What is the relationship between hypoxia, water chemistry and otolith manganese content?

Previous studies have shown an increase in otolith Mn caused by exposure to hypoxic water masses. The mechanism leading to the increases in otolith Mn is still unclear, but might possibly be due to the larger amount of available Mn left in the water column under hypoxia. Thus, this study aimed to examine the relationship between hypoxia, water Mn and otolith Mn through marbled flounder (Pseudopleuronectes yokohamae, Günther) captured from Tokyo Bay and reared under different water Mn at laboratory. Otoliths from the Bay showed a higher (Mn/Ca)_otolith than outside, together with a seasonal trend of high (Mn/Ca)_otolith at the start of translucent zones (which form in the summer), supporting the occurrence of summer hypoxia in Tokyo Bay. Nonetheless, juveniles reared under control (Mn 0.50 μmol l⁻¹), middle (Mn 6.94 μmol l⁻¹) and high (Mn 10.4 μmol l⁻¹) treatments of water Mn concentrations showed a disproportional smaller increase in (Mn/Ca)_otolith. Comparing the laboratory experiment with the field data, (Mn/Ca)_water under hypoxia in Tokyo Bay could reach a low level similar to control treatment, yet (Mn/Ca)_otolith of the Bay showed a higher value than the high treatment. These results revealed an elevated (Mn/Ca)_otolith towards hypoxia, but also suggested that changes in water Mn might not be directly recorded by otolith Mn.