Potential Impacts of N2-fixing Trichodesmium on Heterotrophic Bacterioplankton Turnover Rates and Organic Carbon Transfer Efficiency in the Subtropical Oligotrophic Ocean System

Abstract

Bacterial production (BP), primary production (PP) and N2-fixing cyanobacteria, Trichodesmium spp. abundance (i.e., [Tricho#]) were measured in the N. Pacific west-boundary Kuroshio Current in summer and autumn, 2001. The warm (28 - 29°C) upper water column (0 - 30 m) in summer was low in nitrate (NO₃; 0.08 ± 0.08 µM) with Trichodesmium spp. (192 - 2,115 trichomes L⁻¹ ) observed at all sampling stations. Averaged NO₃ in the upper 30 m were higher (0.39 ± 0.27 µM) in autumn but with no Trichodesmium observed except 1 station with an abundance of 962 trichomes L⁻¹ in surface waters. PP (21 ± 7 µgC L^-1 d^-1 ) and phytoplankton turnover rate (Pµ ; 0.48 ± 0.19 d^-1 ) in summer were at least 1.5-fold higher than those recorded in autumn. BP (summer, 2.5 ± 0.6 µg C L^-1 d^-1 ; autumn, 2.4 ± 0.3 µg C L^-1 d^-1 ) showed no seasonal difference, but bacterial turnover rate (Bµ ; 0.11 ± 0.03 d^-1 ) was 57% higher iegative trend of [Tricho#] vs. NO₃ and higher transmittance in summer indicate that NO₃ depletion and better light availability might trigger the development of N₂-fixing cyanobacteria. Bµ changed positively with [Tricho#] suggesting that bacterial growth might be enhanced by the supply of "new" inorganic (i.e., NH₄) and/or dissolved organic nitrogen from N₂-fixing cyanobacterium. From the negative relationship of BP : PP ratio (7 - 21%) vs. [Tricho#], we deduce that the occurrence of high Trichodesmium abundance might potentially reduce the magnitude of NH₄ competition between algae and bacteria. In the open ocean, organic carbon transfer efficiency, either to higher trophic levels or to system exportation, can be