ABSTRACT

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.

摘要

叶绿素a指示的浮游植物产量怀卡托河上最后一个水电站湖泊 Karāpiro 的浓度显示出高季节性变化，但长期呈下降趋势，尽管河流中含有丰富的养分（N 和 P）。2005 年之前对该河流系统的八座水坝的早期研究发现，只有 Ohakuri 湖和 Arapuni 湖在夏季发生了热分层，并且假设热分层可以通过增加停留时间来提高浮游植物的生产力。期间。最近的一项研究（2013 年至 2014 年）还发现，Karāpiro 湖在电站进水口深度附近与温跃层形成了热分层。在这项研究中，我们发现热分层通过将浮游植物限制在光线充足的上层而促进了它们的生长。然而，浓度。我们发现，沿着湖轴的风应力会引发内部淤积，随着温跃层上下扫过发电站的进水口，这会导致下游水质发生显着的阶梯式变化。