Morphological analysis of phytoplankton as a tool to assess ecological state of aquatic ecosystems: the case of Lake Arancio, Sicily, Italy

Phytoplankton requires a sufficient supply of light and nutrients to grow. At the same time it is largely entrained in water motion and is subject to grazing from planktonic herbivores. The pelagic life of lake phytoplankton is based on 4 pillars: living in suspension, light harvesting, nutrient uptake, and escape from grazing. Environmental variability and the uneven distribution of resources among and within the different aquatic ecosystems exert a selective pressure on these organisms, which are formed by a single cell or by colonies with either relatively low or high numbers of cells. Phytoplankton displays an amazing morphological variability representing an adaptation to spatial and temporal variability of their environments. Among functional traits of phytoplankton, morphological features such as shape and size represent an unavoidable starting point to fully understand the relationships between this group of organisms and its environment. The molding effect of environmental variables on phytoplankton morphology is particularly evident in highly dynamic environments such as reservoirs or ponds. The analysis of phytoplankton morphology can thus supply important information on the ecological conditions of a given waterbody. To support this hypothesis, data are presented on the main morphological features of phytoplankton from a 20-year limnological investigation carried out in a Mediterranean man-made lake. The results show that phytoplankton morphological variability reflects different environmental scenarios, and that the morphological features of phytoplankton represent a powerful tool to assess the ecological or trophic state of aquatic ecosystems.