Sources and bioavailability of phosphorus fractions in freshwaters: a British perspective

Abstract

This paper seeks a perspective on the forms of phosphorus which promote aquatic eutrophication, with the particular quest of establishing their sources. A short background traces the development of understanding of nutrient enrichment and the suppositions about the relative contributions of agriculture, sewage and detergent residues. Most aquatic systems, and their primary producers, are naturally deficient in biologically-available phosphorus. Aquatic plants have evolved very efficient phosphorus uptake mechanisms. The biomass responses to an increase in the supply of phosphorus are stoichiometrically predictable. The most bioavailable forms of phosphorus are in solution, as orthophosphate ions, or are readily soluble or elutable from loose combinations. Ready bioavailability coincides well with what is measurable as molybdate-reactive (MRP) or soluble-reactive phosphorus (SRP). Most other forms, including phosphates of the alkaline earth metals, aluminium and iron are scarcely available at all. Orthophosphate ions sorbed to metal oxides and hydroxides are normally not biologically available either, except through weak dissociation (' desorption '). The production of alkaline phosphatase provides organisms with an additional mechanism for accelerating the sequestration of phosphate from organic compounds. Bioavailable phosphate is liberated when redox-or alkali-sensitive metal hydroxides dissolve but these processes are minor contributors to the biological responses to nutrient enrichment.Most of the familiar eutrophication is attributable to the widespread application of secondary sewage treatment methods to the wastes emanating from a burgeoning and increasingly urbanised human population. The use of polyphosphate-based detergents, now in decline, has contributed to the problem. In aquatic systems, the additional phosphorus raises the biological supportive capacity, sometimes to the capacity of the next limiting factor (carbon, light, hydraulic retention or of another nutrient). At high orthophosphate loadings, the straight stoichiometric yield relationship between biomass yield and phosphorus availability is lost.Movements of phosphorus and its recycling within aquatic systems do not prevent the slow gravitation of phosphorus to the bottom substrata. The phosphorus retentivity of sediments depends upon their chemical composition. While oxide-hydroxide binding capacity in the surface sediments persists, they act as a sink for phosphorus and a control on further cycling. Iron-rich and clay-rich sediments perform best in these conditions ; calcareous sediments least so. Eutrophication may lead to the exhaustion of sediment P-binding capacity. Non-sorbed phosphate is readily recyclable if primary producers have access to it. Recycling is most rapid in shallow waters (where sediment disturbance, by flow, by wind action and through bioturbation, is frequent) and least in deep ventilated sediments.The contributions of phosphorus from catchments are assessed. The slow rate of weathering of (mostly apatitic) minerals,...

Dynamics of key plant nutrients (N & P) in hokersar, a typical wetland of Kashmir Himalaya, India

“…These nutrients limit primary production especially in freshwater ecosystems [50,51]. Bioavailability of these nutrients in aquatic ecosystems varies among living organisms which mostly comprises of dissociated orthophosphate ions (H 2 PO 4 -, HPO 4 2-and PO 4 3-) in the case of P [52] and nitrite (NO 2 -), nitrate (NO 3 -) and ammonium (NH 4 + ) ions in the case of N [24]. Bottom-up control by phytoplankton also equally influences N and P, indicating co-limitation in aquatic ecosystems [53].…”

Surficial sediment phosphorus fractions along a biogeochemical gradient in Nyanza (Winam) Gulf, northeastern Lake Victoria and their possible role in phosphorus recycling and internal loading

“…This high PP concentration in Lake Victoria will result in high P sedimentation and consequent P enrichment of the bottom sediments (Hecky 1993). The sedimented phosphorus may be permanently buried or may undergo biological and chemical transformation to the bio-available SRP which is recycled back into the water column (Reynolds and Davies 2001) and allowing lake sediments to act as an internal source of P and to potentially sustain P concentrations in overlying waters.…”

A revaluation of lake‐phosphorus loading models using a Bayesian hierarchical framework

“…These might include the amount of aluminum, iron, and calcium in lake sediments; dominance of the zooplankton community by efficient grazers like Daphnia, which can increase the downward flux of phosphorus-containing particulate matter incorporated in fecal pellets; the form in which phosphorus is supplied (i.e., inorganic or organic, dissolved or particulate), which determines if it will be readily incorporated into the biologic cycle of lakes or settle to the sediments; sediment resuspension in shallow, wind-exposed lakes; and bioturbation of material likely to have passed through benthic animals or fish, which creates a ''gluing'' of the particles into larger flocs (Reynolds and Davies 2001;Brett et al 2005aBrett et al , 2005bCarpenter 2005;Jensen et al 2006;Brett and Benjamin 2008). Moreover, Nu¨rnberg and LaZerte (2004) indicated that the ratio of watershed-to-lake area can be an important predictor in lakes with complex morphology, while further improvements may be obtained by explicitly considering surrogate variables of soil geochemistry (humic and fulvic acids) such as DOC or lake water color (Nu¨rnberg and Shaw 1998).…”