Environmental Feature Causing a Bloom of the Novel Dinoflagellate Heterocapsa circularisquama (Dinophyceae) in Uranouchi Bay, Kochi Prefecture, Japan

doi:2003.18.4.281

Algae > Volume 18(4); 2003 > Article

Algae 2003;18(4): 281-288. doi: https://doi.org/10.4490/ALGAE.2003.18.4.281

Seok Jin Oh, Yukihiko Matsuyama, Sumio Oda, Takamitsu Moriyama, Takuji Uchida

National Research Institute of Fisheries and Environment of Inland Sea, Fisheries Research Agency, Kochi Prefectural Office of Agriculture, Forestry and Fishery, Kochi Prefectural Deep Seawater Laboratory, Maruyama, Murotomisaki, Muroto, Hokkaido National Fisheries Research Institute

ABSTRACT

To study the environmetna features causing a bloom of the novel dinoflagellate Heterocapsa circularisquama (Dinophyceae), hydrographic and chemical aspects were measured in the Uranouchi Bay, Kochi Prefecture, Japan, from January to December, 1997. The cell density of H. circularisquama increased rapidly in early October, and dropped sharply in mid-October. Growth rate of H. circularisquama during bloom period appeared 1.50 division/day under high water temperature (25℃) and salinity (32psu) conditions. Although the result from hydrographic aspect indicated good condition for their growth, dissolved inorganic phosphorus (DIP) concentration in surface layer before bloom formation was less than 0.70 μM, which is lower than their half saturation constant (Ks). Dissolved inorganic nitrogen (DIN): DIP ratio was >30, indicating potential P-limitation. However, before bloom formation period of H. circularisquama, DIP concentrations were high in bottom layer (>4.0μM). Some studies reported that H. circularisquama had the ability to migrate vertically and to utilize dissolved organic phosphorus (DOP). Thus, DIP in bottom layer might have been utilized by H. circularisquama for their growth. DOP might have weakly affected their growth because of low reactive DOP concentrations owing to low DOP concentration (ca. 0.39μM). Thus if nutrient condition of bottom layer in Uranouchi Bay is not improved, the outbreaks of H. circularisquama red tides may became and annual feature.

Key words: dissolved inorganic phosphorus (DIP), dissolved organic phosphorus (DOP), Heterocapsa. circularisquama, Uranouchi Bay, vertical migration