DOI: 10.5061/dryad.v9s4mw70n
title: Abundance trends for river macroinvertebrates vary across taxa, trophic group and river typology

description: There is mounting evidence that terrestrial arthropods are declining
 rapidly in many areas of the world. It is unclear whether freshwater
 invertebrates, which are key providers of ecosystem services, are also
 declining. We addressed this question by analysing a long-term dataset of
 macroinvertebrate abundance collected from 2002 to 2019 across 5,009
 sampling sites in English rivers. Patterns varied markedly across
 taxonomic groups. Within trophic groups, we detected increases in the
 abundance of carnivores by 19% and herbivores by 14.8%, whilst we
 estimated decomposers have declined by 21.7% in abundance since 2002. We
 also found heterogeneity in trends across rivers belonging to different
 typologies based on geological dominance and catchment altitude, with
 organic lowland rivers having generally higher rates of increase in
 abundance across taxa and trophic groups, with siliceous lowland rivers
 having the most declines. Our results reveal a complex picture of change
 in freshwater macroinvertebrate abundance between taxonomic groups,
 trophic levels and river typologies. Our analysis helps with identifying
 priority regions for action on potential environmental stressors where we
 discover macroinvertebrate abundance declines.
description: Abundance data for riverine macroinvertebrates in England were
 extracted from the EA’s ecological monitoring database. The data were
 filtered to only include three-minute kick-sample data as the primary
 method for sampling freshwater invertebrates (approximately 99% of
 samples). Three-minute kick-samples are a standardised,
 internationally-recognised, semi-quantitative approach to assessing
 macroinvertebrate ecology and water quality in rivers using invertebrate
 diversity indicators. Prior to the implementation of the European Union
 Water Framework Directive in 2000, abundance estimates were based on
 categories (0–9, 10–99, 100–999 etc.). In 2002, the EA started recording
 more exact abundance estimates and enacting improved quality control
 procedures, whereby one in every ten samples was independently
 re-analysed. Hence, although the original dataset covered sampling years
 from 1991, our analysis was restricted to the years 2002–2019. Data were
 further filtered to only include sites sampled for a minimum of three
 years out of a total of eighteen in both spring (March to May) and autumn
 (September to November). After filtering the dataset according to these
 criteria, our final dataset from 2002–2019 included 67,757 individual
 macroinvertebrate samples from 5,009 sites (out of 10,136 sites in the
 original dataset). This equates to an average of 3,764 samples a year,
 covering 2,774 waterbodies distributed across the 10 river basins defined
 under the European Union Water Framework Directive in England: Anglian,
 Humber, North West, Northumbria, Severn, Solway Tweed, South East, South
 West and Thames (Fig. 1). The final dataset provides a wide national
 distribution of sites representative of the main river conditions, albeit
 with a bias towards mid to lower perennial reaches (reflective of the
 purpose of the monitoring programmes instigated for environmental quality
 monitoring, rather than a river’s intrinsic biodiversity). 
 The identification of macroinvertebrates in the original
 database, including within individual samples, is given at a mixture of
 taxonomic levels. We pooled the data at two different levels: 1) wider
 taxonomic groups (non-insect freshwater macroinvertebrates: annelids,
 molluscs, Turbellaria and crustaceans, and individual insect orders:
 Ephemeroptera, Trichoptera, Plecoptera, Coleoptera, Diptera, Megaloptera,
 Hemiptera and Odonata); and 2) taxonomic families, representing
 observations for which this level of identification was available. We also
 pooled data considering main trophic groups (carnivores, herbivores and
 decomposers).