Sediment phosphorus speciation and mobility under dynamic redox conditions

Abstract

Abstract. Anthropogenic nutrient enrichment has caused phosphorus (P) accumulation in many freshwater sediments, raising concerns that internal loading from legacy P may delay the recovery of aquatic ecosystems suffering from eutrophication. Benthic recycling of P strongly depends on the redox regime within surficial sediment. In many shallow environments, redox conditions tend to be highly dynamic as a result of, among others, bioturbation by macrofauna, root activity, sediment resuspension and seasonal variations in bottom-water oxygen (O 2 ) concentrations. To gain insight into the mobility and biogeochemistry of P under fluctuating redox conditions, a suspension of sediment from a hypereutrophic freshwater marsh was exposed to alternating 7-day periods of purging with air and nitrogen gas (N 2 ), for a total duration of 74 days, in a bioreactor system. We present comprehensive data time series of bulk aqueous-and solidphase chemistry, solid-phase phosphorus speciation and hydrolytic enzyme activities demonstrating the mass balanced redistribution of P in sediment during redox cycling. Aqueous phosphate concentrations remained low (∼ 2.5 µM) under oxic conditions due to sorption to iron(III) oxyhydroxides. During anoxic periods, once nitrate was depleted, the reductive dissolution of iron(III) oxyhydroxides released P. However, only 4.5 % of the released P accumulated in solution while the rest was redistributed between the MgCl 2 and NaHCO 3 extractable fractions of the solid phase. Thus, under the short redox fluctuations imposed in the experiments, P remobilization to the aqueous phase remained relatively limited. Orthophosphate predominated at all times during the experiment in both the solid and aqueous phase. Combined P monoesters and diesters accounted for between 9 and 16 % of sediment particulate P. Phosphatase activities up to 2.4 mmol h −1 kg −1 indicated the potential for rapid mineralization of organic P (P o ), in particular during periods of aeration when the activity of phosphomonoesterases was 37 % higher than under N 2 sparging. The results emphasize that the magnitude and timing of internal P loading during periods of anoxia are dependent on both P redistribution within sediments and bottom-water nitrate concentrations.

Speciation of Phosphorus from Agricultural Muck Soils to Stream and Lake Sediments

“…3). Low recovery rates of between 12 and 45% for TP using a NaOH‐EDTA extraction with calcareous soils (Turner et al, 2003; Doolette et al, 2011; Cade‐Menun and Liu, 2014; McLaren et al, 2014) and lake or stream sediments (Giles et al, 2015; Li et al, 2015; Parsons et al, 2017) have been reported. At alkaline pH and in the presence of Ca minerals, the extraction of P by NaOH‐EDTA may be inhibited by the formation of Ca‐P complexes (Celi et al, 2000).…”

Internal phosphorus loading in Canadian fresh waters: a critical review and data analysis

“…The presence of high nitrate in surface water acts against internal P loading (Matthews et al 2013;Parsons et al 2017). The reasons for this appear to be threefold.…”

“…First, nitrate availability at the SWI reduces the likelihood of reductive dissolution of iron (oxyhydr)oxides during short periods of bottom water anoxia. This is due to nitrate's higher energetic efficiency as an electron acceptor than iron (e.g., Hansen et al 2003;Parsons et al 2017); as nitrate must be consumed prior to the onset of iron reduction (Reed et al 2011), longer periods of anoxia are required to cause release of iron-bound P. Second, heterotrophic nitrate reduction (denitrification) coupled to the oxidation of organic matter may deplete the sediment of labile organic matter, decreasing the probability, or rate, of reductive dissolution of iron oxyhydroxides at greater sediment depth due to electron donor limitation. Third, nitrate reduction can be coupled directly to the oxidation of upward-diffusing iron(II) from more reduced sediments (e.g., Straub et al 1996;Hauck et al 2001;Melton et al 2012).…”

“…Invertebrate tube-dwelling organisms can also disturb sediments during burrow construction. Population densities for such bioturbating organisms may reach 50 000 individuals·m -2 (Pelegrí and Blackburn 1995;Parsons et al 2017), potentially resulting in significant sediment mobilisation. Tube-dwelling invertebrates have also been shown to enhance solute transport between pore water and surface water via bioirrigation, increasing fluxes of SRP (Matisoff and Wang 1998;Chaffin and Kane 2010).…”

Coupling Water Column and Sediment Biogeochemical Dynamics: Modeling Internal Phosphorus Loading, Climate Change Responses, and Mitigation Measures in Lake Vansjø, Norway

“…The goal is to deliver an open‐access tool for the combined modeling of water column and sediment that uses a coherent biogeochemical reaction network. Consequently, we updated Matsetlab with reactions of P, aluminum (Al), calcium (Ca), and MyLake with reactions of N, Fe, Al, Ca, and S (Ahlgren et al, ; Canavan et al, ; Couture, Shafei, et al, ; Dijkstra et al, ; Dittrich et al, ; Doan et al, ; Gudimov et al, ; Katsev & Dittrich, ; Katsev et al, ; Li et al, ; Parsons et al, ; Testa et al, ; Van Cappellen & Wang, ).…”