support the theory of anoxic microlayer formation at the sediment-water interface.