René Gächter

The role of microorganisms in mobilization and fixation of phosphorus in sediments

AbstractCycling of phosphorus (P) at the sediment/water interface is generally considered to be an abiotic process. Sediment bacteria are assumed to play only an indirect role by accelerating the transfer of electron from electron donors to electron acceptors, thus providing the necessary conditions for redox-and pH-dependent, abiotic sorption/desorption or precipitation/dissolution reactions.Results summarized in this review suggest that (1)in eutrophic lakes, sediment bacteria contain as much P as settles with organic detritus during one year(2)in oligotrophic lakes, P incorporated in benthic bacterial biomass may exceed the yearly deposition of bioavailable P several times(3)storage and release of P by sediment bacteria are redox-dependent processes(4)an appreciable amount of P buried in the sediment is associated with the organic fraction(5)sediment bacteria not only regenerate PO4, they also contribute to the production of refractory, organic P compounds, and(6)in oligotrophic lakes, a larger fraction of the P settled with organic detritus is converted to refractory organic compounds by benthic microorganisms than in eutrophic lakes. From this we conclude that benthic bacteria do more than just mineralize organic P compounds. Especially in oligotrophic lakes, they also may regulate the flux of P across the sediment/water interface and contribute to its terminal burial by the production of refractory organic P compounds.

Transformation of phosphorus species in settling seston and during early sediment diagenesis

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 hypolimnion• changes in BD-P were highly significantly correlated with changes in reductant soluble iron (BD-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• CaCO3 sedimentation contributed little to P sedimentation but significantly to the permanent burial of P.

Measurement of denitrification in sediments with the 15N isotope pairing technique

Due to increasing nitrogen concentrations in surface waters ([6][1]) and eutrophication of coastal waters ([14][2], [26][3], [51][4]), the quantification of denitrification rates in sediments of lakes, rivers, and estuaries has gained importance. Different methods for measuring denitrification have

Speciation of riverine particulate phosphorus during rain events

Suspended sediments collected during rain events were analysed to assess the maximum potential bioavailability of particulate phosphorus (PP). Physical (separation by particle size) and chemical (sequential extraction) fractionation techniques were applied. Time differentiated sampling during rain events revealed that changes in the concentrations of soluble and particle bound phosphorus, and in the proportion of different PP phases, are due to the changing contribution of various sources of runoff and to flow related variations in particle size. Size fractionation and the extraction of PP phases, can help to distinguish resuspended sediments from sediments coming directly from outside the channel. In light of a former study, investigating PP sedimentation and transformations within the sediments of Lake Sempach, our results lead to the conclusion that, at least 25% (particulate inorganic and reductive soluble P) and at most 70% of the allochthonous PP supply transported during a single rain event, may become bioavailable after early diagenesis in the lake sediments. The uncertainty is due to the unknown time span necessary for the diagenesis, at the lake sediment surface, of particulate organic phosphorus of allochthonous origin.

Sources and sinks of nitrous oxide (N2O) in deep lakes

As reported from marine systems, we found that also in15 prealpine lakes N2O concentrations werestrongly correlated with O2 concentrations. Inoxic waters below the mixed surface layer, N2Oconcentrations usually increased with decreasingO2 concentrations. N2O is produced in oxicepilimnia, in oxic hypolimnia and at oxic-anoxicboundaries, either in the water or at the sediment-waterinterface. It is consumed, however, incompletely anoxic layers. Anoxic water layers weretherefore N2O undersaturated. All studied lakeswere sources for atmospheric N2O, including thosewith anoxic, N2O undersaturated hypolimnia.However, compared to agriculture, lakes seem not tocontribute significantly to atmospheric N2Oemissions.

