Relationship between Fluorescent Dissolved Organic Matter, Phytoplankton and Zooplankton in Urban Lakes

Urban lakes are vulnerable ecosystems, sensitive to eutrophication, which can impact public health. Phytoplankton and zooplankton can produce significant amounts of fluorescent dissolved organic matter (FDOM), contributing to microbial food webs and enriching lake waters. However, little is known about the relationship between FDOM, phytoplankton and zooplankton at open and closed lakes, relative to the impact of human activities. This study aims to determine the variation in FDOM characteristics from open and closed urban lakes with varied biophysical and socioeconomical conditions. The findings suggested that only a small fraction of FDOM is produced by phyto- and zooplankton in the open and closed urban lakes. Although both types of lakes were dominated by Chlorophyceae, Cyanobacteria and Bacillariophyceae, the FDOM from closed lakes displayed significant correlation only with Bacillariophyceae, while FDOM from open lakes correlated only with Chrysophyceae. Results suggested that, at closed lakes, Bacillariophyceae were potentially responsible for the production of autochthonous protein-like DOM or that the input of protein-like fraction stimulated the growth of Bacillariophyceae. At open lakes, FDOM correlated better with zooplankton (testates and copepods, in particular) compared to phytoplankton, indicating that, here, FDOM is mainly produced by zooplankton. However, external and local sources of pollution (commercial activities, stormwater runoff, agricultural wastes, sewage) and the ecosystem potential of accumulation determined the status of these urban lakes. Commercial activities near the lakes were the main source for FDOM and phytoplankton production. Closed lakes are particularly vulnerable, due to their small lake size and water volume, allowing substantial accumulation of pollutants and presenting a high ecological risk. No human induced changes were seen at zooplankton, which were potentially prevented by current management practices.