Seasonal and long-term variations of phytoplankton and protozooplankton in two nearby subtropical coastal environments in Brazil: which are the drivers?
摘要
In the Patos Lagoon estuary (PLE) and the adjacent oceanic Cassino Beach surf zone (CBSZ), physical and chemical factors and the biovolume and composition of phytoplankton and protozooplankton were evaluated monthly over a 26-year period. Water temperature was similar in both environments, while salinity was lower and the concentration of dissolved inorganic nutrients was higher in the PLE; their similar seasonal cycle in both environments was associated with the main freshwater discharge, which was influenced by the El Niño–Southern Oscillation. The main driver of the plankton composition and biovolume was salinity, followed by nitrate and silicate; temperature was more significant in the PLE. Phytoplankton composition differed in both environments: In the PLE, freshwater and euryhaline species of diatoms and cyanobacteria were the main groups responsible for most of the variability, and the diatom Asterionellopsis in the CBSZ. The effects of global rising temperature and eutrophication on the plankton communities was evidenced by the increasing long-term trends of water temperature and inorganic nutrient concentrations, which coincided with the increasing trend of the photosynthetic species of diatoms, cyanobacteria, and green algae biovolume. The decreasing trend of Asterionellopsis in the CBSZ, likely influenced by mud deposition events on the beach, is a signal of environmental change potentially disrupting the food web. The decreasing trend of ciliates and the distinct trends of mixotrophic and heterotrophic dinoflagellates in both environments should be further investigated to understand the complex behavior of their feeding habits and contributions to top-down processes.