Zooplankton vertical migration and the active transport of dissolved organic and inorganic nitrogen in the Sargasso Sea

In the past decade a series of studies have shown that the active transport of inorganic carbon and nutrients by diel vertical migration of zooplankton is an important component of the biological pump. Less well known is how excretion of dissolved organic material (DOM) by migrating zooplankton enhances this export by creating a migratory DOM pump. We measured ammonia (NH4+) and dissolved organic nitrogen (DON) excretion by common vertically migrating zooplankton at the US JGOFS Bermuda Atlantic Time-series Study (BATS) station in the Sargasso Sea. The inclusion of DON excretion in our study builds on published research on migratory transport and excretion of NH4+ and dissolved organic carbon (DOC) at BATS. On average, excretion of DON by migrating organisms is 32% (range=15-66%) of the total dissolved nitrogen (TDN) excreted, significantly augmenting previously documented vertical flux of inorganic nitrogen by migrant zooplankton. This process is discussed in the context of the BATS region, where TDN transport by migratory zooplankton is compared to nitrogen export via sinking of PON and to primary production. Estimates of active transport of TDN (NH4+ +DON) by migrators averaged 13%, 18%, and 27% of the mean monthly sinking PON flux (for the period 1989-1999) at 150, 200, and 300 m, respectively. However, maximum transport of TDN by migrators was from 1.6 to 3.5 times greater than the sinking PON flux at these three depths. On average, the migratory TDN flux was a small proportion (2%) of primary production, but reached a maximum of 19%. Comparison of N excretion data from this study with C and P metabolism data from our other studies in the region indicates that molar remineralization ratios of C, N, and P by migrators differ among species and are not always at the classic Redfield ratio. This may have implications for non-Redfield cycling of nutrients in the western Sargasso Sea. (C) 2002 Elsevier Science Ltd. All rights reserved.