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

Abstract

The growth dynamics of a residual Ruppia cirrhosa meadow of the eutrophic Valle Smarlacca lagoon, Italy, were monitored over an annual cycle and related to the cycling of iron, sulphur and phosphorus in the rhizosphere sediments. The Ruppia biomass and rates of gross primary production increased during spring and early summer, but declined rapidly thereafter. This biomass decline was particularly evident for the below-ground biomass. Microelectrode profiling demonstrated a clear seasonal trend in sediment redox potential, with the sediments becoming increasingly reduced during spring and summer. In addition, high concentrations of toxic free sulphides (up to 7 mM) were detected in the porewater during summer and autumn. Radial oxygen release by the seagrass roots rather than iron precipitation appears to be the principal detoxification mechanism for sulphides in the rhizosphere. However, in summer this system becomes saturated, leading to the accumulation of free sulphides. This results in a positive feedback regime, with the sulphides causing root/rhizome mortality, which further reduces oxygen release to the rhizosphere and thereby favours sulphate reduction activity and further accumulation of sulphides. A redox-dependent exchange of phosphate between the iron-bound, exchangeable and porewater pools was observed. However, these changes were quantitatively of little importance due to the small pool sizes and the very low concentrations 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 communities through changes in sediment redox and sulphide concentrations. These 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.