Results
Effects of foundation species on mean ecosystem parameters - The
presence and absence of Myriophyllum and Dreissenaaffected a wide range of ecosystem parameters. During the first nutrient
addition, ponds with Myriophyllum or Dreissena alone had
lower chlorophyll fluorescence, i.e. lower algal biomass, than ponds
with neither species, consistent with their anticipated negative effects
on the phytoplankton community (Fig. 2). However, following both
disturbance periods, the co-occurrence of these species had strong
non-additive antagonistic effects on algae abundance, illustrated by
their positive effects on mean chlorophyll and phycocyanin fluorescence.
Furthermore, after the first disturbance period, and throughout the
remainder of the experiment, the presence of Myriophyllumincreased the concentration of DOM in the ecosystems, independent ofDreissena presence (i.e. in M and MD treatments). The presence ofMyriophyllum and Dreissena , either alone or in
combination, positively affected dissolved oxygen saturation throughout
most of the experiment, however, not during most of the perturbation
periods: each nutrient addition dramatically increased dissolved oxygen
saturation to levels (between 150 and 200 %) that were not
significantly different across all species contrasts.
Effects of foundation species on variance of ecosystem parameters
- We found only weak effects of Myriophyllum andDreissena presence on variance patterns (CV, Fig. 3) and early
warning indicators (Supplement). Overall, we found strong increases in
CV across all treatment combinations and ecosystem parameters
immediately after the nutrient additions, which reflects the sudden
changes in the mean in response to the disturbances. Prior to the first
nutrient additions, the pond ecosystems with either Myriophyllumor Dreissena alone were less variable in chlorophyll
fluorescence. After the second nutrient pulse, ecosystem where both
species were present variance of chlorophyll and phycocyanin
fluorescence were significantly higher than when species were alone, or
absent. There were almost no effects of foundation species (i.e. their
presence or absence) on the variance of DOM fluorescence. There were
indications of Myriophyllum presence affecting CV of dissolved
oxygen saturation, however, only to weak effect and with variable sign.
As expected, each nutrient addition led to a temporary increase in AC
across all treatment contrasts and parameters (visible as spikes in the
time-series, Fig. S1), but we did not identify treatment specific
differences in AC. Similarly, there were no consistent treatment
specific differences in GEV or Skewness in any of the ecosystem
parameters (Fig. S2 and Fig. S3).
Ecosystem metabolism - GPP and NEP, as well as R were strongly
affected by nutrient perturbation and seasonal dynamics, but less so by
the presence or absence of foundation species (Fig. 4). Each nutrient
addition led to correlated increases of GPP, NEP and R, which reverted
within days after the maximum was reached. During each of these peaks,
there were only little differences across all species contrasts and all
metabolism metrics. During spring 2017, at the beginning of the second
Phase, all pond ecosystems containing Myriophyllum orDreissena had lower NEP and higher R than ecosystems devoid of
foundation species. We found a similar pattern towards the end of the
experiment, after the second nutrient addition in Phase 3, where both
GPP and NEP were lower and R higher when foundation species were
present. Overall, there were only weak effects on variance patterns of
ecosystem metabolism (Fig. 5): there was a tendency for MD ponds to have
higher CV of GPP and NEP than ponds without any foundation species,
especially around the second perturbation Phase. Interestingly, nutrient
perturbation led to increasing CV of GPP and NEP, but not R, which had a
relatively static CV of approximately 0.8 throughout the entire
experiment.