DOI: 10.5061/dryad.n8pk0p2vh
title: Data from: Infection prevalence and density of a pathogenic trematode parasite decrease with stream order along a river continuum

description: In lotic ecosystems, the River Continuum Concept (RCC) provides a
 framework for understanding changes in environmental factors and
 free-living communities, yet how parasite populations shift along river
 continua remains less clear. We quantified infections by a pathogenic
 trematode parasite (Nanophyetus salmincola) in &gt;14,000 host snails
 across 130 stream reaches spanning 165 km in the Willamette River Basin in
 western Oregon, USA. Environmental factors – including flow volume,
 temperature, benthic algae, canopy cover, woody debris, and land cover –
 changed predictably with stream order, consistent with the RCC. From
 first- to eighth-order reaches, infection prevalence decreased by ~42-fold
 and infected snail density, a measure of disease risk to fish hosts,
 decreased ~3-fold. Infected snail density, but not prevalence, was
 positively associated with snail biomass density, and individual infection
 probability increased strongly with host size. Shifts in snail population
 characteristics across stream orders, however, did not explain the
 observed changes in N. salmincola populations, suggesting that
 environmental variables and corresponding changes in non-snail hosts
 explain the downstream decrease in infections. Our findings show
 predictable spatial variation in disease risk to vertebrate hosts from N.
 salmincola and indicate the RCC can help explain shifts in parasite
 populations in lotic ecosystems.
description: Study location – Our survey design included 130 stream reaches
 each measuring 5 m in length, with 18 to 20 reaches in the mainstem
 Willamette River and each of six focal sub-basins. Three focal sub-basins
 drained from the western foothills of the Cascade Range (the Middle Fork
 Willamette, McKenzie, and Santiam Rivers) and three drained from the
 eastern foothills of the Coast Range (the Coast Fork Willamette, Mary’s,
 and Luckiamute Rivers).  Field surveys – Surveys of
 snail population characteristics (density, sizes, biomass density) and
 environmental factors were conducted from 10-Jul to 01-Oct-2019. Snail
 densities were measured using quadrats spaced at 1-m intervals (0.25 m2;
 n=5 quadrats per reach) within a maximum depth of ~2 m in each reach.
 Biomass densities were estimated by measuring all snails (shell length)
 and using length-to-dry mass conversion equations (see "Snail
 surveys" for details). For each reach, Strahler stream order was
 determined using Google Earth Pro satellite imagery, elevation was
 recorded using GPS, water temperature was measured using a YSI instrument,
 and % canopy cover was measured using a spherical densiometer. Relative
 levels of riparian vegetation density, benthic algal abundance, and
 in-stream woody debris were classified on a numerical scale of 1 to 5
 (low-high) based on observations by the first author. We obtained mean
 annual discharge (m3 s-1), and quantitative measures of channel gradient
 and confinement (rescaled to values from 1 to 5) using an existing online
 stream classification system (McManamay and DeRolph 2018). We also
 obtained measures of percent land cover within a 1-km buffer around each
 reach using the National Land Cover Database (MRLC 2019). Land cover was
 grouped into three categories: forested (deciduous, evergreen, and mixed),
 agricultural (cultivated crops, hay/pasture), and urbanized (developed;
 high to low intensity).<br>     <br> Parasite quantification –
 We opportunistically collected &gt;100 Juga snails measuring
 &gt;10 mm in shell length from each reach for dissection. All snails
 were measured, cracked with pliers, and then examined under a dissecting
 microscope (8X–35X). N. salmincola was identified based on unique
 morphological features of the microcercous cercariae, including a bluntly
 conical tail and presence of a stylet. For immature infections, wet mounts
 were examined under a compound microscope (4X–100X) to aid
 identifications. Infections by other trematode species were recorded,
 categorized by morphological features, and opportunistically vouchered for
 future molecular identifications. In the current study, we observed at
 least 12 trematode morphotypes in the dissected Juga snails, but we focus
 on N. salmincola due to its relatively well understood ecology and its
 importance in causing disease in species of conservation concern.
 Information on other trematode taxa in Juga from three of our 130 sites
 that were characterized with molecular methods is also presented in
 Preston et al. (2021).<br>     <br> Snail surveys – We used
 randomized transect-quadrat sampling to quantify snail biomass within each
 reach. Quadrats were spaced at 1-m intervals (0.25 m2; n=5 quadrats per
 reach) within a maximum depth of ~2 m. The distance from the wetted edge
 for each quadrat was randomized using a random number generator. If stream
 current made sampling impractical at a specific location, a maximum safe
 distance from the wetted edge was used as an upper limit for the random
 quadrat locations. All snails within each quadrat were hand-collected
 (using snorkeling in deeper locations), counted, and measured for total
 shell length (mm). Snail density and size measurements were converted to
 biomass densities (g m-2) using a length-to-dry biomass conversion
 equation obtained from measuring, drying and weighing 172 locally
 collected Juga snails from 3 to 34 mm in shell length (DM =
 0.00002*L2.65). All biomass densities in this paper refer to dry tissue
 biomass excluding the snail shell. <br>     <br> Estimating
 trematode densities – To estimate infected snail density at each stream
 reach, we first used reach-specific binomial logistic regressions on N.
 salmincola infection status from dissection data, with snail size as the
 only predictor. The reach- and size-specific N. salmincola infection
 probabilities from the logistic regressions were then applied to each
 snail recorded in the quadrats to assign an estimated infection status.
 The densities of infected snails were summed within quadrats to
 extrapolate the reach-level values of infected snail density.
description: Please see FALKE_and_Preston_2021_Ecosphere_readme.txt for usage
 notes, metadata, and additional information, including citations of
 literature and databases. Additional inquiries about the dataset and usage
 may be directed via email to Landon Falke.