Seawater carbonate chemistry and coverage and dry weight of Ecklonia radiata during experiments, 2010

Predictions about the ecological consequences of oceanic uptake of CO2 have been preoccupied with the effects of ocean acidification on calcifying organisms, particularly those critical to the formation of habitats (e.g. coral reefs) or their maintenance (e.g. grazing echinoderms). This focus overlooks the direct effects of CO2 on non-calcareous taxa, particularly those that play critical roles in ecosystem shifts. We used two experiments to investigate whether increased CO2 could exacerbate kelp loss by facilitating non-calcareous algae that, we hypothesized, (i) inhibit the recovery of kelp forests on an urbanized coast, and (ii) form more extensive covers and greater biomass under moderate future CO2 and associated temperature increases. Our experimental removal of turfs from a phase-shifted system (i.e. kelp- to turf-dominated) revealed that the number of kelp recruits increased, thereby indicating that turfs can inhibit kelp recruitment. Future CO2 and temperature interacted synergistically to have a positive effect on the abundance of algal turfs, whereby they had twice the biomass and occupied over four times more available space than under current conditions. We suggest that the current preoccupation with the negative effects of ocean acidification on marine calcifiers overlooks potentially profound effects of increasing CO2 and temperature on non-calcifying organisms.

有关海洋吸收二氧化碳（oceanic uptake of CO₂）所引发生态后果的预测，长期以来均聚焦于海洋酸化（ocean acidification）对钙化生物（calcifying organisms）的影响，尤其是那些对栖息地构建（如珊瑚礁）或维持（如植食性棘皮动物）至关重要的类群。这一研究视角却忽略了二氧化碳对非钙化类群（non-calcareous taxa）的直接作用，尤其是那些在生态系统相位转换中发挥关键作用的类群。本研究通过两组实验，探究了二氧化碳浓度升高是否会通过促进非钙化藻类加剧海带林（kelp forests）退化——我们提出如下假说：其一，该类藻类会抑制城市化海岸带海带林的恢复；其二，在未来中等程度的二氧化碳升高及伴随的温度上升条件下，这类藻类能够形成更广泛的覆盖度与更高的生物量。我们对发生生态系统相位转换（phase-shifted system）的系统（即由海带林主导转变为藻席（algal turfs）主导的系统）中的藻席进行移除实验，结果显示海带幼体的数量显著增加，这表明藻席确实能够抑制海带的定植与补充。未来二氧化碳浓度升高与温度上升会产生协同交互作用，显著提升藻席的丰度：相较于当前环境条件，藻席的生物量可达两倍，占据的有效空间则超过四倍。本研究表明，当前学界对海洋酸化对海洋钙化生物负面影响的过度关注，忽略了二氧化碳浓度与温度上升对非钙化生物可能造成的深远影响。