Variations in chemical speciation and reactivity of phosphorus between suspended-particles and surface-sediment in seasonal hypoxia-influenced Green Bay

Abstract

Water, suspended-particles, and surface-sediment samples were collected from Green Bay, Lake Michigan, for the measurements of phosphorus (P) species, including dissolved/particulate-P, inorganic/organic-P, and five different forms of particulate-P, namely exchangeable- or labile-P (Ex-P), iron-bound-P (Fe-P), biogenic-apatite and/or CaCO3-associated-P (CFA-P), organic-P (Org-P) and detrital-apatite-P (Detr-P) to elucidate their reactivity and transformation pathways in the water column. Suspended particles contained mainly Ex-P (25 ± 15%), Fe-P (28 ± 12%) and Org-P (29 ± 7%). In contrast, Detr-P (34 ± 10%) and Org-P (36 ± 12%) were the predominant P species in surface sediment. Contents of Ex-P, Fe-P, Org-P and CFA-P decreased consistently from suspended-particles to surface-sediment, but an increase was observed for the Detr-P, indicating a net loss of Ex-P, Fe-P, Org-P and CFA-P from particulate into dissolved phase. Such active regeneration of P in the water column between particulate and dissolved phases may serve as an internal phosphate source in Green Bay, especially under hypoxic conditions. Degradation of organic matter in south central bay areas seemed to promote hypoxia and enhance the reductive-dissolution of Fe-P and preservation of Org-P under low-oxygen conditions in the central bay. Overall, Ex-P, Fe-P, CFA-P and Org-P species, which comprised up to 50–90% of total particulate-P, can be collectively considered as potentially-bioavailable-P (BAP). Under low-phosphate (0.022 ± 0.014 μM in Green Bay) and summer low-oxygen/hypoxic conditions, suspended-particles may release up to 71% of their BAP before deposited in sediment although the BAP regeneration decreased along the south-north transect in Green Bay.