René Gächter

In oligotrophic lakes with low nutrient concentrations, primary production is low, hypolimnetic oxygen (O₂) concentration is generally high, and a large fraction of the settled phosphorus (P) is permanently buried in the sediments. In the deep hypolimnion, ammonia (NH4+) usually occurs in trace concentrations and nitrate (NO3−) neither accumulates nor decreases during summer stagnation. This suggests that the NH4+ released during decomposition of organic material is oxidized to NO3− and then denitrified to a large extent within the sediment.

Areal oxygen (O₂) consumption in deeper layers of stratified lakes and reservoirs depends on the amount of settling organic matter. As phosphorus (P) limits primary production in most lakes, protective and remediation efforts often seek to reduce P input. However, lower P concentrations do not always lead to lower O₂ consumption rates. This study used a large hydrochemical dataset to show that hypolimnetic O₂ consumption rates in seasonally stratified European lakes remain consistently elevated within a narrow range (1.06 ± 0.08 g O₂ m⁻² d⁻¹) as long as areal P supply (APS) exceeded 0.54 ± 0.06 g P m⁻² during the productive season. APS consists of the sum of total P present in the productive top 15 m of the water column after winter mixing plus the load of total dissolved P imported during the stratified season, normalized to the lake area. Only when APS sank below this threshold, the areal hypolimnetic mineralization rate (AHM) decreased in proportion to APS. Sediment trap material showed increasing carbon:phosphorus (C:P) ratios in settling particulate matter when APS declined. This suggests that a decreasing P load results in lower P concentration but not necessarily in lower AHM rates because the phytoplankton community is able to maintain maximum biomass production by counteracting the decreasing P supply by a more efficient P utilization. In other words, in-lake organic matter production depends only on APS if the latter falls below the threshold of 0.54 g P m⁻² and correspondingly, the atomic C:P ratio of the settling material exceeds ~155.

Monitoring of four eutrophic Swiss lakes undergoing oligotrophication during more than 25 years (i.e., gradually decreasing nutrient loading, productivity, and associated symptoms of eutrophication) revealed that phosphorus (P) net sedimentation rates (the fraction of a lake’s total P content that is buried within its sediments each year) and P export rates (the fraction of the lakes’ total P content that is exported via the outlet each year) increased as the lakes’ P contents decreased. These findings are of scientific as well as practical interest because they imply that, contrary to the hitherto prevailing view, the P concentration of eutrophic lakes will decrease more than proportional to the reduction of their external P load, and faster than predicted by the linear (eutrophic state-based) models.

(2011). From Switzerland to Canada: a diary of two Swiss Postdoctoral Fellows and Richard A. Vollenweider. Aquatic Ecosystem Health & Management: Vol. 14, No. 2, pp. 179-181.

The concentration of dissolved reactive phosphorus (DRP) in rivers can change intermittently within minutes depending on the weather and water discharge (Q), or activities in the watershed. Accordingly, accurate estimation of the annual DRP load requires frequent sampling or even continuous monitoring, which is laborious and cost-intensive. We present the design and laboratory evaluation of a new, robust, low-cost, low-tech device based on passive samplers (P-traps). The traps use Fe-(oxy)hydroxide coated quartz sand as an adsorbent enclosed in a vertical grid of individual cells separated from the river water by filter membranes. They are inexpensive, easy to handle, resistant to repeated desiccation and immersion and exposable for several months. They permit estimation of discharge dependant time weighted average DRP concentrations (C–Q relationships) and annual P loads of rivers characterized by highly variable DRP concentrations with a relative accuracy of ± 3%.

Concentration of dissolved reactive phosphorus ([DRP]) in rivers changes periodically (daily, weekly, seasonally) and is dependant on the weather and discharge Q. Accordingly, accurate estimation of the annual DRP load requires intensive sampling if not even continuous monitoring, which is laborious and expensive. We present a new, elaborated low cost technique based on passive samplers (P-traps), describing their design and chemical analysis. P-traps use iron(oxy)hydroxide as a sorbent, are inexpensive, easy to handle, and can be exposed for several months. We compare average DRP concentrations obtained from spot samples and P-traps and discuss the applicability and accuracy of the suggested method to measure annual P loads of rivers characterized by highly variable DRP concentrations.

The nitrogen (N) removal potential of constructed wetlands is increasingly used to lower the N load from agricultural nonpoint sources to inland and coastal waters. To determine the removal efficiency and key factors limiting wetland N removal, N fluxes were studied in a small constructed wetland in Central Switzerland. With an isotope mass balance approach integrating the natural isotope signature of nitrate (NO₃^-), ammonium (NH₄⁺), and particulate nitrogen (PN), the N transformations such as assimilation, mineralization, nitrification, and denitrification were quantified. On average, the wetland removed 45 g m^-2 yr^-1 N during the studied 2.5 years, corresponding to a removal efficiency of 27%. Denitrification contributed 94% to the N removal, while only 6% of the removed N accumulated in the sediments. Denitrification was most efficient during periods with an oxic water column overlying anoxic sediments, as NH₄⁺ released during mineralization of sediment organic matter was completely nitrified and subsequently denitrified at the sediment−water interface. During water column anoxia, NH₄⁺ accumulated in the water and fueled assimilation by duckweed and internal recycling. The NO₃−N isotope signature in the wetland mainly reflected the mineralization of sediment organic matter and subsequent nitrification, while denitrification at the sediment−water interface produced no fractionation.