High rates of ammonium recycling drive phytoplankton productivity in the offshore Mississippi River plume

J Hwang
ERM Druffel
S Griffin
KL Smith
RJ Baldwin

Abstract

[ 1] A 6-year time series of Delta(14) C, delta(13) C, and C/N measurements in deep sinking particulate organic matter ( POM) is presented for an abyssal site, Station M in the northeast Pacific Ocean. The Delta(14)C values revealed that sinking POM at 3450 m depth ( 650 m above bottom) contained old carbon despite its presumed short transit time in the water column. The isotopic and chemical properties of the sinking POM varied with time and appear to be controlled by more than one major process. In 1993, 1994, and late 1996, isotopic signatures and C/N molar ratios indicate negligible or vertically homogeneous influence of resuspended particles from the bottom or particles laterally transported from the margin to the study site. However, during early 1995 and 1998, Delta(14)C values were lower than those during other periods and C/N values at three deep depths were not equal, indicating that the study site was influenced by resuspended sediments more severely than during other periods. During mid-1995 to mid-1996, delta(13)C values decreased abruptly while Delta(14)C values increased slightly, and C/N values were extremely high ( up to -80) at 50 and 600 m above bottom; these results suggest input of degraded, modern, terrestrial organic matter. The periods of anomalous isotopic signatures, as well as vertically heterogeneous C/N values [ Smith et al., 2001], were correlated with high discharge periods of California rivers with a time lag of 2 to 4 months. The correlation suggests that regional meteorological events are important in controlling the biogeochemical properties of particles at Station M by varying the intensity of resuspension and transport of organic matter from the continental margin.