Invasive Species Mediate Insecticide Effects on Community and

Mar 22, 2018 - Faculty of Fisheries and Protection of Waters, South Bohemian Research Center of Aquaculture and Biodiversity of Hydrocenoses, Universi...
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Ecotoxicology and Human Environmental Health

Invasive species mediate insecticide effects on community and ecosystem functioning. Andreia CM Rodrigues, Ana L Machado, Maria D Bordalo, Liliana Saro, Fátima C. P. Simão, Rui MJ Rocha, Oksana Golovko, Vladimir Zlabek, Carlos Barata, Amadeu M.V.M. Soares, and João L. T. Pestana Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b00193 • Publication Date (Web): 22 Mar 2018 Downloaded from http://pubs.acs.org on March 24, 2018

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Abstract Art:

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Title: Invasive species mediate insecticide effects on community and ecosystem functioning.

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Andreia C. M. Rodrigues1,3*, Ana L. Machado1, Maria D. Bordalo1, Liliana Saro1, Fátima C. P.

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Simão1, Rui J. M. Rocha1, Oksana Golovko2, Vladimír Žlábek2, Carlos Barata3, Amadeu M. V. M.

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Soares1, João L. T. Pestana1

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Santiago, 3810-193 Aveiro, Portugal

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Departamento de Biologia & CESAM, Universidade de Aveiro, Campus Universitário de

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South Bohemian Research Center of Aquaculture and Biodiversity of Hydrocenoses, Vodnany,

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Czech Republic

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Spain

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*Correspondence and requests for materials should be addressed to A.C.M.R. (email:

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[email protected])

University of South Bohemia in Ceske Budejovice, Faculty of Fisheries and Protection of Waters,

Department of Environmental Chemistry (IDAEA-CSIC), Jordi Girona, 18-26, 08034 Barcelona,

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ABSTRACT:

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Anthropogenic activities increase pesticide contamination and biological invasions in freshwater

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ecosystems. Understanding their combined effects on community structure and on ecosystem

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functioning presents challenges for an improved ecological risk assessment. This study focuses on

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an artificial stream mesocosms experiment testing for direct and indirect effects of insecticide

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(chlorantraniliprole – CAP) exposure on the structure of a benthic macroinvertebrate freshwater

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community and on ecosystem functioning (leaf decomposition, primary production). To understand

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how predator identity and resource quality alter the community responses to chemical stress, the

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mediating effects of an invasive predator species (crayfish Procambarus clarkii) and detritus

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quality (tested by using leaves of the invasive Eucalyptus globulus) on insecticide toxicity were

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also investigated. Low concentrations of CAP reduced the abundance of shredders and grazers,

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decreasing leaf decomposition and increasing primary production. Replacement of autochthonous

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predators and leaf litter by invasive species decreased macroinvertebrate survival, reduced leaf

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decomposition and enhanced primary production. Structural equation modeling (SEM) highlighted

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that CAP toxicity to macroinvertebrates was mediated by the presence of crayfish or eucalypt leaf

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litter which are now common in many Mediterranean freshwaters. In summary, our results

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demonstrate that the presence of these two invasive species alter the effects of insecticide exposure

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on benthic freshwater communities. The approach used here also allowed to mechanistically

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evaluate the indirect effects on ecosystem functional endpoints of these stressors and of their

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interaction, emphasizing the value of incorporating biotic stressors in ecotoxicological experiments.

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INTRODUCTION:

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Freshwater ecosystems provide essential resources for humans and are considered rich habitats

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with a great diversity of organisms. Nevertheless, these ecosystems are frequently exposed to

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several anthropogenic pressures that lead to their degradation and loss of biodiversity, making their

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recovery one of the priority actions of governments and management policies nowadays 1,2. A great

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challenge for risk assessment is set by the combinations of pesticides and natural stressors that are

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probable to co-occur and whose combined effects on freshwater communities are difficult to

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predict 3–5. Hence, there is a growing awareness that pesticide effects may be misjudged due to

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interactions with biotic stressors and, increasing the complexity of ecotoxicology testing is

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essential for a better ecological risk assessment 3,6,7. Moreover, biological invasions are nowadays

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of the most significant causes of biodiversity loss and ecosystem alterations within freshwaters 8,

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which will further add complexity to the assessment of contaminants effects. Because shifts in

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species composition have been shown to affect community dynamics 9 and ecosystem functions,

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such as litter decomposition and productivity 10, and thus it is critical to investigate the potential

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role of invasive species in altering the community context as well as becoming an important

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mediator of contaminant effects on natural ecosystems 4,11.

