Short-term ecophysiological and biochemical responses of Cystoseira tamariscifolia and Ellisolandia elongata to changes in solar irradiance and nutrient levels were analyzed in situ in oligotrophic coastal waters by transferring macroalgae collected at 0.5 and 2.0 m depth and exposing them to 2 irradiance levels (100 and 70% of surface irradiance) and nutrient conditions (nutrient-enriched and non-enriched). Both species were affected by changes in irradiance and nutrient levels. Few interactive effects between these 2 physical stressors were found, suggesting major additive effects on both species. C. tamariscifolia collected at 0.5 m and exposed to 70% irradiance had the highest maximal electron transport rate (ETR max ), saturated irradiance (Ek ETR ) and chl a content and the lowest antioxidant activity. Under the same conditions, E. elongata had increased Ek ETR , antheraxanthin and β-carotene content. At 100% irradiance, C. tamariscifolia collected at 2.0 m had higher maximal quantum yield (F v /F m ), photosynthetic efficiency (α ETR ), ETR max , maximal non-photochemical quenching (NPQ max ), saturation irradiance for NPQ (Ek NPQ ), and antheraxanthin and polyphenol content increased, whereas in E. elongata only α ETR increased. In nutrient-enriched conditions, phenolic compounds, several carotenoids and N content increased in C. tamariscifolia at both depths. E. elongata from 2.0 m depth at 100% irradiance and nutrient-enriched conditions showed increased N content and total mycosporine-like amino acids (MAAs). Our results show rapid photophysiological responses of C. tamariscifolia to variations in in situ irradiance and nutrient conditions, suggesting efficient photoacclimation to environmental changes. In E. elongata, F v /F m and ETR max did not change in the transplant experiment; in contrast, N content, pigment and MAAs (biochemical variables) changed. The responses of these macroalgae to nutrient enrichment indicate oligotrophic conditions at the study site and environmental stress.

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Previous studies of the impact of increased CO 2 on macroalgae have mainly been done in laboratories or mesocosm systems, placing organisms under both artificial light and seawater conditions. In this study, macroalgae were incubated in situ in UV-transparent cylinders under conditions similar to the external environment. This system was tested in a short-term study (5.5 h incubation) on the effect of 2 partial pressures of CO 2 (pCO 2 ): air (ambient CO 2 ) and the pCO 2 predicted by the end of the 21st century (700 µatm, high CO 2 ), on photosynthesis, photosynthetic pigments and photoprotection in calcifying (Ellisolandia elongata and Padina pavonica) and non-calcifying (Cystoseira tamariscifolia) macroalgae. The calcifying P. pavonica showed higher net photosynthesis under high CO 2 than under ambient CO 2 conditions, whereas the opposite occurred in C. tamariscifolia. Both brown algae (P. pavonica and C. tamariscifolia) showed activation of non-photochemical quenching mechanisms under high CO 2 conditions. However, in P. pavonica the phenol content was reduced after CO 2 enrichment. In contrast to phenols, in E. elongata other photoprotectors such as zeaxanthin and palythine (mycosporine-like amino acid) tended to increase in the high CO 2 treatment. The different responses of these species to elevated pCO 2 may be due to anatomical and physiological differences and could represent a shift in their relative dominance as key species in the face of ocean acidification (OA). More in situ studies could be carried out to evaluate how macroalgae will respond to increases in pCO 2 in a future OA scenario. The in situ incubator system proposed in this work may contribute towards increasing this knowledge.

Summary:The canopy-forming macroalga Gelidium corneum (Hudson) J.V. Lamouroux plays a major role in the functioning of the subtidal ecosystem of the Cantabrian Sea (northern Spain). Despite its importance, little is known about the factors that may potentially affect the distribution pattern of its epiphytic flora. Here we examine two indirect factors: coastal orientation (N and NW) and depth (3 and 7 m), as proxies for wave exposure and light availability, respectively. We test their effects on the total epiphytic load, alpha diversity (species richness, Shannon, Simpson and evenness measures) and multivariate structure of the epiphytic flora growing on G. corneum in subtidal waters off the Basque coast. Plocamium cartilagineum, Dictyota dichotoma and Acrosorium ciliolatum were found to be the most common epiphytes. Significant interactive effect of coastal orientation and depth were observed for species composition and abundance of epiphytic flora. Increased wave exposure resulted in a lower epiphyte load and a less diverse community, suggesting that under high hydrodynamic conditions epiphytes were more likely to become dislodged from hosts. However, light availability only had a significant effect on the distribution of epiphytes below a certain threshold of wave action, with the epiphytic load being 30-40% greater on shallow bottoms.Keywords: epibionts; host frond; light availability; spatial variability; macroalgae; wave action. Flora epífita deGelidium corneum (Rhodophyta: Gelidiales) en relación a la exposición al oleaje y la profundidad Resumen: El alga formadora de copa Gelidium corneum (Hudson) J.V. Lamouroux tiene un papel fundamental en el funcionamiento de los ecosistemas submareales del mar Cantábrico (norte de España). A pesar de su importancia, se sabe poco sobre los factores que afectan a la distribución de su flora epífita. En este estudio, examinamos dos factores indirectos: la orientación de la costa (N y NO) y la profundidad (3 y 7 m), como factores representativos de la exposición al oleaje y la disponibilidad de luz, respectivamente. Este estudio testa sus efectos sobre la carga total de epífitos, la alfa-diversidad (riqueza de especies, Shannon, Simpson y equitatividad) y la estructura multivariable de la flora epífita que crece sobre G. corneum en las aguas submareales de la costa vasca. Los epífitos más comunes fueron Plocamium cartilagineum, Dictyota dichotoma y Acrosorium ciliolatum. Se observó un efecto interactivo de la orientación de la costa y la profundidad para la composición de especies y la abundancia de la flora epífita. En las localidades con mayor exposición al oleaje la carga epífita era menor y la comunidad menos diversa, sugiriendo que bajo condiciones de un elevado hidrodinamismo los epífitos eran más susceptibles de ser desprendidos de su hospedador. Sin embargo, la disponibilidad de luz solo tuvo un efecto significativo en la distribución de los epífitos por debajo de ciertos umbrales de la acción del oleaje, siendo la carga de epifitos un 30-40% mayor en los fondos ...

Recent declines in the foundation species Gelidium corneum (Hudson) J.V. Lamouroux in pristine coastal stretches of the Southeastern Bay of Biscay have been documented in which individuals showed a high bleaching incidence. Among the potential factorsthat may be responsible for these changes in G. corneum abundance, it has been suggested that high levels of irradiance may be imposing stressful environmental conditions for shallow populations. Given that light is reduced exponentially as depth increases, in this investigation we examined the influence of depth on the stress of G. corneum by comparing the performance of several biochemical parameters and thallus size in plants collected at three different depths. The results revealed that plants growing in shallow waters showed lower antioxidant activity, lower concentrations of photosynthetic pigments (Chl‐a, total carotenoids, phycoerythrin), lower internal nitrogen levels (higher C:N ratio) and shorter thallus lengths than those living in deeper waters. Some of these results may be an expression of photoacclimation, but the low antioxidant activity and total carotenoid concentration detected in plants living in shallow waters suggest that the photoprotective mechanisms of G. corneum may have failed to offset photodamage. If so, our findings support previous studies that suggest that recent changes in solar radiation are partially responsible for G. corneum declines. Nevertheless, given that the metabolic performance of macroalgae is determined by the inter‐play of irradiance with other factors, including temperature and nutrient availability, further research is needed to reach a better understanding of the factors causing stress in G. corneum.