Dynamics of bacterial respiration and related growth efficiency, dissolved nutrients and dissolved oxygen concentration in a subarctic coastal embayment

Abstract

Temporal variations in dissolved oxygen, nutrient concentrations and oxygen utilization rates (<0.7 m fraction) were recorded for almost two years in the subarctic Funka Bay, Japan. Dissolved inorganic nitrogen ranged from 0 to 26 M, phosphate from 0 to 2 M and silicate from 1 to 64 M. There was evidence of nitrogen limitation. Dissolved oxygen concentration ranged from 130 to 440 M, and decreased to <150 M in the bottom layer in summer. The rate of oxygen utilization, attributed to bacterial respiration, ranged from 0.6 to 9.3 M day^–1 at 10 m depth, and from 0.8 to 5.0 M day^–1 at 90 m depth, and was the principal mechanism causing the decrease in dissolved oxygen in the bottom layer in the summer. Bacterial growth efficiencies calculated for the 10 m and 90 m depths were similar: 1.6–17.2% and 1.4–23.6%, respectively. With the bacterial growth efficiencies <25%, the bacteria in Funka Bay acted as a net sink of carbon, where >75% of the organic matter flux through bacteria could be mineralized to CO₂.