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Subsurface chlorophyll maximum layers (SCMLs) are nearly ubiquitous in stratified water columns and exist at horizontal scales ranging from the submesoscale to the extent of oligotrophic gyres. These layers of heightened chlorophyll and/or phytoplankton concentrations are generally thought to be a consequence of a balance between light energy from above and a limiting nutrient flux from below, typically nitrate. Here we present multiple lines of evidence demonstrating that iron (Fe) limits or with light co-limits phytoplankton communities in SCMLs along a primary productivity gradient from coastal to oligotrophic offshore waters in the southern California Current ecosystem. SCML phytoplankton responded markedly to added Fe or Fe/light in experimental incubations, biogeochemical proxies for Fe limitation peaked in SCML waters, and transcripts of diatom and picoeukaryote Fe stress genes were strikingly abundant in SCML metatranscriptomes. A 40-year time-series indicates that SCMLs in this region display a persistent biogeochemical signal of diatom Fe deficiency during the spring and summer months, particularly within inshore and transition zones. In addition, the spatial extent of SCML Fe limitation has markedly increased during the first decade of the 21st century. We conclude that phytoplankton Fe limitation and Fe/light co-limitation at SCMLs is an important constraint on primary productivity and carbon export in the California Current and potentially SCMLs worldwide.
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