DOI: 10.5061/dryad.xksn02vk7
title: Benthic macroinvertebrates of 63 northern Finnish streams along a dissolved organic carbon gradient

description: Concentrations of dissolved organic carbon (DOC) have increased over the
 past few decades, causing freshwater browning. The impacts of browning on
 biodiversity have been little studied, despite many of the individual
 stressors associated with browning being known to control freshwater
 communities. We explored the responses of benthic invertebrates along a
 wide gradient of DOC concentrations (3.6 mg to 27 mg L-1) in 63 boreal
 streams variously impacted by peatland drainage or peat production. DOC
 was a prime determinant of macroinvertebrate diversity and abundance, with
 the strongest negative response in algal scrapers. Threshold indicator
 taxa analysis indicated a community change at 12-13 mg DOC L-1, with only
 four taxa increasing, while 13 taxa decreased along the DOC gradient. Our
 findings of both a gradual loss and an abrupt change of biodiversity along
 a browning gradient provide a benchmark against which changes to stream
 biodiversity relative to the predicted browning trend can be gauged.
description: Benthic macroinvertebrates
 were sampled in late September 2019 (50 sites) or 2020 (13 sites) with a
 2-min kick-net (Ø 500-μm) sample at each site. A sample of this size
 covers about 1.3 m2 of the stream bed and is known to capture about 75% of
 all species present in a rifﬂe (Mykrä et al. 2006). Samples were preserved
 in 70% ethanol in the ﬁeld and later identiﬁed to species or genus level.
 We
 collected water samples simultaneously with macroinvertebrate sampling and
 they were analysed for dissolved organic carbon (DOC, mg
 L<sup>-1</sup>), water colour (mg Pt
 L<sup>-1</sup>), total phosphorus (TP, µg
 L<sup>-1</sup>), nitrate-N (NO3, µg
 L<sup>-1</sup>), and pH following national standards (National
 Board of Waters 1981) (see Table S1). DOC concentration was measured from
 filtered (Ø 0.45 µm Whatman GF/F) water samples by infrared spectrometry
 with Shimadzu TOC-VCPH analyzer (Shimadzu Scientific Instruments, Kyoto,
 Japan). Absorbance spectra (190-800 nm; 1 nm intervals) were recorded for
 filtered water samples (Ø 0.2 µm PES membrane filter) using Lambda 650
 (Perkin Elmer) spectrophotometer and 1-cm quartz
 cuvettes. Water temperature (°C) was recorded at 1–h
 intervals by HOBO Pendant (Onset, Massachusetts) data loggers for five
 weeks prior to invertebrate sampling. Mean substratum size was determined
 as the weighted average of particle size classes in ten randomly
 distributed 0.25 m<sup>2</sup> plots using a modified
 Wentworth scale from 1 (clay/silt 0.001–0.07 mm) to 10 (large boulder to
 bedrock &gt;512 mm). Substrate diversity was quantified by calculating
 Simpson diversity index based on the relative cover of each Wentworth
 class in each quadrat. Moss cover (%) was estimated in 20 0.25
 m<sup>2</sup> plots distributed randomly across the study
 section. Current velocity (cm s−1; Schiltknecht® MiniAir20) and water
 depth (cm) were measured at three positions along each of five randomly
 placed transects perpendicular to the flow. Stream width (cm) was measured
 at the same transects.