Effect of seawater temperature, pH, and nutrients on the distribution and character of low abundance shallow water benthic foraminifera in the Galápagos

In order to help predict the effects of anthropogenic stressors on shallow water carbonate environments, it is important to focus research on regions containing natural oceanographic gradients, particularly with respect to interactions between oceanography and ecologically sensitive carbonate producers. The Galápagos Archipelago, an island chain in the eastern equatorial Pacific, spans a natural nutrient, pH, and temperature gradient due to the interaction of several major ocean currents. Further, the region is heavily impacted by the El Niño—Southern Oscillation (ENSO) and the Galápagos exhibited widespread coral bleaching and degradation following the strong ENSO events of 1982–1983 and 1997–1998. These findings are coupled with reports of unusually low abundances of time-averaged benthic foraminiferal assemblages throughout the region. Foraminifera, shelled single-celled protists, are sensitive to environmental change and rapidly respond to alterations to their surrounding environment, making them ideal indicator species for the study of reef water quality and health. Here, statistical models and analyses were used to compare modern shallow water benthic foraminiferal assemblages from 19 samples spanning the Galápagos Archipelago to predominant oceanographic parameters at each collection site. Fisher α diversity indices, Ternary diagrams, Canonical Correspondence Analysis, regression tree analysis and FORAM-Index (FI; a single metric index for evaluating water quality associated with reef development) implied a combined impact from ENSO and upwelling from Equatorial Undercurrent (EUC) waters to primarily impact foraminiferal abundances and drive assemblage patterns throughout the archipelago. For instance, repeated ENSO temperature anomalies might be responsible for low foraminiferal density, while chronically high nutrients and low aragonite saturation and low pH—induced by EUC upwelling and La Niña anomalies—likely inhibited post-ENSO recovery, and caused foraminiferal assemblages to exhibit a heterotrophic dominance in the southern archipelago. What resulted are low FI values in the southern collection sites, indicating environments not conducive to endosymbiont development and/or recovery.

为助力预测人为胁迫对浅水碳酸盐环境的影响，将研究重心置于具备自然海洋学梯度的区域尤为关键，尤其是针对海洋学与生态敏感型碳酸盐生产者之间的相互作用。加拉帕戈斯群岛（Galápagos Archipelago）是东赤道太平洋的岛链，受多支主要洋流相互作用影响，形成了自然的营养盐、pH值与温度梯度。该区域还深受厄尔尼诺-南方涛动（El Niño—Southern Oscillation, ENSO）影响，1982–1983年与1997–1998年的强ENSO事件后，加拉帕戈斯群岛出现了大范围珊瑚白化与退化现象。此外，有报告称全区域内时间平均底栖有孔虫组合的丰度异常偏低。有孔虫（Foraminifera）是带壳单细胞原生生物，对环境变化敏感且能快速响应周边环境扰动，因此是评估礁体水质与健康状况的理想指示物种。本研究借助统计模型与分析手段，对比了横跨加拉帕戈斯群岛的19个采样点的现代浅水底栖有孔虫组合与各采样点的主要海洋学参数。研究采用了费希尔α多样性指数（Fisher α diversity indices）、三元图（Ternary diagrams）、典范对应分析（Canonical Correspondence Analysis）、回归树分析以及有孔虫指数（FORAM-Index, FI；一种用于评估与礁体发育相关水质的单一量化指标），结果表明，ENSO与赤道潜流（Equatorial Undercurrent, EUC）上升流的联合影响，是调控群岛内有孔虫丰度、驱动组合分布格局的主要因素。例如，反复出现的ENSO温度异常可能是导致有孔虫密度偏低的原因，而由EUC上升流与拉尼娜（La Niña）异常引发的长期高营养盐、低文石饱和度与低pH值，很可能抑制了ENSO事件后的种群恢复，并导致南部群岛的有孔虫组合呈现异养型主导的特征。最终，南部采样点的FI值偏低，表明该区域环境不利于共生体的发育与/或恢复。