The phosphorus cycle in coastal marine sediments

Abstract

320sediment cores collected at three soft bottom stations; two brackish-marine and one freshwater. One of the marine stations was reduced and azoic, whereas the freshwater and the other marine station had well oxygenated conditions in the bottom waters. Positive redox-turnovers, including anaerobic incubation followed by reaeration, were generated in the cores and the supernatant water.In cores from the oxygenated freshwater and marine stations, dissolved phosphate and ferrous ions were released from the sediment during the anaerobic incubation. At the positive redox-turnover, the concentration of dissolved phosphate in the supernatant water decreased sharply due to scavenging by rapidly formed colloidal ferric hydroxide. Dissolved phosphate was also released during the incubation of the marine sediment from the reduced station. However, in these cores the concentration of iron in the supernatant water was low throughout the experiment and after the redox-turnover phosphate remained dissolved. In a parallel experiment in which iron was added to the supernatant water, dissolved phosphate was scavenged by ferric hydroxide at the positive redox-turnover in a similar way as observed for the two oxygenated stations. The low abundance of dissolved iron in the reduced marine system could be due to a rich supply of sulphide.In freshwater systems the concentration of dissolved phosphate is effectively diminished after a positive redox-turnover due to interaction with iron. In marine systems, which have had prevailed reduced conditions in the bottom waters, iron is immobilised. Consequently, a potent retention mechanism for phosphorus is eliminated. Our results imply that the cycling of phosphorus, in this aspect, differs in fresh and saltwater systems. This difference might have large effects on the availability of phosphorus as a nutrient.Manganese showed a consistent redox-dependent behaviour in all systems, but it did not interact with iron or phosphorus.The concentration of dissolved phosphate in sediment box-cores from the Laurentian Trough in the Gulf of St. Lawrence increased sharply across the sediment-water interface from 2.0 #mol/PO 4 /1 in the bottom water to 6 + 3 tmol PO 4 /1 in the top cm, remained almost constant at this value down to 5-15 cm depth, and then increased rapidly with further depth. In the region of constant concentration, phosphate is buffered by sorption equilibria with the sediment. The production rate of phosphate, the sorption capacity of the sediment, and the thickness of the diffusive boundary layer at the sediment-water interface appear to control the shape of the pore water profile. Even though the buffering places an upper limit on the concentration gradient across the sediment-water interface, and hence on the flux, the phosphate flux to the overlying water is controlled by the production rate of phosphate within the sediment. A model is proposed to relate sediment chemistry to phosphorus fluxes.Approximately half of the net sedimentation flux of phosphorus is not buried but is mobil...

Evaluation of ammonium and phosphate release from intertidal and subtidal sediments of a shallow coastal lagoon (Ria Formosa – Portugal): a modelling approach

“…These findings are reinforced by other studies, in which HPO 4 2-in porewater were positively correlated with bottom water temperature (Jensen et al 1995;Lillebø et al 2004), and negatively correlated with dissolved oxygen (Jensen et al 1995). During the period of lower temperatures, the oxygen availability increases in porewater due to an increase of its solubility (Ohtake et al 1984;Moutin 1992), which turns the iron and sulphur cycles very reactive (Sundby et al 1992;Bally et al 2004;Caetano et al 2003). The low levels of HPO 4 2-found in porewater of muddy sediments (<10 lM) may be partially explained by P-sorption onto iron oxides formed under oxic conditions (Sundby et al 1992;De Jonge et al 1993;Van Raaphorst and Kloosterhuis 1994;Slomp et al 1998;Perkins and Underwood 2001), since there is a large iron availability in the intertidal muddy sediments of this lagoon (Caetano et al 2002).…”

Nitrogen as the limiting nutrient for eutrophication in coastal marine ecosystems: Evolving views over three decades

“…This, in combination with N lost through denitrification, is a major reason that primary production in Narragansett Bay is predominantly N limited (Nixon et al 1980;Howarth 1988). On the other hand, the P released from sediments in Chesapeake Bay is less than the amount released during decomposition (Boynton and Kemp 1985), and there is evidence of P adsorption and storage in surface sediments in several other estuaries (van Raaphorst et al 1988;Koop et al 1990;Sundby et al 1992). Many of the differences in P uptake and release from sediments in both estuaries and lakes may be explained in part by the extent of eutrophication in those systems.…”

Seasonal oxygen, nitrogen and phosphorus benthic cycling along an impacted Baltic Sea estuary: regulation and spatial patterns

“…Recent experiments in the North Sea revealed that anammox rates did not change during exposure to short-term hypoxia (Neubacher et al 2011), whereas prolonged hypoxia significantly stimulated anammox activity (Neubacher et al 2013). Under anoxic conditions, inorganic P is efficiently regenerated, in particular in iron-rich sediments (Sundby et al 1992). It is not clear how bottom water oxygen concentrations and organic matter content and composition affect benthic microbial N transformation pathways and alter the stoichiometry of regenerated N and P in eutrophic seas.…”