Sediment phosphate fractionation and interstitial water phosphate concentration in two coastal lagoons (Albuferas de Adra, SE Spain)

The concentrations of o-phosphate and dissolved inorganic carbon were monitored in the interstitial water of the sediment in two highly eutrophic coastal lagoons, Laguna Honda and Laguna Nueva ( SE, Spain) from July 2000 to August 2001. Additionally, the organic matter concentration and the P-fractions in the sediment were analysed. Despite their proximity, these two lagoons showed many significant differences ( P< 0.05). The P-concentration in the interstitial water of the top sediment was higher in the Laguna Honda (up to 1 mg l(-1)), especially during summer, when both sites registered their highest values. By contrast, the concentration of organic matter and organic P-fractions were higher in the sediment of the Laguna Nueva. The concentration of the organic matter in the vertical profile of the sediment decreased with depth in the Laguna Nueva ( from 17% at 0 cm to 7% at 15 cm, annual mean value) while it increased in the sediment of Laguna Honda ( from 9% at 0 cm to 12% at 15 cm, annual mean value). The P-concentration in the interstitial water and the organic matter in the top sediment of Laguna Honda followed a seasonal pattern and, were both correlated with the temperature of the bottom water (r = 0.706, P < 0.05 and r = - 0.929, P< 0.01, respectively). The inorg-P fractions comprised 63% of the Tot-phosphate in the sediment of Laguna Honda, whereas org-P fractions (68%) dominated in the sediment of Laguna Nueva. The concentrations of Tot-P in the sestonic material collected at three different depths of the water column averaged 2.2 mg g(-1)d. w. in Laguna Honda and 2.5 mg g(-1)d. w. in Laguna Nueva. The average atomic C: P ratio of the sestonic material was significantly higher in Laguna Honda than in Laguna Nueva, but the top sediment in Laguna Nueva had a C: P ratio significantly higher than that of the sestonic material ( average: 416 and 162, respectively). The different biodegradability of the organic matter in the top sediment probably explained the differences found in sediment composition between these two lagoons. We suggest that the P-reclying from the sediment to the interstitial water was faster in Laguna Honda due to the higher biodegradability of the organic matter. By contrast, the lower biodegradability of the organic matter in the top sediment of Laguna Nueva was probably due to a higher contribution of vascular plants which could explain the organic matter accumulation detected in this lagoon.