Traits and stress: keys to identify community effects of low levels of toxicants in test systems

Abstract

Community effects of low toxicant concentrations are obscured by a multitude of confounding factors. To resolve this issue for community test systems, we propose a trait-based approach to detect toxic effects. An experiment with outdoor stream mesocosms was established 2-years before contamination to allow the development of biotic interactions within the community. Following pulse contamination with the insecticide thiacloprid, communities were monitored for additional 2 years to observe long-term effects. Applying a priori ecotoxicological knowledge species were aggregated into trait-based groups that reflected stressor-specific vulnerability of populations to toxicant exposure. This reduces inter-replicate variation that is not related to toxicant effects and enables to better link exposure and effect. Species with low intrinsic sensitivity showed only transient effects at the highest thiacloprid concentration of 100 μg/l. Sensitive multivoltine species showed transient effects at 3.3 μg/l. Sensitive univoltine species were affected at 0.1 μg/l and did not recover during the year after contamination. Based on these results the new indicator SPEAR (mesocosm) was calculated as the relative abundance of sensitive univoltine taxa. Long-term community effects of thiacloprid were detected at concentrations 1,000 times below those detected by the PRC (Principal Response Curve) approach. We also found that those species, characterised by the most vulnerable trait combination, that were stressed were affected more strongly by thiacloprid than non-stressed species. We conclude that the grouping of species according to toxicant-related traits enables identification and prediction of community response to low levels of toxicants and that additionally the environmental context determines species sensitivity to toxicants.

Fig. 1

Abundance (a) and taxa richness (b) of aquatic macroinvertebrates (log(x + 1)-transformed number of individuals per square metre and number of taxa, respectively). Asterisks indicate significant (P < 0.05, ANOVA, Games–Howell post hoc tests) differences from the controls. Vertical dashed lines show contamination events

Fig. 2

Abundance as a percentage of the control values for the four trait-based groups of aquatic macroinvertebrates: nonsensitive multivoltine (a), sensitive multivoltine (b), non-sensitive univoltine (c), and sensitive univoltine (d). Asterisks indicate significant (P < 0.05, ANOVA, Games–Howell post-hoc tests) differences from the controls. Vertical dashed lines show contamination events

Fig. 3

Principal response curves (PRC) (a) and the SPEAR _mesocosm index (b), which indicate the proportion of sensitive taxa, are shown as a percentage of the control values. The large black asterisks at the top of the graphs indicate significant (P < 0.05) effects of the toxicant in the whole model (all concentrations) as tested by Monte Carlo permutation tests followed by RDA for PRC (a) and ANOVA and Games–Howell post hoc tests for SPEAR_mesocosm (b). The small coloured asterisks indicate significant (P < 0.05) effects at particular concentrations tested by the same methods

Fig. 4

Effect of thiacloprid at 0.1 μg/l on species that were characterised by decline or increase of abundance in control mesocosms (right and left of the dashed line, respectively) over the 2 years after the first contamination (between-year change of abundance, factor of multiplication). The horizontal lines indicate the mean values for the two groups (i.e. decline and increase). Asterisks above the line indicate the difference from the control, whereas the asterisk below the line indicates the difference between the two groups (P < 0.05, one-sample t-test)