Seawater carbonate chemistry and growth, reproduction of calcifying and non-calcifying epibionts of a brown macroalga in a laboratory experiment@en

Seaweeds are key species of the Baltic Sea benthic ecosystems. They are the substratum of numerous fouling epibionts like bryozoans and tubeworms. Several of these epibionts bear calcified structures and could be impacted by the high pCO2 events of the late summer upwellings in the Baltic nearshores. Those events are expected to increase in strength and duration with global change and ocean acidification. If calcifying epibionts are impacted by transient acidification as driven by upwelling events, their increasing prevalence could cause a shift of the fouling communities toward fleshy species. The aim of the present study was to test the sensitivity of selected seaweed macrofoulers to transient elevation of pCO2 in their natural microenvironment, i.e. the boundary layer covering the thallus surface of brown seaweeds. Fragments of the macroalga Fucus serratus bearing an epibiotic community composed of the calcifiers Spirorbis spirorbis (Annelida) and Electra pilosa (Bryozoa) and the non-calcifier Alcyonidium hirsutum (Bryozoa) were maintained for 30 days under three pCO2 conditions: natural 460±59 µatm, present-day upwelling1193±166 µatm and future upwelling 3150±446 µatm. Only the highest pCO2 caused a significant reduction of growth rates and settlement of S. spirorbis individuals. Additionally, S. spirorbis settled juveniles exhibited enhanced calcification of 40% during daylight hours compared to dark hours, possibly reflecting a day-night alternation of an acidification-modulating effect by algal photosynthesis as opposed to an acidification-enhancing effect of algal respiration. E. pilosa colonies showed significantly increased growth rates at intermediate pCO2 (1193 µatm) but no response to higher pCO2. No effect of acidification on A. hirsutum colonies growth rates was observed. The results suggest a remarkable resistance of the algal macro-epibionts to levels of acidification occurring at present day upwellings in the Baltic. Only extreme future upwelling conditions impacted the tubeworm S. spirorbis, but not the bryozoans.海藻是波罗的海底栖生态系统的关键物种，它们是众多污着附生生物（如苔藓虫和管蠕虫）的附着基质。其中部分附生生物具有钙化结构，可能受到波罗的海近岸夏季晚期上升流引发的高二氧化碳分压（pCO2）事件的影响。随着全球变化与海洋酸化进程，这类事件的强度与持续时间预计将有所增加。若钙化附生生物受到上升流驱动的短期酸化影响，其种群占比的上升可能会导致污着群落向肉质（非钙化）物种转变。本研究旨在探究筛选出的大型海藻附生生物在其自然微环境——即褐藻藻体表面的边界层——中对二氧化碳分压（pCO2）短期升高的敏感性。将携带有由钙化生物螺旋虫（Spirorbis spirorbis，环节动物门）、膜孔苔虫（Electra pilosa，苔藓动物门）以及非钙化生物粗被软苔虫（Alcyonidium hirsutum，苔藓动物门）组成的附生群落的墨角藻（Fucus serratus）片段，置于三种二氧化碳分压条件下培养30天：自然状态组（460±59 µatm）、当前上升流组（1193±166 µatm）以及未来上升流组（3150±446 µatm）。仅最高浓度的二氧化碳分压会显著降低螺旋虫（S. spirorbis）个体的生长速率与附着能力。此外，已附着的螺旋虫幼体在日间的钙化作用较夜间提升了40%，这或许反映了藻类光合作用的酸化调控效应与藻类呼吸作用的酸化增强效应呈现出昼夜交替的变化规律。膜孔苔虫（E. pilosa）群落的生长速率在中等二氧化碳分压（1193 µatm）条件下显著提升，但对更高浓度的二氧化碳分压无响应。未观测到酸化对粗被软苔虫（A. hirsutum）群落生长速率产生的影响。研究结果表明，大型海藻附生生物对当前波罗的海上升流区域出现的酸化水平具有显著的耐受能力。仅极端未来上升流条件会对管蠕虫螺旋虫（S. spirorbis）产生影响，而不会波及苔藓虫类。