Frédéric Soulignac

Neutral community model explains the bacterial community assembly in freshwater lakes.

Over the past decade, neutral theory has gained attention and recognition for its capacity to explain bacterial community structure (BCS) in addition to deterministic processes. However, no clear consensus has been drawn so far on their relative importance. In a metacommunity analysis, we explored at the regional and local scale the effects of these processes on the bacterial community assembly within the water column of 49 freshwater lakes. The BCS was assessed using terminal restriction fragment length polymorphism (T-RFLP) of the 16S rRNA genes. At the regional scales, results indicated that the neutral community model well predicted the spatial community structure (R(2) mean = 76%) compared with the deterministic factors - which explained only a small fraction of the BCS total variance (less than 14%). This suggests that the bacterial compartment was notably driven by stochastic processes, through loss and gain of taxa. At the local scale, the bacterial community appeared to be spatially structured by stochastic processes (R(2) mean = 65%) and temporally governed by the water temperature, a deterministic factor, even if some bacterial taxa were driven by neutral dynamics. Therefore, at both regional and local scales the neutral community model appeared to be relevant in explaining the bacterial assemblage structure.

Performance Assessment of a 3D Hydrodynamic Model Using High Temporal Resolution Measurements in a Shallow Urban Lake

Urban lakes provide many ecosystem services, e.g., flood control, nature protection, coolness island, recreation. Hydrodynamic models will increasingly be used to enhance these benefits. We present the first validation of a three-dimensional (3D) hydrodynamic model on a small shallow lake with high resolution and high frequency measurements. Lake Créteil, France (area 0.4 km2, mean depth 4.5 m, and catchment area 1 km2) is a former gravel pit and now part of a regional park. The model Delft3D-FLOW was calibrated on a one-month period, with continuous measurements of temperature at five depths at the center of the lake and at three depths at two other stations, and with current speed profiles at the centre of the lake. The model was then verified on 18 1-month periods with similar temperature measurements. The model reproduced very well the temperature dynamics, including the alternation between mixing and stratification periods and internal wave patterns. The mean absolute errors over the five depths at the central point remained below 0.55∘C in spring and summer, the most favorable seasons for phytoplankton growth. Horizontal temperature differences, which rose up to 3∘C at the beginning of stratification periods, were also well reproduced, as well as current speeds. These results are very promising for assessing nutrient and pollutant diffusion, settling and resuspension, as well as for understanding how phytoplankton blooms start in small shallow lakes.

An Integrated Approach for Assessing the Impact of Urban Stormwater Discharge on the Fecal Contamination in a Recreational Lake Near Paris

Urban stormwater discharges contribute to the fecal contamination of recreational lakes. It is essential to assess the spatial and temporal distribution of fecal bacterial indicators in the receiving waterbodies, to prevent public health risks. This study develops for the first time in continental waters an integrated monitoring and modelling approach, linking the SWMM and Delft-3D models and including a detailed monitoring of the stormwater discharge and at different points of the lake, for assessing Escherichia coli (E.coli) dynamics in stormwater discharges and the receiving urban lake. This integrated approach is applied to a recreational shallow lake and its adjacent urban catchment, with a single stormwater outlet which discharges into the lake. The SWMM model parameters are calibrated and validated with continuous measurements of the flow rate and mean concentration of E. coli in the stormwater discharge measured during each rainfall event. Using the simulated flow rate and E. coli concentration at the sewer outlet, the Delft3D-FLOW-WAQ model simulates E. coli transport in the urban lake with previously calibrated hydrodynamic parameters and default values of E. coli parameters. Comparing simulations with E. coli concentrations measured at different points in the lake, this integrated modelling approach yields promising results. Further studies will focus on the development of automatic model coupling and parameter optimisation, as well as on the evaluation of long-term impacts and management scenarios.