In recent decades, several lake models of varying complexity have been developed and incorporated into numerical weather prediction systems and climate models. To foster enhanced forecasting ability and verification, improvement of these lake models remains essential. This especially applies to the limited simulation capabilities of biogeochemical processes in lakes and greenhouse gas exchanges with the atmosphere. Here we present multi-model simulations of physical variables and dissolved gas dynamics in a temperate lake (Harp Lake, Canada). The five models (ALBM, FLake, LAKE, LAKEoneD, MTCR-1) considered within this most recent round of the Lake Model Intercomparison Project (LakeMIP) all captured the seasonal temperature variability well. In contrast, none of the models is able to reproduce the exact dates of ice cover and ice off, leading to considerable errors in the simulation of eddy diffusivity around those dates. We then conducted an additional modeling experiment with a diffusing passive tracer to isolate the effect of the eddy diffusivity on gas concentration. Remarkably, sophisticated k−ε models do not demonstrate a significant difference in the vertical diffusion of a passive tracer compared to models with much simpler turbulence closures. All the models simulate less intensive spring overturn compared to autumn. Reduced mixing in the models consequently leads to the accumulation of the passive tracer distribution in the water column. The lake models with a comprehensive biogeochemical module, such as the ALBM and LAKE, predict dissolved oxygen dynamics adequate to the observed data. However, for the surface carbon dioxide concentration the correlation between modeled (ALBM, LAKE) and observed data is weak (∼0.3). Overall our results indicate the need to improve the representation of physical and biogeochemical processes in lake models, thereby contributing to enhanced weather prediction and climate projection capabilities.

With the aim to improve current knowledge on physical processes in tropical and subtropical reservoirs, we explored the dynamics of prevailing hydrodynamic processes in a small- to medium-sized reservoir based on 1-year continuous measurements of temperature, flow velocity, and turbulence. The mixing regime of the reservoir is characterized as warm polymictic, and extended periods of comparably strong density stratification were observed during all seasons, interrupted by several mixing periods between autumn and winter. The stratification was affected by lateral, density-driven flows from the main river inflow. A forebay through which the river enters the reservoir influenced the density currents and reduced the occurrence of underflows. Flow velocities in the middle of the reservoir were generally small, and higher velocities (currents >3.5 cm s⁻¹) were driven by wind. These currents were more frequent during stratified than during mixed periods. No basin-scale internal waves (seiches) were observed, but, high-frequency internal waves propagating, with periods between 2 and 17 min were present in all seasons. Dissipation rates of turbulent kinetic energy were generally small (10⁻¹⁰–10⁻⁹ W kg⁻¹). Wind-driven currents were associated with enhanced dissipation rates at the bottom-boundary layer but represented only 10% of the total monitored period. We identified several aspects in which the observed hydrodynamics of the reservoir differed from those of more commonly studied temperate aquatic systems. We expect that our findings are representative for a large number of small- to medium-sized lakes and for a growing number of reservoirs located in the tropical and subtropical zone.

Chlorophyll-a (chla) is an important parameter to assess water quality in lakes and reservoirs, since it is a proxy for phytoplankton biomass and primary production. The increasing availability of data with high spatial and temporal resolution allows assessing short-term dynamics and small-scale variations of chla within larger water bodies. In freshwater reservoirs, the nutrient concentrations and the physical conditions that control phytoplankton growth vary along their longitudinal extend. Here, we analyze how the flow paths of the inflowing river water into density-stratified reservoirs affect the vertical and longitudinal distribution of nutrients and chla. We combine spatially resolved and high-frequency measurements of chla from satellite remote sensing and in-situ sensors, with numerical simulations using a three-dimensional hydrodynamic model to assess the influence of density currents on chla dynamics along a subtropical drinking water reservoir in the south of Brazil. Chla did not have pronounced seasonal dynamics (4.9 ± 1.2 µg L⁻¹, at the continuous measurement station); instead, spatial variability along the reservoir was more pronounced (4.4 ± 2.1 µg L⁻¹, all monitored points within the euphotic zone). Most of the nutrients from the inflowing river were consumed in the upstream region, and phytoplankton in the lacustrine zone depended on internal loading. Temporal variability was observed only in the upstream region, and phytoplankton produced in that area was transported downstream by density currents, resulting in large concentrations of chla below the euphotic zone. The results of a hydrodynamic model that simulates the present state are in good agreement with the observations. Two simulated scenarios, where the density current patterns were altered, resulted only in slight variations in density currents, indicating that the influence of the main inflow was of minor relevance in chla concentrations in downstream regions of the reservoir. Our results highlight the importance of two-dimensional hydrodynamic processes in regulating phytoplankton dynamics in reservoirs.

Floating photovoltaic (FPV) systems are designed for free water surface installations to provide a feasible solution for places with no availability of land areas and to avoid land-use conflicts caused by conventional solar energy farms. However, lakes and reservoirs are essential for ecosystem services like water supply and biodiversity support. In this regard, there was a lack of long-term and high-frequency monitoring data of important parameters influenced by FPV installation such as photosynthetically active radiation (PAR) and dissolved oxygen (DO). Temperature, despite being more reported, there is no consensus on increase or decrease values below the FPV. In this study, we conduct a comprehensive field assessment to accurately quantify temperature variations (at the weather station, modules, between the modules and the water surface and within the water), DO, and PAR of a FPV installed in a water supply reservoir. High-frequency monitoring sensors indicated that despite the 94.7% of radiation reduction below the FPV compared to the lake reference station, slight differences in water temperature and dissolved oxygen were found between measurements in monthly and daily averages in a system that covers less than 1% of the reservoir water surface. In addition, a microclimate due to the module warming is created between the module and water surface with temperature 12% greater than the weather station measurements. The fluctuation throughout the different time frequencies showed that the processes of the reservoirs are influenced by the FPV according to complex interactions among meteorological conditions, FPV configuration, and site-specific factors.