Abstract. Calcified macroalgae are critical components of marine ecosystems worldwide, but face considerable threat both from climate change (increasing water temperatures) and ocean acidification (decreasing ocean pH and carbonate saturation). It is thus fundamental to constrain the relationships between key abiotic stressors and the physiological processes that govern coralline algal growth and survival. Here we characterize the complex relationships between the abiotic environment of rock pool habitats and the physiology of the geniculate red coralline alga, Corallina officinalis (Corallinales, Rhodophyta). Paired assessment of irradiance, water temperature and carbonate chemistry, with C. officinalis net production (NP), respiration (R) and net calcification (NG) was performed in a south-western UK field site, at multiple temporal scales (seasonal, diurnal and tidal). Strong seasonality was observed in NP and nighttime R, with a P max of 22.35 µmol DIC (g DW) −1 h −1 , E k of 300 µmol photons m −2 s −1 and R of 3.29 µmol DIC (g DW) −1 h −1 determined across the complete annual cycle. NP showed a significant exponential relationship with irradiance (R 2 = 0.67), although was temperature dependent given ambient irradiance > E k for the majority of the annual cycle. Over tidal emersion periods, dynamics in NP highlighted the ability of C. officinalis to acquire inorganic carbon despite significant fluctuations in carbonate chemistry. Across all data, NG was highly predictable (R 2 = 0.80) by irradiance, water temperature and carbonate chemistry, providing a NG max of 3.94 µmol CaCO 3 (g DW) −1 h −1 and E k of 113 µmol photons m −2 s −1 . Light NG showed strong seasonality and significant coupling to NP (R 2 = 0.65) as opposed to rock pool water carbonate saturation. In contrast, the direction of dark NG (dissolution vs. precipitation) was strongly related to carbonate saturation, mimicking abiotic precipitation dynamics. Data demonstrated that C. officinalis is adapted to both long-term (seasonal) and short-term (tidal) variability in environmental stressors, although the balance between metabolic processes and the external environment may be significantly impacted by future climate change.

<p><strong>Abstract.</strong> Calcified macroalgae are critical components of marine ecosystems worldwide, but face considerable threat both from climate change (increasing water temperatures) and ocean acidification (decreasing ocean pH and carbonate saturation). It is thus fundamental to constrain the relationships between key abiotic stressors and the physiological processes that govern coralline algal growth and survival. Here we characterize the complex relationships between the abiotic environment of rock pool habitats, and the physiology of the geniculate red coralline alga, <i>Corallina officinalis</i> (Corallinales, Rhodophyta). Paired assessment of irradiance, water temperature and carbonate chemistry, with <i>C. officinalis</i> net production (<i>NP</i>), respiration (<i>R</i>) and net calcification (<i>NG</i>) was performed in a south-west UK field site, at multiple temporal scales (seasonal, diurnal and tidal). Strong seasonality was observed in <i>NP</i> and night-time <i>R</i>, with a <i>P<sub>max</sub></i> of 22.35 μmol DIC gDW<sup>−1</sup> h<sup>−1</sup>, <i>E<sub>k</sub></i> of 300 μmol photons m<sup>−2</sup> s<sup>−1</sup> and <i>R</i> of 3.29 μmol DIC gDW<sup>−1 </sup. h<sup>−1</sup> determined across the complete annual cycle. NP showed a significant exponential relationship with irradiance (R<sup>2</sup> = 0.67), although was temperature dependent given ambient irradiance > <i>E<sub>k</sub></i> for the majority of the annual cycle. Over tidal emersion periods, dynamics in <i>NP</i> highlighted the ability of <i>C. officinalis</i> to acquire inorganic carbon despite significant fluctuations in carbonate chemistry. Across all data, <i>NG</i> was highly predictable (R<sup>2</sup> = 0.80) by irradiance, water temperature and carbonate chemistry, providing a <i>NG<sub>max</sub></i> of 3.94  μmol CaCO<sub>3</sub> gDW<sup>−1</sup> h<sup>−1</sup>, and <i>E<sub>k</sub></i> of 113 μmol photons m<sup>−2</sup> s<sup>−1</sup>. Light-<i>NG</i> showed strong seasonality and significant coupling to NP (R<sup>2</sup> = 0.65), as opposed to rock pool water carbonate saturation. In contrast, the direction of dark-<i>NG</i> (dissolution vs. precipitation) was strongly related to carbonate saturation, mimicking abiotic precipitation dynamics. Data demonstrated that <i>C. officinalis</i> is adapted to both long-term (seasonal) and short-term (tidal) variability in environmental stressors, although the balance between metabolic processes and the external environment may be significantly impacted by future climate change.</p>

Supp. Figure 1: Average carbonate chemistry (TA, DIC, pH, pCO 2 , HCO 3-, CO 3 2-, Ω arg and Ω cal) recorded at the start (grey bars) and end (white bars) of daytime tidal emersion periods during December 2013 (Dec '13), and March (Mar '14), July (Jul '14) and September (Sep '14) 2014 (Average ± SE, n = 12). Numbers denote % change in parameters in relation to start emersion values. Supp Figure 2: Average carbonate chemistry (TA, DIC, pH, pCO 2 , HCO 3-, CO 3 2-, Ω arg and Ω cal) recorded at the start (grey bars) and end (white bars) of night-time tidal emersion periods during March (Mar '14), July (Jul '14) and September (Sep '14) 2014 (Average ± SE, n = 12). Numbers denote % change in parameters in relation to start emersion values.