Changes in Microbial Plankton Assemblages Induced by Mesoscale Oceanographic Features in the Northern Gulf of Mexico

Mesoscale circulation generated by the Loop Current in the Northern Gulf of Mexico (NGOM) delivers growth-limiting nutrients to the microbial plankton of the euphotic zone. Consequences of physicochemically driven community shifts on higher order consumers and subsequent impacts on the biological carbon pump remain poorly understood. This study evaluates microbial plankton <10 μm abundance and community structure across both cyclonic and anti-cyclonic circulation features in the NGOM using flow cytometry (SYBR Green I and autofluorescence parameters). Non-parametric multivariate hierarchical cluster analyses indicated that significant spatial variability in community structure exists such that stations that clustered together were defined as having a specific ‘microbial signature’ (i.e. statistically homogeneous community structure profiles based on relative abundance of microbial groups). Salinity and a combination of sea surface height anomaly and sea surface temperature were determined by distance based linear modeling to be abiotic predictor variables significantly correlated to changes in microbial signatures. Correlations between increased microbial abundance and availability of nitrogen suggest nitrogen-limitation of microbial plankton in this open ocean area. Regions of combined coastal water entrainment and mesoscale convergence corresponded to increased heterotrophic prokaryote abundance relative to autotrophic plankton. The results provide an initial assessment of how mesoscale circulation potentially influences microbial plankton abundance and community structure in the NGOM.

墨西哥湾北部（Northern Gulf of Mexico, NGOM）的环流水（Loop Current）所催生的中尺度环流，可为透光层（euphotic zone）内的微型浮游微生物供给其生长所需的限制性营养盐。目前，物理化学驱动的群落结构演替对高阶消费者造成的影响，及其后续对生物碳泵（biological carbon pump）的作用机制仍不甚明晰。本研究采用流式细胞术（flow cytometry，采用SYBR Green I与自发荧光参数），针对墨西哥湾北部气旋式与反气旋式环流特征区域内，粒径<10 μm的微型浮游生物的丰度与群落结构展开评估。非参数多元层级聚类分析（Non-parametric multivariate hierarchical cluster analyses）结果显示，群落结构存在显著空间异质性：聚类为组的站位具有特定的"微生物特征谱"，即基于微生物类群相对丰度定义的统计均质群落结构特征。基于距离的线性模型（distance based linear modeling）分析表明，盐度，以及海面高度异常与海面温度的组合，是与微生物特征谱变化显著相关的非生物预测变量。微生物丰度提升与氮素可利用性之间的相关性提示，该开阔海域内的微型浮游生物受氮素限制。沿岸水夹带与中尺度辐合共同作用的区域，异养原核生物（heterotrophic prokaryote）的丰度相较于自养浮游生物（autotrophic plankton）显著升高。本研究结果首次评估了墨西哥湾北部中尺度环流对微型浮游生物丰度与群落结构的潜在影响。