Seagrass meadows enhance benthic microalgal growth in a coral reef ecosystem under global warming scenario- a mesocosm study

The effects of presence of seagrass bed on microalgal assemblages under increasing CO2 concentrations and temperature were examined using mesocosms that simulate coral reef ecosystem. We assessed changes in chlorophyll a and diversity of benthic algal communities grown with/without seagrass bed, at 25 oC and 28 oC, under high CO2 conditions (800~1000 µatm). Total alkalinity, pCO2, and the aragonite saturation state, were all similar between seagrass and control groups, in both temperature treatments. Phytoplankton chl a concentration were similar between seagrass and control groups, however the benthic microalgal chl a concentrations were significantly higher in seagrass group, at both 25 oC and 28 oC. Chlorophyte decreased whereas diatoms increased under 28 oC without seagrass bed, and algal parasites Syndiniales increased at both groups under 28 oC. Our results suggest that seagrass bed will enhance benthic microalgal primary productivity, and the ecosystem favor chlorophyte over diatom growth.

本研究采用模拟珊瑚礁生态系统的中型实验生态系（mesocosms），探究了二氧化碳浓度与温度均升高的背景下，海草床的存在对微藻类群组成的影响。我们设置了25℃、28℃两个温度梯度，以及800~1000 µatm的高二氧化碳条件，评估了有、无海草床培养的底栖藻类群落的叶绿素a含量与多样性变化。两种温度处理下，海草组与对照组的总碱度、二氧化碳分压（pCO2）以及文石饱和状态均无显著差异。浮游植物叶绿素a浓度在海草组与对照组间无显著差异，但在25℃与28℃条件下，海草组的底栖微藻叶绿素a浓度均显著高于对照组。在无海草床且温度为28℃的处理中，绿藻门（Chlorophyte）丰度下降，而硅藻门（Diatoms）丰度上升；且在28℃条件下，两组的寄生性藻类旋沟藻目（Syndiniales）丰度均有所增加。本研究结果表明，海草床可提升底栖微藻的初级生产力，且该生态系统更有利于绿藻门生长而非硅藻门。