Community composition has greater impact on the functioning of marine phytoplankton communities than ocean acidification

Abstract

Ecosystem functioning is simultaneously affected by changes in community composition and environmental change such as increasing atmospheric carbon dioxide (CO2 ) and subsequent ocean acidification. However, it largely remains uncertain how the effects of these factors compare to each other. Addressing this question, we experimentally tested the hypothesis that initial community composition and elevated CO2 are equally important to the regulation of phytoplankton biomass. We full-factorially exposed three compositionally different marine phytoplankton communities to two different CO2 levels and examined the effects and relative importance (ω(2) ) of the two factors and their interaction on phytoplankton biomass at bloom peak. The results showed that initial community composition had a significantly greater impact than elevated CO2 on phytoplankton biomass, which varied largely among communities. We suggest that the different initial ratios between cyanobacteria, diatoms, and dinoflagellates might be the key for the varying competitive and thus functional outcome among communities. Furthermore, the results showed that depending on initial community composition elevated CO2 selected for larger sized diatoms, which led to increased total phytoplankton biomass. This study highlights the relevance of initial community composition, which strongly drives the functional outcome, when assessing impacts of climate change on ecosystem functioning. In particular, the increase in phytoplankton biomass driven by the gain of larger sized diatoms in response to elevated CO2 potentially has strong implications for nutrient cycling and carbon export in future oceans.

A data–model synthesis to explain variability in calcification observed during a CO<sub>2</sub> perturbation mesocosm experiment

“…Briefly, our results not only support the findings of Eggers et al (2014), they provide additional insight to the problem of resolving a CO 2 response in the presence of variability in measurements. One added message compared to Eggers et al (2014) is that our mass flux estimates are shown to differ more between the different calcification solutions than between the different CO 2 treatments. This situation exemplifies that simulation results (e.g.…”

“…Instead, mesocosms with similar initial conditions should be exposed to different levels of OA. Such favourable starting conditions were met in the mesocosm experiment described in Engel et al (2005) and Delille et al (2005), as well as in the experiment of Eggers et al (2014).…”

“…In fact, the optimisations yielded estimates that suggest fairly similar initial coccolithophore biomass concentrations between all nine mesocosms. Eggers et al (2014) stressed that variations in initial plankton composition can be responsible for large differences in the responses observed on community level, thereby masking any possible CO 2 effect on photosynthesis or calcification. Briefly, our results not only support the findings of Eggers et al (2014), they provide additional insight to the problem of resolving a CO 2 response in the presence of variability in measurements.…”

“…Eggers et al (2014) stressed that variations in initial plankton composition can be responsible for large differences in the responses observed on community level, thereby masking any possible CO 2 effect on photosynthesis or calcification. Briefly, our results not only support the findings of Eggers et al (2014), they provide additional insight to the problem of resolving a CO 2 response in the presence of variability in measurements. One added message compared to Eggers et al (2014) is that our mass flux estimates are shown to differ more between the different calcification solutions than between the different CO 2 treatments.…”

“…Thus, small deviations in the initial relative mass distribution of photoautotrophs, zooplankton, and detritus between replicate mesocosms can translate into some pronounced variability in measurements even under similar environmental conditions (e.g. Eggers et al, 2014).…”