Title:

Drivers of Temporal Variability in Ocean Iron Stress in HNLC Regions using Ocean Color and Climate Modes

Determining the seasonal, interannual, and decadal trends in iron-stressed regions helps quantify the variability in marine nutrient limitations. Seasonally resolved time-series measurements of dissolved iron from three high nutrient low chlorophyll (HNLC) regions (Equatorial Pacific, Subarctic N. Pacific, and Southern Ocean), are limited to nonexistent. Here we compare the temporal variability of nonphotochemical quenching satellite-based fluorescence quantum yields (NPQ-corrected φsat), a remote sensing-based estimate of phytoplankton iron stress, across these HNLC regions using two decades (2003-2023) of monthly ensemble MODIS-Aqua satellite data and different modes of climate variability. Average NPQ-corrected φsat increased in order of the Southern Ocean (1.51%), Subarctic Pacific (2%), and Equatorial Pacific (2.17%). Seasonal variability in iron stress was 11.56% globally, 5.6% in the Equatorial Pacific, 8.51% in the Subarctic N. Pacific, and 19.56% in the Southern Ocean. The drivers of the observed temporal variability of NPQ-corrected φsat diagnosed iron stress were investigated. Iron stress in the Equatorial Pacific was negatively correlated with indices of ocean-atmosphere oscillations (Niño 3.4 & 4, Pacific Decadal Oscillation (PDO), Multivariate ENSO Index Version 2 (MEI.v2), and Southern Oscillation Index (SOI)), which unexpectedly indicates less iron stress in warmer sea surface temperatures (SST). The Subarctic N. Pacific region was positively correlated with the North Pacific Gyre Oscillation (NPGO), indicating more iron stress during warmer SST anomaly. The Southern Ocean exhibited seasonally iron-replete zones perhaps linked to iron-laden dust input that is spatially heterogeneous and concentrated around land masses. Seasonally, peak iron stress occurred in July near maximum austral sea ice, and peak iron replete conditions occurred following sea ice melt. Iron stress in the Subarctic N. Pacific varies seasonally with a peak in late summer coinciding with the annual peak in surface chlorophyll and SST. Overall, the dominant mechanisms driving iron stress variability in the investigated HNLCs are regional environmental properties (advection, river input, atmospheric dust deposition, sea ice, and stratification), more so than long-term climate oscillations.

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