Factors influencing summer phytoplankton biomass in a large river system with impoundments: retention time, zooplankton grazing, thermal stratification and internal seiching in a hydro lake

Phytoplankton production as indicated by chlorophyll a concentrations in Lake Karāpiro, the last hydro power station lake on the Waikato River, shows high seasonal variability but a long term trend of decrease, despite an abundance of nutrients (N and P) in the river. Early studies pre-2005 of the river system’s eight hydro dams identified that only Lakes Ohakuri and Arapuni thermally stratified in summer, and it was hypothesised that thermal stratification could enhance phytoplankton productivity through increased retention time. During. a more recent study (2013–2014), Lake Karāpiro was also found to be thermally stratified with the thermocline around the depth of the power station intake. In this study we found that thermal stratification enhanced phytoplankton growth by confining them to the well-lit epilimnion. However, growth was limited as the nutrient supply became depleted and zooplankton grazing was found to adversely impact phytoplankton biomass, with highest grazing pressure in spring and lowest in late summer to autumn, consistent with an increase in chlorophyll a concentration. We found that wind stress along the lake axis can induce an internal seiche, which causes a substantial stepwise change in water quality downstream as the thermocline sweeps up and down across the power station intake.