Seawater carbonate chemistry and calcium carbonate of Padina spp., photosynthesis of Padina pavonica in nature CO2 gradients experiment

Predicting the impacts of ocean acidification on coastal ecosystems requires an understanding of the effects on macroalgae and their grazers, as these underpin the ecology of rocky shores. Whilst calcified coralline algae (Rhodophyta) appear to be especially vulnerable to ocean acidification, there is a lack of information concerning calcified brown algae (Phaeophyta), which are not obligate calcifiers but are still important producers of calcium carbonate and organic matter in shallow coastal waters. Here, we compare ecological shifts in subtidal rocky shore systems along CO2 gradients created by volcanic seeps in the Mediterranean and Papua New Guinea, focussing on abundant macroalgae and grazing sea urchins. In both the temperate and tropical systems the abundances of grazing sea urchins declined dramatically along CO2 gradients. Temperate and tropical species of the calcifying macroalgal genus Padina (Dictyoaceae, Phaeophyta) showed reductions in CaCO3 content with CO2 enrichment. In contrast to other studies of calcified macroalgae, however, we observed an increase in the abundance of Padina spp. in acidified conditions. Reduced sea urchin grazing pressure and significant increases in photosynthetic rates may explain the unexpected success of decalcified Padina spp. at elevated levels of CO2. This is the first study to provide a comparison of ecological changes along CO2 gradients between temperate and tropical rocky shores. The similarities we found in the responses of Padina spp. and sea urchin abundance at several vent systems increases confidence in predictions of the ecological impacts of ocean acidification over a large geographical range.

预测海洋酸化对沿海生态系统的影响，需先明晰其对大型藻类（macroalgae）及其牧食者的作用——二者是岩岸生态系统运转的核心支撑。尽管钙化珊瑚藻（calcified coralline algae，红藻门Rhodophyta）对海洋酸化尤为易感，但学界对钙化褐藻（calcified brown algae，褐藻门Phaeophyta）的相关研究仍存在信息缺口。此类藻类并非专性钙化生物，却是浅海沿岸水域碳酸钙与有机质的重要生产者。本研究对比了地中海与巴布亚新几内亚两处海底火山喷口（volcanic seeps）形成的二氧化碳梯度下，潮下带（subtidal）岩岸系统的生态变化，重点关注优势大型藻类与牧食性海胆（sea urchin）。在温带与热带系统中，沿二氧化碳梯度，牧食性海胆的丰度均出现显著下降。钙化大型藻类团扇藻属（Padina，网地藻科Dictyoaceae，褐藻门Phaeophyta）的温带与热带物种，其碳酸钙（CaCO₃）含量随二氧化碳浓度升高而降低。但与其他钙化大型藻类的研究结果不同，本研究发现酸化环境下团扇藻属（Padina spp.）的丰度反而有所提升。海胆牧食压力的降低与光合速率（photosynthetic rates）的显著提升，或可解释脱钙化的团扇藻属物种在高二氧化碳环境下的意外繁盛。本研究首次实现了温带与热带岩岸沿二氧化碳梯度的生态变化对比。我们在多个喷口系统中观察到的团扇藻属丰度与海胆种群响应的相似性，提升了全球大地理范围内海洋酸化生态影响预测的可信度。