Journal Article

Contrasting Supply Dynamics of Dissolved Iron and Nitrate Shape the Biogeography of Nutrient‐Limiting Conditions in the North Pacific

Chai, Fei, Ma, Wentao, Liang, Bo, Wang, Yuntao, Wang, Tao, Zhao, Zihui, Xiu, Peng

2024

Overview

The North Pacific is known with iron limitation for phytoplankton growth in the subarctic region and nitrogen limitation in the subtropical gyre. Although the growth rate of phytoplankton is determined by the concentration of limiting nutrient, the supply ratio of iron to nitrogen is suggested to be essential to this biogeographic pattern. However, the underlying dynamics determining the ratio remain largely unknown. We investigated mechanisms of dissolved iron (dFe) and nitrate (NO3−) transport to the euphotic zone of the North Pacific using an eddy‐resolvable biogeochemical model. We show that lateral advection and atmospheric deposition are dominant drivers for dFe transport, resulting in high Fe:N supply ratio in both subarctic and subtropical regions. Conversely, significant vertical supplies of NO3− through upwelling and diffusion processes markedly reduce the supply ratio in the subarctic region. These dynamics combined lead to high Fe:N supply ratio in the gyre and low ratio in the subarctic, ultimately driving high nitrogen fixation condition in the gyre and the iron‐limited phytoplankton growth condition in the subarctic region. Plain Language Summary Iron is a critical trace element for the photosynthesis and nitrogen fixation of phytoplankton in the ocean. In North Pacific Subarctic region, although there is plenty of nitrate, the growth of phytoplankton is limited due to the lack of iron. In the North Pacific Subtropical Gyre (NPSG), nutrient supply to the surface is restricted due to ocean stratification, but diazotrophs can fix nitrogen from the atmosphere. However, their growth is also constrained by iron availability. Understanding how nutrients like iron reach the ocean's surface is vital for predicting the productivity of marine life. Our research employed advanced computer models to explore how dissolved iron is transported in the North Pacific. We discovered that lateral transport by ocean currents, followed by atmospheric deposition, is the primary pathway for iron delivery to the sunlit layer of the NPSG. In the Subarctic Gyre, atmospheric deposition and vertical advection are the major sources of iron. However, we found different transport patterns for nitrate, revealing that physical process‐controlled supply ratio of iron to nitrate may determine where different types of phytoplankton thrive in the surface ocean. This research helps understand the complex processes that supply nutrients to ocean surface. Key Points Lateral transport and atmospheric deposition dominate supplies of dissolved iron (dFe) to the euphotic zone of the North Pacific Upwelling and vertical diffusion control nitrate (NO3−) supply in subarctic region Lateral dFe and vertical NO3− transports determine the stoichiometric supply ratio and shape the biogeographic pattern

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Advection / Biogeography / budget analysis / Cold regions / Computer models / Constraining / Deposition / Diffusion processes / dissolved iron / Euphotic zone / Growth conditions / Growth rate / Iron / lateral transport / Limiting factors / Mathematical models / Nitrates / Nitrogen / Nitrogen fixation / Nitrogen-fixing bacteria / Nitrogenation / North Pacific subtropical gyre / Nutrient availability / Nutrient concentrations / Nutrient cycles / Nutrient dynamics / Nutrients / Ocean circulation / Ocean currents / Ocean models / Ocean surface / Oceans / Photosynthesis / Phytoplankton / Plankton / Stratification / Trace elements / Trace elements (nutrients) / Upwelling / Vertical advection