A robust numerical procedure for biogeochemical interpretation and analysis of measured concentration profiles of solutes in sediment pore water has been developed. Assuming that the concentration-depth profile represents a steady state, the rate of net production or consumption as a function of depth can be calculated, together with the flux across the sediment-water interface. Three kinds of vertical transport can be included in the analysis: molecular diffusion, bioturbation, and irrigation. The procedure involves finding a series of least square fits to the measured concentration profile, followed by comparisons of these fits through statistical F-testing. This approach leads to an objective selection of the simplest production-consumption profile that reproduces the concentration profile. Because the numerical procedure is optimized with respect to speed, one prediction can typically be done in a few minutes or less on a personal computer. The technique has been tested successfully against analytical solutions describing the transport and consumption of 0, in sediment pore water. In other tests, measured concentration profiles of O,, NO;, , NH:, and ZCO, have been interpreted using the new procedure.

Field data obtained from 18 European estuaries using the isotope pairing technique were analyzed for trends in relationship between activity of benthic microalgae and coupled nitrification-denitrification. Kruskal-Wallis tests and analyses of covariance performed on the field dataset showed strong statistical evidence for the hypothesis that sediments colonized by microalgae whose activity exceeds community respiration display lower rates of coupled nitrification-denitrification than do heterotrophic sediments. In fully heterotrophic sediments, 90% of the measurements fell within the range 0-92 mol N m Ϫ2 h Ϫ1 with a median of 20.3 mol N m Ϫ2 h Ϫ1. In highly autotrophic sediments, 90% of the measurements fell within the range 0-34 mol N m Ϫ2 h Ϫ1 , and the median was 4.2 mol N m Ϫ2 h Ϫ1 . The hypothesis was tested experimentally using 15 N and microsensor (NO ) techniques in prepared Ϫ 3 microcosms with and without algal activity. The results of the experimental studies were consistent with the hypothesis derived from the field data analysis. For the 15 N study, coupled nitrification-denitrification in alga-colonized sediments was between 4 and 51% of the activity in sediments without algae activity, depending on the N load. For the microsensor study, there was no indication of net NO production in alga-colonized sediments before furthermore showed that compared to heterotrophic sediment, the presence of active microalgae might reduce the population of nitrifying bacteria capable of having an active metabolism. These bacterial populations could display diurnal variations in activity correlated with the diurnal variations in O 2 penetration depth, however. The results showed that induction of nitrogen limitation of the nitrifying bacteria population is a major controlling mechanism of coupled nitrification-denitrification in alga-colonized sediments.

Summary Cable bacteria (CB) are Desulfobulbaceae that couple sulphide oxidation to oxygen reduction over centimetre distances by mediating electric currents. Recently, it was suggested that the CB clade is composed of two genera, Ca. Electronema and Ca. Electrothrix, with distinct freshwater and marine habitats respectively. However, only a few studies have reported CB from freshwater sediment, making this distinction uncertain. Here, we report novel data to show that salinity is a controlling factor for the diversity and the species composition within CB populations. CB sampled from a freshwater site (salinity 0.3) grouped into Ca. Electronema and could not grow under brackish conditions (salinity 21), whereas CB from a brackish site (salinity 21) grouped into Ca. Electrothrix and decreased by 93% in activity under freshwater conditions. On a regional scale (Baltic Sea), salinity significantly influenced species richness and composition. However, other environmental factors, such as temperature and quantity and quality of organic matter were also important to explain the observed variation. A global survey of 16S rRNA gene amplicon sequencing revealed that the two genera did not co‐occur likely because of competitive exclusion and identified a possible third genus.

Shallow lakes may play an important role for the nitrogen (N) balance in drainage basins by processing, transferring and retaining N inputs. An increase in the frequency of storm‐induced short‐term N pulses and increased water temperatures are both likely outcomes of climate change, potentially affecting the N processing in lakes. An experiment with a K15NO3− pulse addition (increase in NO3− concentration from c. 0.1 to 2 mg/L) was carried out in 12 mesocosms with relatively low (applies to Danish lakes) total N (TN) and total phosphorus (TP) concentrations (c. 0.3 mg N L−1 and 0.04 mg P L−1) to assess the effects of an N pulse on N processing and storage in shallow lake ecosystems. The mesocosms have a hydraulic retention time of approximately two and a half months, and at the time of the experiment, they had been adapted to contrasting temperatures for a period of 10 years: ambient, T3 (heating according to the Intergovernmental Panel on Climate Change 2007 A2 scenario, +3.7–4.5°C, depending on season) and T5 (heating with A2 + 50%, +4.9–6.6°C). Macrophytes and filamentous algae retained up to 40% and 30% of the added 15N, respectively, reflecting their high biomass in the mesocosms. Macrophytes and filamentous algae constituted between 70% and 80% of the biomass of all primary producers during the experiment in the T3 and ambient treatments and between 20% and 40% in T5. By comparison, less than 1% of the added 15N diffused to the sediment and less than 5% was lost to the atmosphere as N2 gas. Snails represented the long‐term storage of 15N, retaining up to 6% of the tracer and with detectable enrichment 100 days after tracer addition. We found no significant differences among the temperature treatments in the 15N turnover after pulse dosing. However, a larger percentage of 15N was stored in macrophytes in the ambient and T3 mesocosms, reflecting higher biomasses than in T5 where filamentous algae were more abundant. Macrophytes and filamentous algae rather than temperature were therefore key controllers of N processing during the summer N pulse in these shallow, relatively low TP lakes.