The results of a field experiment that investigated the in situ association of total coliforms (TC) and E. coli (EC) with suspended inorganic particles in a drinking water reservoir are presented. The experimental program measured TC and EC at various locations and at multiple depths in a medium sized Australian reservoir subject to continuous inflow forcing. Particle concentrations and size distributions were measured using a LISST (Laser In Situ Scattering Transmissometer) profiler. Correlations between the particle measurements and the bacteria were calculated to provide in situ evidence for the association of TC and EC with suspended inorganic particles. Both TC and EC correlated most strongly with fine particles between 3.2 and 4.5 μm. Development of a simple Lagrangian model of the inflow for particles and microbes additionally provided insight into the relative roles of dilution, sedimentation, and inactivation for the bacteria. The model results and the correlations support the theory that high association with the small particles was due to their high number concentration and the effective surface area available for attachment. It was also evident that the majority of bacteria (> 80%) were physically associated with the suspended particles and allowed a quantitative estimate of the net sedimentation rate. Bacterial attachment and the concomitant increase in settling should be considered when modeling bacterial dynamics, during design of monitoring programs and when implementing pathogen risk management strategies.

Steady-state models for the prediction of P retention coefficient (R) in lakes were evaluated using data from 93 natural lakes and 119 reservoirs situated in the temperate zone. Most of the already existing models predicted R relatively successfully in lakes while it was seriously under-estimated in reservoirs. A statistical analysis indicated the main causes of differences in R between lakes and reservoirs: (a) distinct relationships between P sedimentation coefficient, depth, and water residence time; (b) existence of significant inflow-outflow P concentration gradients in reservoirs. Two new models of different complexity were developed for estimating R in reservoirs: [graphic removed] , where τ is water residence time (year), was derived from the Vollenweider/Larsen and Mercier model by adding a calibrated parameter accounting for spatial P non-homogeneity in the water body, and is applicable for reservoirs but not lakes, and [graphic removed] , where [Pin] is volume-weighted P concentration in all inputs to the water body (μg l-¹), was obtained by re-calibrating the OECD general equation, and is generally applicable for both lakes and reservoirs. These optimised models yield unbiased estimates over a large range of reservoir types.

Lake Vänern is Sweden's largest freshwater reservoir. It has been significantly affected by mercury contamination during the latter half of the 20th century. The aim of this study was to analyse the spatial and vertical mercury distribution, whereas 46 sediment cores were sampled in 2001 and analysed for total mercury. Several of these cores were dated presenting sediment accumulation rates varying from 6-8 mm yr-¹ outside major rivers to ~2 mm yr-¹ in the deeper areas. This was made using ¹³⁷ Cs, which was verified by ²¹⁰ Pb dating. Cluster analysis was used to identify five areas with similar accumulation and mercury concentration regimes. In areas far from shore, surface concentrations ranged from 0.1 to 0.5 ppm Hg, while the deeper layers in contaminated areas held concentrations up to 11 ppm Hg. In total, ~50 tonnes of mercury accumulated in the lake's sediment between ~1940-2001; almost 80% (or 37 tonnes) originate from before the mid 70's when the recovery period began, and at least 30 tonnes can be attributed to the former point source - a chlor-alkali industry.

Lake sediments are a potential source of mercury (Hg) for aquatic biota. Here, we investigated the predictive power of (a) key parameters for lake catchment morphometry and (b) organic matter composition of sediments in an effort to account for observed variations of total (THg) and methyl (MeHg) mercury concentrations in lake sediments. Using regression models we demonstrate that the morphometric parameters lake depth as well as inclination of catchment soils and lake bottoms can significantly predict variations of THg concentrations, but not MeHg, at profundal lake sediments. Although THg and MeHg concentrations in sediments could not be predicted by specific organic matter sources, as elucidated by atomic C/N ratios, our data suggest that wetland-derived total organic carbon (TOC) contained less THg concentrations than TOC derived from mostly forested watersheds. However, TOC concentrations could significantly predict MeHg concentrations and the proportion of methylated Hg at all sediment stations. Finally, from an ecotoxicological point of view, we propose that concentrations of TOC at surface lake sediments, rather than parameters of catchment morphometry, may predict dietary sources of MeHg for benthic consumers and consequently perhaps for organisms at higher trophic levels.