Autotrophic production and elemental fluxes at 26°N, 155°W in the North Pacific subtropical gyre

Abstract

Autotrophic carbon and nitrogen assimilation rates and the fluxes of particulate carbon (PC) and particulate nitrogen (PN) into sediment traps positioned near the base of the euphotic zone were measured at 26°N, 155°W in the North Pacific subtropical gyre. Carbon assimilation uncorrected and corrected for grazing and algal respiration losses averaged 484 ± 81 and 692 ± 115 mg C m −2d −1, respectively. Carbon:nitrogen assimilation ratios were always higher than the Redfield ratio and averaged 7.1 ± 0.6 g C g −1N. Absolute growth rates were estimated to be 69 ± 10% of nutrient-saturated values. A comparison of conical and cylindrical sediment traps indicated that the conical traps underestimated PN and PC fluxes by factors of 30 and 40, respectively. This undertrapping probably occurred in part because the effective trapping area of the conical traps is significantly less than the mouth area of the trap for slowly sinking particles. However, several lines of evidence suggest that the major cause of the discrepancy was the breakup of marine snow aggregates that encountered the sloping sides of the conical traps. The f ratios calculated from PN fluxes into the cylindrical traps varied by about a factor of three and averaged 19 ± 10%. part of the temporal variability in the f ratio appeared to be due to the fact that particulate material accumulated in the euphotic zone during storm events and subsequently appeared as a large flux of PN as the storm dissipated. The PC:PN ratio of the material that sedimented at the base of the euphotic zone (9.2 ± 1.1 g C g −1N) was consistently higher than the corresponding autotrophic assimilation rate ratio. Because of this fact net community production exceeded new production in terms of carbon by about 30%. The latter was calculated by multiplying the downward flux of PN by the autotrophic carbon:nitrogen assimilation rate ratio.