Cascading climate effects in deep reservoirs: Full assessment of physical and biogeochemical dynamics under ensemble climate projections and ways towards adaptation

Abstract

We coupled twenty-first century climate projections with a well-established water quality model to depict future ecological changes of Rappbode Reservoir, Germany. Our results document a chain of climate-driven effects propagating through the aquatic ecosystem and interfering with drinking water supply: intense climate warming (RCP8.5 scenario) will firstly trigger a strong increase in water temperatures, in turn leading to metalimnetic hypoxia, accelerating sediment nutrient release and finally boosting blooms of the cyanobacterium Planktothrix rubescens. Such adverse water quality developments will be suppressed under RCP2.6 and 6.0 indicating that mitigation of climate change is improving water security. Our results also suggested surface withdrawal can be an effective adaptation strategy to make the reservoir ecosystem more resilient to climate warming. The identified consequences from climate warming and adaptation strategies are relevant to many deep waters in the temperate zone, and the conclusion should provide important guidances for stakeholders to confront potential climate changes.

Keywords: Climate adaptive strategy; Global warming; Metalimnetic hypoxia; Stratification phenology; Water quality simulation.

Fig. 1

Comparison between simulated and measured DO concentration during the calibration (A and B) and validation (C and D) period. The colour scale, in subfigure B and D, denotes the amount of samples per hexagon, and the straight line has a slope of one with an intercept of zero (1:1 line). The number at bottom-right shows the year of comparison

Fig. 2

Projected mean and variability of water temperature, for Rappbode Reservoir, at the depth of 1 m (top), 10 m (middle) and 60 m (bottom) during 3 periods in the twenty-first century (Period 07–21: from 2007 to 2021, Period 45–59: from 2045 to 2059, Period 85–99: from 2085 to 2099) under RCP2.6, 6.0 and 8.5. The boxplots indicate the medians, upper (lower) quartiles and full ranges of the 15 annual ensemble mean temperatures for each period

Fig. 3

Future projections of DO concentration (mg L⁻¹), for Rappbode Reservoir, under RCP2.6 (left), RCP6.0 (middle) and RCP8.5 (right). The top row indicates the ensemble average results, for every Julian day, in Period 07–21 (from 2007 to 2021), the middle row indicates the results in Period 45–59 (from 2045 to 2059) and the bottom row indicates the results in Period 85–99 (from 2085 to 2099)

Fig. 4

Future projections of the MOM (metalimnetic oxygen minima, left) and hypolimnetic (right) DO concentration, for Rappbode Reservoir, under RCP2.6 (upper), RCP6.0 (middle) and RCP8.5 (bottom). The red lines indicate the annual ensemble average results driven by four climate models, and the blue shaded areas indicate the annual minimum and maximum results from the ensemble. Dashed lines, in the vertical direction, indicate 3 periods in the twenty-first century

Fig. 5

Future projections of diatoms (left) and P. rubescens (right) concentration in the top 15 m, for Rappbode Reservoir, during 3 periods (Period 07–21: from 2007 to 2021, Period 45–59: from 2045 to 2059, Period 85–99: from 2085 to 2099) under RCP2.6 (1st line), RCP6.0 (2nd line), RCP8.5 (3rd line) and RCP8.5 driven by the SW (4th line). Lines indicate the ensemble average results, for every Julian day, driven by four climate models, the shaded areas indicate the minimum and maximum results of the four means over 15 years of the four ensemble members

Fig. 6

Future projections of phosphate concentration in the top 15 m, for Rappbode Reservoir, during 3 periods (Period 07–21: from 2007 to 2021, Period 45–59: from 2045 to 2059, Period 85–99: from 2085 to 2099) driven by the current and SW strategy under RCP8.5. The thick lines indicate the annual ensemble average results driven by four climate models and the thin lines indicate the annual minimum and maximum results of the four means over 15 years of the four ensemble members

Fig. 7

Future projections of the volumetric oxygen consumption (VOC, including sediment- and water-borne oxygen consumption) rate, for Rappbode Reservoir, in the upper (top) and lower (below) 35 m driven by the current and SW strategy under RCP8.5. The thick lines indicate the annual ensemble average results driven by four climate models and the thin lines indicate the annual minimum and maximum results from four means of the ensemble members

Fig. 8

Phosphate release rate from sediment driven by the current (left) and surface (right) withdrawal strategy under RCP8.5 (seasonal ensemble averages over 15 simulated years as indicated in the left panels)

Fig. 9

Conceptual framework visualizing the “cascading climate effect” on reservoir ecosystems, under the surface and bottom withdrawal strategy. The causal chain shown at the bottom summarizes the mode of action from physical variables towards biogeochemical and ecological water quality variables