Interactive effects of nitrate concentrations and carbon dioxide on the stoichiometry, biomass allocation and growth rate of submerged aquatic plants

The partial pressure of carbon dioxide (CO₂) in freshwater ecosystems is likely to be affected by climate change, but little is known of its potential effects on aquatic plants when interacting with other stressors such as nitrate pollution. We set a laboratory experiment to test the effects of CO₂ and nitrate availability on growth, biomass allocation, chlorophyll content, and nitrogen, carbon and starch content in four species of submerged aquatic plants. Plants were grown under low CO₂ (0.02 mm CO₂ and 0.83 mm HCO₃ ⁻) and high CO₂ (0.33 mm CO₂ and 0.54 mm HCO₃ ⁻) at five nitrate concentrations (0.1, 0.5, 1, 2 and 4 mg N/L). Growth rates were stimulated in all species by increasing nitrogen and CO₂ availability. Leaf dry matter content (LDMC), leaf starch content and biomass allocation to roots increased with increasing CO₂, but decreased with increasing nitrogen availability. Leaf chlorophyll and nitrogen content in roots and leaves were significantly higher in the low than in the high CO₂ treatments, and generally increased with increasing nitrogen availability. The starch content of leaves correlated positively with LDMC, but negatively with chlorophyll content. There were strong interactive effects of CO₂ and nitrogen availability on plant growth parameters, tissue nitrogen, chlorophyll and starch content. CO₂ availability had a stronger effect on biomass allocation to roots than nitrogen. The effects of CO₂ on LDMC, tissue nitrogen, starch and chlorophyll content were more pronounced under low than under high nitrogen concentration. Our laboratory study suggests that changes in the composition of dissolved inorganic carbon driven by climate change can interact with environmental stressors such as nitrate pollution and affect growth, biomass allocation and physiology of submerged aquatic plants. This is likely to have significant implications for the structuring role of macrophytes in freshwater ecosystems.