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 a vailability 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...

Efeito de diferentes níveis de enriquecimento por fósforo sobre a estrutura da comunidade perifítica em represa oligotrófica tropical (São Paulo, Brasil)

“…No fi toplâncton, Ankistrodesmus pertence à associação de espécies características de condições eutrófi cas e hipereutrófi cas (Reynolds 1984). Trabalhos sobre fi toplâncton relataram, ainda, a ocorrência frequente de Dictyosphaerium pulchellum em sistema eutrófi co (Rosen 1981) e seu usual aumento populacional com a eutrofi zação (Margalef 1983).…”

“…É frequentemente reportado como nutriente limitante do crescimento algal em diferentes regiões climáticas do globo (Hecky & Kilham 1988, Reynolds & Davies 2001, podendo atuar de forma indireta sobre vários processos, como sobre o fl uxo e dinâmica do nitrogênio em ecossistemas oligotrófi cos (Dodds & Priscu 1990) e até mesmo levando à limitação por outros nutrientes, como a sílica (Schelske & Stoermer 1971).…”

“…Estudos sobre fontes e biodisponibilidade de fósforo na água doce reportam esse elemento como agente primário do processo de eutrofi zação, pois o aumento adicional da biomassa de produtores primários está diretamente ligado à aceleração antropogênica do aporte das frações de fósforo biologicamente disponíveis (Reynolds & Davies 2001). Como o enriquecimento por fósforo é frequentemente documentado como o fator que, usualmente, mais afeta a integridade dos ecossistemas de águas doces, indicadores biológicos rápidos vêm sendo propostos a fi m de detectar impactos relacionados ao aporte desse nutriente (Newman et al 2003).…”

Iron colloids reduce the bioavailability of phosphorus to the green alga Raphidocelis subcapitata

“…For practical purposes, the bioavailable P is often 54 measured by colorimetric methods and reported as soluble reactive phosphorus (SRP) or 55 dissolved reactive phosphorus (DRP), an approach followed in water quality monitoring 56 programmes worldwide. However, it has been shown that SRP often overestimates the 57 bioavailable P fraction (Boström et al, 1988;Reynolds and Davies, 2001;Rigler, 1968). 58…”

Calibração de modelo para a simulação de vazão e de fósforo total nas sub-bacias dos Rios Conrado e Pinheiro - Pato Branco (PR)

“…A carga maior na sub-bacia 12, separada do conjunto formado pelas sub-bacias contíguas ao exutório, deve-se à existência de maior área agrícola contínua. Segundo Reynolds & Davies (2001), as lavouras aumentam drasticamente as perdas de nutrientes em relação às áreas florestadas. Na figura 4, observa-se a distribuição mensal da exportação total de P simulada no exutório da bacia.…”