Individual and combined effect of organic eutrophication (DOC) and ocean warming on the ecophysiology of the Octocoral Pinnigorgia flava

Abstract

Dissolved organic carbon (DOC) enrichment and ocean warming both negatively affect hard corals, but studies on their combined effects on other reef organisms are scarce. Octocorals are likely to become key players in future reef communities, but they are still highly under-investigated with regard to their responses to global and local environmental changes. Thus, we evaluated the individual and combined effects of DOC enrichment (10, 20 and 40 mg L^-1 DOC, added as glucose) and warming (stepwise from 26 to 32 °C) on the widespread Indo-Pacific gorgonian Pinnigorgia flava in a 45-day laboratory experiment. Oxygen fluxes (net photosynthesis and respiration), as well as Symbiodiniaceae cell density and coral growth were assessed. Our results highlight a differential ecophysiological response to DOC enrichment and warming as well as their combination. Individual DOC addition did not significantly affect oxygen fluxes nor Symbiodiniaceae cell density and growth, while warming significantly decreased photosynthesis rates and Symbiodiniaceae cell density. When DOC enrichment and warming were combined, no effect on P. flava oxygen fluxes was observed while growth responded to certain DOC conditions depending on the temperature. Our findings indicate that P. flava is insensitive to the individual effect of DOC enrichment, but not to warming and the two stressors combined. This suggests that, if temperature remains below certain thresholds, this gorgonian species may gain a competitive advantage over coral species that are reportedly more affected by DOC eutrophication. However, under the expected increasing temperature scenarios, it is also likely that this octocoral species will be negatively affected, with potential consequences on community structure. This study contributes to our understanding of the conditions that drive phase shift dynamics in coastal coral reef ecosystemds.

Keywords: Coral reefs; Global warming; Octocorals; Organic Carbon Enrichment; Photosynthesis; Respiration.

Figure 1. P. flava O₂ fluxes response to individual DOC addition.

O₂ production (A) and consumption (B) rates (mg O₂ m⁻² h⁻¹) of P. flava under simulated DOC organic eutrophication over time. The control: 2–3 mg L⁻¹ (grey), and the treatment conditions low: 10 mg L⁻¹ (light blue), medium: 20 mg (teal) and high: 10 mg L⁻¹ (dark blue) are represented accordingly. Individual DOC enrichment did not alter O₂ fluxes of the P. flava fragments at any of the DOC treatment concentrations assessed. Bars values indicate mean ± s.e.m. for n = 3 corals per treatment.

Figure 2. P. flava O₂ fluxes response to DOC enrichment and warming.

P. flava response in terms of (A) O₂ production and (B) consumption rates (mg O₂ m⁻² h⁻¹) to increased temperature and prolonged DOC addition over time. The temperature control: 2–3 mg L⁻¹ at 26 °C (white), the combined increased temperature treatments including the DOC control: 2–3 mg L⁻¹ (grey), and the DOC treatments, low: 10 mg L⁻¹ (light blue), medium: 20 mg (teal) and high: 10 mg L⁻¹ (dark blue) are represented accordingly.Under increased temperatures O₂ production rates were negatively affected at 32 °C, and significantly different for the contrasts between 26–32 °C and 28–32 °C. However, no significant effects were found on P. flava O₂ consumption rates, regardless of temperature. Asterisks mark statistically significant differences (P < 0.05; LMM), and the quantity of asterisks displayed indicate the corresponding p-value significance codes. The bars values indicate mean ± s.e.m. for n = 3 corals per treatment.

Figure 3. P. flava Symbiodiniaceae density in response to DOC enrichment and warming.

P. flava Symbiodiniaceae cell densities (cells g⁻¹ wet.wt) by the end of the experiment (day 45), corresponding to increased temperature of 32 °C and prolonged DOC addition. The graph presents the temperature control: 2–3 mg L⁻¹ at 26 °C (white), the combined increased temperature treatments including the DOC control: 2–3 mg L⁻¹ (grey), and the DOC treatments, low: 10 mg L⁻¹ (light blue), medium: 20 mg (teal) and high: 10 mg L⁻¹ (dark blue). Significant reduction of Symbiodiniaceae cell densities was observed by the end of the experimental term when increased temperature reached 32 °C, for all the treatments where the fragments had been exposed to increased temperatures. Asterisks mark statistically significant differences (P < 0.05; LMM), and the quantity of asterisks displayed is proportional to the corresponding p-value significance code. The bars values indicate mean ± s.e.m. for n = 3 corals per treatment.

Figure 4. P. flava growth as change in surface area in response to DOC enrichment and warming.

Coral surface area changes (cm²) corresponding to every temperature condition at each DOC treatment. The bar graph shows growth under individual DOC addition and warming as a coupled stressor (increasing red intensity scale). Temperatures per each DOC treatment are shown as 26 °C (light pink), 28 °C (salmon), 29 °C (red) and 32 °C (dark red). The first four bars correspond to the temperature control condition (controlT) with no DOC addition, constantly at 26 °C, but at each temporal step in which the rest of the system reached the corresponding temperature treatment targets. Thus, these four bars’ outlines display increasing red intensities accordingly. While neither DOC nor temperature had any effect on growth, their interaction had a significant effect with the effect of DOC on growth varying depending on the temperature value. Asterisks mark statistically significant differences (P < 0.05; LMM) while the bars values indicate mean ± s.e.m. for n = 3 corals per treatment. The number of asterisks displayed on top of the lines indicate the corresponding p-value significance code.