Active heterotrophic nanoflagellates in the hypoxic water-column of the eutrophic Masan Bay, Korea

Abstract

To investigate if heterotrophic nanoflagellates (HNF) in the hypoxic zone of the coastal water-column were actively grazing on bacteria, and to understand what factors were controlling HNF in the hypoxic zone, field work was carried out in summer 1999 m the eutrophic Masan Bay, Korea. Samples ranging from 0.22 to 3.87 mg O21-1 were assayed in chambers in which oxygen concentration was adjusted with N2 gas to the in situ pO2 of samples. Ratios of HNF-to-bacteria abundance varied from 6 to 31 × 10-5 in the hypoxic zone, and those at the surface usually ranged from 7 to 33 × 10-5, with some high values (46 to 115 × 10-5) observed in August. Biomass-carbon ratios of HNF-to-bacteria varied from 1.0 to 6.9 × 10-2 in the hypoxic zone and were generally 2- to 3-fold lower in the hypoxic zone than at the surface. Biovolumes of HNF in the hypoxic zone (6 to 24 μm3) were mostly similar to those in the surface layer. Interestingly, per cell clearance rates of hypoxic HNF were statistically higher than or similar to those in the surface layer. Thus, an active microbial loop seemed to operate in the hypoxic zone as well. Aeration of hypoxic samples to oxygen saturation did not cause any significant changes in grazing rates of hypoxic HNF, indicating non-susceptibility of hypoxic HNF to changes in oxygen exposure. It seems that most HNF in the hypoxic zone used aerobic metabolism. Further, incubation of hypoxic HNF with temperatures 4 to 6°C greater and lower than in situ temperature resulted in a low Q10 value (1.6) in HNF grazing rate in the hypoxic sample compared to the Q10 value (3.2) in the surface sample. This shows the significantly different response of HNF to temperature in the hypoxic zone. The positive relationship of the HNF grazing rate with bacterial abundance in the hypoxic zone suggests that HNF were largely controlled by bacterial availability.