-Aquatic plants play a substantial role in almost all freshwater habitats throughout the world. Even though submerged aquatic plants dominantly spread by the dispersal of vegetative plant fragments, most aquatic plant species show a broad distribution range. Here we studied the differences in the regeneration capacity and the regeneration type of fragments (by root and/or shoot growth) of eight submerged plant species (Ceratophyllum demersum, Egeria najas, Elodea canadensis, Elodea nuttallii, Hydrilla verticillata, Myriophyllum aquaticum, Myriophyllum heterophyllum and Myriophyllum spicatum) under different water nutrients in sediment-free conditions. Overall, M. spicatum showed the highest regeneration (82 ¡ 2%) in this study, followed by C. demersum (73 ¡ 2%) and M. aquaticum (47 ¡ 4%), whereas M. heterophyllum showed the lowest (1 ¡ 1%). The shoot fragments of E. canadensis, H. verticillata, E. najas and E. nuttallii regenerated by 40 ¡ 2, 23 ¡ 2, 16 ¡ 2 and 7 ¡ 1%. The nitrate concentration affected the regeneration capacities of E. najas (P = 0.05), M. spicatum (P = 0.013) and C. demersum (P = 0.001), whereas phosphate had no significant effect. Additionally, the different nutrient concentrations had a significant effect on the portion of the regeneration types within E. canadensis, E. nuttallii and H. verticillata. Summarizing, submerged plants differ significantly in their regeneration capacity, and water nutrients have a potential effect on the regeneration of submerged plant fragments. This might influence the further colonization and spread of the species under field conditions.
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.
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