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In freshwater ecosystems and within detritus based food chains, density and trait mediated effects

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of predation and resource quality (leaf litter) may shape ecosystem functioning 12,13. These natural

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biotic stressors and the effects of non-indigenous predators and leaf litters are thus potential

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mediators of changes on organisms and populations susceptibility to chemicals 14–17.

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Predators of invertebrate detritivores are known to influence trophic dynamics and elemental

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cycling through effects on leaf litter decomposition, a key ecosystem process in many freshwater

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systems 16. However, the mechanisms of such influences are not well understood and there is an

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increased debate about the strength of non-trophic interactions and habitat complexity 18. Among

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these predators, omnivorous macroinvertebrates are usually top consumers that may decouple

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cascading trophic chains through both direct effects on the food resource and indirect effects on

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intermediate consumers 19,20. For this study we selected the red swamp crayfish, Procambarus

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clarkii, native to northeastern Mexico and southcentral USA that has become a dominant predator

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in several European rivers 21. This crayfish has many properties of a successful invader (plastic life

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cycle, generalist and opportunistic feeding habits, behavioral plasticity when coping with other

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predator species), allowing rapid dispersion and high tolerance towards environmental extremes

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22,23

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Regarding leaf litter, the replacement of native riparian vegetation by exotic plantations that is

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occurring worldwide poses an extra challenge to freshwater benthic detritivore communities,

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especially in low order streams 24,25. The input of allochthonous leaf litter of different nutritional

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quality, litter fall timing and quantity has the potential to alter benthic invertebrate assemblages and

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the structure as well as functioning of these communities 16,26. Eucalyptus globulus, which is widely

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planted in Portugal, has demonstrated invasive behaviour, since it can grow aggressively outside

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plantations 27,28. Nowadays, E. globulus is frequently found on the riparian vegetation throughout

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southern European countries, replacing native species such as Alnus glutinosa and changing

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organic matter dynamics 29,30. Eucalyptus leaves have lower nutritional quality for freshwater

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detritivores and higher quantities of essential oils as well as polyphenols (that are toxic to most

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freshwater organisms), than leaves of European native species (e.g., A. glutinosa) 31–33..

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The present study intends to evaluate the combined effects of insecticides and invasive species on

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freshwater benthic macroinvertebrate community structure and function, with special focus on

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detritus-based stream food webs 34. Chlorantraniliprole (CAP), which belongs to the anthranilic

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diamides pesticides group, was used as model insecticide. CAP readily won the pesticides market

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due to successful action against target species at low doses and high selectivity toward ryanodine

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receptor of insects 35,36. Nevertheless, feeding and development impairment have already been

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reported for freshwater non-target invertebrates exposed to environmentally relevant CAP

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concentrations 35,37–40.

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To test the hypothesis that direct and indirect effects of environmentally relevant insecticide

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exposure on macroinvertebrates communities and on ecosystem functioning, can be mediated by

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predator identity and/or resource quality, we performed mesocosms experiments where these

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different stressors were combined. For that, we exposed a natural benthic invertebrate community

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to different combinations of stressors: insecticide exposure (0 vs 2 µg/L CAP), predator identity

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(native odonate Cordulegaster boltonii vs invasive crayfish P. clarkii) and different leaf litter

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(leaves from the native A. glutinosa vs the invasive E. globulus). Direct and indirect effects of

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insecticide exposure and the presence of invasive species as well as of their interactions, were

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assessed on community structure (detritivore abundances) and on ecosystem functional endpoints

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(leaf decomposition and primary production). We predicted that pesticide exposure, replacement of

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dragonfly predator by crayfish and of alder leaf litter by eucalypt leaf litter, and their combination

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would negatively affect leaf decomposition while enhancing primary production either though

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density or trait mediated effects in invertebrate community. Primary producers and predator species

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are, in general, more tolerant to this insecticide 35 so, no direct effects of CAP were expected for

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these groups.

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Structural Equation Modeling (SEM) was used to mechanistically discriminate and relate indirect

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effects and interactions. Through path analysis we can quantify the relative weight of direct,

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indirect and interaction effects of the studied stressors on community structure, linking changes in

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different invertebrate functional groups to alterations on ecosystem functional parameters.