Nutrient transfer from soil to surface waters: differences between nitrate and phosphate

Abstract.Nitrate (NO3–) and soluble reactive phosphorus (SRP), the two major dissolved N and P species available to aquatic biota, respond differently to varying water discharge rates (Q) in agricultural drainage pipes and rivers (Fig. 1): SRP concentrations are positively related to Q, whereas NO3– concentrations decrease with increasing discharge rates. In addition, NO3-N concentrations exceed (in mass units) SRP concentrations up to 700-fold even though the liquid manure applied to agricultural fields has a N:P ratio equal to only about 5. Preferential flow of rainwater across the soil column and different affinities of the two nutrients for the soil matrix explain these differences in behaviour and mobility:i. Concentrations of substances that have a high sorption affinity for the soil matrix (such as SRP) tend to increase in drainage pipes and streams as water discharge increases.ii. Concentrations of species that are not retarded by sorption processes (such as NO3–) and, hence, do not accumulate in the topsoil, tend to be negatively related to discharge rate.Differences in the availability and pool size of NO3– and SRP in the topsoil explain the different hysteresis patterns if NO3– and SRP concentrations are plotted versus the corresponding discharge rate during precipitation events (Fig. 2).

Transport of Cu, Zn and Cd in a small agricultural catchment.

Abstract Concentrations of dissolved and particulate metals (Cu, Zn and Cd) were determined in water samples collected during several rain events from the River Kleine Aa, a first order tributary to Lake Sempach (Central Switzerland). Metal contents were also measured in soil cores obtained from fertilized and unfertilized areas of the catchment and in liquid manure samples. The contribution of farming activities to the metal loads was evaluated. Because total metal concentrations were linearly related to water discharge, rain events significantly contribute to the yearly metal load. During such events, metals were predominantly associated to particles, but dissolved copper constituted up to half of the Cu load at low flow rates. Dissolved Cu concentrations exceeded dissolved Zn concentrations at low discharge rates, and in experimental water extracts of the grassland soil. The dynamic behavior of dissolved metals and dissolved organic carbon were linked in the river water and in the soil extracts. Metal concentration of suspended particles decreased with increasing discharge to a constant level with Cu and Zn contents similar to those of the grassland topsoil. Their Cd content was, however, lower than in the soil. These observations suggest that the grassland topsoil is the main source of dissolved and particulate trace metals in the river water. Farming activities have caused metal accumulation, since the metal contents in the grassland soil were highest at the surface and higher than in the forest soil at any depth. Based on a metal budget of the drainage area and on metal profiles of forest and grassland soil, it is concluded that frequent application of liquid manure with high metal concentrations was mainly responsible for the high Cu and Zn content of the soil and the elevated loss rates to the river, whereas air pollution mainly explained the elevated Cd load of the drainage basin and the river water.

Lake restoration. Why oxygenation and artificial mixing cannot substitute for a decrease in the external phosphorus loading

AbstractData obtained from Lakes Baldegg and Sempach, two artificially mixed and oxygenated lakes, show that it is technically possible to significantly improve redox conditions, even in large eutrophic lakes.It is discussed why- the well-known release of phosphorus from anoxic sediments does not necessarily result from an abiotic reduction of phosphorus containing iron compounds,- net phosphorus retention by sediments is not simply linearly related to the phosphorus content of a lake,- artificial oxygenation of a previously anoxic hypolimnion does not permanently increase phosphorus retention capacity of lake sediments. From this it is concluded that improvement of hypolimnic redox conditions by lake internal measures such as aeration or oxygenation may accelerate the rate of recovery induced by a reduction of the external phosphorus loading, but that oxygenation per se will hardly be able to cause a reduction of trophic state.

