Seagrass response to CO2 contingent on epiphytic algae: indirect effects can overwhelm direct effects

Abstract

Increased availability of dissolved CO2 in the ocean can enhance the productivity and growth of marine plants such as seagrasses and algae, but realised benefits may be contingent on additional conditions (e.g. light) that modify biotic interactions between these plant groups. The combined effects of future CO2 and differing light on the growth of seagrass and their algal epiphytes were tested by maintaining juvenile seagrasses Amphibolis antarctica under three different CO2 concentrations representing ambient, moderate future and high future forecasts (i.e. 390, 650 vs. 900 µl l(-1)) and two light levels representing low and high PAR (i.e. 43 vs. 167 µmol m(-2) s(-1)). Aboveground and belowground biomass, leaf growth, epiphyte cover, tissue chemistry and photosynthetic parameters of seagrasses were measured. At low light, there was a neutral to positive effect of elevated CO2 on seagrass biomass and growth; at high light, this effect of CO2 switched toward negative, as growth and biomass decreased at the highest CO2 level. These opposing responses to CO2 appeared to be closely linked to the overgrowth of seagrass by filamentous algal epiphytes when high light and CO2 were combined. Importantly, all seagrass plants maintained positive leaf growth throughout the experiment, indicating that growth was inhibited by some experimental conditions but not arrested entirely. Therefore, while greater light or elevated CO2 provided direct physiological benefits for seagrasses, such benefits were likely negated by overgrowth of epiphytic algae when greater light and CO2 were combined. This result demonstrates how indirect ecological effects from epiphytes can modify independent physiological predictions for seagrass associated with global change.

Epiphyte-cover on seagrass (Zostera marina L.) leaves impedes plant performance and radial O2 loss from the below-ground tissue

“…This might seem obvious, but the DBL-induced impedance of O 2 exchange with the water-column of plants with epiphyte-cover, could also have resulted in an enhancement in the aerenchymal O 2 level (seen as the build-up in the surface O 2 concentration on Figure 5) and thereby a concomitant higher ROL from the root-apex, but this effect was apparently overruled by lower seagrass photosynthesis due to epiphyte shading and/or inorganic carbon limitation due to increased DBL thickness. Burnell et al (2014) recently demonstrated that high incident photon irradiance (∼200 µmol photons m −2 s −1 ) in combination with elevated water-column CO 2 concentrations (up to 900 µl L −1 , representing future predictions of enhanced water-column CO 2 levels) had a negative effect on seagrass biomass and leaf growth, as compared to low light conditions. The observed negative growth response to combined high CO 2 and light conditions appeared to be closely related to overgrowth of seagrass leaves with filamentous algal epiphytes.…”

Effects of ocean acidification on Posidonia oceanica epiphytic community and shoot productivity

“…) and was observed between epiphytes and the seagrass Amphibolis antartica under high‐light and elevated CO 2 (Burnell et al . ). Among epiphytic groups, coralline algae and foraminifera are calcifying along with several species of polychaetes that form calcified tubes and some bryozoans that have calcified surfaces.…”

“…; Burnell et al . ) and reproduction (Palacios & Zimmerman ). However, benefits for seagrasses may largely depend on biological interactions that are difficult to mimic in the laboratory.…”

Climate Change Impacts on Seagrass Meadows and Macroalgal Forests: An Integrative Perspective on Acclimation and Adaptation Potential

“…This also applies to seedlings, as P. oceanica seedlings grown under elevated CO 2 improved photosynthetic performance, and developed larger carbon storage in belowground tissues, having thus more resources to tolerate and recover from stressors. However, elevated CO 2 also favors filamentous algae, which can overgrow seagrass seedlings, leading to reduced growth (Burnell et al, 2014). Moreover, lower N content and increased sucrose levels in seedlings growing under high pCO 2 lead to higher herbivory pressure (Hernán et al, 2016).…”