We performed combined in situ measurements of bottom boundary-layer turbulence and of diffusive oxygen fluxes at the sediment-water interface in a medium-sized mesotrophic lake. The turbulence was driven by internal seiching with a period of 18 h. This periodic forcing, a prominent feature of enclosed water bodies, led to distinct deviations of the structure and the dynamics of the bottom boundary layer from the classical law-of-the-wall theory. A major feature was a phase lag between the current velocity and the turbulent energy dissipation of approximately 10% of the seiching period (1.5-2 h). The oxygen flux into the sediment was controlled by the diffusive boundary layer, the thickness of which varied between 0.16 and 0.84 mm during the course of a seiching period, and was strongly affected by the periodic bottom boundary-layer turbulence. The rate of dissipation of turbulent energy in the bottom boundary layer allowed us to define the Batchelor length for dissolved oxygen, which quantifies the smallest scales of oxygen fluctuations and provides an appropriate scaling for the diffusive boundary-layer thickness and the corresponding oxygen fluxes. An analysis of the governing time scales revealed the importance of turbulence in controlling the small-scale spatial heterogeneity of the diffusive fluxes. Higher turbulence causes the diffusive boundary layer (DBL) to follow the sediment topography more smoothly, resulting in an increased area-averaged flux due to the greater effective surface area.After surface zones, the bottom boundary layer (BBL) is the second prime site for animals, plants, and microorganisms in natural waters. From a physical and geochemical point of view, the importance of the BBL is twofold. First, the BBL is a major energy sink for basin-scale currents due to bottom friction and also due to the breaking of propagating internal waves on sloping bottoms (Imberger 1998). Consequently, the level of turbulence is enhanced in the BBL compared with the interior water body. Second, the BBL controls the exchange of solutes and particles between water and sediment. The sediment surface is usually an enormous sink of oxygen due to the processes caused by the decomposition of organic matter. Furthermore, the redissolution and subsequent vertical transport of ions and other solutes supply primary producers with nutrients and affect the stability of the water column by chemical (salinity) stratification (Wüest and Gloor 1998).Especially in eutrophic and mesotrophic systems with 1 Corresponding author (andreas.lorke@eawag.ch). AcknowledgmentsWe thank C. Dinkel and M. Schurter for their great help in the field. D. McGinnis kindly improved the English. We gratefully acknowledge the helpful criticism of two unknown reviewers.

Abstract. There is mounting evidence that organic or inorganic enrichment of aquatic environments increases the risk of infectious diseases, with disease agents ranging from helminth parasites to fungal, bacterial, and viral pathogens. The causal link between microbial resource availability and disease risk is thought to be complex and, in the case of so-called ''opportunistic pathogens,'' to involve additional stressors that weaken host resistance (e.g., temperature shifts or oxygen deficiencies). In contrast to this perception, our experiment shows that the link between resource levels and infection of fish embryos can be very direct: increased resource availability can transform benign microbial communities into virulent ones. We find that embryos can be harmed before further stresses (e.g., oxygen depletion) weaken them, and treatment with antibiotics and fungicides cancels the detrimental effects. The changed characteristics of symbiotic microbial communities could simply reflect densitydependent relationships or be due to a transition in life-history strategy. Our findings demonstrate that simple microhabitat changes can be sufficient to turn ''opportunistic'' into virulent pathogens.

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The intensity and pathways of mineralization of sedimentary organic matter were investigated in eutrophic Lake Zug, Switzerland. In a depth transect (25-180 m) from oxic to anoxic bottom water we recorded in situ sediment pore-water concentration profiles of O 2 , NO { 3 , and NH z 4 with a benthic lander system equipped with both oxygen and ion-selective electrodes. Anaerobic sedimentary mineralization ranged from 13.1 to 34.9 mmol carbon (C) m 22 d 21 and increased linearly with water depth, as determined from the NH z 4 flux rates in the anoxic pore water and the molar C : nitrogen (N) ratio of the surface sediment. A parallel increase of the total organic carbon concentration of the sediment was attributed to lateral transfer of resuspended sedimentary matter. Denitrification was estimated from nitrate profiles and contributed only 1.5-3.2% to the total organic carbon mineralization at any water depth. Aerobic respiration and oxidation of reduced compounds were calculated from O 2 microprofiles and pore-water data of dissolved Mn(II), Fe(II), S(-II), and CH 4 . When the O 2 concentration exceeded 0.15 mmol L 21 in the sediment overlying water, 41-58%, or 12.4-18.1 mmol C m 22 d 21 , was mineralized aerobically, whereas at lower concentrations (,0.04 mmol L 21 ), .92% of organic carbon was mineralized anaerobically. Total benthic mineralization of organic carbon was 26.9-34.9 mmol C m 22 d 21 . A budget including particulate as well as dissolved reduced compounds in the sediment indicated that .95% of the anaerobic mineralization was due to methanogenesis. Oxidation of CH 4 consumed 39-56% of the O 2 at the sediment-water interface. Oxygen exposure times for these sediments were estimated to be on the order of weeks to months. These time spans are too short to change the reactivity spectrum of sedimentary organic matter.

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