This special issue is dedicated to Professor Dr. Walter Geller on the occasion of his 65th birthday anniversary (11. December 2009). Walter Geller joined the editorial board of Limnologica in 1992. Since his doctoral thesis in 1975 “Die Nahrungsaufnahme von Daphnia pulex in Abhängigkeit von der Futterkonzentration, der Temperatur, der Körpergröße und dem Hungerzustand der Tiere” his main interest research was the interaction of biotic and abiotic factors in lake ecosystems with a special focus on physical in-lake processes. In the beginning of his scientific work investigations on the plankton communities in lakes were in the foreground, especially about Daphnia and its feeding ecology (e.g. 6 publications about Daphnia among 12 papers between 1975 and 1989). With the progress of his careers, the focus of Walter Geller’s scientific interest shifted more and more towards general water quality aspects in context with the formation of pit lakes after the closure of lignite mines in East Germany when he was appointed Head of the Institute for Inland Water Research in Magdeburg in 1992. A substantial point in his life was the DAAD visiting professorship at the Universidad Austral de Chile in Valdivia in 1987, which provided the base for the long-standing cooperation and collaboration with biologists and limnologist from South America, especially from Argentina, Chile, and Brazil.

This short introductory paragraph mirrors the broad range of basic and applied limnological issues of Walter Geller’s interests, which is also reflected in the brief biography and list of publications attached at the end of this editorial. Consequently, our intention was to collect scientific papers, which cover this broad thematic range of Walter Geller’s work contemporarily including the international stations of his scientific career.

This special issue starts with two contributions about the combined influence of biotic and abiotic factors on plankton dynamics. Bouchnak and Steinberg (2010) found a surprising and unexpected positive influence of humic substances on the longevity of Daphnia magna under low quality food conditions. The authors show two stressors not necessarily act synergistically, but antagonistically and explain this effect in part by fine particulate and colloidal humic substances as an additional food source and by potential interactions of dissolved humic substances in the anti-aging signaling pathway. The complex and varying factors, which are responsible for the plankton dynamics of Bosmina in Lake Constance are described by Straile and Müller (2010). They found varying influences by climate variability associated with the North Atlantic Oscillation as well as by changes from eutrophic to oligotrophic conditions in Lake Constance.

A series of three papers focuses on the role of sediments and sedimentary processes on the ecological status of lakes. Sediment cores are used for paleolimnological studies to resolve the history of lakes by different proxies. A key factor for such studies is the estimation of a time scale and of sedimentation rates for the core segments of interest. The thickness of the sediment above an artificially imported marl layer from 1995 was measured in Lake Arendsee (Saxony Anhalt, Germany) by Scharf et al. (2010). Calculated sedimentation rates were clearly correlated with the bottom contour of the lake. Kleeberg et al. (2010) show in their contribution the strong influence of the soil (from the catchment) and sediment quality onto the phosphorus cycle in the initial stage of lake development. In the subsequent stage, increase in import of organic matter will result in the accumulation of carbon and the growth of aquatic macrophytes, which afterwards will be the main driver for the sedimentary phosphorus cycle by mobilizing and transferring phosphorus to other biota. The study of the anthropogenic influence on the degradation of an urban lake exemplified on the Lake Pampulha in Belo Horizonte (Minas Gerais, Brazil) by Friese et al. (2010) exhibits the strong effect of nutrients like organic carbon, phosphorus, and sulfur for the oxygen-free conditions of this lake. A combination of water, sediment, and pore water chemistry with mineralogical determinations and Principal Component Analyses helped to identify the anthropogenic contributions responsible for the bad ecological status of the lake.

The next three contributions report on limnological research from South America. Pinto-Coelho et al. (2010) point at the difficulties to recover the spatial distribution of pH, electric conductivity, and dissolved oxygen in a small tropical reservoir, which receives effluents from an oil refinery. Interestingly, a minimum of 400 measuring points were necessary to minimize the statistical error to an acceptable value. A strong correlation of temperature and mixotrophic ciliates in a deep oligotrophic lake (327 m) of the southern hemisphere was observed by Woelfl et al. (2010) showing the dominance of Chlorella-bearing ciliates in deep North-Patagonian lakes. The vertical and temporal distribution of phytoplankton in a natural acidic volcanic lake of Patagonia is described by Beamud et al. (2010). The survival of algae below the euphotic zone in this lake is explained as a result of hypolimnetic nutrient assimilation.

The second part of this special issue contains 7 papers, which address the limnology of acidic pit lakes from lignite mining from different points of view (physical, chemical, biological, and microbiological) representing the main topic of Walter Geller’s research within the last 15 years before retirement. In the paper of Schultze et al. (2010) an overview is given about the formation, morphometry, and water quality of pit lakes from the Central German lignite mining district. About 140 pit lakes exist in this district and acidification by pyrite oxidation and salinization by intrusion of saline groundwater are the main reasons for the detraction of some of these lakes. To approach the water exchange between a lake, groundwater, and atmosphere quantitatively, Boehrer et al. (2010) deployed pressure sensors for the measurement of water storage changes in a small acidic mine pit lake. The resulting resolution of 2 kg/m² could not be easily achieved by other methods, which makes lake water storage measurements a useful alternative for meteorological data acquisition (i.e. precipitation/evaporation). The strong influence of the lake morphometry on biological processes (e.g. species distribution and richness) is demonstrated by Weithoff et al. (2010). The authors compared two acidic mine pit lakes with similar chemical conditions but different in surface area and depth.

According to German laws and European regulations (EU-Water Framework Directive) acidic mine pit lakes do not meet water quality standards and have to be managed (remediated or neutralized) before integrated into the hydrological system. Nixdorf et al. (2010) compared three kinds of remediation measures (flooding with alkaline river water, liming, and microbiological reduction processes) using a hydrogeochemical model to demonstrate the long-time efficiency for each treatment. The study was exemplified on the pit lake 117 of the Lausatian lignite mine district and showed, e.g., the importance of pyrite formation instead of FeS for the effectiveness of biological treatments. The possibility to raise the pH of acidic pit lake water by microbiological mediated iron reduction processes at the sediment–water interface was studied by Wendt-Potthoff et al. (2010). The authors found enhanced ferric iron and sulfate reduction if organic matter was applied leading to elevated pH in the sediment. The successful neutralization of an acidic pit lake by flooding with alkaline river water is reported from the “Goitsche” pit lake in the Central German lignite mine district (Herzsprung et al., 2010). Currently, the high ferric iron content in the sediment and pore water prevents the lake from eutrophication since all imported phosphorus is adsorbed onto iron oxides. Finally, the contribution of Rönicke et al. (2010) shows the change in the plankton composition in the course of the chemical neutralization of a formerly acidic mine pit lake by addition of soda ash. Although the diversity of the plankton community increased, some groups were still missing one year after neutralization.

We are grateful to all authors for their generous support and hard work to create this special issue. A rigorous peer-review system including at least two anonymous reviews per each contribution established a scientific collection of high quality. The reviewers from Argentina, Australia, Canada, China, Czech Republic, Germany, UK, USA, Mexico, Spain, and Uruguay did a great job for which we are very thankful. Finally, we would like to thank Prof. Dr. Rainer Koschel, Editor in Chief of LIMNOLOGICA, for his encouragement, Edith Tesch from the editorial office for her great support and technical assistance, and the publisher (Elsevier, The Netherlands) for their willingness to publish this special issue.