Carbon and nitrogen isotopic composition of sedimenting particulate material at Station Papa in the subarctic northeast Pacific

Abstract

The δ 13 C and δ 15 N of sedimenting particulate organic matter (POM) collected biweekly by sediment trap at 3800 m at Ocean Station Papa (OSP: 50°N, 135°W) in the NE subarctic Pacific from 1982 to 1990 changed significantly on seasonal and annual time-scales. δ 13 C POM ranged from −25.3 to −22.0‰, and δ 15 N POM ranged from 0.24 to 7.75‰, over the entire period, isotopically depleted values occurring mainly in summer and heavier values occurring in winter. Extreme depletion in δ 13 C POM values also occurred in the winters of 1982–83 and 1988–89 and in the late summer of 1985; these were not matched by significant changes in δ 15 N POM . The changes in isotope ratios are related to the annual changes in settling particulate fluxes only in a general way; flux maxima in particulate organic carbon (POC) and particulate organic nitrogen (PON) occurred in mid- and late summer in most years, but there is not a one-to-one correlation between these changes and the isotope variations. In some years, the isotope ratios begin to changed before the summer increase in settling fluxes. A one-year record of sedimenting δ 13 C POM and δ 15 N POM (1989–90) at three depths (200, 1000 and 3800 m) at the same location showed a marked carbon isotope enrichment with depth, consistent with decomposition or/and food-chain enrichment during particle settling, but very small changes in the nitrogen isotope composition of the same particles, suggesting that C and N are released from sedimenting particles via different pathways. Suspended δ 13 C POM and δ 15 N POM values increased significantly with depth and were higher than those of sedimenting POM below the euphotic zone, suggesting that the two types of POM do not interact in deep water. Seasonal variations in surface temperature, irradiance, phytoplankton growth rate, species composition and carbon fixation pathways do not appear to be important controls of the observed changes in δ 13 C POM ; the increases in temperature, irradiance and phytoplankton growth rate in the summer months should lead to enriched δ 13 C POM values rather than the observed isotopically light values in this season. Likewise, the summer increase in the growth of diatoms should produce isotopically heavy particles in view of the reported 13 C -enrichment of this group, and this should be augmented by the decrease in [CO 2] (aq) due to the summer increase in surface temperature. Opposing these possible controls, the planktonic food chain becomes shorter in the spring/early summer months, probably due to the migration to the surface of copepodites and their direct grazing on phytoplankton, and this results in 13 C enrichment relative to the winter months when the food chain is more complex. The relationship between sedimenting δ 15 N POM and the macro-nutrients at OSP is complicated by the fact that ammonium and urea are important phytoplankton substrates, and generally more so than nitrate. The change from isotopically heavy winter values to lighter summer values begins before the major drawdown of the surface nitrate, and the observed change is opposite to that observed in other ocean regimes, probably because nitrate never reaches limiting levels in this High Nutrient Low Chlorophyll regime. The change of phytoplankton composition from winter to summer also should lead to 15 N -enrichment, the opposite of the trends observed. Finally, the changes in sedimenting δ 15 N POM at OSP do not appear to be related to phytoplankton growth rate. We conclude that the nitrogen isotopic signal in settling POM at OSP reflects seasonal changes in food-web structure, the simpler (shorter) spring/summer plankton community causing a smaller isotopic fractionation from the nutrient substrate to sedimenting POM.