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

Abstract

Summary Biological interactions can alter predictions that are based on single‐species physiological response. It is known that leaf segments of the seagrass Posidonia oceanica will increase photosynthesis with lowered pH, but it is not clear whether the outcome will be altered when the whole plant and its epiphyte community, with different respiratory and photosynthetic demands, are included. In addition, the effects on the Posidonia epiphyte community have rarely been tested under controlled conditions, at near‐future pH levels. In order to better evaluate the effects of pH levels as projected for the upcoming decades on seagrass meadows, shoots of P. oceanica with their associated epiphytes were exposed in the laboratory to three pH levels (ambient: 8.1, 7.7 and 7.3, on the total scale) for 4 weeks. Net productivity, respiration, net calcification and leaf fluorescence were measured on several occasions. At the end of the study, epiphyte community abundance and composition, calcareous mass and crustose coralline algae growth were determined. Finally, photosynthesis vs. irradiance curves (PE) was produced from segments of secondary leaves cleaned of epiphytes and pigments extracted. Posidonia leaf fluorescence and chlorophyll concentrations did not differ between pH treatments. Net productivity of entire shoots and epiphyte‐free secondary leaves increased significantly at the lowest pH level yet limited or no stimulation in productivity was observed at the intermediate pH treatment. Under both pH treatments, significant decreases in epiphytic cover were observed, mostly due to the reduction of crustose coralline algae. The loss of the dominant epiphyte producer yet similar photosynthetic response for epiphyte‐free secondary leaves and shoots suggests a minimal contribution of epiphytes to shoot productivity under experimental conditions. Synthesis. Observed responses indicate that under future ocean acidification conditions foreseen in the next century an increase in Posidonia productivity is not likely despite the partial loss of epiphytic coralline algae which are competitors for light. A decline in epiphytic cover could, however, reduce the feeding capacity of the meadow for invertebrates. In situ long‐term experiments that consider both acidification and warming scenarios are needed to improve ecosystem‐level predictions.

Seagrass (Posidonia oceanica) seedlings in a high-CO2 world: from physiology to herbivory

“…Sucrose is the principal end-product of leaf photosynthesis54; and the higher content found in our study is thus likely resulting from the increased photosynthetic activity during the early development of the seedlings. In general, higher CO 2 availability increases photosynthetic activity in seagrasses30343555 which sometimes translates into increases in aboveground biomass or growth333539. However this increase in photosynthesis and thus in carbon incorporation is not always allocated to aboveground growth323445.…”

“…In insects, for instance, sugars increase stimulation to taste91 and can mask the deterrent effect of other compounds92. Additionally, we performed the feeding experiments only with the sea urchin P. lividus , whereas different herbivore species may have responded differently to CO 2 -driven changes in plant chemical composition4574 or epiphyte abundance or composition since it is expected that fleshy epiphytes may increase93 and calcareous epibionts would decrease their abundance under low pH conditions30. While we only performed the feeding experiments under ambient CO 2 water conditions, studies to date with adult sea urchins do not suggest strong changes in feeding rates9495, nor in preferences (S.R.…”

“…Indeed, even though most seagrass species have carbon-concentrating mechanisms, they seem to be limited by the current CO 2 concentration, as increased CO 2 availability in the water enhances photosynthesis272829. However, how this predicted increase in photosynthesis would translate into increase in plant performance or abundance remains unclear although long term experiments and studies in CO 2 vents found increased productivity and density respectively at low pH (7.6 and 7.3)3031. Besides, there are no conclusive patterns about long-term effects of elevated p CO 2 in carbon budget or chemical composition in most seagrasses32333435.…”

The response of seagrass (<i>Posidonia oceanica</i>) meadow metabolism to CO<sub>2</sub> levels and hydrodynamic exchange determined with aquatic eddy covariance

“…These results contrast with the increased productivity of seagrass at elevated CO2 and reduced pH (Beer and Koch, 1996;Invers et al, 2001;Palacios and Zimmerman, 2007). Nevertheless, P. oceanica is also not stimulated under comparable, 260 intermediate, ocean acidification scenarios at CO2 vents (Hall-Spencer et al, 2008;Cox et al, 2015;Cox et al, 2016). Thalassia testudinum is also not stimulated by intermediate CO2 enrichment in mesocosms (Campbell and Fourqurean, 2013).…”

Variability in patterns of macro-epiphytic leaf community of Posidonia oceanica in the Islands of Kuriate: Western coast of Tunisia

“…Moreover, epiphytes of seagrass are considered as sensitive indicators of natural and anthropogenic disturbance. Changes in the abundance and composition of their assemblages occur in relation to variation in environmental conditions [8,9]. In addition, they provide an early warning of ecosystem change revealing first-order changes in organism function since molecular, biochemical, and/or cellular changes triggered by pollutants are measurable in cells, tissues, and/or cellular fluids [10,11].…”