Life-history responses of a freshwater rotifer to copper pollution

Abstract

In organisms with dormant stages, life-history responses to past pollution can be studied retrospectively. Here, we study such responses in a rotifer (Brachionus calyciflorus) from the once heavily copper-polluted Lake Orta (Italy). We extracted resting eggs from sediments, established clonal lineages from hatchlings, and exposed newborns of these lineages to one of three copper concentrations that each mimicked a specific period in the lake's pollution history. For each rotifer, we daily collected life-table data. We then estimated treatment-specific vital rates and used a stage-structured population model to project population growth rate λ. We also estimated elasticities of λ to vital rates and contributions of vital rates to observed Δλ between copper treatments. As expected, λ decreased with increasing copper concentration. This decrease resulted mostly from a decline in juvenile survival rate (S_J ) and partly from a decline in the survival rate of asexually reproducing females (S_A ). Maturation rate, and with one exception fecundity, also declined but did not contribute consistently to Δλ. λ was most elastic to S_J and S_A , indicating that survival rates were under stronger selection than maturation rate and fecundity. Together, our results indicate that variation in juvenile survival is a key component in the rotifers' copper response. The consistent decrease in S_J with increasing copper stress and the sensitivity of λ to that decrease also suggest that juvenile survival is a useful indicator of population performance under environmental pollution.

Keywords: copper; life history; pollution; population growth; rotifer; vital rate.

FIGURE 1

Life‐cycle diagram for B. calyciflorus rotifers and corresponding population projection matrix. (a) Schematic representation of the brachionid life cycle including juveniles (subscript J) as well as mictic and amictic adults (subscripts M and A). S_J , S_A , and S_M are survival rates, T_J is maturation rate, A_J is the probability of being an amictic individual, and F_A is the fecundity of amictic adults. Arrow‐headed solid and broken lines represent stage transitions and reproductive events, respectively. Note that the reproductive type (mictic or amictic) is unknown at the juvenile stage. As a consequence, juveniles are pooled into a single stage. (b) Population projection matrix derived from the life cycle

FIGURE 2

Vital‐rate estimates. Rotifers from the peak‐pollution and the recovery population were subjected to one of three copper concentrations: 0, 40, or 80 µg Cu L^–1. Lower and upper edges of boxes depict the first and third quartiles, respectively; lines inside the boxes represent medians; whiskers extend to maximally 1.5 times the interquartile range; dots beyond whiskers indicate outliers. Survival rates S_J , S_A , and S_M and maturation rate T_J are daily rates; A_J is the probability of being an amictic individual; and F_A is the daily number of female offspring per amictic adult. Vital‐rate abbreviations are as in Figure 1

FIGURE 3

Asymptotic population growth rate λ. Rotifers from the peak‐pollution and the recovery population were subjected to one of three copper concentrations: 0, 40, or 80 µg Cu L^–1. Estimates of λ are based on daily collected life‐table data. Values above and below the dotted line indicate positive and negative population growth, respectively. Boxplot conventions are as in Figure 2

FIGURE 4

Elasticity of asymptotic population growth rate λ. Rotifers from the peak‐pollution and the recovery population were subjected to one of three copper concentrations. From left to right within vital rates, boxplot triplets represent treatments 0, 40, and 80 µg Cu L^–1 (cf. arrows). Elasticity values are proportional changes in λ caused by proportional changes in vital rates. Vital‐rate abbreviations and boxplot conventions are as in Figures 1 and 2, respectively

FIGURE 5

LTRE contributions to differences in asymptotic population growth rate λ. Rotifers from the peak‐pollution and the recovery population were subjected to one of three copper concentrations (cf. subscripts to λ). Shown are the contributions of each vital rate to the overall treatment effect (Δλ) resulting from increasing the copper concentration from 0 to 40 µg Cu L^–1 (top row) and from 40 to 80 µg Cu L^–1 (bottom row). When Δλ is negative (top right panel and bottom panels), negative (positive) contributions of vital rates support (oppose) Δλ. Vital‐rate abbreviations and boxplot conventions are as in Figures 1 and 2, respectively