Vertical distribution of transparent exopolymer particle (TEP) concentration in the oligotrophic western tropical North Pacific

Seawater samples were collected during May-June 2013 from 32 stations west of Guam (10 degrees 30' N-16 degrees N, 140 degrees 30' E-144 degrees E) at depths of 5-300 m to describe the vertical profile of transparent exopolymer particle (TEP) concentrations. TEP concentration varied from 18 to 69 mu g gum xanthan equivalents per liter, which was a similar level to those observed in other open ocean areas, and the vertical profile of the TEP concentration had a subsurface maximum and minimum which was not associated with phytoplankton abundance. The TEP subsurface maximum occurred from the bottom of the mixed layer to the top of the subsurface chlorophyll maximum (SCM) where dissolved oxygen was supersaturated, indicating that the TEP maximum is produced based on active photosynthesis. The TEP subsurface minimum occurred from the bottom of the SCM to 200 m depth, and was characterized by negative preformed-nitrate water which is produced by decomposition of carbon-rich and nitrogen-poor dissolved organic matter. Additionally, water turbulence, which promotes abiotic TEP production, hardly occurred in the TEP subsurface minimum layer because stratification in this layer was most developed in the vertical profiles. Therefore, our results suggest that the low abiotic TEP production may have induced the TEP minimum. Hence, the TEP concentration in the oligotrophic ocean varied both with phytoplankton activities and also abiotic processes.