Iron, sulphur and phosphorus cycling in the rhizosphere sediments of a eutrophic Ruppia cirrhosa meadow (Valle Smarlacca, Italy)

The growth dynamics of a residual Ruppia cirrhosa meadow of the eutrophic V alle Smarlacca lagoon, Italy, were monitored over an annual cycle and relat ed to the cycling of iron, sulphur and phosphorus in the rhizosphere sedime nts. The Ruppia biomass and rates of gross primary production increased dur ing spring and early summer, but declined rapidly thereafter. This biomass decline was particularly evident for the below-ground biomass. Microelectro de profiling demonstrated a clear seasonal trend in sediment redox potentia l, with the sediments becoming increasingly reduced during spring and summe r. in addition, high concentrations of toxic free sulphides (up to 7 mM) we re detected in the porewater during summer and autumn. Radial oxygen releas e by the seagrass roots rather than iron precipitation appears to be the pr incipal detoxification mechanism for sulphides in the rhizosphere. However. in summer this system becomes saturated, leading to the accumulation of fr ee sulphides. This results in a positive feedback regime. with the sulphide s causing root/rhizome mortality, which further reduces oxygen release to t he rhizosphere and thereby favours sulphate reduction activity and further accumulation of sulphides. A redox-dependent exchange of phosphate between the iron-bound, exchangeabl e and porewater pools was observed. However. these changes were quantitativ ely of little importance due to the small pool sizes and the very low conce ntrations of labile ferric iron in the sediments throughout the year. Overall. the data indicate that in addition to light competition with other primary producers, eutrophication may also negatively impact seagrass comm unities through changes in sediment redox and sulphide concentrations. Thes e interactions between eutrophication, water column primary production and bacterial metabolism in the sediment could play a part in the catastrophic collapses of seagrass communities which have been observed in many coastal areas on a worldwide scale. (C) 2001 Elsevier Science B.V. All rights reser ved.