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.

Water abiotics measurementsVariables: ID, Species, Density and Replicate are explained in metadata document. Each ID is an experimental unit. The variable Date gives date at which the following variables were measured: Temperature, pH1, Oxygen, Conductivity, Alkalinity, pH2, Turbidity, TON, NH4, PO4, NO2, NO3abiotics.txtepiphytonEpiphyton data from PVC strips. Variable mesocosmID links to metadata.txt file. Variable Sample links to original raw spectrophotometer file. Variable date gives the date. The following four variables were used to calculate the epiphyton and pheopigment density per surface area (last threecolumns): 665-0, 750-0, 665-a, 750-a, chl-a (ug), pheo (ug), surface (cm2)filamentousalgaeGives the amount of filamentous algal biomass that was removed per mesocosm (links to metadata.txt) for each date (columns).gasdataRaw output of Los Gatos measurements on greenhouse gas. Use gasmetadata.txt to infer when actual measurements took place.gasmetadataIs used to make sense of gasdata.txt in that it holds the timepoints (Time-In) and some metadata on experimental units (mesocosmID, Block). The variable Seconds is the recalculated timepoint on gasdata.txt using the time-in.overall-metadataHolds the metadata on the experimental units used in the experiment, comprising of a number for each mesocosm (mesocosmID), the name of the plant that grew in the mesocosm (plantName), a codified plant name (plantCode), its initial density (density), the replicate number assigned (replicate), the spatial position in the experimental mesocosm configuration (X, Y), the date at which the mesocosm was harvested (harvestDate) and whether a species is native or non-native (plantOrigin)phytoplanktonData on the phytoplankton measurements (twice per week). First metadata (ID Species Density Replicate), then actual data of Phytopam results: Diluted, GAIN, green (ug/l), yield, blue (ug/l), yield, brown (ug/l), yield, sumchl, DATEplantsData on plant biomass with variables mesocosmID (links to metadata.txt), DM.g.end (gives plant dry weight at harvest), PVI-after-acclimation (gives plant PVI three weeks after being planted), FM.g.start (fresh plant biomass at planting) and DMC (dry matter content).