Zooplankton egg bank: characterization and effect of biotic factors on hatching

Many zooplankton organisms facing harsh environmental conditions producing resting eggs. Resting eggs accumulate in the sediment and create a resting egg bank. Knowledge on the egg bank structure, morphology and morphometry of the eggs as well as the effect of cues on hatching, contributes to the understanding zooplankton dynamics in lakes. Here we described the composition and structure of zooplankton egg bank from a shallow tropical lake and analyzed the effect of some biotic factors on hatching rates. In order to describe the structure and composition of the resting egg bank, we determined the richness, density, morphology and morphometry of the resting eggs isolated by the sugar flotation method. Diapausing eggs were measured and their external features studied by microphotography under optical microscope and SEM. To analyze the effect of biotic factors on hatching rates, we exposed the resting structures and the entire sediment to three biotic factors: a chlorophyte (Scenedesmus acutus), a cyanobacteria (Microcystis sp.) and a predatory rotifer (Asplanchna girodi). A total of 25 zooplankton species hatched from the sediments. Our results show that the medium density of the healthy-looking diapausing eggs was of 7.6 +/- 2 diapausing eggs center dot cm(-3) and that rotifers are the predominant group in the egg bank. Medium conditioned with the chlorophyte and the cyanobacteria resulted in a higher hatching rate. The ornamentations of rotifer diapausing eggs present different features, such as wrinkles and spines, which can help to taxonomic identification without the need of promoting hatching. This study provides information on the structure and composition of the egg bank of a shallow lake and the effect of biotic factors on hatching rates. The results show the dominance of rotifers over cladocerans and copepods and a positive effect of certain biotic factors on hatching.