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The term meromixis was introduced more than 80 years ago to denote lakes that do not annually mix completely. Since then our understanding of meromictic lakes has considerably advanced. Physical processes support the difference in water density between deep (monimolimnion ) and surface (mixolimnion ) waters in meromictic lakes; such lakes reveal complex biogeochemical interactions that contribute to the maintenance of meromixis. This Conclusion chapter of the book on Ecology of Meromictic Lakes presents the general overview of physical, chemical and biological properties of meromictic lakes, based partly on the foregoing 12 chapters. The broad spectrum of meromixis supported by different processes in lakes located in different regions and climates is presented. We stress the importance of undisturbed sediments in meromictic lakes as paleolimnological archives and demonstrate how this information can be used to reconstruct lake history and development over longer time periods. The effects of switch from holomictic to meromictic regime and vice versa on the food web and biological community are demonstrated by some of the case study lakes. However, we do not have the mathematical or modeling tools to understand all the causal factor for processes that switch the mixing regimes of lakes. Man-made meromictic lakes (pit lakes) can be used as lake management tools to control water quality in lakes . We discuss that numerical simulations and model forecasts are important tools to predict the mixing regimes in both man-made and natural meromictic lakes. Biogeochemical reactions play important role in cycling of nutrients in meromictic lakes. Both anoxygenic photosynthesis and chemolithoautotrophy are exceptionally important for fixation of inorganic carbon in organic matter in meromictic lakes. These carbon fixation activities link the carbon cycle with the sulphur, nitrogen and iron cycles. Studies of meromictic lakes—aquatic systems where physics, biogeochemistry and ecology interact intensively—give us new insights into the limnology of inland lakes.

We start with the recent developments and reasons why the meromictic lakes should get more attention in limnological literature. Thereafter, we define monimolimnion , mixolimnion and some other terms related to meromixis. The definition of Hutchinson for meromictic lakes holds for more than 60 years while some modification is required. We differentiate between a meromictic lake and a non-meromictic ones, despite the fact that for deep waters the patterns of recirculation in non-meromictic lakes may be similar to meromixis. We also refer briefly to the classification schemes of meromixis and list processes involved in sustaining stratification. Regions on the Globe that produce salinity gradients in surface waters; mining areas, where geochemistry is more actively operative; and areas where waters of different compositions meet may promote the development of such conditions in lakes. Meromixis is a much more dynamic process than it is often understood. If monimolimnion gets eroded, the meromixis can terminate, while changes in the hydrology can turn a lake meromictic. Finally, we briefly introduce here the structure of this book, which has three parts. Part I focuses on physical (Chap. 2), chemical (Chap. 3) and biological (Chap. 4) properties of meromictic lakes. Part II presents eight case studies in separate chapters including the regional studies on different meromictic lakes in five continents. Part III (Chap. 13) is based mainly on the brief conclusion summaries derived from the preceding 12 chapters with editors’ remarks. Lastly, this book is an attempt to update our available knowhow and expertise on the existing state-of-the-art information on meromictic lakes and the literature references that can form the basis for future studies on meromictic lakes.

Case studies and typical examples for meromictic lakes are used to provide a review of the biology and ecology of these ecosystems. Water column in meromictic lakes is not entirely mixed. These lakes are chemically and/or thermally stratified for several years and have several specific ecological features. The chemocline —the habitat created between the mixolimnion on top and monimolimnion below—is characterised by the existence of complex bacterial communities, autotrophic and heterotrophic protists and metazooplankton, commonly dominated by rotifers , high rates of oxygenic and anoxygenic photosynthesis and some biogeochemical processes . In these lakes, the sulphur, carbon and nitrogen cycles are partially coupled. However, a large number of bacterial and archaeal taxa, especially in anoxic waters, are still unidentified. An unaccomplished important task is to both investigate the uncultivated microbial diversity and access metabolic potential of the bacterial communities in meromictic lakes. The different components of the chemocline communities represent the ingredients of microbial loop that probably links the production of organic matter in anoxic waters with the classical grazer food web . However, in most of such lakes, the food web is not quite quantified. The classical grazer food web in meromictic lakes is often truncated, especially because fish and other predators are often absent. Meromixis has several effects on the grazer food web. First, the lack of mixing favours the loss of nutrients into the monimolimnion, which thus controls nutrient availability and the development of the phytoplankton . Because there is virtually no annual mixing in meromictic lakes, spring algal blooms can be less pronounced. The anoxic monimolimnion prevents zooplankton from vertical migrations that change the nature of food web interactions. The relatively large size of the monimolimnion and prevailing anoxic conditions adversely affect the biota. With the development of anoxic monimolimnion, the size of the photic and aerobic zones decreases, benthic community is altered and habitat for zooplankton and fish is reduced. The zooplankton community in meromictic lakes varies in the species composition and abundance. Depending on salinity and chemical composition of the mixolimnion, the zooplankton may include certain typical cladocerans and copepods . If the salinity increases, the zooplankton can shift to Artemia dominated community, typical of hypersaline lakes. Concluding, complex trophic links, coupling of nutrients cycles and anoxic and oxic food web components are peculiar features that make meromictic lakes natural laboratories to study the complexity of the food webs and biological interactions.