Variability of the sediment phosphate composition of a temporary pond (Donana National Park, SW Spain)

This study reports on the spatial and temporal variability of the phosphate composition in the sediment of a temporary pond over a period of 3 years using the EDTA-method for P-fractionation. Sediment samples were collected at three different sites (open-water, littoral and flood plain) to compare the effect of the length of the wet/dry phase on the sediment phosphate composition, with special emphasis on the potential bioavailability of the P-fractions. Fine sediments (< 0.1 mm) were rich in organic matter ( 9 - 25%) and contained high mean concentrations of Tot-P (182 - 655 mg kg(-1) d. w.), especially in the flood plain sediment. The sediment P composition was dominated by P-organic fractions at all sites ( 64 - 94%). The average C/N ratios were 8.8, 6.0 and 5.9 for sediments of the flood plain, littoral and open-water sites, respectively. The flood plain sediment was significantly poorer in iron-bound P ( FeOOH≈P), but richer in the P-organic fractions extracted by EDTA than the sediment of the open-water site ( P < 0.01). The percentage of organic matter increased significantly in the sediment of the open-water site at the end of each dry season ( P<0.05), while it decreased in the sediment of the flood plain site ( P<0.01). In all sediments, the fraction of Fe( OOH)approximate toP decreased at the end of each dry season and some of these changes were significant ( P < 0.05). The decrease in the fraction of Fe( OOH)≈P was not related to changes in the sediment redox potential. Although the flood plain site was dry longer than the open-water site during the study period, the differences between the sediment composition of both sites were probably due to the effect of plant growth on the dry sediments of the flood plain site rather than to a direct effect of desiccation.