Stressor dominance and sensitivity‐dependent antagonism: Disentangling the freshwater effects of an insecticide among co‐occurring agricultural stressors

Abstract

1. Pesticide concentrations are correlated with regional declines in stream invertebrate diversity. Experimental studies have identified that pesticides can have strong and persistent negative effects on aquatic ecosystems. These effects may occur at concentrations orders of magnitude lower than laboratory toxicity studies predict.Synergism among stressors is one explanation for observed laboratory-field differences. However, the true effect of pesticides on stream invertebrates remains uncertain, given interactions between stressors and natural environmental conditions. 2. We experimentally examined multiple-stressor effects on stream invertebrate assemblages and leaf litter breakdown using 24 independent ~900 L re-circulating outdoor mesocosms in a semi-orthogonal design. Two pulses of the pesticide malathion (C 10 H 19 O 6 PS 2 ) were delivered at low and high concentrations (Pulse 1: low at 0.1 and high at 1 µg/L; Pulse 2: at 2.5 and 25 µg/L). These were crossed with a treatment combining stressors commonly associated with agricultural development; nitrogen, phosphorus and sediment (kaolinite). Malathion degradation was rapid (<24 hr half-life) in all treatments, likely becauseof photolysis, hydrolysis, the presence of biofilms and sorptive processes. There were significant differences in invertebrate assemblages between treatments, where malathion contributed to 48% and 87% of deviance during Pulse 1 and 2 respectively. Malathion had strong negative effects during Pulse 2, with declines occurring between control and high pesticide treatments in Ephemeroptera, Plecoptera and Trichoptera abundances (Cohen's d = 3.08), invertebrate richness (d = 3.57) and total abundance (d = 3.31). Despite the dominant effects of malathion on invertebrates, agricultural stressorsinhibited rates of leaf litter breakdown (p < 0.05), and weakly mitigated malathion toxicity in mesocosms (e.g. PERMANOVA, P ≈ 0.1). Malathion breakdown analysis indicated sediment addition reduced malathion concentrations through alkaline |

Interactions with freshwater biofilms cause rapid removal of common herbicides through degradation – evidence from microcosm studies

“…11 Through leaching or spray dri, pesticides commonly enter surface waters where they pose a risk to non-target aquatic organisms. 12 For example, pesticides negatively affect the abundance of invertebrates, 13 as well as the density, antioxidant defence and photosynthetic efficiency of diatoms, 14 although the latter may reect shortterm inhibition. 15 Herbicides are commonly more watersoluble than insecticides and fungicides and thus comprise a large fraction of the pesticides frequently detected in surface water monitoring.…”

Changes in community composition determine recovery trajectories from multiple agricultural stressors in freshwater ecosystems

“…The majority of studies assessing the effects of multiple stressors usually describe changes in community composition following disturbance (Barmentlo et al, 2018;Bray et al, 2019;Halstead et al, 2014;Matthaei et al, 2010;, and only few focused on describing post-disturbance recovery trajectories Rico et al, 2018). However, whether changes in community composition in one trophic level (or organism group) influence ecological responses of other trophic levels and their recovery dynamics is rarely studied.…”

“…Particularly, the freshwater realm has been highlighted as heavily impacted by a large number of disturbances, such as chemical pollution (Arenas-Sánchez et al, 2019; Rico et al, 2019), eutrophication (Woodward, 2012) and altered flow regimes (Matthaei, Piggott, & Townsend, 2010). The majority of studies assessing the effects of multiple stressors usually describe changes in community composition following disturbance (Barmentlo et al, 2018; Bray et al, 2019; Halstead et al, 2014; Matthaei et al, 2010; Piggott et al, 2015; Piggott, Townsend, & Matthaei, 2015), and only few focused on describing post-disturbance recovery trajectories (Arenas-Sánchez et al, 2019; Barmentlo et al, 2019; Rico et al, 2018). However, whether changes in community composition in one trophic level (or organism group) influence ecological responses of other trophic levels and their recovery dynamics is rarely studied.…”

“…Pesticides reaching freshwater ecosystems act as selective stressors targeting different organisms based on their physicochemical properties and toxicological mode of action, leading to non-random effects on communities (De Laender et al, 2016). Pesticides are known to reduce diversity (Bray et al, 2019), produce shifts in trophic interactions (Schrama et al, 2017) and affect ecosystem functioning (Spaak et al, 2017). Linking knowledge about the toxicological properties of pesticides and the implications for the whole food web is crucial to predict the net effects of complex chemical mixtures on aquatic ecosystems.…”

“…Therefore, shifts in community composition imposed by changes in environmental conditions may also determine changes in single and multiple stressor effects. Nutrient enrichment is a common agricultural stressor, which contributes to eutrophication and to modify environmental conditions (Piggott, Townsend, and Matthaei 2015; Bray et al 2019; Barmentlo et al 2019). Increased nutrient concentrations can push ecosystems to a regime shift (Scheffer & Van Nes, 2007), modify community structure (Barmentlo et al 2019) and decrease biodiversity, favouring a more homogenized community across a variety of ecosystems (Donohue et al, 2009; Isbell et al, 2013; Licursi, Gómez, & Sabater, 2016).…”