Complexing capacity of the nutrient medium and its relation to inhibition of algal photosynthesis by copper

The toxic effect of copper on phytoplankton production is investigated in waters having different complexing capacities. It is demonstrated that a water’s complexing capacity does not guarantee that an equivalent amount of copper could be tolerated without adversely affecting algal production. Possible explanations for these findings are offered and discussed.It is deduced that ionic copper probably is already toxic to planktonic algae at concentrations of about 10−10 mole/l.Zusammenfassung1.Membranfiltrierte Proben aus natürlichen Gewässern haben die Fähigkeit, zugeführte Kupferionen durch Komplexierung oder kolloidale Adsorption zu maskieren. Es ist möglich, diese Kapazität («Komplexierungskapazität») polarographisch zu bestimmen. Es werden mit der angewandten Methode jedoch nur Ligandern erfasst, die Kupferkomplexe mit einer KomplexbildungskonstantenKf≳1010 bilden.2.Es wird gezeigt, dass Kupfer schon toxisch wirkt, lange bevor die so gemessene minimale Komplexierungskapazität der Probe voll ausgeschöpft ist.3.Bei Gesamtkupferkonzentrationen von weniger als 10−6 Mol/l kann im Moment die Konzentration der mit den unbekannten Liganden oder Kolloiden im Gleichgewicht stehenden freien Kupferionen weder analytisch noch rechnerisch exakt bestimmt werden.4.Sollte es aber zutreffen, dass nur freie Kupferionen [Cu(H2O)6]2+ die Photosynthese hemmen, so muss angenommen werden, dass Konzentrationen von weniger als etwa 10−10 Mol/l bereits toxisch wirken.5.Es ist denkbar, dass saisonale Veränderungen der Kupferkonzentration und der Komplexierungskapazität die jahreszeitliche Sukzession des Phytoplanktons beeinflussen.RésuméLes eaux naturelles filtrées sur membrane sont capables de masquer les ions de cuivre par l’adsorption colloïdale ou par formation de complexes. Il est possible de déterminer cette capacité («complexing capacity») attribuée aux ligands formant des complexes de cuivre d’une constanteKf≳1010 avec une méthode polarographique.On a démontré que le cuivre est toxique bien avant que cette capacité ne soit épuisée.Pour l’instant il n’existe ni méthode chimique ni mathémathique pour déterminer la concentration précise du [Cu(H2O)6]2+.S’il s’avérait que ce ne sont que les ions libres de cuivre [Cu(H2O)6]2+ qui inhibent la photosynthèse, il faudrait admettre que des concentrations de ≲10−10 Mol/l sont déjà toxiques.Il est bien possible que la variation saisonnière des concentrations du cuivre et de la capacité des eaux de masquer des ion métalliques influence la succession du phytoplancton.

Nitrogen cycling across the sediment-water interface in an eutrophic, artificially oxygenated lake

Processes controlling the nitrogen (N) exchange between water and sediment in eutrophic Lake Sempach were studied using three different independent methods: benthic flux chambers, interstitial water data and hypolimnetic mass balances. The sediments released NH4+ (1.1–16.1 mmoles m−2 d−1), NO2- (0.1–0.4 mmoles m−2 d−1) and dissolved organic N (<0.25 mmoles m−2 d−1). A net NO3- consumption (2.4–11.1 mmoles m−2 d−1) related to the NO3- concentrations in the overlying water was observed in all benthic chamber experiments. The flux of the reactive species NO3- and NH4+ was found to depend on hydrodynamic conditions in the water overlying the sediment. For this reason, benthic chambers overestimated the fluxes of inorganic N compared to the other methods. Thus, in short-term flux chamber experiments the sediment may either become a sink or a source for inorganic N depending on the O2 concentration in the water overlying the sediment and the stirring rate. As demonstrated with a15NO3- experiment, nitrate-ammonification accounted for less than 12% of the total NO3- consumption. After six years of artificial oxygenation in Lake Sempach, a decrease in hypolimnetic total inorganic nitrogen (TIN) was observed in the last two years. The occurrence of dense mats of H2S-oxidizingBeggiatoa sp. indicated micro-aerobic conditions at the sediment surface. Under these conditions, a shorter distance between the ecological niches of nitrifying and denitrifying bacteria, and therefore a faster NO3--transport, can possibly explain the lowering of TIN by enhanced net denitrification.