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METHODS

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Macroinvertebrate and leaf collection.

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The Mau river (Sever do Vouga, 40º 44’ 21.2’’ N, 8º 24’ 6.9’’ W, Portugal) was selected for

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invertebrate sampling as this stream presents a good ecological quality status 41. Prior to the

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experiment and to determine the composition of benthic macroinvertebrate communities in the

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chosen aquatic system, four quantitative samples were taken using an enclosed surber sampler

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(0.09 m2, 500 µm mesh), in winter (January 2016), from slow-flow riffle habitats.

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Macroinvertebrate samples were preserved in 70% ethanol and were identified to family level,

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using a stereomicroscope. The density of each taxon in the natural habitat was then calculated,

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based on the mean values of the four samples. These density values were then used as reference to

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distribute the different taxa among the artificial streams, according to their natural densities.

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Juveniles of Procambarus clarkii Girard were collected using funnel traps in channels of Ria de

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Aveiro, central Portugal. Organisms were acclimated to laboratory conditions for at least one

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month before the experiment (15 ºC, 16 h light: 8 h dark photoperiod).

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Alnus glutinosa (L.) Gaertn leaves were collected soon after senescence from the riparian

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vegetation at São Pedro de Alva (40º 16’ 37’’N, 8º 11’ 51.72’’ W), central Portugal. Leaves of

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Eucalyptus globulus Labill. were abscised during autumn at Boialvo (40º 30’ 4.2’’ N, 8º 20’ 37.0’’

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W), central Portugal. Both types of leaves were air dried and stored in darkness.

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Description of the mesocosm experiments.

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The present study was conducted in an indoor modular mesocosms system, in a total of 24 glass

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artificial streams (individual streams: 2 m length, 0.200 m width, 0.225 m depth) on recirculated

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water regime. Flow in each artificial stream was maintained at a constant rate of 3.75 L/min,

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similar to the hydraulic conditions present in nearby natural riffles. The aqueous media used was

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artificial pond water (APW) 42, enriched with phosphate, nitrate and silicate to meet realistic

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concentrations of these minerals in Mau river water 41. The mesocosm room was maintained at

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constant temperature of 15 ± 1 ºC and with a photoperiod of 16 h light: 8 h dark, to simulate field

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conditions. The artificial streams were filled with enriched APW medium (~ 280 L) and

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commercial river sand (~ 2 cm layer), which was previously heated at 500 ºC for 4 h to avoid

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contamination with possible adsorbed compounds. The varied granulometry of the commercial

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river sand suits the different needs of the species in our community.

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Alder and eucalyptus leaves were previously conditioned for 14 days in separate plastic buckets

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with aerated natural river water, which allowed leaching33 and a similar microbial conditioning 43.

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Leaves were then dried at 50 ºC for 4 days, weighed and placed in the respective experimental

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streams in leaf packs with a coarse mesh (10 mm mesh size). The mean initial weight of alder and

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eucalypt leaves available per artificial stream was 1660 ± 54.4 mg and 1696.6 ± 68.3 mg (mean ±

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SD), respectively. Ten unglazed ceramic tiles (20 cm2) were added along each artificial stream as

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standardized substrate for posterior periphyton sampling. Stones from the Mau River were scraped

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with toothbrushes and washed with water from the site to obtain a final volume of ~ 5 L of

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concentrated biofilm. In the same day of collection, 100 mL of this concentrated inoculum of

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biofilm was added to each artificial stream and recirculated through the experimental streams for

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15 days to allow for biofilm colonization.

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On the 15th day (day 0 of exposure period), benthic macroinvertebrates were collected by kick-

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sampling in riffle and pool habitats using a 500 µm mesh pond net in Mau river in February 2016.

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Macroinvertebrates were transferred in river water to the laboratory, immediately sorted by taxa

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and placed into the artificial streams based on the previously calculated densities. Individuals of the

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same taxa were selected by eye based on their similar size and distributed in equal numbers per

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artificial stream. A total number of 217 macroinvertebrates were thus distributed per artificial

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stream, representing 17 taxa, with the main representative functional feeding groups in terms of

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abundance being shredders (62 individuals), collectors (95 individuals) and grazers (49 individuals

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per artificial stream) (SI Table S1). The macroinvertebrate community was exposed for 7 days to

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the combinations of: insecticide (0 or 2 µg/L CAP), predator identity (the native Cordulegaster

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boltonii or the invasive Procambarus clarkii) and leaf litter from two different tree species (the

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native A. glutinosa or the invasive E. globulus) (Figure 1). Each treatment combination was

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replicated in 3 artificial streams. One odonatan or one crayfish were allocated per artificial stream,

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with mean fresh weight of 712 ± 114 mg and 744 ± 177 mg (mean ± SD), respectively.

