Impact of native and non-native aquatic plants on methane emission and phytoplankton growth

Freshwater plants affect the ecosystem functioning of shallow aquatic ecosystems. However, because native plants are threatened by environmental change such as eutrophication, global warming and biological invasions, continued ecosystem functioning may be at risk. In this study, we explored how the growth of native and non-native plant species in eutrophic, warm conditions impacts two plant ecosystem functions: regulation of phytoplankton growth and methane emission. We expected that plants would inhibit phytoplankton growth, while for methane emission both inhibition and stimulation are possible. We conducted an outdoor experiment using monocultures of four native and four non-native freshwater plant species planted at three different densities, as well as a no-plant control. Monocultures of each species were planted in 65 L mesocosms and after three weeks of acclimatisation each mesocosm was inoculated with phytoplankton. Subsequently, we added nutrients twice a week for eight weeks, before harvesting the plant biomass. During these eight weeks, we measured chlorophyll-a concentration thirteen times and the diffusive methane emissions once after four weeks. The mesocosms amplified the temperature of a warm summer so that plants were exposed to higher-than-average temperatures. We found that five plant species lost biomass, two species increased their biomass only at the highest initial plant density (native Myriophyllum spicatum and non-native Lagarosiphon major) and a single species increased its biomass at all densities (on average 14 times its initial mass; amphibious non-native Myriophyllum aquaticum). Overall, the mean biomass change of non-natives was positive, whereas that of natives was negative. This difference in biomass change between native and non-native plants did not relate to overall differences in phytoplankton mass or diffusive methane emissions. In mesocosms where submerged plant species gained biomass, chlorophyll-a concentration was lower than in the no-plant control and mesocosms with biomass loss. Diffusive methane emissions were highest in mesocosms where plants lost considerable biomass, likely because it increased substrate availability for methanogenesis. However, mesocosms where plant biomass increased had emissions similar to the no-plant control, hence we found no inhibitory effects of plant presence on diffusive methane emission. We conclude that plant growth in eutrophic, warm conditions varies strongly with plant identity. Our results furthermore suggest that plant identity determines whether the replacement of native by non-native freshwater plants will alter ecosystem functions such as regulation of phytoplankton growth and methane emission.