DOI: 10.6078/d1kh7c
title: Energy limitation or sensitive predators? Trophic and non-trophic impacts of wastewater pollution on stream food webs

description: Impacts of environmental stressors on food webs are often difficult to
 predict because trophic levels can respond in divergent ways, and biotic
 interactions may dampen or amplify responses. Here we studied food-web
 level impacts of urban wastewater pollution, a widespread source of
 degradation that can alter stream food webs via top-down and bottom-up
 processes. We selected ten pairs of sampling locations (i.e., twenty sites
 total) in ten small-to-medium tributaries of the Ebro River (NE Iberian
 Peninsula; see "Related article" for locations). These streams
 were located in low-mountain habitats (365 m – 950 m a.s.l.) under the
 influence of Mediterranean climate and flow regimes (mean daily discharge
 0.05 ± 0.09 m3·s-1). Hydrology in the region is characterized by dry
 summer periods with reduced flows, and rainfall episodes in spring and
 late fall. We surveyed the sites in early spring (April 15 - 23, 2016) to
 avoid spring floods; the last scouring flood preceding our sampling took
 place 5 months before sampling (November 2nd, 2015). Each sampling
 location was upstream or downstream of an urban wastewater effluent.
 Sewage impacts represented the main stressor influencing downstream
 community composition. The present dataset contains  (i) aquatic
 invertebrate and vertebrate community composition, and (ii) Stable Isotope
 Analysis data (13C/12C and 15N/14N) for each taxa and reach.
description: <strong>Characterization of faunal community
 composition:</strong> We sampled macroinvertebrate and vertebrate
 communities at each site. In sites dominated by cobbles (n=5), we
 collected five macroinvertebrate samples using a Surber sampler (30 x 30
 cm, mesh size 250 µm), covering the different microhabitats. In streams
 where sand dominated (n=5), we obtained the five samples using a core
 sampler (Ø = 25 cm). Samples were preserved in 4% formalin, and
 invertebrates were sorted, counted, and identified to the lowest taxonomic
 level possible in the laboratory (usually genus). We measured body length
 of the first twenty-five individuals of each taxon, from which we derived
 biomass (mg dry weight) using published length-mass relationships (e.g.,
 Burgherr and Meyer, 1997). We sampled aquatic vertebrates (fish,
 amphibians and reptiles) at each site via 3-pass depletion of a 100 m
 reach delimited by blocking nets, using a portable generator and an
 Electracatch WFC4 with pulsed direct current (DC) output at a frequency of
 50 Hz and 1.5 amps. All vertebrates were identified at the species level,
 measured (total length), and weighed. Three individuals by site and size
 class were euthanized and frozen in the field (-20 ºC, for SIA). Species
 of conservation concern (e.g., the Mediterranean pond turtle,
 <em>Mauremys leprosa</em>) were measured and released
 immediately. <strong>Stable Isotope Analyses
 (SIA): </strong>We collected samples for SIA at each reach,
 targeting the diversity of trophic levels and feeding strategies. We
 collected and sorted macroinvertebrates in the field to the lowest level
 possible, and immediately froze them. In the laboratory, invertebrates
 were thawed and identified mostly to species level (except for Diptera,
 where we used a combination of genus and subfamily; and Oligochaeta,
 family level). Up to three independent samples (each 0.4 mg of dry weight)
 were prepared for every taxon to be processed for SIA. These samples
 included one or several individuals from the same taxa, size, and reach.
 Invertebrate gut contents and calcified structures such as shells were
 removed prior to processing. We obtained a muscle tissue sample from the
 flanks of each vertebrate. We dried samples at 60ºC until they reached
 constant weight (usually for 24 hours), and we later ground them using a
 mortar and pestle. Samples were analyzed for stable isotope ratios
 (13C/12C and 15N/14N) on a Flash 1112 elemental analyzer connected to a
 Delta C isotopic ratio mass spectrometer with Conflo III interface (Thermo
 Scientific, Inc.). Analytical precision from multiple runs was 0.1‰ for
 δ13C, and 0.2‰ for δ15N. Stable isotope results were expressed as the
 difference between sample ratios and standards (PeeDee Belemnite limestone
 for δ13C and atmospheric nitrogen for δ15N). We used these stable isotope
 data to examine the mechanisms of CSS-slope change, and potential niche
 compression (see related article). Complementary
 data:   (1) Concentrations of
 pharmaceutical products can be found in: Mandaric, L., J. R. Mor, S.
 Sabater and M. Petrovic. 2018. Impact of urban chemical pollution on water
 quality in small, rural and effluent-dominated Mediterranean streams and
 rivers. Science of the Total Environment 613–614: 763–772. doi:
 10.1016/j.scitotenv.2017.09.128 (2)
 Gross primary production (GPP) values for each site can be found in:
 Pereda, O., D. von Schiller, G. García-Baquero, J. R. Mor, V. Acuña, S.
 Sabater, and A. Elosegi. 2021. Combined effects of urban pollution and
 hydrological stress on ecosystem functions of Mediterranean streams.
 Science of The Total Environment, 753, 141971.
description: <strong>Mor_et_al_Ecology_community_composition: </strong>Vertebrate and invertebrates abundance (ind m<sup>-2</sup>; "Abd_m2") and mean species body size per site (mg; "CWMBS"). <em>Up</em> (upstream) and <em>Do</em> (downstream) denote pairs of sites located upstream-downstream of wastewater effluents.  <strong>Mor_et_al_Ecology_SIA_fauna:</strong> Stable isotope analyses (SIA) data by taxon and site. <em>Up</em> (upstream) and <em>Do</em> (downstream) denote pairs of sites located upstream-downstream of wastewater effluents. “Species” category denotes lowest taxonomic level possible (usually species or genus).