Potential drivers and consequences of regional phosphate depletion in the western subtropical North Pacific

In regions of the nitrogen limited low latitude ocean, phosphate can also be depleted to levels initiating stress responses in marine microbes. Here, we associate a broad region of phosphate depletion in the subtropical North Pacific with different levels of phosphorus stress. Nutrient and aerosol addition experiments demonstrated primary nitrogen limitation of the bulk phytoplankton community, with supply of aerosols relieving this limitation. At northern sites with depleted phosphate, alkaline phosphatase activities were enhanced, indicating elevated phosphorus stress. Analysis of satellite- and model-derived aerosol loading showed that aerosol deposition was elevated in these regions. Surface rate measurements suggested that the regional enhancement in phosphate depletion was predominantly driven by elevated nitrogen fixation, likely stimulated by the coincident supply of aerosol iron. Such observations are important for predicting future biogeochemical responses in the subtropical North Pacific to changing aerosol supply.

Automated summary

In nitrogen limited low latitude ocean regions, phosphate depletion can induce stress responses in marine microbes. This study associates a broad region of phosphate depletion in the subtropical North Pacific with different levels of phosphorus stress, and demonstrates the primary nitrogen limitation of the phytoplankton community with the relief provided by aerosol supply. Enhanced alkaline phosphatase activities indicate elevated phosphorus stress in northern sites with depleted phosphate. Increased aerosol deposition, likely stimulated by aerosol iron supply, can drive the regional enhancement in phosphate depletion through elevated nitrogen fixation. These findings are crucial for predicting future biogeochemical responses in the subtropical North Pacific to changes in aerosol supply.