Active transport of particulate organic carbon and nitrogen by vertically migrating zooplankton in the Sargasso Sea

Abstract

Diel vertically migrating zooplankton can contribute significantly to dissolved carbon and nutrient export by respiring and excreting surface-ingested particulate organic matter below the mixed layer. Active export of particulate organic carbon (POC) and particulate organic nitrogen (PON) due to defecation at depth has rarely been considered in export budgets. We measured the gut passage time (GPT) of common migrant species at the Bermuda Atlantic Time-series Study (BATS) site, using the gut fluorescence method, to determine whether GPT is slow enough to allow active export of POC and PON to depth. Mean GPT for the copepods Pleuromamma xiphias and Euchirella messinensis was 191 and 114 min, respectively, and for the euphausiids Thysanopoda aequalis and Euphausia brevis (analyzed together) was 41 min, exceeding previously reported GPT for non-migrating zooplankton by a factor of 6. Between 18 and 81% of the initial gut pigment was retained in zooplankton guts upon descent below a mixed layer of 150 m. By comparing pigment ingestion rates (gut fluorescence technique) with total ingestion rates (CHN analysis of fecal material), we estimated that 71 to 85% of the migrant diet originated from non-plant material, which we included in our estimates of active POC/PON export. We applied the mean weight-specific active POC/PON export rate for the species examined to the total migratory zooplankton biomass using data from the BATS zooplankton time-series. Mean active POC (PON) flux at BATS was 0.94 mgC m-2 d-1 (0.18 mgN m-2 d-1) and the maximum was 5.27 mgC m-2 d-1 (1.02 mgN m-2 d-1), corresponding to a mean of 3% (4%) and a maximum of 18% (20%) of the mean gravitational POC (PON) flux measured by sediment traps at 150 m. Migrants also contributed significantly to passive flux via production of sinking fecal pellets during the night in surface waters. This passive flux exceeded active POC flux by ∼10-fold. Freshly released feces by migrators at depth could be a valuable food source for mesopelagic organisms, in contrast to feces produced in surface waters which decompose while settling through the water column.