Data from: 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.