Reservoir Response to the Asian Monsoon with an Emphasis on Longitudinal Gradients

ABSTRACT

Measured responses to monsoon inflow into Taechung Reservoir, Korea support loading theory. During an intense monsoon in 1993 there was nearly twice the total phosphorus (TP) and three times the non-volatile suspended sediments (NVSS) relative to a weak monsoon in 1994. Across both monsoon seasons, water residence time was negatively correlated with TP and NVSS (r > −0.92). Despite lower TP levels, volumetric chlorophyll values (Chl) were larger by ~ 45% during the weak monsoon. This increased response to the limiting nutrient was attributed to longer residence time and reduced suspended load. The reservoir showed sharp longitudinal zonation during the intense monsoon in 1993 with a light limited riverine zone created by strong advective inflow. With a downlake decline in the suspended load the photic depth doubled and autochthonous production became important. Within the transition and lacustrine zones Chl was responsive to nutrients and volatile solids dominated the seston. An interflow resulted in weak stratification and moved oxygenated water, at mid-depths in the water column, the length of the mainstem. During the weak monsoon in 1994 there was no apparent riverine zone within the sampled reach and lacustrine conditions dominated. High yields of Chl per unit of TP suggested bloom conditions were sustained throughout the reservoir. Low flow resulted in strong stratification and the anoxic zone extended through > 30% of the water column within the lacustrine zone. These data demonstrate that large inter-annual differences in water quality and associated longitudinal gradients within an individual reservoir are regulated by monsoon intensity.