Changes in phytoplankton assemblage caused by anoxic conditions revealed with mesocosms in Omura Bay, western Japan

Abstract

We artificially reproduced a scenario where the phytoplankton community changes under increased regeneration of nutrients from the sediment due to anoxic conditions using a mesocosm in Katagami Bay, one of many smaller bays in Omura Bay. Two instrumented mesocosms were used in the field experiment, whereas the surrounding waters acted as a control. The plastic bags used in the first mesocosm (M1) was completely transparent, whereas the second mesocosm (M2) was designed so that the bottom two-thirds of the plastic bag was opaque and only the top third was transparent. Photon flux density of M2 was about the same value in the surface layer as compared with M1 and Control, but it was approximately 1/2 in the middle layer and approximately 1/10 in the bottom layer. In M2, hypoxic water mass appears in the bottom layer on day 2 and then dissolved inorganic phosphate increased from day 3 to end of the experiment. At the start of the experiment, a diatom (Dactyliosolen flagilismus) was dominant in all compartments. Diatoms continued to dominate in M1 and the control, but in M2, a dinoflagellate (Heterocapsa circularisquama) became dominant at the end of the experiment. This study clearly shows that nutrient elution by water stratification can affect floral changes from diatom to flagellate dominance in summer in Omura Bay.