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

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 through32 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/FeS2. The release of soluble P from sulfidic sediments to oxygenated overlying waters only occurred during periods of solid FeS/FeS2 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/FeS2 concentrations. During this period, increasing ASC-Fe concentrations correlated with increases in ASC-P concentrations and decreases in pore-water PO . The seasonal correlations 32 4 between Fe-S-P indicate that Fe redox chemistry controls sediment P flux to the overlying water column. Nutrient cycling in estuarine systems is of primary concern due to its direct effect on primary productivity. Excessive nutrient loading in estuaries has led to increased organic matter production, and its subsequent decay has created extensive suboxic or anoxic zones and toxic sulfide levels in many productive estuarine systems (e.g., Officer et al. 1984). In many coastal environments, nitrogen species have traditionally been described as the limiting nutrient (e.g., Mortimer et al. 1999). However, with increased urbanization, fertilizer use, and runoff, many systems such as inland bays and estuarine waters may now be phosphorus limited on a seasonal basis instead. The removal of phosphate from estuarine waters can increase the dissolved N : P to above the generalized 16 : 1 Redfield ratio, limiting primary productivity. However, if sufficient PO is released from the sediments and allowed 32 4 1 Present address: School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia 30332. 2 Corresponding author. Acknowledgments This project was funded by the NOAA office of Sea Grants (NA16RG0162-03) to G.W.L. and by the Delaware Center for the Inland Bays to K.S.P. J.H. was partially supported by a grant from the Science Foundation of Argentina. R.E.T. was partially supported by a grant from the Stichting Molengraaff Fonds. to diffuse back into to the overlying water, sufficient nutrients are available for additional organic matter production. Such a mechanism may explain the secondary algal growth observed in nitrate rich environments such as coastal bays (Timmons and Price 1996; Cerco and Seitzinger 1997) following the onset of anoxic conditions in shallow sediments. Although seasonal phosphorus cycling in these intercoastal environments is not well documented, data have been amassed on the mechanisms responsible for P cycling in near shore/continental margin sediments (e.g., Krom and Berner 1981; Klump and Martens 1987; Sundby et al. 1992; Gunnars and Blomqvist 1997; Anschutz et al. 1998; Golterman 2001). In these sediments, phosphorus has been classified into four distinct fractions: organic P, Fe bound P, authigenic P minerals (such as carbonate fluorapatite, CFA) and detrital P minerals (Ruttenberg and Berner 1993). However, only organic P (nonrefractory organics, Ingall and Jahnke 1997) and Fe bound P (Krom and Berner 1981) appear to be involved in P regeneration with subsequent release to pore waters and overlying waters via the decomposition of organic matter (Ingall and Jahnke 1997) and the reduction of solid iron oxides (Krom and Berner 1981). McManus et al. (1997) showed that P release to overlying waters was related to Fe bound P rather than organic matter decomposition. In a detailed study on Fe bound P, Anschutz et al. (1998) showed that PO is primarily associated with amorphous Fe 32 4 1347 Fe-S-P cycling in shallow bay sediments (ascorbate leachable, ASC-Fe) and not crystalline Fe in a variety of sediments. In addition, they found that CFA is not leachable by the methods used to measure Fe bound P. The extent of P release is thought to be controlled either by the redox conditions at the sediment–water interface (Krom and Berner 1981) and/or by the formation of authigenic P minerals (CFA) in supersaturated pore waters (Ruttenberg and Berner 1993; Slomp et al. 1996). In continental margin sediments, McManus et al. (1997) found negligible P release from sediments where the O2 penetration depth exceeded 5 mm, whereas Ingall and Jahnke (1997) found sharp decreases in the P flux from sediments when bottom waters were fully oxygenated. In both cases, the removal mechanism appears to be P adsorption on iron oxides in the oxic/suboxic sediments. Alternatively, P removal due to mineral precipitation may occur when pore-water phosphate levels are increased quickly due to iron reduction events (Slomp et al. 1996). However, with only sparse field data existing on seasonal P cycling in sediments (Klump and Martens 1987; Slomp et al. 1998, including no specific data on seasonal controls of redox variations in the solid Fe fractions: amorphous oxides, crystalline oxides, and iron sulfides), our understanding of how phosphorus is annually recycled in shallow water systems is very limited. In experimental studies on sediment cores, Gunnars and Blomqvist (1997) showed that the reduction of FeOOH in freshwater and brackish sediments was predominately responsible for the release of PO to the overlying water column. 32 4 In freshwater sediments, the released Fe :P ratio was found to equal 1. Since FeOOH reduction of organic matter produces a much higher Fe :P ratio (Eq. 1), the released P was 1 424FeOOH 1 (CH O) (NH ) H PO 1 756H 2 106 3 16 3 4 2 1 21 22 ⇔ 106HCO 1 16NH 1 424Fe 1 HPO 3 4 4