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Figure 1. Schematic representation of experimental design.

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End of the experiment assessment.

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Water physico-chemical parameters (temperature, pH, conductivity and dissolved oxygen) of the

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artificial streams were assessed every two days and remained similar across all treatments

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throughout the trial, with mean values of: 15.3 ± 0.2 ºC, pH of 8.0 ± 0.4, 810 ± 13.7 µS/cm, 9.7 ±

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0.3 mg O2/L.

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After 7 days of exposure period, leaf packs were carefully rinsed in distilled water, sediment

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particles removed with a soft paint brush, and the attached invertebrates retrieved. Remaining leaf

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material was dried at 50 ºC for 4 days and weighed. Ceramic plates were removed from each

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artificial stream, scrubbed with toothbrushes and rinsed with water to individual plastic flasks.

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Samples for periphyton identification were fixed with 5 % formaldin and stored in darkness.

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Samples for chlorophyll measurement were kept in the dark, at 4 ºC for 24 h. These samples were

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then filtered through GF/C filters (1.2 µm; Whatman Inc.) and stored in darkness at – 20 ºC until

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analysis. Extraction was performed with 90 % acetone and chlorophyll a measured

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spectrophotometrically following Jeffrey and Humphrey (1975). Chlorophyll a concentration was

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used as an estimation of periphyton primary production. All the macroinvertebrates per artificial

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stream were collected by sieving the sediment and then preserved in 70 % ethanol for later

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identification, using a stereomicroscope.

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Insecticide contamination.

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A stock solution of CAP (analytical standard, CAS No. 500008-45-7, Sigma-Aldrich) was prepared

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in analytical grade acetone, protected from light to avoid degradation. Water was spiked to a

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nominal concentration of 2 µg/L CAP in experimental streams (keeping acetone below 0.01 %).

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The selected nominal CAP concentration represents a realistic measured concentration in

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freshwaters 40 and also a sub-lethal concentration for most of the detritivore freshwater

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invertebrates 35,37,38. Qi and Casida 45 found no significant binding activity of CAP to ryanodine

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receptor of a decapoda species. Also, Barbee et al. 46 tested the sensitivity of P. clarkii to CAP and

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reported a no-observed effect concentration of 480 µg/L. The native representative predator

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Cordulegaster boltonii showed similar tolerance to CAP exposure: > 640 µg/L (acute exposure)

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and > 96 µg/L (feeding inhibition test) (personal observation, data not shown). Nominal CAP

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concentrations in overlying water (sampled at 2 h, 96 h and 7 days of exposure) were verified by

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chemical analysis. All samples were frozen at − 20 °C and sent in dry ice for analysis. Liquid

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chromatography mass spectrometry (LC-MS) was used to quantify CAP on water samples (detailed

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description of the chemical analysis in SI).

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Statistical analysis.

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A t-test compared the number of organisms per stream on day 0 and day 7 in the control streams

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(no-CAP and no invasive species presence) to exclude effects on survival of organisms due to

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factors other than the tested stressors.

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Differences in macroinvertebrate community structure among treatments (CAP exposure, predator

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identity and leaf litter type) were first tested globally by Permutational Multivariate Analysis of

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Variance (PERMANOVA) using the absolute abundance of all the present taxa. Bray-Curtis

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dissimilarity was used as a distance measure and the significance of the analysis was tested using

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Monte Carlo permutation method with 999 permutations 47. These analyses were performed on R

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software 3.2.2 version, from The R Foundation for Statistical Computing (Vienna, Austria), using

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the ‘vegan’ package 48.

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Additionally, for functional analysis, macroinvertebrates were grouped into three functional

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feeding groups according to Usseglio‐Polatera et al.49 and Tachet et al.50: shredders

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(Sericostomatidae, Lepidostomatidae, Calamoceratidae and Limnephilidade), collectors

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(Ephemeridae and Chironomidae) and grazers/scrapers (Habrophlebiidae, Baetidae and

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Heptageniidae). The abundance of individuals in each feeding group per artificial stream was

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calculated. These functional groups include the main representative taxa of our community,

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accounting for ~ 88 % of initial number of the organisms per stream, and also three important

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feeding groups regarding leaf decomposition/ grazing and nutrient cycling 51.

