Combined Gel Probes for the In Situ Determination of Dissolved Reactive Phosphorus in Porewaters and Characterization of Sediment Reactivity

Abstract

Diffusive equilibrium in thin films (DET) and diffusive gradients in thin films (DGT) were applied in situ to obtain high spatial resolution dissolved reactive phosphorus (DRP) sediment porewater profiles in two lagoons of the Gippsland Lakes (SE Australia) during summer. Although the DRP depth profiles were different in each lake, highlighting the sensitivity of DRP to the redox state of the sediment spatial DRP variations obtained from DET and DGT showed striking similarities with depth in each lake. Comparison of DRP concentrations obtained from DGT and DET allows an assessment of the reactivity of the sediment. A dynamic numerical model of DGT-induced flux in sediments (DIFS) quantified reactivity kinetics. Sediment response time (Tc) related to sedimentary phosphorus resupply resulting from DGT-induced lowering of the porewater DRP concentration was calculated. Values of Tc ranged from 4128 to 183 400 s and from 55 to 149 400 s for Lakes Victoria and Wellington, respectively. These results indicate the limited capacity of the sediment to quickly resupply DRP to the porewater, especially in surface sediment which was always characterized by the highest Tc. Adsorption and desorption rate constants were also calculated from DIFS with values ranging from 2 x 10(-3) to 2 x 10(-2) day(-1) and from 0.3 to 21 day(-1) for Lake Victoria and from 2 x 10(-4) to 0.8 day(-1) and from 0.6 to 1558 day(-1) for Lake Wellington, respectively. Diffusive fluxes estimated from the in situ DRP gradient at the sediment-water interface by DET were 558 and 1.2 micromol m(-2) day(-1) in lakes Victoria and Wellington, respectively. Despite only a single measurement, these fluxes highlight a substantial contribution of P from the sediment to the water column in summer for Lake Victoria which is likely to contribute to the recurring blooms of blue-green algae. These calculated fluxes obtained with minimal sediment disturbance and high spatial resolution were substantially different (by a factor of 6-180) from other reported values using more conventional quantitative assessments such as diffusive fluxes (from core slicing) and benthic chambers.

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

“…Phosphorus concentrations across the SWI obtained from pore water peepers (or emerging tools such as DGT (diffusive gradients in thin films) and DET (diffusive equilibration in thin films) (e.g., Monbet et al 2008;Matisoff et al 2016) are used to estimate sediment P flux based on the thin film boundary layer model. This approach assumes that diffusion is the primary process responsible for internal P loading.…”

“…The most common method for quantifying L gross is a sediment incubation experiment, in which an intact core of sediment is incubated in the laboratory (or a lander in the field, as described by Orihel and Rooney 2012) and the concentration of P in overlying water is measured at designated intervals, from which the rate of change is used to calculate L gross . One can also estimate L gross directly in the field by deploying benthic flux chambers (e.g., Hendzel et al 1994) or measuring pore water P concentrations across the sediment-water interface (SWI) and applying a diffusion coefficient to calculate L gross based on Fick's first law of diffusion (e.g., Carignan and Lean 1991;Monbet et al 2008). L gross can be interpreted as a near-instantaneous input of "new" P to overlying water that, under P-limiting conditions and sufficient light, could stimulate an algal bloom.…”

Variations in Phosphorus Speciation in Pilot Scale Subsurface Flow Wetlands Constructed with Blast Furnace Slag and Gravel

“…In order to get a better understanding of the P removal processes, different forms of P and their dynamics were studied by researchers [7 -10], such as soluble reactive P [11], dissolved P [12] and organic P [13]. However, all this research treated particulate P as a simple speciation, or just categorized it as inorganic P and organic P. Nor did they explain the transformation of particulate P inside the wetlands.…”

Geochemical behavior of metals–sulfide–phosphorus at SWI (sediment/water interface) assessed by DGT (Diffusive gradients in thin films) probes

“…After deployment at SWI, DGT probes were treated and analyzed according to references (Monbet et al, 2008;Naylor et al, 2004). Chelex-100 gel and ferrihydrite gel were sliced at 2 mm intervals.…”