Phosphorus is the limiting factor for primary production in most freshwater ecosystems. In many areas, diffuse P losses from intensively cultivated land cause severe eutrophication of surface waters. We investigated the P export from two drainage systems under intensively used grassland in a catchment of the Swiss Plateau. Flow rate and nutrient concentrations were measured with a high temporal resolution during discharge events. During most flow peaks, P concentrations strongly increased with increasing flow rates. Concentrations of soluble‐reactive P (SRP) reached up to 155 µmol L−1. Phosphorus was mainly transported as soluble‐reactive and particulate P. Organic P compounds, as well as P associated with colloids between 0.05 and 0.45 µm in effective diameter, were of minor importance. Estimated P loads from the drainage systems were 227 g SRP ha−1 within a period of 2.5 mo at site I and 1290 g ha−1 during 6 mo at site II. Estimation uncertainty was large (±21 and ±36% for the two sites, respectively) due to the weak correlation between discharge and concentration for all data from a given site. Water‐extractable P in the soil was concentrated in the uppermost layer of the profiles or, for short periods after spreading of manure, deposited on the vegetation. The discharge‐concentration relationship indicated that P was transported through preferential flow paths extending from close to the surface to the drains. Sprinkling experiments with a blue dye confirmed this conclusion. At one site, we observed preferential flow in a downhill direction within the saturated zone.

Sequential P extraction was combined with electron microscop and X-ray spectroscopy to characterise various P species and to study their transformation in settling seston and in recent sediment. During early diagenesis most of the particulate P formed in the water was redissolved. No net transformation into species that would resist dissolution was observed. It was shown that • the phosphorus (P) content and the P flux of settling particles varied seasonally over one order of magnitude • particles became enriched with reductant soluble P (BD-P) while settling through the hypo-.limnion • changes in BD-P were highly significantly correlated with changes in reductant soluble iron (BO-Fe) • bacteria oxidising Fe and Mn seemed to be mainly responsible for this increase in P concentration • other fractions including organic P did not change during sedimentation • most of the organic P and of the Fe bound P and 70% of TP was released from the sediment during early diagenesis • the sediment surface did not act as a trap for P migrating upwards from deeper sediment layers • CaC0 3 sedimentation contributed little to P sedimentation but significantly to the permanent burial of P.

In order to improve the trophic state of Lake Sempach, a eutrophied lake in central Switzerland, its external phosphorus (P) load has been decreased and its hypolimnion has been artificially oxygenated to lower the lake‐internal P recycling. Based on more than 15 yr of experience, we conclude that the reduction of the external P load resulted in a corresponding decrease of the lake’s P concentration. However, contrary to initial expectations, increased hypolimnetic dissolved oxygen concentrations neither (1) reduced the P release from sediments during summer nor (2) resulted in anincreased permanent P retention. These observations warrant a reevaluation of the well‐accepted management strategy of decreasing the lake internal P cycling by maintaining an aerobic hypolimnion and sediment surface. We hypothesize that oxygenation only results in an increased permanent benthic P burial if, because of the depressed sulfide production, more ferrous phosphate (e.g., vivianite) and less FeS is deposited in the anoxic sediment. Hence, it is not the oxic sediment surface that directly affects the permanent redox‐dependent sediment P retention but the molar ratio of the available reactive Fe(II) : S2‐ : PO4 in the anoxic sediment. This ratio is driven by the settling rate and the nature of organic matter and particulate iron, as well as the supply of oxygen, nitrate, and sulfate to the sediment.

Cycling of phosphorus at the sediment‐water interface is traditionally considered to be controlled by pH‐ and redox‐dependent, abiotic processes, such as formation and dissolution of FeOOH‐PO4 complexes. In this study, however, a large part of total P in sediments of Lake Sempach, an 87‐m deep eutrophic lake, was estimated to be incorporated in bacterial biomass. Laboratory experiments indicated that sediment microorganisms can rapidly take up and release soluble reactive P (SRP), depending on redox conditions, and that sterilization of oxic sediments decreased their SRP sorption capacity. In an in situ experiment conducted in the lake, bacteria also contributed considerably to SRP fixation when water enclosed within a sediment flux chamber was reoxygenated following anoxia. Moreover, in that experiment and in data sets from several other lakes, anoxic releases of Fe and P from sediments appeared to be partly uncoupled. As part of an ongoing revision of the classical model for P exchange across the sediment‐water interface, these results provide direct evidence that fixation and release of SRP may be controlled partly by redox‐dependent changes in microbial physiology, as well as by production and decomposition of microbial biomass.

Hypolimnetic aeration was used in conjunction with reductions in phosphorus (P) inputs in order to lower the P concentrations in two eutrophic lakes. Based on more than 10 years of experience with artificial mixing and hypolimnetic oxygenation we conclude that the lakes' internal P cycling was not affected by increased hypolimnetic dissolved oxygen (D.O.) concentrations. We show that irrespective of oxic conditions in the hypolimnion the sediment/water interface remained anoxic due to unchanged high sedimentation rates of organic matter. This may explain why oxygenation did not increase the P retention capacity of the sediment. However, the presented information indicates that, contrary to general assumption, anoxic sediment surface and high P release rates from lake sediments may not be cause-effect related but simply two parallel symptoms of one common cause:  excessive organic matter and P sedimentation exhausting the stock of hypolimnetic D.O. and exceeding the P retention capacity of the sediment after diagenesis. Therefore, it is not surprising that fighting one symptom (an anoxic hypolimnion and sediment surface) does not solve the other one (a high P release rate). These findings and considerations based on more than 10 years of experimental experience warrant a reevaluation of the well accepted theoretical management strategy of limiting lake internal P cycling by maintaining an aerobic hypolimnion and sediment surface.