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Macroinvertebrates abundances per functional feeding group, leaf decomposition and periphyton

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primary production data were analyzed by three-way analysis of variance (ANOVA). This

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procedure identified statistically significant differences between treatments with contamination by

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CAP, resource quality and predator identity as fixed factors. Normality of data was assessed using

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the Shapiro-Wilk normality test and residual plots, plus homoscedasticity was verified with the

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Brown-Forsythe test. The significance level was set at 0.05 and calculations were performed using

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GraphPad Prism version 7.00 for Windows (GraphPad Software, La Jolla California USA).

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A Structural Equation Modelling (SEM) model was used to investigate mechanistic pathways that

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may explain the direct, indirect and total effects 52,53 of CAP exposure, predator identity and leaf

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litter type on community composition (abundance of shredders, collectors and grazes), and on

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ecosystem functions (leaf decomposition and primary production). By using SEM to interpret the

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mesocosm results, a refined hypothesized model was obtained to evaluate the effects of insecticides

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and invasive species at the community and ecosystem levels. Path coefficients (partial multiple

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regression coefficients), with the values of each variable standardized to a common metric, allow to

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compare the relative importance of each effect. All response variables were previously normalized

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so that their responses had equal weight in the analysis. Since we tested for effects of absence/

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presence of invasive species and their interaction with insecticide exposure (single concentration)

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against control treatments (autochthonous species and no insecticide exposure) all exogenous

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variables (CAP, crayfish and eucalypt leaves) in the model were set as either 0 or 1. The same for

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interaction variables with 1 representing the treatments were 2 stressors were simultaneously

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present (CAP and eucalyptus, CAP and crayfish or eucalyptus and crayfish). Indirect effects of a

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variable on another were calculated as the sum of the products of direct path coefficients 54.

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Strength and sign of the total effect of a stressor on one variable were calculated as the sum of both

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direct and indirect effects 54. The fit of the path model for our data was tested using four goodness-

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of-fit measures: the χ2 test value testing the similarity between the observed and the predicted

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covariance matrices; the root of mean square error of approximation (RMSEA) where values ≤

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0.05 indicate a good fit; the comparative fit index (CFI) where a value ≥ 0.95 combined with the

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standardized root-mean-squared residual (SRMR) value ≤ 0.08 is indicative of a good fit 55.

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Preliminary SEM models testing for the interactions between functional feeding groups were

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designed and tested, being the final model selected based on a likelihood ratio test (p < 0.05) and

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the goodness of fit values described above. These analyses were performed with R software 3.2.2

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version, from The R Foundation for Statistical Computing, using the ‘lavaan’ package 56.

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RESULTS

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Chemical fate of CAP and exposure concentrations.

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Measured concentrations of CAP are presented in Table 1. After 7 days measured concentrations of

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CAP were still on average 77 % of initial ones, which means that CAP was quite stable in the

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studied mesocosms and in agreement with previous reported stability of CAP in water under lower

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temperatures and pH of 8.0 57.

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Table 1. Chlorantraniliprole concentrations measured in overlying water (µg/L; mean ± SD) after 2

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h, 96 h and 7 days of experimental exposure.

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Measured concentration (µg/L) 2h 96 h 7 days (n = 3) (n = 1) (n = 3)

Alder + C. boltonii

1.63 ± 0.21

1.6

1.29 ± 0.37

Alder + P. clarkii

1.40 ± 0.26

1.5

1.16 ± 0.47

Eucalypt + C. boltonii

1.65 ± 0.14

1.6

1.23 ± 0.36

Eucalypt + P. clarkii

1.50 ± 0.14

1.6

1.11 ± 0.43

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Global analysis of effects on the macroinvertebrate community structure and composition.

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No significant differences (t4 = 1.732, p = 0.158, η2 = 0.429) between invertebrates’ abundances at

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day 0 and day 7 were observed in the streams simulating natural conditions (A. glutinosa as leaf

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litter and predation by the native C. boltonii).

