Dr. Ryota Nakajima

A biological oceanographer

High fish abundance in the confluence of two large rivers in the Amazon

Ryota and his team report a high fish larvae abundance in the confluence zone between black and white water rivers in the Amazon River system.
In the center of the Amazon River basin, white water of the Amazon River meets with black water of the Negro River, creating a conspicuous visible boundary spanning over 10 km along the Amazon River, which can even be seen from space. Local fishermen acknowledge the confluence of black and white water rivers is rich in fish abundance, and indeed there are many freshwater dolphins there. So Ryota was wondering what’s going on in the confluence and suspecting there were many prey zooplankton here.
His study found that the confluence boundary offers benefits of both high prey concentration (zooplankton) from the black water river and low predation risk from the white water river for fish larvae, explaining the high abundance of fish larvae in the confluence zone. That’s why the dolphins like the confluence zone!

Typhoon enhances the production of bacteria and phytoplankton

Kenji Tsuchiya and his colleagues investigated responses of bacteria and phytoplankton to physical-chemical environments induced by typhoon passages.

The study showed that the passage of typhoon Malou in a coastal water of Japan in 2010 caused an abrupt decline of salinity and a large increase in the amount of nutrients, immediately enhancing bacterial production.

The study also sowed that phytoplankton production exceeded bacterial production two days after Malou passage, and then reached a maximum five days later.

The study team considers that sediment resuspension induced by typhoon passage enhanced bacterial productivity abruptly just after the passage at an inshore station.

The bacterial response could be regulated by difference in relative contribution of nutrient sources after the passage of typhoon.

 

Tsuchiya T, Kuwahara VS, Hamasaki K, Tada Y, Ichikawa T, Yoshiki T,  Nakajima R, Imai A, Shimode S, Toda T (2015) Typhoon-induced response of phytoplankton and bacteria in temperate coastal waters. Estuarine, Coastal and Shelf Science 167: 458-465.