[A-CG43] Water and sediment dynamics from land to coastal zones

[ACG43-P08] Phytoplankton blooms driven by freshwater input from small-scale rivers

*Shinichiro Kida¹ (1.Research Institute for Applied Mechanics, Kyushu University)

Keywords:River plume, phytoplankton bloom

Coastal areas surrounding Japan are influenced by freshwater inputs from numerous small-scale rivers. The transport and width of each river are small, but their combined transport can become comparable to that of a single mid-scale river. The presence of multiple small-scale rivers can therefore result in contiguous freshwater plumes and establish a density front along the coastline. Small-scale rivers are also driven by weather events that occur on a weekly timescale, making the freshwater input occur in the form of pulses rather than a steady input. The dispersion of freshwater and the response of phytoplankton in the ocean for a group of small-scale rivers may differ from that of a classical river plume.
An idealized numerical model was used to understand the dynamics and biological response to freshwater inputs from a group of small-scale rivers. When the freshwater input lasts only for a day, model experiments show a moderate increase in phytoplankton and zooplankton along the coast; the magnitude of phytoplankton blooms enhance downstream while zooplankton bloom occurs as patches near river mouths. The spatial pattern, growth speed, and time scale of plankton bloom was found to differ compared to that forced by a single river or continuous freshwater input.