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PERMANOVA results showed that significant alterations of macroinvertebrate community were

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due to crayfish presence (F1,23 = 25.42, p = 0.001), eucalyptus presence (F1,23 = 3.65, p = 0.027) and

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due to the predator - leaf - CAP interaction (F1,23 = 2.89, p = 0.049). No significant effects of CAP

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or two-way interactions were revealed by this analysis (detailed information in SI Table S2).

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Changes on community structure and function.

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The mean values of abundance for the three functional groups, changes in leaf decomposition and

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primary production (measured as chlorophyll a concentration) after 7 days of exposure are

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presented in Figure 2a-e. The presented SEM model provided good fit to the data: χ2 = 0.82, df = 6,

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p = 0.99; RMSEA = 0.00, GFI = 1.00 and SRMR = 0.005, with the changes on the abundances of

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shredders, collectors and grazers, leaf decomposition and primary production being well explained

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by the model (R2 range: 0.561–0.974, Figure 3). In general, results of Three-way ANOVA (Table

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2) and SEM analysis (Table 3) agree in the main direct effects of CAP, resource quality and

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predator identity to the three functional groups of macroinvertebrates and leaf decomposition.

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Shredders abundance decreased significantly (p < 0.05) upon exposure to CAP, by the presence of

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eucalypt leaves and crayfish (Table 2). The SEM analysis showed that both CAP exposure and

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eucalypt leaves contribute similarly to the observed decrease in shredders abundance with a

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standardized path coefficient (r) = - 0.662 (Figure 3, Table 3). SEM analysis also identified a

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negative effect caused by crayfish, i.e. decreases in shredders abundance (r = - 0.397, Figure 3,

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Table 3).

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The presence of crayfish and eucalypt leaf litter also induced negative direct effects on collectors (r

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= - 1.312 and r = - 0.270, respectively) while no direct effects of CAP exposure were detected

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either by SEM analysis nor by linear models (Figure 3, Table 2 and 3). Crayfish presence (r = -

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1.172) and CAP exposure (r = - 0.486) also had negative effects on grazers (Figure 3 Table 2,3).

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SEM analysis revealed that CAP direct effects on collectors were stronger in the presence of

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eucalypt leaf litter (interaction effect, r = - 0.329, Figure 3, Table 3). On the other hand, crayfish

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direct negative effects on grazers and collectors were reduced under CAP exposure and the

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presence of crayfish (CAP × Crayfish, interaction effects: r = + 0.421 for both invertebrate groups,

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Figure 3, Table 3). These negative effects of crayfish on collectors were also greatly reduced in

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treatments with eucalyptus leaf litter (Eucalyptus x Crayfish, interaction effects r = + 0.467, Figure

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3, Table 3).

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Figure 2. Effects of chlorantraniliprole (CAP) exposure and leaf litter type (Alnus glutinosa or

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Eucalyptus globulus) on: the abundance (no. of individuals) of macroinvertebrates per feeding

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group (a) shredders, (b) collectors and (c) grazers; (d) leaf decomposition as leaf mass loss (mg)

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and (e) primary production, as chlorophyll a concentration (mg/cm2), in streams mesocosms with a

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native and invasive predator (Cordulegaster boltonii or Procambarus clarkii, left and right

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columns, respectively), after 7 days exposure. All values are presented as mean ± SE.

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Table 2. Three-way ANOVA results testing for effects of effects of chlorantraniliprole (CAP)

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exposure, crayfish (C) and eucalyptus (E) presence, and their interactions on shredders, collectors

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and grazers abundances, leaf decomposition and primary production. Factor

df

F

p

R2

CAP

1, 16

5.59

0.031

0.110

Eucalypt

1, 16

14.31

0.002

0.281

Crayfish

1, 16

5.59

0.031

0.110

C × CAP

1, 16

2.012

0.175

0.039

E × CAP

1, 16

0.894

0.358

0.018

E×C

1, 16

0.224

0.643

0.004

CAP × E × C

1, 16

6.36

0.023

0.125

Collectors

CAP

1, 16

2.521

0.132

0.021

abundance

Eucalypt

1, 16

4.28

0.055

0.036

Crayfish

1, 16

75.84

< 0.001

0.640

C × CAP

1, 16

4.28

0.055

0.036

E × CAP

1, 16

7.00

0.018

0.059

E×C

1, 16

8.613

0.010

0.073

CAP × E × C

1, 16

0.021

0.887

1.8*10-4

CAP

1, 16

0.754

0.398

0.010

Eucalypt

1, 16

0.754

0.398

0.010

Crayfish

1, 16

48.2

< 0.001

0.633

C × CAP

1, 16

1.572

0.228

0.021

Endpoint Shredders abundance

Grazers abundance

16 ACS Paragon Plus Environment

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Environmental Science & Technology

