https://doi.org/10.22541/essoar.174120423.33438965/v1

Shallow, eutrophic lakes often exhibit high and extremely variable phytoplankton
biomass. This variability makes drinking water supplies from shallow lakes particularly
vulnerable to rapid change, as phytoplankton blooms can strongly impact treatment processes.
We use 39 years of data to assess the interacting roles of climate and flow management in
driving change in chlorophyll a (an indicator of phytoplankton biomass) in a shallow,
hydrologically managed drinking water reservoir. Generalized additive modelling shows a
significant increase in phytoplankton biomass (1992–1997), leading to a doubling of average
chlorophyll a concentrations. Interestingly, we also see a shift towards smaller spring blooms
and larger summer blooms. Our results show an impact of nutrients, climate, and flow source on
phytoplankton biomass. Specifically, the increase in phytoplankton biomass coincided with
periods with greater precipitation and associated nutrient-rich inflows from an agriculturally
dominated catchment, along with strong El Niño events that potentially contributed to a warm,
stable water column during the growing season. During high flows from the local catchment,
flows from the upstream supply reservoir are typically reduced to prevent downstream flooding.
However, flows from the supply reservoir have a diluting impact due to its lower nutrient
concentrations. Thus, where reservoir flow sources vary in chemistry, considering water quality
in flow management could help to ameliorate bloom severity and reduce drinking water
treatment costs. Understanding interacting management and climatic drivers of reservoir water
quality can also contribute to advance planning and mitigation of risks to water treatment.

This is a preprint and has not been peer reviewed. Data may be preliminary.

https://doi.org/10.22541/essoar.174120423.33438965/v1 ​