Seasonal changes in the abundance and composition of picophytoplankton in relation to the occurrence of ’Kyucho’ and bottom intrusion in Uchiumi Bay, Japan

• Published in: Marine ecology. Progress series (Halstenbek) 2005-01, Vol.298, p.59-67
• Main Authors: Katano, Toshiya, Kaneda, Atsushi, Takeoka, Hidetaka, Nakano, Shin-ichi
• Format: Article
• Language: English
• Subjects: Animal and plant ecology; Animal, plant and microbial ecology; Biological and medical sciences; Cell growth; Eucarya; Fundamental and applied biological sciences. Psychology; Nitrates; Nitrogen; Oceans; Phosphates; Phytoplankton; Prochlorococcus; Sea water; Sea water ecosystems; Seas; Surface water; Synechococcus; Synecology; Water temperature
• ISSN: 0171-8630; 1616-1599
• DOI: 10.3354/meps298059

Uchiumi Bay experiences intermittent physical events of ’Kyucho’ and bottom intrusion. AKyuchois an intrusion of warm surface water from the Kuroshio in the Pacific Ocean. Bottom intrusion, which contains a large amount of nitrates, phosphates, and silicates, slips through just above the continental shelf. We investigated seasonal changes in the abundance ofProchlorococcus, Synechococcus, and eukaryotic picophytoplankton while monitoringKyuchoand bottom intrusion from March to October 2002.Kyuchoand bottom intrusion frequently occurred from June to September. Relatively high concentrations of nitrate + nitrite (>0.8 μmol N l–1) and phosphate (>0.1 μmol P l–1) were found when bottom intrusion occurred. The cell densities ofProchlorococcuswere relatively high (>1 × 10⁴ cells ml–1) whenKyuchooccurred. Those ofSynechococcuswere high (2 to 30 × 10⁴ cells ml–1) during the period of thermal stratification except in July, when bottom intrusion occurred. The cell densities of eukaryotic picophytoplankton were high (2 to 8 × 10⁴ cells ml–1) in May and July. To examine the effects on picophytoplankton growth of the nutrients supplied by bottom intrusion, we conducted nutrient-enrichment experiments. The growth rates ofProchlorococcusandSynechococcuswere not stimulated by the addition of any kinds of nutrients. The growth rates ofProchlorococcuswere negative in most cases. In July, the growth rate of eukaryotic picophytoplankton was stimulated by nitrate and phosphate additions. Thus,Prochlorococcusdetected in Uchiumi Bay might have been transported byKyuchofrom the Pacific Ocean and could therefore not grow vigorously.Synechococcusmay have been flushed out by bottom intrusion, and its growth was not limited by the nutrient concentrations. Eukaryotic picophytoplankton was abundant in spring, and its growth might have been limited by the nutrient concentrations in some cases. These results suggest thatKyuchoand bottom intrusion have different effects on the abundance and growth rate of the 3 picophytoplankton groups.