Leaf decomposition

Primary production

E × CAP

1, 16

4.502

0.049

0.059

E×C

1, 16

1.34

0.264

0.018

CAP × E × C

1, 16

3.014

0.102

0.040

CAP

1, 16

1.186

0.192

0.003

Eucalypt

1, 16

649.2

< 0.001

0.964

Crayfish

1, 16

0.002

0.968

2.5*10-6

C × CAP

1, 16

0.002

0.967

2.6*10-6

E × CAP

1, 16

2.771

0.116

0.004

E×C

1, 16

0.236

0.634

3.5*10-4

CAP × E × C

1, 16

3.203

0.093

0.005

CAP

1, 16

0.331

0.573

0.004

Eucalypt

1, 16

9.677

0.007

0.122

Crayfish

1, 16

35.74

< 0.001

0.450

C × CAP

1, 16

0.384

0.544

0.005

E × CAP

1, 16

12.36

0.003

0.156

E×C

1, 16

2.869

0.110

0.036

CAP × E × C

1, 16

2.005

0.176

0.025

306 307

Decomposition of eucalypt leaf litter was significantly lower when compared with alder leaves (r =

308

- 0.943, Figure 3). Three-way ANOVA’s and SEM results are in agreement since non-significant

309

direct effects of CAP or predator identity were obtained (Figure 2,3, Table 2 and 3). However,

310

SEM analysis also show weak negative effects of CAP exposure on leaf decomposition (Table 3,

311

Figure 3). Negative indirect effects of the presence of crayfish through effects on shredders and

312

collectors’ abundances on leaf decomposition were also identified (r = - 0.182, Table 3).

313

ANOVA’s showed that both eucalypt leaf litter and crayfish presence induced positive effects on

314

primary production (i.e. higher chlorophyll a concentration, Table 2), with SEM analysis indicating

315

positive albeit non-significant total effects of all stressors on primary production (Table 3).

316

Moreover, SEM analysis identify eucalypt leaf litter presence as the main stressor affecting primary

317

production (Figure 3, Table 3). These positive effects of eucalypt leaf litter on primary producers

17 ACS Paragon Plus Environment

Environmental Science & Technology

Page 18 of 37

318

were stronger when crayfish were present (Eucalyptus × Crayfish, r = + 0.632, Figure 3, Table 3)

319

but were reversed under CAP exposure (Eucalyptus × CAP, r = - 0.353, Figure 3, Table 3). A

320

complete description of direct, indirect and total effects is presented in Table 3.

321 322

Figure 3. Graphical representation of the supported SEM model for chlorantraniliprole (CAP), leaf

323

litter and predator identity and their combined effects. Standardized path coefficients and variation

324

explained by the model are given for response variables. Thickness of the arrows is proportional to

325

the strength of the standardized path coefficient. * p < 0.05, ** p < 0.001.

326 327

Table 3. Estimates for standardized path coefficients (r) of the structural equation model for direct,

328

indirect, and total effects of chlorantraniliprole (CAP) exposure, crayfish (C) and eucalyptus (E) presence,

329

and their interactions on macroinvertebrate abundance, leaf decomposition and primary production. (SE:

330

standard error) Significant (p < 0.05) values are stressed in bold.

18 ACS Paragon Plus Environment

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Environmental Science & Technology

Standardized coefficients Endpoint

Predictor

Estimate

SE

Z-value

P-value

Direct effects

Shredders

CAP

-0.333

0.118

-2.828

0.005

-0.662

Eucalypt

-0.333

0.118

-2.828

0.005

-0.662

Crayfish

-0.200

0.118

-1.697

0.090

-0.397

C × CAP

0.133

0.136

0.980

0.327

0.229

E × CAP

0.200

0.136

1.470

0.142

0.344

E×C

-0.067

0.136

-0.490

0.624

-0.115

CAP

-0.104

0.068

-1.530

0.126

-0.199

Eucalypt

-0.141

0.068

-2.074

0.038

-0.270

Crayfish

-0.687

0.068

-10.098