Direct effects of elevated dissolved CO2 can alter the life history of freshwater zooplankton

Abstract

Dissolved CO₂ levels (pCO₂) are increasing in lentic freshwaters across the globe. Recent studies have shown that this will impact the nutritional quality of phytoplankton as primary producers. However, the extent to which freshwater zooplankton may also be directly affected remains unclear. We test this in three model species representative of the main functional groups of primary consumers in freshwaters; the water flea Daphnia magna, the seed shrimp Heterocypris incongruens and the rotifer Brachionus calyciflorus. We experimentally exposed individuals to three pCO₂ levels (1,500; 25,500 and 83,000 ppm) to monitor changes in life history in response to current, elevated and extreme future pCO₂ conditions in ponds and shallow lakes. All species had reduced survival under the extreme pCO₂ treatment, but the water flea was most sensitive. Body size and reproduction were reduced at 25,500 ppm in the water flea and the seed shrimp and population growth was delayed in the rotifer. Overall, our results show that direct effects of pCO₂ could impact the population dynamics of freshwater zooplankton. By differentially modulating the life history of functional groups of primary consumers, elevated pCO₂ has the potential to change the evolutionary trajectories of populations as well as the ecological functioning of freshwater communities.

Figure 1

Kaplan–Meier curves showing survival probability over time of three freshwater zooplankton species subjected to a control (C = 1,520 ppm), an elevated (T1 = 25,609 ppm) and an extreme (T2 = 83,201 ppm) pCO₂ treatment. Log-rank test results are shown for (a) individual survival of the water flea D. magna, (b) individual survival of the seed shrimp H. incongruens and (c) survival of clonal populations of the rotifer B. calyciflorus. Plus signs ( +) indicate right censored data. Significant differences are indicated with asterisks: *p < 0.05, **p < 0.01, ***p < 0.001.

Figure 2

Life history responses of three freshwater zooplankton species subjected to a control (C = 1,520 ppm; blue circle), an elevated (T1 = 25,609 ppm; orange triangle) and an extreme (T2 = 83,201 ppm; red square) pCO₂ treatment including water flea (D. magna) (a) somatic growth rate per experimental interval (day 1–6 = int. 1, 6–17 = int. 2, 17–24 = int. 3), (b) body size in time in µm; seed shrimp (H. incongruens) (c) somatic growth rate per experimental interval (day 0–7 = int. 1, 7–17 = int. 2, 17–24 = int. 3), (d) body size in time in µm; and rotifer (B. calyciflorus) (e) population growth rate per experimental interval (day 0–6 = int. 1, 6–16 = int. 2, 16–23 = int. 3) and (f) population size in time. Error bars represent standard errors. Significant differences are indicated with asterisks: *p < 0.05, **p < 0.01, ***p < 0.001.

Figure 3

Lifetime fecundity responses of three freshwater zooplankton species subjected to a control (C = 1,520 ppm), an elevated (T1 = 25,609 ppm) and an extreme (T2 = 83,201 ppm) pCO₂ treatment. Water flea (D. magna) and seed shrimp (H. incongruens) responses represent total number of neonates per female. Rotifer (B. calyciflorus) response represents total number of females with eggs per population. Error bars represent standard errors. Significant differences are indicated with asterisks: *p < 0.05, **p < 0.01, ***p < 0.001.