AUTHOR=He Hu , Chen Kunquan , Du Yingxun , Li Kuanyi , Liu Zhengwen , Jeppesen Erik , Søndergaard Martin 

TITLE=Increased Nitrogen Loading Boosts Summer Phytoplankton Growth by Alterations in Resource and Zooplankton Control: A Mesocosm Study

JOURNAL=Frontiers in Environmental Science

VOLUME=Volume 9 - 2021

YEAR=2021

URL=https://www.frontiersin.org/journals/environmental-science/articles/10.3389/fenvs.2021.772314

DOI=10.3389/fenvs.2021.772314

ISSN=2296-665X

ABSTRACT=The effectiveness of controlling nitrogen (N) to manage eutrophication of aquatic ecosystems remains debated. To understand the mechanisms behind phytoplankton growth in shallow lakes (resource and grazing effects) under contrasting N loading scenarios, we conducted a 70-day mesocosm experiment in summer. The mesocosms contain natural plankton communities deriving from a 10-cm layer of lake sediment and 450 L of lake water. We also added two juvenile crucian carp (Carassius carassius) in each mesocosm to simulate presence of the prevailing omni-benthivorous fish in subtropical lakes. Our results showed that N addition increased not only water N levels but also total phosphorus (TP) concentrations, which togeth-er elevated the phytoplankton biomass and caused strong dominance of cyanobacte-ria. Addition of N significantly lowered the herbivorous zooplankton to phytoplank-ton biomass ratio and promoted the phytoplankton yield per nutrient (Chl-a : TP or Chl-a : TN ratio), indicating that summer N addition likely also favored phytoplank-ton growth through reduced grazing by zooplankton. Accordingly, our study indi-cates that summer N loading may boost eutrophication via both resource and grazing control in shallow lakes. Thus, alleviation of eutrophication in shallow eutrophic lakes requires a strategic approach to control both nutrients (N & P) appropriately.