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The annual seasonal abundance and spatial distribution of four widespread pelagic copepods, the Palaeartic calanoid Eudiaptomus gracilis, the cyclopoids Mesocyclops leuckarti and Thermocyclops oithonoides, and the Holartic Cyclops scutifer were investigated in Lake Gjerstadvann, an oligotrophic boreal lake. Important ecological traits such as life cycles, pelagic microhabitats and wintering strategies varied strongly between the investigated copepods, and influenced seasonal succession in the plankton community. Fish predation did not seem to affect copepod abundances, except perhaps the two lage-sized, less abundant species, the Palaeartic calanoid Heterocope saliens and the Holartic cyclopoid Cyclops abyssorum. Life cycles varied from one (C. scutifer) to three (M. leuckarti and E. gracilis) complete generations per year, primarily related to habitat temperatures. Wintering took place as late instars (C. scutifer, C. abyssorum) or cop V and adults (E. gracilis) in the plankton, late instars in profundal (T. oithonoides) or littoral (M. leuckarti) sediment diapause, and embryonic diapause in sediment egg bank (H. saliens). C. scutifer and C. abyssorum exhibited delayed development in the profundal waters during winter, which could be characterised as so-called "active diapause". C. scutifer, T. oithonoides, and C. abyssorum in Lake Gjerstadvann were probably negatively affected by acidified waters. M. leuckarti seemed to be the most acid-tolerant of these species being able to endure pH slightly below 5.0, whereas T. oithonoides was usually absent at such pH levels. The calanoid species H. saliens and E. gracilis were extremely tolerant towards acidic environments. The yearly differences in population abundance as indicated by the fluctuations in the diapausing populations were probably due to environmental variations in water chemistry occurring during the most vulnerable ontogenetic stages, i.e., eggs and nauplii. Even if the pelagic ecosystem in boreal and oligotrophic lakes may appear homogeneous, a whole array of life histories and dormancy patterns has evolved among copepods.
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