Vertical distribution, transport, and exchange of carbon in the northeast Pacific Ocean: evidence for multiple zones of biological activity

A sediment trap experiment was conducted to investigate the production, decomposition, and transport of organic matter from 0 to 2000 m at a station 100 km northeast of Point Sur, California. Parameters measured included (1) rates of autotrophic production of carbon, (2) vertical depth distributions of total carbon, nitrogen, and living biomass, and (3) downward flux of organic carbon, nitrogen, ATP, RNA, and fecal pellets. Metabolic activity and microbial growth rates (RNA and DNA synthesis) were also estimated in situ, for both the ‘suspended’ (i.e., samples captured in standard water bottles) and ‘sinking’ (i.e., samples captured in sediment traps) particles. Daily depth-integrated rates of primary production averaged 564 mg C m−2, of which 10 to 15% was removed from the euphotic zone by sinking, assuming steady-state conditions. The profiles of suspended carbon, nitrogen, C:N ratios, and ATP conformed to previously published concentration-depth profiles from the region. The vertical flux profiles of organic matter, however, revealed two important features that were not evident in the suspended particulate matter profiles. First, there was an obvious mid-water depth increase (i.e., an increase in organic carbon and nitrogen flux with increasing depth) between 700 and 900 m, suggesting horizontal advection or in situ production. Similar flux profiles were also observed for ATP, RNA, and total fecal pellets. Second, the C:N ratios for the sediment trap materials collected at mid-ocean depths (600 to 1200 m) were low compared to values measured for ‘suspended’ particulate organic materials collected from comparable depths, supporting the in situ production hypothesis. An observed maximum in the rate of RNA and DNA synthesis for microorganisms associated with particles collected at 700 m confirmed that the flux anomalies were the result of in situ microbiological processes rather than horizontal advection. We hypothesize that the in situ activity measured at 700 m is the result of a chemolithotrophic-based carbon production system supported by the presence of reduced inorganic compounds (e.g., NH4+, HS−) found in association with the sinking particles. “New carbon production” (a value equivalent to the increased downward flux of carbon) between 700 and 900m was 15 mg C m−2 d−1, or 2 to 1% of the daily integrated primary production. These regions of intense biological metabolic activity, growth, and organic matter diagenesis may have a profound influence on the oceanic carbon cycle and on the observed steady-state distributions of various non-conservative properties of seawater.

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