3. Sunlight and motion

Water transparency powerfully indicates the state of health of a lake ecosystem. The water’s turbidity defines the physical habitat characteristics of lakes, strongly influencing their chemistry, biology, and ecosystem services. ‘Sunlight and motion’ explains the methods for measuring water transparency and how the penetration of sunlight into water declines with depth. The different colours, hues, and brightness levels of lakes are due to the materials dissolved and suspended within them. The layering of different temperature water in lakes is also described. This stratification varies greatly with the seasons. Mixing of the waters is mainly due to slow waves at and just below the surface as well as deeper currents.

7. Lakes and us

Humans are not only part of the lake ecosystem through activities including lake shore development and transportation of goods and people, but we can also do great damage to a lake and its ability to provide essential services. ‘Lakes and us’ considers the impact of dam-building and the threats facing lakes due to the proliferation of algae and water plants caused by eutrophication and toxins formed by cyanobacterial blooms. It asks whether a eutrophic ‘dead lake’ can be restored to its original, near-pristine condition; and concludes that an integrated, system-level perspective on lakes is now vital in managing the world’s freshwater resources in the face of rapid, global, climate change.

4. Life support systems

There is an astonishing variety and abundance of microscopic life that underpins the ecology of natural waters. The aquatic microbiome—made up of viruses, bacteria, archaea, and microbial eukaryotes—is central to the healthy functioning of lake ecosystems and their responses to environmental change. ‘Life support systems’ describes the key input of energy from the sun and the precarious balance between oxygen gains and losses. It also outlines the key features of the carbon and nitrogen cycles and the important roles that the microbiomes play; and it describes the two different zones of production: the inshore or littoral zone, and the offshore limnetic or pelagic zone.

6. Extreme lakes

‘Extreme lakes’ describes a range of lakes that are of great scientific interest because of their unusual physical, chemical, and biological features. Salt water lakes occur in many parts of the world and are often highly productive, with a simplified food chain supporting large flocks of birds. Polar and alpine lakes are strongly affected by snow and ice, and are therefore sensitive to small changes in temperature across the freezing–melting threshold of water. Other extreme lakes include those that have acid and alkaline waters; geothermal hot-water lakes, and water bodies that periodically erupt, disgorging liquid and gas. In some, only the hardiest of ‘extreme-loving’ microbes can survive and grow.

2. Deep waters

‘Deep waters’ explains the role of François A. Forel in the birth and development of limnology, a new integrative science he defined as ‘the oceanography of lakes’. In his comprehensive monograph on Lake Geneva (1892), Forel subdivided the science of lakes into different disciplines and subjects. There were two aspects of Forel’s new science that set his thinking apart from many of his contemporaries. Studying the physical, chemical, and biological properties of lakes produced an integrated picture of the ecosystem. The second aspect recognized that human beings were also part of the ecosystem. The creation of lakes, their underwater shape, and the importance of lake sediments analysis is also discussed.

1. Introduction

What is a lake? At first glance, this seems like such an easy question: a lake is simply a body of water surrounded by land. But this sterile, physical definition is only a beginning, and there are so many other more interesting ways to consider the nature and meaning of lakes. The ‘Introduction’ outlines the aim of this VSI: to provide a condensed overview of scientific knowledge about lakes, their functioning as ecosystems that we are part of and depend upon, and their responses to environmental change. It considers the physical, biological, and chemical features of lakes as well as their importance in environmental sciences, engineering, and society.

5. Food chains to fish

‘Food chains to fish’ considers the nature of pelagic and benthic food webs and the coupling between them. Life at the bottom of lakes in the benthic zone is mostly made up of worms, molluscs, and amphipods. In the open lake waters, three groups of zooplankton play a leading role in the transfer of carbon and energy from the base of the food web (phytoplankton and bacteria) to pelagic fish: rotifers, cladocerans, and copepods. Although certain fish species remain within specific zones of the lake, others swim between zones and access multiple habitats. The tracking of elements through the food chain can help understand the impacts of chemical pollution.