Submerged macrophytes can counterbalance the negative effects of rising temperature and eutrophication by inhibiting the photosynthetic activity of cyanobacteria and adjusting their morphology and physiology

Climate-driven changes in temperature and high nutrient inputs from anthropogenic activities significantly impact the interactions between submerged macrophytes and phytoplankton, potentially causing regime shifts in shallow freshwater lakes. Cyanobacteria, in particular, are predicted to become dominant among the phytoplankton under climate change, contributing to the collapse of submerged macrophytes. This study aimed to explore how submerged macrophytes and phytoplankton respond to rising temperatures and nutrient addition, how the presence of submerged macrophytes influences the photosynthetic activity of cyanobacteria, and how submerged macrophytes adapt their morphology and physiology to cope with excess phytoplankton. We conducted a fully factorial experiment consisting of two temperature conditions (low and high temperature), two nutrient conditions (with and without nutrient addition) and two plant conditions (plant presence and absence). We analysed changes in phytoplankton biomass and diversity (taxonomic and functional) in response to rising temperatures and nutrient addition. The maximum photochemical efficiency of the photosystem II (Fv/Fm) and the maximum relative electron transport rates (rETRmax) of cyanobacteria were determined. We measured the response of macrophytes to environmental changes by measuring antioxidant enzymes, such as superoxide dismutase (SOD) and catalase (CAT), as well as morphological indicators like plant height, weight, and the relative growth rate. In the absence of submerged macrophytes, rising temperatures led to higher total phytoplankton biomass and cyanobacteria dominance alongside reduced taxonomic and functional diversity. Conversely, when submerged macrophytes were present and no nutrients were added, rising temperatures did not lead to increased phytoplankton biomass. Macrophytes can suppress undesirable cyanobacterial proliferation by competing for nutrients and inhibiting Fv/Fm. Furthermore, under conditions of environmental stress, such as high chlorophyll a concentrations due to nutrient inputs, submerged macrophytes adapted by increasing height and the activity of antioxidant enzymes, such as SOD and CAT. Within the context of climate warming and increased nutrient inputs, phytoplankton growth, especially of cyanobacteria, may be favoured. However, our study demonstrated the critical role of submerged macrophytes in inhibiting phytoplankton, especially when nutrients were controlled. By using these physiological indicators to assess the life activity of organisms, our research provided new insights into macrophyte-phytoplankton relationships during regime shifts in aquatic ecosystems.