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-days 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 together elevated the phytoplankton biomass and caused strong dominance of cyanobacteria. Addition of N significantly lowered the herbivorous zooplankton to phytoplankton 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 phytoplankton growth through reduced grazing by zooplankton. Accordingly, our study indicates that summer N loading may boost eutrophication via both changes in resource and grazing control in shallow lakes. Thus, alleviation of eutrophication in shallow eutrophic lakes requires a strategic approach to control both nutrients (N and P) appropriately.
Highlights
Nitrogen (N) and phosphorus (P) are key limiting factors determining autotroph growth in both terrestrial and aquatic ecosystems (Elser et al, 2007; Conley et al, 2009)
We found that summer N addition increased the N levels and P availability for phytoplankton growth
We found that N addition lowered the zooplankton-phytoplankton biomass ratio and promoted the phytoplankton yield, expressed as the chlorophyll a to total phosphorus (TP) and Total nitrogen (TN) ratios, indicating that N addition favored phytoplankton growth through reduction of zooplankton grazing
Summary
Nitrogen (N) and phosphorus (P) are key limiting factors determining autotroph growth in both terrestrial and aquatic ecosystems (Elser et al, 2007; Conley et al, 2009). A higher phytoplankton biomass due to summer N enrichment augments the P release from P-rich sediments via various mechanisms including increasing diffusion, bacteria activity, pH, and likely degradation of settled algae (Xie et al, 2003; Gao et al, 2014; Chen et al, 2018; Ma et al, 2018; Ma et al, 2021) This released P, in turn, hinders a change of the system towards P limitation at persistent external N loading and, it creates a positive feedback loop benefitting phytoplankton growth (Figure 1). The dual effect of oxidized nitrogen on sediment P release makes prediction of the effects of summer N loading in shallow lakes uncertain
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