Although phosphorus is an abundant element on Earth, its low availability often constrains the growth and/or biomass of aquatic biota. Introducing large quantities of available P into the biosphere, humans have opened up the relatively closed biogeochemical cycle of P, resulting in the eutrophication of many types of aquatic ecosystems worldwide. A thorough understanding of the P cycle is needed, therefore, to both understand the structure and functioning of aquatic ecosystems and to preserve the quality of our aquatic resources.

In this review, we deal first with the often misused concept of ‘nutrient limitation’. The rather general use of P uptake kinetics as an indicator of nutrient deficiency requires a discussion on methodology. Since metabolic rates and nutrient demands scale with the size of organisms, coexistence of aquatic osmotrophs relies on unique adaptations and is controlled by the whole network of ecological interactions. Some of these adaptations and interactions are reviewed, with a focus on P cycling. Finally, a case study demonstrates that the complicated P cycle must be simplified to extremes to predict eutrophication-related changes in a shallow lake.