| Summary: | The aim of this study was to determine the variations of δ¹³C and δ¹⁵N of particulate organic matter (POM) and the factors that control the isotopic fractionation from inorganic substrates to zooplankton and lead to variations in the isotopic composition of suspended and sinking POM in the subarctic northeast Pacific Ocean. Along a transect from Station L (48°39'N, 126°40'W) to Station P (50°N, 145°W), surface δ¹⁵NO₃⁻ decreased from 11.2‰ to 7.6‰, while [NO₃⁻] increased from 3 to 12 pM. The δ¹⁵N trends for bulk POM, < 5 pm POM, 50-253 pm POM, < 253 pm POM and zooplankton were similar to that for nitrate. The fractionation factor (e) for the nitrogen isotopes was 5‰. Trophic enrichment of ¹⁵N was observed with mesozooplankton being isotopically heavier than suspended POM by 3.9‰ at Station P and 2.2‰ at Station L. The range of δ¹⁵N among six zooplankton groups was 3.7‰. At Station P, sinking δ¹³C[sub POM] and δ¹⁵N[sub POM] at 3800 m between 1982 and 1990 show significant annual changes. The δ¹³C[sub POM] ranges from -25.3‰ to -22.0‰ and the δ¹⁵N[sub POM] ranges from 10.2‰ to 7.7‰, isotopically depleted values occurring in summer and heavier values occurring in winter. The change in trophic length may be the principal controlling factor of the variations. The δ¹³C[sub POM] at 200,1000 and 3800 m show a ¹³C enrichment with depth. Suspended POM δ¹³C and δ¹⁵N increased with depth and were higher than those of sinking POM, suggesting that the two types of POM did not interact in deep water. At Station L, high frequency variations of δ¹³C and δ¹⁵N in summer are due to the periodic injection of subsurface nitrate by upwelling, which leads to diatom blooms, resulting in a heavier δ¹³C and a lighter δ¹⁵N in sinking POM. The sinking δ¹⁵N[sub POM] value (8%c) is much heavier than that of nitrate (4%c) at 500 m, suggesting that the lateral input of lower nitrate δ¹⁵N from the open ocean (3‰) was important. This study develop our understanding of the isotopic biogeochemistry of carbon and nitrogen in the ocean. Science, Faculty of Earth, Ocean and Atmospheric Sciences, Department of Graduate
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