Damien Bouffard

Application of remote sensing for the optimization of in-situ sampling for monitoring of phytoplankton abundance in a large lake

Directives and legislations worldwide aim at representatively and continuously monitoring the ecological status of surface waters. In many countries, chlorophyll-a concentrations (CHL) are used as an indicator of phytoplankton abundance and the trophic level of lakes or reservoirs. In-situ measurements of water quality parameters, however, are time-consuming, costly and of unknown but naturally limited spatial representativeness. In addition, the variety of the involved lab and field measurement methods and instruments complicates comparability and reproducibility. Taking Lake Geneva as an example, 1234 satellite images from the MERIS sensor on the Envisat satellite from 2002 to 2012 are used to quantify the spatial and temporal variations of CHL concentrations. Based on histograms of spring, summer and autumn CHL estimates, the spatial representativeness of two existing in-situ measurement locations is analysed. Appropriate sampling frequencies to capture CHL peaks are examined by means of statistical resampling. The approaches proposed allow determining optimal in-situ sampling locations and frequencies. Their generic nature allows for adaptation to other lakes, especially to establish new survey programmes where no previous records are available.

Effects of climate change on deepwater oxygen and winter mixing in a deep lake (Lake Geneva): Comparing observational findings and modeling

Low concentrations of dissolved oxygen remain a global concern regarding the ecological health of lakes and reservoirs. In addition to high nutrient loads, climate-induced changes in lake stratification and mixing represent additional anthropogenic meanace resulting in decreased deep-water oxygen levels. The analysis of 43 years of monitoring data from Lake Geneva shows no decreasing trend neither in the areal hypolimnetic mineralization rate, nor in the extent of hypoxia. Instead, hypoxic conditions are predominantly controlled by deep mixing in winter and much less by the trophic variations over the past decades. To reproduce winter mixing, the one-dimensional hydrodynamic model SIMSTRAT was specially adapted to deep lakes and run for several climate scenarios. The simulations predicted a decrease in the maximum winter mixing depth from an average of ∼172 m for 1981–2012 to ∼136 m and ∼127 m in response to predicted atmospheric temperatures between 2045–2076 and 2070–2101 according to Intergovernmental Panel on Climate Change scenarios. Concurrently, events with complete homogenization of temperature and oxygen in winter will decrease by ∼50%. Consequently, the hypolimnetic oxygen concentrations will significantly decrease. These results demonstrate that changes in deep mixing can have stronger impact than eutrophication on the deep-water oxygen levels of oligomictic lakes. This article is protected by copyright. All rights reserved.

Efficiency of turbulent mixing in the abyssal ocean circulation

Turbulent mixing produced by breaking of internal waves plays an important role in setting the patterns of downwelling and upwelling of deep dense waters and thereby helps sustain the global deep ocean overturning circulation. A key parameter used to characterize turbulent mixing is its efficiency, defined here as the fraction of the energy available to turbulence that is invested in mixing. Efficiency is conventionally approximated by a constant value near one sixth. Here we show that efficiency varies significantly in the abyssal ocean and can be as large as approximately one third in density stratified regions near topographic features. Our results indicate that variations in efficiency exert a first-order control over the rate of overturning of the lower branch of the meridional overturning circulation.

Sediment resuspension mechanisms and their contributions to high‐turbidity events in a large lake

High-resolution field data, collected during April to October of 2008–2009, were analyzed to investigate the quantitative contribution of sediment resuspension to high-turbidity events in central Lake Erie. Resuspension events were distinguished within high-turbidity events according to turbidity, fluorescence and acoustic backscatter timeseries, as well as satellite images. We observed 16 high-turbidity events, causing a total duration of ∼20 d (out of 344 d) with elevated nearbed turbidity (> 10 NTU). Of these events, 64% were correlated with algal biomass, with the remaining 18%, 5%, and 4% being attributed to sediment resuspension by surface waves, storm-generated currents and enhanced nearbed turbulence induced by high-frequency internal waves, respectively. This is the first time that resuspension by enhanced nearbed turbulence from high-frequency linear internal wave degeneration has been observed in a large lake. Resuspension was parameterized as a function of the instantaneous critical bottom velocity, bottom shear stress and the Shields parameter. From the in situ measurements, we suggest an extended Shields diagram for silty bed material that can be used to predict resuspension in other aquatic systems with similar sediment composition (∼20% cohesive sediment).

