Effects of human-driven water stress on river ecosystems: a meta-analysis

Abstract

Human appropriation of water resources may induce water stress in freshwater ecosystems when ecosystem needs are not met. Intensive abstraction and regulation cause river ecosystems to shift towards non-natural flow regimes, which might have implications for their water quality, biological structure and functioning. We performed a meta-analysis of published studies to assess the potential effects of water stress on nutrients, microcontaminants, biological communities (bacteria, algae, invertebrates and fish), and ecosystem functions (organic matter breakdown, gross primary production and respiration). Despite the different nature of the flow regime changes, our meta-analysis showed significant effects of human-driven water stress, such as significant increases in algal biomass and metabolism and reduced invertebrate richness, abundance and density and organic matter decomposition. Water stress also significantly decreased phosphate concentration and increased the concentration of pharmaceutical compounds. The magnitude of significant effects was dependent on climate, rainfall regime, period of the year, river size and type of water stress. Among the different causes of water stress, flow regulation by dams produced the strongest effects, followed by water abstraction and channelization.

Figure 1

Effects of water stress on water chemistry, biota (bacteria, algae, invertebrates and fish) and ecosystem function, given by the response ratio (R = Impacted/Control; ±95% Confidence Limit, CL). The dashed line (mean effect size = 1) indicates no effect. Mean effect size >1 indicates an increase, while mean effect size <1 indicates a decrease due to water stress. The effect of water stress is significant when the 95% CL does not overlap 1 (black symbols). Variables do not significantly differ when their 95% CL overlap. Values in the parentheses indicate sample size.

Figure 2

Effects of water stress on chemical descriptors, given by the response ratio (R = Impacted/Control; ±95% Confidence Limit, CL). The dashed line (mean effect size = 1) indicates no effect. Mean effect size >1 indicates an increase, while mean effect size <1 indicates a decrease due to water stress. The effect of water stress is significant when the 95% CL does not overlap 1 (black symbols). Levels within a given moderator (same symbol) do not significantly differ when the 95% CL overlap. Values in the parentheses indicate sample size. DIN, dissolved inorganic nitrogen; Pharm, pharmaceutical compounds.

Figure 3

Effects of water stress on algal biomass as a function of climate, rainfall pattern, period of the year, river size, type of water stress and nutrient status, given by the response ratio (R = Impacted/Control; ±95% Confidence Limit, CL). The dashed line (mean effect size = 1) indicates no effect. Mean effect size >1 indicates an increase, while mean effect size <1 indicates a decrease due to water stress. The effect of water stress is significant when the 95% CL does not overlap 1 (black symbols). Levels within a given moderator (in bold) do not significantly differ when the 95% CL overlap. Values in the parentheses indicate sample size.

Figure 4

(Top) Effects of water stress on invertebrate abundance, density, diversity and richness. (Bottom) Effects of water stress on macroinvertebrate richness as a function of climate, rainfall pattern, period of the year, river size and the type of water stress, given by the response ratio (R = Impacted/Control; ±95% Confidence Limit, CL). The dashed line (mean effect size = 1) indicates no effect. Mean effect size >1 indicates an increase, while mean effect size <1 indicates a decrease due to water stress. The effect of water stress is significant when the 95% CL does not overlap 1 (black symbols). Levels within a given moderator (in bold) do not significantly differ when the 95% CL overlap. Values in the parentheses indicate sample size.

Figure 5

Effects of water stress on ecosystem functions (organic matter breakdown, gross primary production and respiration), given by the response ratio (R = Impacted/Control; ±95% Confidence Limit, CL). The dashed line (mean effect size = 1) indicates no effect. Mean effect size >1 indicates an increase, while mean effect size <1 indicates a decrease due to water stress. The effect of water stress is significant when the 95% CL does not overlap 1. Levels within a given moderator (same symbol) do not significantly differ when the 95% CL overlap. Values in the parentheses indicate sample size.

Figure 6

Effects of water stress on organic matter breakdown, as modulated by climate, rainfall pattern, period of the year, river size, type of water stress and nutrient status, given by the response ratio (R = Impacted/Control; ±95% Confidence Limit, CL). The dashed line (mean effect size = 1) indicates no effect. Mean effect size >1 indicates an increase, while mean effect size <1 indicates a decrease due to water stress. The effect of water stress is significant when the 95% CL does not overlap 1 (black symbols). Levels within a given moderator (in bold) do not significantly differ when the 95% CL overlap. Values in the parentheses indicate sample size.