Limnetica 38

Laguna de La Cruz is a small meromictic karstic lake located in Cuenca (Central Spain), whose stratification and thermal mixing patterns largely determine the structure and composition of its phytoplankton populations. In this lake, the succession in the oxic zone of the water column starts, with the autumnal mixing, with some diatom species, such as Cyclotella delicatula, of the functional codon B, dominating the community throughout the oxic water column during the winter. This species persists in spring, but then linked to deeper layers. In spring, at the beginning of the thermal stratification, picocyanobacteria, cryptophytes, and dinoflagellates develop in upper layers, corresponding, respectively, to the functional coda Z, Y, and Lo, with the first two being progressively restricted to the oxycline as the stratification advances. As stratification advances, dense populations of chlorophytes belonging to the functional groups F, X, and J develop in the epilimnion and the metalimnion, while towards the end of the summer Cosmarium species, belonging to the functional group NA, are selected and become dominant until the autumnal mixing favors again the diatoms. As a whole, the strong stratification of the water column allows for the appearance of different niches with a differential vertical distribution of the phytoplankton during the stratification periods, which favors its diversity within the whole water column. However, several types of disturbances, such as episodes of heavy rainfall, horizontal movements of water bodies and, in particular, the whitening that occurs every year towards the middle of summer, alter the successional pattern to some extent