High-resolution observations on fine-scale spatial and temporal heterogeneity of phytoplankton communities using FlowCAM

To understand the fine-scale spatio-temporal phytoplankton dynamics with reference to the environmental properties of the water column, high-frequency samplings every 4 h, at 0.5-2 m depth intervals using a submersible pump was conducted in Tateyama Bay, Japan, for 24 h from 12 h of 12 May to 08 h of 13 May 2017. The FlowCAM, which is an automatic device, was used to identify, count and size the phytoplankton. As a result, the phytoplankton distribution significantly varied within a few meters and a short timescale（several hours）of a day. For example, Thalassiosira sp. and Prorocentrum minimum were detected at all sampling depths and times rather evenly, but the water-column total abundance of Dactyliosolen fragilissimus and Scrippsiella trochoidea significantly decreased after the 2nd set of the samplings. Speciesspecific vertical distributions were various but related to the condition（strength and depth）of the thermocline at each sampling time. These phytoplankton species-specific distributions and variations in the composition reflect the eco-physiological characteristics and size structure of phytoplankton and the short-term hydrodynamic events. The high value of chlorophyll-a in the bottom layers was not only from the phytoplankton but also from the fluorescence of the aggregate particles.