Kinetic control of dissolved phosphate in natural rivers and estuaries: A primer on the phosphate buffer mechanism

Abstract

The primary mode of interaction of dissolved phosphate with fluvial inorganic suspended particles is via a reversible two-step sorption process. The first step, adsorption/desorption on surfaces, has fast kinetics (minutes-hours). The second step, solid-state diffusion of adsorbed phosphate from the surface into the interior of particles, has slow kinetics (days-months) and is dependent on the time history of the previous surface sorption and the chemistry of the solid diffusional layer. Natural clay particles with a surficial armoring of reactive iron and aluminum hydroxyoxides resulting from chemical weathering of rocks and soils have a high capacity for absorbing phosphate in the second step and for maintaining low "equilibrium phosphate concentrations" in solution. Extrapolation of laboratory sorption and extraction experiments with natural soils and suspended sediments to the environment suggests that the phosphate concentrations of unperturbed turbid rivers (SPM > 50 mg liter I) are controlled near the dynamic equilibrium phosphate concentration of their particles (EPC, = 0.2-l .5 PM) and that fluvial suspended particles "at equilibrium" contain up to 10 pmol-P g-l that is desorbable. Release of this phosphate from particles entering the sea produces the characteristic shape and magnitude of input profiles of dissolved phosphate observed in unperturbed estuaries. On a global scale, fluvial particulates could transport from 1.4 to 14 x 1O'O mol yr-I of reactive phosphate to the sea, some 2-5 times more than that in the dissolved load alone.

Relationship between nutrients and plankton biomass in the turbidity maximum zone of the Pearl River Estuary

“…PO 4 3 − concentrations in the TMZ varied in a narrower range relative to those in the non-TMZ. This was likely due to the PO 4 3 − buffering mechanism in TMZ, where the ambient PO 4 3 − concentrations were the result of the equilibrium between desorption and adsorption of PO 4 3 − onto particles (Froelich, 1988). The highest PIP concentration was found at the bottom layer of the station B1 and accompanied by the low PO 4 3 − concentration, which was probably attributed to the PO 4 3 − buffering mechanism.…”

Particle Transport, Nutrient Cycling, and Algal Community Structure Associated with a Major Winter-Spring Sediment Resuspension Event in Southern Lake Michigan

“…One possible explanation for the higher plume APP would be rapid adsorption of dissolved P by the clay particles. Depletion of the dissolved P pool within the water column would generate a potential for dissolution/re-equilibration of the large inventory of sediment P mineral phases, effectively creating a nutrient pump from the sediments (Froelich 1988) when they are resuspended. The estimate of approximately 1,000 kg of excess TPP in the plume over background, almost 2 weeks after the initiation of the plume, implies that this is an important process in sediment-water nutrient exchange.…”

Planktonic and whole system metabolism in a nutrient-rich estuary (the Scheldt estuary)

“…This approach provides system scale estimates and is easily implemented as it is based on usually well studied and available variables. Gordon et al (1996) recommended dissolved inorganic phosphorus (DIP) as the main variable to be used to compute net ecosystem production (NEP) rates using the LOICZ budgeting procedure but in turbid estuaries, processes of adsorption and desorption to and from suspended particles and sediment (Froelich 1988) can introduce a bias in the estimates. DIC can also be used to compute NEP but it requires knowledge on other processes, besides primary production and respiration, that are likely to have an effect on DIC in coastal systems, such as CO 2 fluxes at the airwater interface and CaCO 3 precipitation and dissolution (Gordon et al 1996).…”