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1. Introduction
Planktonic Cyanobacteria and unicellular eukaryotes belonging to different functional groups constitute key components of aquatic ecosystems [1]. Among the unicellular plankton there are species that negatively influence the ecosystem [2, 3]. Several of these microorganisms lack distinct morphological features. Even if taxonomically useful morphological features are present, they may get lost throughout sampling, preservation, and examination procedures [4] making identification by traditional microscopic methods difficult. Molecular techniques have spawned new ways to access the diversity of the microbial world. Yet, molecular techniques have limitations [5]. Therefore, a combination of molecular techniques and microscopy methods is required in order to uncover the diversity of the microbial world [6].
Mass fish kills are known to occur in eutrophic lakes. They have been attributed mostly to hypoxic/anoxic conditions or uncommonly high/low temperatures. Other factors, related or not to the eutrophication, include floods, droughts, cyclonic storms, habitat loss, low water flow, and abrupt water level fluctuations [7]. Due to the changes of the grazing pressure, fish kills may lead to considerable changes in the food web structure of the lake ecosystem, with diminishing consequences for the possibilities of using the lake for recreation, fishing, or as a source of drinking water. Although such mass mortality events are well documented in the literature, to the best of our knowledge, there is no such data on newly reconstructed lakes.
In freshwater, the haptophyte Prymnesium parvum is considered one of the most dangerous microorganisms and is responsible for adverse effects on aquatic organisms [8] and in particular for several fish kill incidents [9]. It poses a serious threat to several ecosystems since it survives in a wide range of salinities and blooms in coastal and brackish inland waters worldwide [10, 11]. In Lake Koronia, Greece, P. parvum coincided with a mass death of birds and fish [2, 12]. The dinoflagellate Pfiesteria species can harm fish in coastal waters [13, 14] and has caused fish kills under certain circumstances in North Carolina, USA [13]. No Pfiesteria-induced fish kills have ever been reported in Mediterranean coastal waters, while the only, and most unusual, inland ecosystem where Pfiesteria has been reported is Ace Lake, Antarctica [15].
While acute fish kills due to toxic algae are well studied, another less obvious impact of...