DIAGENESIS OF ORGANIC-MATTER IN A WETLAND RECEIVING HYPEREUTROPHIC LAKE WATER .1. DISTRIBUTION OF DISSOLVED NUTRIENTS IN THE SOIL AND WATER COLUMN

A recently constructed marsh from previously drained agricultural land currently receives nutrient-laden water from adjacent hypereutrophic Lake Apopka, located in central Florida. Lake water is allowed to cycle through the marsh, allowing settlement of particulate organic matter, which forms a floc sediment layer on the native peat soil surface. The water leaving the marsh is returned to the lake after a retention time of 3 to 12 d. This study determined changes in temporal and spatial distribution of selected nutrients in the soil-water column of the marsh during the first 13 mo of operation. In situ distribution of selected chemical species (H+, NH4+, soluble P, SO42-, dissolved organic C, dissolved inorganic C, CH4, Ca, Mg, Fe, Mn, Al) were measured using soil pore water equilibrators at 3, 8, and 13 mo after marsh creation. Initial flooding of the agricultural soils resulted in high concentrations of NH4+ (11 mg N L-1) and soluble P (31 mg P L-1) as a result of solubilization and anaerobic decomposition. Initially rapid soluble P flux (mean = 2.4 mg P m-2 d-1) occurred from soil to the water column, although lower flux (mean = 0.8 mg P m-2 d-1) occurred after 10 additional months of operation. In contrast, initial flux of NH4+ into the water column was generally lower (mean = 3.4 mg N m-2 d-1) than observed after 10 additional months (mean = 8.2 mg N m-1 d-1). Microbial degradation and nutrient regeneration from settled labile organic matter appeared to support nutrient flux to the water column. After 13 mo of flooding, 75% of the variability of NH4+-N and 65% of the variability of soluble P contained in the water and floc sediment was explained by (DIC + CH4)-C mineralized from settled organic matter. Anaerobic conditions in both the floc sediment and peat soil layers (indicated by increased amounts of dissolved CH4 and Fe, and by SO42- reduction) had significant effects on nutrient retention and release in the soil-water column.