Giant lacustrine pockmarks with subaqueous groundwater discharge and subsurface sediment mobilization

Subsurface fluid flow in oceans and lakes affects bathymetric morphology, sediment distribution, and water composition. We present newly discovered giant lacustrine pockmarks in Lake Neuchatel (up to 160m diameter and 30m deep) that rank among the largest known pockmarks in lakes. Our multidisciplinary study reveals 60m of suspended sediment inside a pockmark. The sediment suspension is 2.6 degrees warmer and isotopically lighter in O-18(H2O) by 1.5 than the ambient lake water, documenting currently active fluid flow by karstic groundwater discharge from the Jura Mountain front into the Swiss Plateau hydrological system. Strikingly, the levees of the pockmarks comprise subsurface sediment mobilization deposits representing episodic phases of sediment expulsion during the past. They strongly resemble subsurface fluid flow features in the marine realm. Comparable processes are expected to also be relevant for other carbonate-dominated mountain front ranges, where karstic groundwater discharges into lacustrine or marine settings.

Impacts of hydrodynamics and benthic communities on phytoplankton distributions in a large, dreissenid-colonized lake (Lake Simcoe, Ontario, Canada)

Abstract We quantified the vertical and horizontal variation of chlorophyll a (Chl-a) to test how benthic filter feeders (dreissenid mussels), rooted macrophytes, and hydrodynamics influence phytoplankton distributions in a large lake (Lake Simcoe, Canada). Water column Chl-a did not differ significantly among sites of different depth, distance offshore, or rooted macrophyte biomass, but among the nearshore sites (7.5–10 m deep) it was higher where mussel biomass was greater. This counterintuitive result may be explained by wind-driven horizontal circulation during our specific study periods together with the patchy distribution of the mussels in the lake. Information on mixing depths, vertical eddy diffusivity, and mussel biomass was used to predict when and where the grazing pressure of mussels would likely deplete near-bottom phytoplankton. Chl-a depletion was frequently predicted at sites with moderate or high mussel biomass and sufficient thermal stratification to impede vertical mixing but never at sites without thermal stratification. Observations were consistent with predictions in most cases. The results suggested that mussels at depths of 7.5–15 m (a depth range of generally high mussel biomass in the lake) may frequently suffer food limitation due to near-bottom depletion during the early and middle stratified season. A deep Chl-a maximum was documented and may be important for mussel nutrition at such times.

Physical effects of thermal pollution in lakes

Anthropogenic heat emissions into inland waters influence water temperature and affect stratification, heat and nutrient fluxes, deep water renewal, and biota. Given the increased thermal stress on these systems by growing cooling demands of riparian/coastal infrastructures in combination with climate warming, the question arises on how to best monitor and manage these systems. In this study, we investigate local and system-wide physical effects on the medium-sized perialpine Lake Biel (Switzerland), influenced by point-source cooling water emission from an upstream nuclear power plant (heat emission ∼700 MW, ∼18 W m−2 lake wide). We use one-dimensional (SIMSTRAT) and three-dimensional (Delft3D-Flow) hydrodynamic numerical simulations and provide model resolution guidelines for future studies of thermal pollution. The effects on Lake Biel by the emitted excess heat are summarized as: (i) clear seasonal trend in temperature increase, locally up to 3.4°C and system-wide volume mean ∼0.3°C, which corresponds to one decade of regional surface water climate warming; (ii) the majority of supplied thermal pollution (∼60%) leaves this short residence time (∼58 days) system via the main outlet, whereas the remaining heat exits to the atmosphere; (iii) increased length of stratified period due to the stabilizing effects of additional heat; (iv) system-wide effects such as warmer temperature, prolonged stratified period, and river-caused epilimnion flushing are resolved by both models whereas local raised temperature and river short circuiting was only identifiable with the three-dimensional model approach. This model-based method provides an ideal tool to assess man-made impacts on lakes and their downstream outflows.

