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Spatial variability of ammonium concentrations along repeat transects were examined on the Chukchi shelf during 2012–2018. Two distinct near-bottom high ammonium pools (>1 μmol/kg) near 67.5°N and 72.5°N of the transects were identified in all years. The accumulation of ammonium in the regions is driven primarily by a combination of biogeochemical processes (e.g., dynamic bacterial remineralization of organic matter) and physical controls (e.g., strong density-contrast barrier limits upward mixing of ammonium). The ammonium pool on the shelf may became larger in the expectation of the stronger bacterial remineralization following elevate primary production, and may have potential impact on the structure and productivity of ecosystem on the Chukchi shelf. • High ammonium concentrations were measured in near-bottom water of the Chukchi shelf during summer of 2012–2018. • The accumulation of ammonium may be caused by a combination of intense remineralization and inhibited vertical diffusion. • The ammonium pool on the shelf may became larger with elevated primary production and enhanced bacterial remineralization. [ABSTRACT FROM AUTHOR]