Zooplankton dynamics in the hyper-eutrophic Nakdong River system (Korea) regulated by an estuary dam and side channels

Zooplankton in the main channel of the Nakdong River and in three tributari es was sampled from June 1994 to September 1995. Planktonic rotifers (Brach ionus spp.. Keratella spp.. and Polyarthra spp.). cyclopoid nauplii and sma ll cladocerans (Bosmina longirostris) were numerically dominant. There was considerable longitudinal variation of zooplankton biomass in the main chan nel as well as spatial heterogeneity among the major tributaries. In the mi ddle region of the main channel, between river kilometer (RK) 170 and 150 a bove the estuary dam, total zooplankton abundance sharply increased from le ss than 100 ind. L-1 to more than 1,000 ind. L-1. In a downstream direction toward the estuary dam. phytoplankton biomass increased while total zoopla nkton biomass decreased. However, as shown by the increasing transport of z ooplankton biomass, zooplankton was diluted in the reach of the estuary dam . Advective effects from major tributaries appear to be: the contributory f actor for the higher zooplankton biomass in the middle region. Overall. rat her the external factors (flushing, retention) than internal factors (e.g.. phytoplankton) appear to be responsible for changes in zooplankton abundan ce toward the river mouth.