The influence of bottom boundary layer hydrodynamics on sediment focusing in a contaminated bay

Understanding the dynamics and fate of particle bound contaminants is important for mitigating potential environmental, economic and health impacts linked to their presence. Vidy Bay, Lake Geneva (Switzerland), is contaminated due to the outfall and overflow from the wastewater treatment plant of the City of Lausanne. This study was designed to investigate the fate of particle-bound contaminants with the goal of providing a more complete picture of contaminant pathways within the bay and their potential spread to the main basin. This goal was achieved by investigating the sediment transport dynamics, using sediment traps and radionuclide tracers, and ascertaining how local bottom-boundary hydrodynamic conditions (temperature, turbidity, current velocity and direction) influence these dynamics. Results of the study indicated that sedimentation rates and lateral advections increased vertically with proximity to the lakebed and laterally with proximity to shore, indicating the presence of sediment focusing in the bay. Hydrodynamic measurements showed the persistent influence of a gyre within the bay, extending down to the lake bed, while just outside of the bay circulation was influenced by the seasonal patterns of the main basin. Calculated mean displacement distances in the bay indicated that suspended particles can travel ∼3 km per month, which is 1.7 times the width of the Vidy Bay gyre. This results in a residence time of approximately 21 days for suspended particles, which is much greater than previously modelled results. The calculated mobility Shield parameter never exceeded the threshold shear stress needed for resuspension in deeper parts of the bay. In such, increased lateral advections to the bay are not likely due to local resuspension but rather external particle sources, such as main basin or shallow, littoral resuspensions. These external sources coupled with an increased residence time and decreased current velocity within the bay are the precipitating factors in sediment focusing. While the spread of contaminants from the bay may occur through the transport of fine suspended sediments in shallower zones of the bay (<60 m) by longshore littoral currents, results suggest that particle-bound contaminants are likely to remain within the bay.

Three-Dimensional Simulation of Lake Ontario North-Shore Hydrodynamics and Contaminant Transport

The hydrodynamics and contaminant transport in the nearshore region of Lake Ontario, from Port Hope to Cobourg, were simulated. The model results were comprehensively validated against observations of water level, temperature, and currents collected during April-September, 2010. The model generally agrees well with the observations. A RMS error of similar to 2 degrees C was simulated and normalized Fourier norm (between 0.45 and 1.17), indicating that both the thermal stratification and currents are well-simulated, respectively, and are comparable to other model applications. Internal Kelvin waves were not observed and the internal Poincare wave oscillation was observed offshore, but not modeled. Rather, up-welling and down-welling events caused by southwesterly and northeasterly winds, respectively, were both modeled and observed to be the dominant large-scale hydrodynamic processes. The episodic events lasted for 4-5 days with the upwelling front extending similar to 10 km offshore. The up-welling and down-welling events generated geostrophic alongshore currents or coastal jets of similar to 20 cm . s(-1). The influence of these dynamics on the transport of river and wastewater-treatment plant plumes, toward drinking water intakes, was investigated using tracer release simulations. Tracer concentrations in the range of 10-0% (i.e., 70-90% dilution) were found at the Port Hope and Cobourg drinking water intakes. The tracer concentrations were primarily influenced by the proximity of the intakes to the plume origins and the wind direction, which governs the direction of the alongshore currents resulting from the up-welling and downwelling events. These results will help municipalities better understand the transport of contaminants in the nearshore zone relative to drinking water intakes

Homogenization of lake cyanobacterial communities over a century of climate change and eutrophication

Human impacts on biodiversity are well recognized, but uncertainties remain regarding patterns of diversity change at different spatial and temporal scales. Changes in microbial assemblages are, in particular, not well understood, partly due to the lack of community composition data over relevant scales of space and time. Here, we investigate biodiversity patterns in cyanobacterial assemblages over one century of eutrophication and climate change by sequencing DNA preserved in the sediments of ten European peri-Alpine lakes. We found species losses and gains at the lake scale, while species richness increased at the regional scale over approximately the past 100 years. Our data show a clear signal for beta diversity loss, with the composition and phylogenetic structure of assemblages becoming more similar across sites in the most recent decades, as have the general environmental conditions in and around the lakes. We attribute patterns of change in community composition to raised temperatures affecting the strength of the thermal stratification and, as a consequence, nutrient fluctuations, which favoured cyanobacterial taxa able to regulate buoyancy. Our results reinforce previous reports of human-induced homogenization of natural communities and reveal how potentially toxic and bloom-forming cyanobacteria have widened their geographic distribution in the European temperate region.Analysis of sedimentary DNA from ten European lakes reveals that over the past 100 years cyanobacterial communities have become compositionally and phylogenetically more similar, in line with homogenization of lake environmental conditions.