Luncheon Seminar #235: Contrasting Nitrogen and Nitrifier Dynamics in the Euphotic Zone across the Mid-Atlantic Bight Shelfbreak Front

Abstract

The hypothesis that frontal systems enhance the abundance of nitrifying microbes and nitrogen recycling was investigated in July 2019 along repeat transects across the Mid-Atlantic Bight shelfbreak front. A suite of parameters was measured, including concentrations of ammonium, particulate and dissolved organic nitrogen, ammonia monooxygenase subunit A (amoA) genes of shallow water ammonia−oxidizing archaea (AOA) and bacteria (Nitrosospira; AOB), and 16S rRNA genes of nitrite−oxidizing bacteria (Nitrospina; NOB). Ammonium concentrations measured by a shipboard high-resolution (nanomolar-level detection) assay revealed a typical cross-shelf gradient. High ammonium concentrations accumulated in subsurface waters on the shelf (up to >2 µmol L⁻¹), which may have resulted from a temporary imbalance in regeneration from sinking particles and subsequent consumption via oxidation/assimilation. In contrast, peak ammonium concentrations in both the shelfbreak frontal zone and slope sea waters were one to two orders of magnitude lower (∼100−200 nmol L⁻¹). This can be attributed to tight coupling between ammonium regeneration and utilization in these waters. The shelfbreak front acted as an ecological boundary where AOA and AOB amoA and NOB 16S rRNA gene numbers were partitioned, with the former most abundant in slope water and the latter two highest in shelf waters, likely due to relative substrate availability. Surprisingly, at some slope stations, the supply of deep-water nutrients via isopycnal lifting induced by Gulf Stream intrusion caused unexpectedly high phytoplankton biomass, which may have stimulated both ammonium regeneration and nitrification. This study suggests that marginal sea-western boundary current interactions have a significant but under-appreciated influence on nitrogen biogeochemistry.

报告人简介

祝依凡，2013年本科毕业于中国海洋大学化学专业，2015年硕士毕业于5123导航海洋化学专业，2016─2017 担任 MEL 科研助理，2023年于美国欧道明大学 (Old Dominion University) 获海洋学博士学位。博士课题基于时间序列观测、DNA 分子、稳定同位素、卫星遥感、通量计算等手段研究美国东海岸河口有害藻华，近岸大陆架以及大洋氮循环。目前发表SCI论文十余篇，多篇第一作者/通讯作者文章发表于Marine Chemistry, Estuarine, Coastal and Shelf Sciences, Limnology & Oceanography: Methods等期刊。