DOI: 10.5061/dryad.4b8gthtc0
title: Pervasive decline of subtropical aquatic insects over 20 years driven by water transparency, non-native fish and stoichiometric imbalance

description: Insect abundance and diversity are declining worldwide. Although recent
 research found freshwater insect populations to be increasing in some
 regions, there is a critical lack of data from tropical and subtropical
 regions. Here we examine a 20-year monitoring data set of freshwater
 insects from a subtropical floodplain comprising a diverse suite of
 rivers, shallow lakes, channels and backwaters. We found a pervasive
 decline in abundance of all major insect orders (Odonata, Ephemeroptera,
 Trichoptera, Megaloptera, Coleoptera, Hemiptera and Diptera) and families,
 regardless of their functional role or body size. Similarly, Chironomidae
 species richness decreased over the same time period. Increased invasions
 of non-native insectivorous fish, water transparency and changes to water
 stoichiometry (i.e., N:P ratios) over time were the main drivers of this
 pervasive insect decline. All these drivers represent human impacts caused
 by reservoir constructions. This work sheds light on the importance of
 long-term studies for deeper understanding of human-induced impacts on
 aquatic insects. We highlight that anthropogenic impact monitoring and
 mitigation actions are pivotal in maintaining freshwater ecosystem
 integrity.
description: We analyzed a 20-year
 (2000-2019) data set from a Long-Term Ecological Research program
 (PELD-Sitio PIAP), carried out in the Upper Paraná River Floodplain,
 Brazil (20º40'–22º50'S; 53º10'–53º24´W). We took four
 annual samples during summer, spring, autumn, and winter (except for year
 2001, 2003, 2016, 2017, 2018, and 2019, which we sampled twice a year,
 i.e., in summer and winter) of insects, environmental variables and
 non-native fish in 12 independent environments, comprising three rivers,
 six shallow lakes, two channels and one backwater. Importantly, all
 samplings were performed simultaneously at the same sites, following a
 standard protocol. Aquatic insect larvae were
 collected following a standard methodology [19]: three samples were
 obtained from each environment, including two samples at both sides and
 one in the center, using a Petersen sampler (0.0345 m²). The collected
 insects were identified to order (Coleoptera, Megaloptera, Hemiptera,
 Trichoptera, Odonata and Ephemeroptera) or family level (Ephemeroptera:
 Baetidae, Caenidae, Leptophlebiidae; Diptera: Dolichopodidae, Chaoboridae,
 Ceratopogonidae, Culicidae and Chironomidae). Chironomidae larvae were
 additionally identified to morphospecies level. We calculated insect
 abundance (order, family) and Chironomidae species richness per m²
 captured in each environment during each sampling over 20 years. These
 insect orders and families comprised all key functional feeding groups,
 including predators, shredders, scrappers, grazers, gatherers, filter
 feeders, and spanned a wide range of body sizes, from very small (e.g.,
 Culicidae, Chaoboridae, Chironomidae) to very large organisms (e.g.,
 Megaloptera, Ephemeroptera, Trichoptera). Simultaneously to insect, we took
 water samples from each aquatic environment to quantify nutrient
 concentrations (total phosphorus and total nitrogen; μg
 L<sup>-1</sup>) and turbidity (NTU). Total nitrogen (N) was
 analyzed with the persulfate method and determined in a spectrophotometer
 in the presence of cadmium, using a flow-injection system. Total
 phosphorus (P) was measured according to Golterman et al. Turbidity was
 measured using a turbidimeter (LaMotte, Chestertown, MD, U.S.A). We also
 measured water level (m) using a fixed water level ruler. All these
 variables indicate human-induced disturbance, including damming (low
 turbidity and depth) and underlying changes in nutrient
 dynamics.