A biological tool for indicating hypoxia in coastal waters: calcareous walled-type to naked-type cysts of Scrippsiella trochoidea (Dinophyceae)

Scrippsiella trochoidea produces a calcareous walled cyst with spines during its life history. It has been recently reported that the calcareous wall can be decalcified under acidified conditions in coastal areas linked to hypoxia caused by bacterial activities. In this study, in order to determine whether the calcareous cysts can be a biological tool for indicating an in situ hypoxic environment, the morphology of S. trochoidea cysts in the surface sediments of Ise Bay, Japan, was examined in relation to dissolved oxygen concentrations in the bottom water. The surface sediments were collected from the inner to outer parts of the bay in May 2014 and June 2018. The living cysts of S. trochoidea were counted separately into two morphotypes: cyst with calcareous wall (calcareous walled-type cyst) and without the wall (naked-type cyst). The proportions (%) of naked-type cyst abundance in the total living cyst population showed an increasing tendency in the hypoxic environment of the inner and central parts of Ise Bay, but not in the normoxic environment at the mouth. This result supports the supposition that the naked-type cyst of S. trochoidea can be used as a biological indicator for the assessment of hypoxia in coastal areas.