Diel vertical distribution of planktonic microcrustaceans (Crustacea: Cladocera, Copepoda) in a natural shallow lake from Transylvania, Romania

The diel vertical migration (DVM) of planktonic cladocerans and copepods was investigated in Lake Ştiucii, a natural shallow lake from north-western Romania. The aim of this study was to test the existence of migration patterns for cladocerans, adult copepods and copepod development stages (copepodites and nauplii) in four sampling seasons: October 2004; January, April and July 2005. The main abiotic and biotic factors influencing the microcrustacean day and night distributions were also analyzed: water temperature, dissolved oxygen, transparency; the distribution of planktonic algae at midday and at midnight in July and the number of larvae of the dipteran predator Chaoborus present in the plankton samples. For most microcrustaceans, nighttime biomass exceeded the daylight values in the water column, indicating a strategy of predator avoidance by staying near to the bottom of the lake during the day. Normal DVM patterns (up at night and down during the day) were observed for cladocerans and adult copepods, while for copepodites and nauplii no clear patterns were depicted. Negative correlations between daytime vertical profiles of planktonic algae and microcrustacean biomass revealed that both cladocerans and copepods migrated away from phytoplankton-rich waters during the day, to escape visual predators like fish. The return in the water column during the night was influenced by invertebrate predation pressure, exerted by Chaoborus larvae.