When sunlight penetrates the ice layer covering lakes in winter, it warms the top water layer and sets up convection, with several potentially contrasting effects on phytoplankton. While convective mixing keeps cells in suspension and prevents sedimentation losses, it may also transport phytoplankton well below the euphotic zone. We investigated diurnal variations in the vertical distribution of phytoplankton under ice and just after ice-off in Lake Onego (Russia), a lake with moderate to high colored dissolved organic carbon (CDOM) levels. We showed that diurnal variation in convection under ice restricts phytoplankton access to light in the morning hours to a narrow euphotic zone, whereas cells are mixed through a deep aphotic layer in the afternoon. After ice-off, low chlorophyll a was found on the open-water side of the thermal bar as convection distributed cells throughout the water column. By contrast, the inshore side had significantly higher concentrations of chlorophyll a (p < 0.001) because the mixing depth brought about by diurnal microstratification was reduced, resulting in greater access to light in the afternoon. Overnight, convective cooling broke down microstratification, which redeveloped the next day. Our work highlights the importance of studying diurnal variation in light availability for photoautotrophic growth, both under ice and after ice-off in lakes characterized by high CDOM.

Contrasting the paradigm that methane is only produced in anoxic conditions, recent discoveries show that oxic methane production (OMP, aka the methane paradox) occurs in oxygenated surface waters worldwide. OMP drivers and their contribution to global methane emissions, however, are not well constrained. In four adjacent pre-alpine lakes, we determine the net methane production rates in oxic surface waters using two mass balance approaches, accounting for methane sources and sinks. We find that OMP occurs in three out of four studied lakes, often as the dominant source of diffusive methane emissions. Correlations of net methane production versus chlorophyll-a, Secchi and surface mixed layer depths suggest a link with photosynthesis and provides an empirical upscaling approach. As OMP is a methane source in direct contact with the atmosphere, a better understanding of its extent and drivers is necessary to constrain the atmospheric methane contribution by inland waters.

Permanent ponds are valuable freshwater systems and biodiversity hotspots. They provide diverse ecosystem services (ES), including water quality improvement and supply, food provisioning and biodiversity support. This is despite being under significant pressure from multiple anthropogenic stressors and the impacts of ongoing global change. However, ponds are largely overlooked in management plans and legislation, and ecological research has focused on large freshwater ecosystems, such as rivers or lakes. Protection of ponds is often insufficient or indirectly provided via associated habitats such as wetlands. This phenomenon is likely exacerbated due to lacking a full-scale understanding of the importance of ponds. In this review, we provided a detailed overview of permanent ponds across Europe, including their usages and the biodiversity they support. By discussing the concepts of pondscape and metacommunity theory, we highlighted the importance of connectivity among and between ponds and identified fluxes of emerging insects as another ES of ponds. Those insects are rich in essential nutrients such as polyunsaturated fatty acids (PUFA), which are delivered through them to the terrestrial environment, however the extent and impact of this ES remains largely unexplored. Several potential stressors, especially related to ongoing global change, which influence pond diversity and integrity were discussed. To conclude this review, we provided our insights on future pond management. Adaptive measures, taking into account the pond system per se within the pondscape, were found to be the most promising to mitigate the loss of natural ponds and restore and conserve natural small water bodies as refuges and diversity hotspots in increasingly urbanized landscapes.

Toxic cyanobacteria, such as Planktothrix rubescens, came to dominate temperate lakes in the mid‐to‐late 20th century, as a result of eutrophication. Even after decades of re‐oligotrophication, where phosphorus levels were reduced by 1–2 orders of magnitude, P. rubescens remains present in various lakes. In this study, we examine the persistence and changes of P. rubescens deep chlorophyll maximum (DCM) in Lake Hallwil (Switzerland) over 35 yr of steadily decreasing phosphorous concentrations. Although lake transparency increased and the euphotic layer deepened during this period, the P. rubescens population maximum moved even deeper. It is now found ca. 7.7 m deeper than its shallowest position in the 2000s, and this depth no longer coincides with the depth of maximal water column stability. P. rubescens neutral buoyancy has now driven it beneath the stable metalimnion into the hypolimnion, where buoyancy regulation is restricted due to reduced metabolic activity at low light and low temperature. If P. rubescens DCM continues to deepen each year, it will eventually reach a region of lower stability in the hypolimnion where turbulent conditions are strong enough to disperse the DCM. We also explore the mechanisms that ensure P. rubescens ongoing presence in peri‐alpine lakes despite strong re‐oligotrophication and ongoing climate change. We find that P. rubescens in the lake is mainly sustained by growth during fully mixed conditions in winter, not during stratification in summer. This may contradict another commonly made prediction that periods of longer stratification will promote future blooms of this cyanobacterium.