Iron‐sulfur‐phosphorus cycling in the sediments of a shallow coastal bay: Implications for sediment nutrient release and benthic macroalgal blooms

Abstract

We conducted a study to determine the seasonal relationship between iron, sulfur, and phosphorus in the upper sediments and pore waters of a shallow intercoastal bay. From April 1999 to September 2000, sediment cores were collected from Rehoboth Bay, Delaware. Analyses of the sediments in the upper 4 cm revealed that redox conditions controlled Fe-S-P concentrations in the sediments, pore waters, and overlying water. Monthly sampling showed a marked decrease in the reactive solid phase P pool (ascorbate leachable fraction, ASC-P) and sharp increases in soluble P (measured as PO ) in pore waters and overlying waters, as the conditions became more reducing through-3Ϫ 4 out the summer months. These changes were paralleled by decreases in the amorphous Fe(III) (ascorbate leachable fraction, ASC-Fe) and total Fe(III)oxyhydroxide pools [dithionite extracted fraction, Fe(III) oxide ] and increases in solid FeS/FeS 2 . The release of soluble P from sulfidic sediments to oxygenated overlying waters only occurred during periods of solid FeS/FeS 2 production, which indicates that Fe(III) oxides act as a barrier to diffusive P flux. During these anoxic conditions, the regenerative P appears to induce secondary benthic algal blooms and promotes eutrophication in these inland bays through late summer. By the late fall and into early spring, sulfide production diminished and oxic conditions were reestablished as indicated by increases in solid amorphous and crystalline Fe(III) oxides and decreases in FeS/FeS 2 concentrations. During this period, increasing ASC-Fe concentrations correlated with increases in ASC-P concentrations and decreases in pore-water PO . The seasonal correlations 3Ϫ 4 between Fe-S-P indicate that Fe redox chemistry controls sediment P flux to the overlying water column.

Benthic primary production and nutrient cycling in sediments with benthic microalgae and transient accumulation of macroalgae

“…Phosphate turnover in the absence of macroalgae-The relation between O 2 uptake in the dark and PO flux was bilization in the sediment on a number of factors other than mineralization, such as the redox state of the sediment (e.g., Ruttenberg 1992;Jensen et al 1998;Rozan et al 2002). The O 2 penetration in darkness into the sediment, and thus the redox state of the sediment, was lower in summer than in winter (data not shown).…”

Coastal hypoxia and sediment biogeochemistry

“…High fluxes of phosphate usually occur concurrently with high fluxes of iron, either naturally or experimentally induced (Sundby et al, 1986), but not always because of iron trapping by sulphides. High phosphate fluxes under hypoxic and anoxic conditions have been reported for ocean-margin settings (Ingall and Jahnke, 1997), large to intermediate coastal hypoxia such as found in the Chesapeake Bay (Kemp et al, 2005;Jordan et al, 2008), the Baltic Sea (Conley et al, 2003) and in smaller coastal systems (Rozan et al, 2002). This oxygen dependency of phosphorus cycling has received much attention since the 1940's (Mortimer, 1941) because of its implication for eutrophication and Earth System dynamics (Wallmann, 2003;Slomp and van Cappellen, 2007).…”

Phosphorus release from coastal sediments: Impacts of the oxidation-reduction potential and sulfide

“…In the presence of sulfide, the Fe 2+ ion can be quickly and effectively removed from pore waters by the formation of FeS and FeS 2 , which also removes the toxic sulfide in the sediment-water system (Taillefert et al, 2000b). The biogeochemical reactions of Fe and S also affect both availability of sedimentary P to aquatic organisms and mobility of P within the sediments (Rozan et al, 2002). In light of the global increase in the areal extent of coastal marine systems that experience oxygen depletion and alterations in benthic P cycling in response to eutrophication, it is of considerable interest to clarify the ORP-dependent P release processes (Diaz and Rosenberg, 2008;Middelburg and Levin, 2009;Qian et al, 2011).…”