Seawater carbonate chemistry and copepod reproduction

The combined upwelling-El Niño (EN) event regulation of the numerically dominant Acartia tonsa (Crustacea, Copepoda) reproduction was examined in a year-round upwelling system (23°S) of the Humboldt Eastern Boundary Upwelling System (EBUS) during the EN 2015. A previous analysis of the environmental regulation of this system is extended here by considering complementary oceanographic information (sea level, stratification indexes) and additional reproductive traits, such as maximum (MaxEPR), median (MedianEPR) and prevalence of egg producing females over a period of six months. Furthermore, field minimum-maximum pH levels were reproduced in three 96-h incubation experiments conducted under variable salinity conditions to evaluate copepod mean EPR, egg size and hatching success. Supporting previous assertions, the warm-high salinity EN 2015 was observed in the study site separately from hydrographic conditions associated with upwelling to non-upwelling regimes. Analysis of similarity-distance (Distance based Linear Model (DistLM)) and normalized data (separate-slope comparison under a General Linear Model (GLM)) showed that reproductive traits were regulated by specific combinations of ambient conditions, and that this regulation was also sensitive to the prevailing hydrographic regime. Thus, upwelling to non-upwelling transitions changing the pH, and EN-associated salinity and stratification shifts, were significantly and strongly linked to almost all reproductive traits (DistLM). Slope comparison (GLM) indicated MaxEPR and MedianEPR variations also underlie the phenology, highlighting the relationship between pH and salinity with biological variations. In conjunction with experimental observations, the current study consistently suggests that pH-variations in the upwelling realm, and EN hydrographic perturbations might underpin responses of plankton populations to climate change in productive EBUS.

本研究于2015年厄尔尼诺（El Niño, EN）事件期间，在洪堡东边界上升流系统（Humboldt Eastern Boundary Upwelling System, EBUS）内一处全年运作的上升流系统（南纬23°）中，探究了上升流与EN联合事件对优势种汤氏纺锤水蚤（Acartia tonsa，甲壳纲，桡足亚纲）繁殖的调控作用。本研究拓展了此前针对该系统环境调控的分析，纳入了补充性海洋学观测数据（海平面、层化指数），并新增了为期6个月的多项繁殖性状指标，包括最大产卵率（MaxEPR）、中位产卵率（MedianEPR）以及怀卵雌性个体的占比。此外，本研究在盐度可变的条件下开展了3组96小时培养实验，复现了野外观测到的pH极值区间，以评估桡足类的平均产卵率、卵径与孵化成功率。研究观测到2015年暖高盐EN事件与研究区内上升流-非上升流相关的水文条件相互独立，这一结果支持了此前的相关论断。通过相似性距离分析（基于距离的线性模型，Distance based Linear Model, DistLM）与标准化数据处理（广义线性模型，General Linear Model, GLM下的分斜率比较），结果显示繁殖性状受环境条件的特定组合调控，且该调控效应受研究区主导水文状况的影响。因此，改变pH的上升流-非上升流转换过程，以及与EN相关的盐度与层化变化，与几乎所有繁殖性状均存在显著且强烈的关联（DistLM分析结果）。斜率比较分析（GLM）结果显示，最大产卵率与中位产卵率的变化同样受物候调控，进一步凸显了pH、盐度与生物学变异之间的关联。结合实验观测结果，本研究一致表明：上升流区域的pH波动与EN相关的水文扰动，可能是驱动高生产力EBUS中浮游生物种群响应气候变化的关键因素。