Development of a thin diatom layer observed in a stratified embayment in Japan

• Published in: Journal of oceanography 2018-08, Vol.74 (4), p.351-365
• Main Authors: Onitsuka, Goh, Yoshikawa, Yutaka, Shikata, Tomoyuki, Yufu, Kei, Abe, Kazuo, Tokunaga, Takahisa, Kimoto, Katsunori, Matsuno, Takeshi
• Format: Article
• Language: English
• Subjects: Aggregation; Biological activity; Biological effects; Chains; Chlorophyll; Chlorophyll a; Coastal waters; Diatoms; Dynamics; Earth and Environmental Science; Earth Sciences; Freshwater & Marine Ecology; Instruments; Irradiance; Kinetic energy; Mineral nutrients; Oceanography; Original Article; Physiological responses; Phytoplankton; Pycnocline; Research vessels; Sea surface; Temperature (air-sea); Thickness; Turbulence; Turbulent mixing
• ISSN: 0916-8370; 1573-868X
• DOI: 10.1007/s10872-018-0466-0

The temporal evolution of a thin phytoplankton layer was observed by field measurements using a research vessel and mooring instruments in the Yatsushiro Sea, a semi-enclosed narrow embayment in Japan, in early August 2013. The subsurface chlorophyll maximum developed into a thin layer within 2 days just below the pycnocline at around 10-m depth, where turbulent mixing (the dissipation rate of turbulent kinetic energy) was weak (low). The layer persisted for 1.5 to 2 days and declined after irradiance drastically decreased at the sea surface. At the peak period, the layer thickness, which is defined as the full-width at half-maximum of the peak in chlorophyll a concentration, ranged from 0.6 to 1.4 m, and the maximum concentration reached 42.3 mg m −3 . The horizontal extent of the layer was approximately 10 km along the longitudinal axis of the bay. The phytoplankton population characterized by the layer was dominated by a chain-forming centric diatom, Chaetoceros spp. The formation mechanisms of the thin diatom layer were investigated using the observed data and a vertical one-dimensional model that includes physical and biological processes. The results suggest that the development of the thin layer was caused by in situ growth and aggregation due to nutrient-dependent sinking of the species under weak turbulence. The study highlights that continuous multidisciplinary observations and understanding species-specific physiological responses to environmental variations are necessary to elucidate drastically fluctuating phytoplankton dynamics in a coastal water.