A first study on the zooplankton-cyanobacteria interactions in Karaoun reservoir (Lebanon) during 2016-2017

The planktonic organisms are characterized by their strong capacity of reproduction as well as a high death rate. The factors that intervene simultaneously and result in the seasonal succession and change in the planktonic community are not well studied, especially in Middle East fresh water bodies. Stratification, nutrients, oxygen and light intensity can lead to an increase in phytoplankton and cyanobacteria biomass. In this study, we monitor the interaction between zooplankton and cyanobacteria in Karaoun Reservoir, the largest freshwater body in Lebanon. Karaoun Reservoir has gone through a degradation of its water quality and eutrophication with a very low phytoplankton biodiversity and a dominance of two toxic cyanobacteria species (Microcystis aeruginosa and Aphanizomenon ovalisporum) forming a thick bloom that persisted during the years 2016 and 2017. This spring bloom assured the development of small cladoceran, rotifers that were replaced progressively by big cladoceran such as Daphnia and by copepods Calanoides. Having strongly contributed to the exhaustion of their food resources and being equally submitted to a strong predation from the zooplankton feeding fish such as the carps, the zooplanktonic species decrease gradually till their total disappearance at the beginning of summer. The lake's zooplankton does not proliferate until the end of spring and disappears completely after. The environment rich in cyanotoxins does not seem to be responsible for this decrease. This was attributed to the grazing by Cyprinus carpio. The cycle of these species pass through a period of spring reproduction which allows the zooplankton to develop without being reduced by these fish species.