DataSheet_8_Interactive effects of salinity, temperature and food web configuration on performance and harmfulness of the raphidophyte Heterosigma akashiwo.pdf

The cosmopolitan raphidophyte Heterosigma akashiwo commonly forms harmful algal blooms (HABs) in diverse estuaries discharging into Algoa Bay, South Africa, potentially leading to hypoxia, fish kills and a decline in key primary consumers. Despite the high environmental variability in these estuaries, little is known about how abiotic factors such as temperature and salinity constrain bloom formation and harmfulness of H. akashiwo. The present study therefore investigates growth, competition, and grazing interactions of H. akashiwo in laboratory experiments in response to two naturally relevant levels of salinity (15, 30) and temperature (16, 22°C), respectively. Experiments were set up with the naturally co-occurring dinoflagellate competitor Heterocapsa rotundata and two estuarine microzooplankton consumers, i.e., nauplii of the copepod Acartia tonsa and the rotifer Brachionus plicatilis. In monoculture, H. akashiwo growth was promoted at high temperature – low salinity conditions, while H. rotundata thrived under low temperature – high salinity conditions. In polyculture, H. akashiwo dominated at high temperature irrespective of the salinity regime, while at low temperature, it only dominated at low salinity and was suppressed by H. rotundata at high salinity. Grazing assays revealed highly negative effects of H. akashiwo on copepod nauplii survival and growth as well as mucus-induced immobilization, especially at high temperatures in combination with low salinity, while the estuarine adapted rotifers showed highest mortalities at the higher salinity level. The presence of H. rotundata significantly alleviated the harmful effects of H. akashiwo on both grazers, and the selectively feeding copepod nauplii actively avoided H. akashiwo when non-harmful prey was present. Overall, this study demonstrates that population dynamics and harmful effects of H. akashiwo are interactively determined by both abiotic conditions and food web configuration, implying competitor and consumer specific tolerances to the abiotic environment and their susceptibility to the harmful alga H. akashiwo.

广布性针胞藻（raphidophyte）赤潮异弯藻（Heterosigma akashiwo）通常会在汇入南非阿尔戈阿湾的各类河口区域形成有害藻华（harmful algal blooms, HABs），可能引发缺氧、鱼群死亡以及关键初级消费者种群数量下降。尽管这些河口的环境非生物因子变异程度极高，但学界对温度、盐度等非生物因子如何制约赤潮异弯藻的藻华形成与危害能力仍知之甚少。为此，本研究通过实验室实验，探究了赤潮异弯藻在两种自然环境相关水平的盐度（15、30）与温度（16、22℃）下的生长、竞争及捕食相互作用。实验设置中纳入了自然共存的双鞭甲藻（dinoflagellate）竞争者圆梨甲藻（Heterocapsa rotundata），以及两种河口微浮游动物消费者：即尖额纺锤水蚤（Acartia tonsa）的无节幼体与褶皱臂尾轮虫（Brachionus plicatilis）。在单培养体系中，赤潮异弯藻的生长在高温-低盐度条件下得到促进，而圆梨甲藻则在低温-高盐度环境下生长旺盛。在混培养体系中，无论盐度条件如何，赤潮异弯藻在高温环境下均占据竞争优势；而在低温环境下，其仅在低盐度条件下占据优势，在高盐度环境下则会被圆梨甲藻抑制。捕食实验结果显示，赤潮异弯藻对桡足类无节幼体的存活与生长具有极强的负面影响，还会引发黏液介导的行动抑制，这种效应在高温结合低盐度的条件下尤为显著；而适应河口环境的轮虫则在较高盐度水平下出现最高死亡率。圆梨甲藻的存在可显著缓解赤潮异弯藻对两类捕食者的有害影响，且具有选择性摄食能力的桡足类无节幼体在存在非有害猎物时，会主动避开赤潮异弯藻。综上，本研究表明赤潮异弯藻的种群动态与危害效应由非生物条件与食物网结构共同交互决定，这意味着竞争者与消费者对非生物环境具有特异性耐受能力，且对有害藻类赤潮异弯藻的易感性存在差异。