Biological and Ecological Features, Trophic Structure and Energy Flow in 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.