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

This paper seeks a perspective on the forms of phosphorus which promote aqu atic eutrophication, with the particular quest of establishing their source s. A short background traces the development of understanding of nutrient e nrichment and the suppositions about the relative contributions of agricult ure, sewage and detergent residues. Most aquatic systems, and their primary producers, are naturally deficient in biologically-available phosphorus. A quatic plants have evolved very efficient phosphorus uptake mechanisms. The biomass responses to an increase in the supply of phosphorus are stoichiom etrically predictable. The most bioavailable forms of phosphorus are in sol ution, as orthophosphate ions, or are readily soluble or elutable from loos e combinations. Ready bioavailability coincides well with what is measurabl e as molybdate-reactive (MRP) or soluble-reactive phosphorus (SRP). Most ot her forms, including phosphates of the alkaline earth metals, aluminium and iron are scarcely available at all. Orthophosphate ions sorbed to metal ox ides and hydroxides are normally not biologically available either, except through weak dissociation (' desorption'). The production of alkaline phosp hatase 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 nutri ent enrichment. Most of the familiar eutrophication is attributable to the widespread appli cation of secondary sewage treatment methods to the wastes emanating from a burgeoning and increasingly urbanised human population. The use of polypho sphate-based detergents, now in decline, has contributed to the problem. In aquatic systems, the additional phosphorus raises the biological supportiv e 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 yi eld and phosphorus availability is lost. Movements of phosphorus and its recycling within aquatic systems do not pre vent the slow gravitation of phosphorus to the bottom substrata. The phosph orus retentivity of sediments depends upon their chemical composition. Whil e oxide-hydroxide binding capacity in the surface sediments persists, they act as a sink for phosphorus and a control on further cycling. Iron-rich an d clay-rich sediments perform best in these conditions; calcareous sediment s 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 sedimen t disturbance, by how, 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, the role of chemical binding in soils and the incorporation and retentivity by forested terrestrial ecos ystems each contribute to the minimisation of phosphorus leakage to drainag e waters. Palaeolimnological and experimental evidence confirms that cleara nce of land and ploughing its surface weakens the phosphorus retentivity of catchments. The phosphorus transferred from arable land to drainage remain s dominated by sorbed fractions which are scarcely bioavailable. Some forms of intensive market gardening or concentrated stock rearing may mobilise p hosphates to drainage but it is deduced that drainage from agricultural lan d is not commonly a major source of readily bioavailable phosphorus in wate r. Careful budgeting of the phosphates in run-off from over-fertilised soil s may nevertheless show that a proportionately small loss of bioavailable p hosphorus can still be highly significant in promoting aquatic plant produc tion. The bioavailable-phosphorus (BAP) load achieving the OECD threshold o f lake eutrophy (35 mg P m(-3)) is calculated to be equivalent to a terrest rial loss rate of approximately 17.5 kg BaP km(-2) year(-1)), or only 1-2% of a typical fertiliser application. The output is shown to be comparable w ith the P yield from secondary treatment of the sewage produced by a reside nt population of 30-44 persons km(-2). With tertiary treatment, the equival ence is with approximately 200 persons km(-2).