Carbon and Nitrogen Isotope Ratios in Sedimenting Particulate Organic Matter at an Upwelling Site off Vancouver Island

Abstract

Variations of δ13Corganicand δ15Ntotalof sedimenting particulate organic matter (POM) were determined at 500m depth at a site (Station L, 48°39′N, 126°40′W) in the La Perouse Bank upwelling area off Vancouver Island, Canada. δ13C values were heavier in summer and lighter in winter, the reverse of the changes in δ15N. These variations are probably governed by the longer trophic transfer lengths in winter than in summer, coupled with a significant difference in δ13C and similar values of δ15N between nanophytoplankton and diatoms. High frequency variations in spring, summer and early autumn were probably caused by the periodic injection of subsurface nitrate by upwelling, which leads to diatom blooms, resulting in heavier δ13C and lighter δ15N in sedimenting POM. Utilization of regenerated ammonium during periods of nutrient depletion could also cause short-term increases in δ15N. At 500m depth, the δ15N value of the POM (c8‰) is heavier than that of the nitrate reservoir (c. 4‰), suggesting that intensive nitrate utilization at the surface and the lateral input of nitrate with lower δ15N from the open ocean are important. POM δ13C data suggest that land input at most comprises 29% of total sedimenting POM.