Transient effects following peak exposures towards pesticides – An explanation for the unresponsiveness of in situ measured functional variables

Highlights

• Biomonitoring tools integrating over long time spans do not catch transient effects.

• Bioassay with daily resolution shows recovery and post-exposure compensation.

• Monitoring for effects of pulsed stress events needs adequate temporal resolution.

Abstract

Invertebrate-mediated leaf litter decomposition is frequently used to assess stress-related implications in stream ecosystem integrity. In situ measures such as the mass loss from leaf bags or the feeding of caged invertebrates deployed for days or weeks may, however, fail to detect transient effects due to recovery or compensatory mechanisms. We assessed the relevance of transient effects using the peak exposure towards an insecticide (i.e., etofenprox) as a model scenario at three levels of complexity. These were 1) the assessment of the decomposition realised by invertebrate communities in stream mesocosms over 21 days via leaf bags, 2) 7-days lasting in situ bioassays quantifying the leaf consumption of Gammarus fossarum, and 3) a laboratory experiment determining the daily feeding rate of the same species over 7 days. Etofenprox did not trigger a significantly altered decomposition by invertebrate communities during the leaf bag assay, while in situ bioassays detected a significant reduction in gammarids’ feeding rate at the highest tested concentration. The laboratory bioassay suggests that observed mismatches might be explained by recovery and post-exposure compensation. As leaf-shredding invertebrates are likely in a vulnerable state following transient effects, biomonitoring for implications of peak exposures and other pulsed stress events must happen at an adequate temporal resolution.

Introduction

The use of functional variables as a measure for the impact of stressors in the integrity of ecosystems is becoming increasingly popular. In this context, the decomposition of allochthonous organic material (i.e., leaf litter in leaf bags) is frequently used to determine effects of point and non-point sources of chemical stressors including wastewater treatment plant effluents and agricultural edge of field spray drift or run off, respectively (Englert et al., 2013, Schäfer et al., 2007). However, as multiple species and trophic levels interactively perform this ecosystem process, differing sensitivities in combination with functional redundancy within local communities might buffer the translation of behavioural or structural into functional responses (Pascoal et al., 2005, Rasmussen et al., 2012). A potential methodological solution to this is the use of in situ bioassays, which involve locally relevant leaf-shredding organisms caged individually or in small groups together with pre-weighed portions of leaves and thus allow only for limited interactions with the stream's community (Matthiessen et al., 1995, Schulz and Liess, 1999). These assays may be of limited ecological relevance if used as exclusive measure for impacts at the functional level. However, they might help to explain effect patterns observed during studies involving leaf bags or even to uncover incidences of stress affecting the functional performance of involved organisms that are not detectable using the leaf bag method (Englert et al., 2013).

For experimental ease and to integrate functional responses over a period of time considered relevant by the experimenter, in situ bioassays and leaf bags are usually deployed in the field for several days and weeks, respectively (Englert et al., 2013). Such durations may, however, allow for recovery or even post-exposure compensation (e.g., by increased leaf consumption) of shredders if detrimental effects are only transient (Agatz et al., 2014). Such phenomena may be particularly relevant for peak exposures towards chemical stressors, which are often observed in agricultural stream ecosystems and might explain the lack of effects in studies assessing these exposure scenarios (Rasmussen et al., 2012, Schulz, 2004).

The present study assessed the relevance of such transient effects for the quantification of functional responses as a consequence of a 6-h pyrethroid insecticide (i.e., etofenprox) peak exposure – in both mesocosm and laboratory experiments. A tiered testing approach was followed, which included in a first step the assessment of potential effects of this exposure scenario on the leaf litter decomposition realised by a complex community of invertebrates in stream mesocosms. This leaf bag study was supplemented by in situ bioassays (i.e., caged organisms within the mesocosms; cf. Hopkin, 1993) quantifying the mortality and feeding rate of a key species in the leaf litter decomposition in central European streams (i.e., Gammarus fossarum; Dangles et al., 2004). Selection of this particular species was further prompted by the reported correlation between the in situ measured feeding rates of gammarids and local leaf litter decomposition (Maltby et al., 2002). In a third step, the etofenprox peak exposures were simulated under laboratory conditions, while the feeding rate of G. fossarum was determined on a daily basis, allowing for the identification of potential post-exposure recovery and compensation (Agatz et al., 2014).