Data_Sheet_1_Effects of Higher CO2 and Temperature on Exopolymer Particle Content and Physical Properties of Marine Aggregates.pdf

We investigated how future ocean conditions, and specifically the interaction between temperature and CO2, might affect marine aggregate formation and physical properties. Initially, mesocosms filled with coastal seawater were subjected to three different treatments of CO2 concentration and temperature: (1) 750 ppm CO2, 16°C, (2) 750 ppm CO2, 20°C, and (3) 390 ppm CO2, 16°C. Diatom-dominated phytoplankton blooms were induced in the mesocosms by addition of nutrients. In aggregates produced in roller tanks using seawater taken from the mesocosms during different stages of the bloom, we measured sinking velocity, size, chlorophyll a, particulate organic carbon and nitrogen, and exopolymer particle content; excess density and mass were calculated from the sinking velocity and size of the aggregates. As has been seen in previous experiments, no discernable differences in overall nutrient uptake, chlorophyll-a concentration, or exopolymer particle concentrations could be related to the acidification treatment in the mesocosms. In addition, in the aggregates formed during the roller tank experiments (RTEs), we observed no statistically significant differences in chemical composition among the treatments during Pre-Bloom, Bloom, and Post-Bloom periods. However, physical characteristics were different and showed a synergistic effect of warmer temperature and higher CO2 during the Pre-Bloom period; at this time, temperature had a larger effect than CO2 on aggregate sinking velocity. In RTEs with warmer and acidified treatment (future conditions), aggregates were larger, heavier, and settled faster than aggregates formed at present-day or only acidified conditions. During the Post-Bloom, however, aggregates formed under present and future conditions had similar physical properties. In acidified tanks at ambient temperature, aggregates were slower, smaller and less dense than those formed at the same temperature but under present CO2 or under warmer and acidified conditions. Thus, the sinking velocity of aggregates formed in acidified tanks at ambient temperature was slower than the other two cases. Our findings point out the potential of ocean acidification and warming to modify physical properties of sinking aggregates but also emphasize the need of future experiments investigating multiple environmental stressors to clarify the importance of each factor.

本研究探讨了未来海洋环境条件，尤其是温度与二氧化碳（CO₂）的交互作用，对海洋聚集体形成过程及其物理特性的影响。实验初期，我们将装有近岸海水的中型生态箱（mesocosm）设置了三组不同的CO₂浓度与温度处理：(1) 750 ppm CO₂、16°C；(2) 750 ppm CO₂、20°C；(3) 390 ppm CO₂、16°C。通过添加营养盐，诱导中型生态箱内形成硅藻占优的浮游植物水华。在滚罐（roller tank）实验中，我们使用浮游植物水华不同阶段取自中型生态箱的海水制备聚集体，并测定了其沉降速率、粒径、叶绿素a（chlorophyll a）、颗粒有机碳与氮含量，以及胞外聚合物颗粒（exopolymer particle）含量；同时依据聚集体的沉降速率与粒径计算其过剩密度与质量。与既往实验结果一致，中型生态箱中未观测到营养盐整体吸收、叶绿素a浓度或胞外聚合物颗粒浓度与酸化处理存在显著关联。此外，滚罐实验（roller tank experiments, RTEs）制备的聚集体在水华前期、水华期与水华后期，各处理组的化学组成均未表现出统计学显著差异。但物理特性存在显著差异：在水华前期，升温与高CO₂浓度表现出协同效应，且此时温度对聚集体沉降速率的影响大于CO₂。在模拟未来环境条件的升温酸化处理组滚罐实验中，聚集体的粒径更大、质量更重，沉降速率也快于当前环境条件组或仅酸化处理组。但在水华后期，当前环境与未来环境条件下形成的聚集体物理特性趋于一致。在常温酸化处理组的滚罐中，聚集体的沉降速率更慢、粒径更小且密度更低，相较于同温度下的当前CO₂条件组，以及升温酸化条件组均是如此。换言之，常温酸化处理组聚集体的沉降速率慢于其余两组。本研究结果表明，海洋酸化与升温有可能改变沉降型海洋聚集体的物理特性，同时也强调未来需开展多环境胁迫因子实验，以厘清各单一因子的具体影响权重。