Quantifying and classifying microplastics and microparticles across aquatic heterotrophs from headwater streams in central Pennsylvania

Microplastics (<5 millimeters) are a prominent contaminant globally, negatively affecting terrestrial, freshwater, and saltwater systems. Virtually no research has investigated microplastic contamination in remote, forested headwater streams in Pennsylvania. At five streams in central Pennsylvania, we assessed microparticle/microplastic contamination across three trophic levels: tertiary consumers (Salvelinus fontinalis), secondary consumers (Rhinichthys atratulus), and primary consumers (Trichoptera, Plecoptera, Ephemeroptera, Odonata, and Megaloptera). From 100-meter unblocked reaches, fish (n = 46 total, n = 21 tertiary consumers, n = 25 secondary consumers) and benthic macroinvertebrates (n = 106 total, Trichoptera: n = 40, Plecoptera: n = 39, Ephemeroptera: n = 22, Odonata: n = 4, Megaloptera: n = 1) were collected using standard wadeable stream sampling procedures. Fish gastrointestinal (GI) tracts and macroinvertebrates were digested in 10% potassium hydroxide (KOH) solutions, and microparticles were identified after vacuum filtration and visualization with a dissecting microscope at 45× magnification. Microparticles were categorized as microplastics at two of our five sites using μFTIR spectroscopy on Salvelinus fontinalis, Rhinichthys atratulus, and Plecoptera. Our efforts resulted in the identification of 159 microparticles and 5 microplastics among all heterotrophs across all sites. Microparticle contamination was highest among secondary consumers with no heterotroph or site effect on microparticle count (p > 0.05). The distribution of microparticle morphology and color combinations varied significantly across the heterotroph groups, and there were negative correlations between brook trout morphological characteristics and microparticle count (e.g. total length: r = –0.62, p = 0.03; weight: r = –0.62, p = 0.03; gastrointestinal tract weight: r = –0.66, p = 0.02). Negative relationships between size characteristics among all fish and microparticle count were expressed by δ15N analysis (r = –0.37, p = 0.03). We believe that the disparity in microparticle contamination by fish size and between trophic levels is driven by divergences in feeding strategies, ontogenetic shifts in feeding behavior, and contrasts in life histories. Overall, our data highlight the presence of microplastic/microparticle contamination in remote, forested headwater streams in Pennsylvania without any known upstream point-source pollution.