Macroinvertebrate data from weir removal study UK

Abstract

Weirs are a globally abundant type of low-head dam that negatively alter river ecosystems and the removal of weirs is an increasingly popular river restoration measure in many countries. However, it is unclear how effective weir removal is in enhancing the macroinvertebrate communities of urban rivers, which typically experience a range of anthropogenic pressures and are ecologically depauperate. This Before-After Control-Impact Paired study tested whether the removal of a weir on an urban river in the UK resulted in a desired outcome; a shift in the aquatic macroinvertebrate community towards a more natural state. Macroinvertebrates were sampled before and after removal, above and below the structure and at control sites over four years. The removal of the weir had a major impact on upstream habitat, with the replacement of the deep slow-flowing impoundment with much shallower, faster flowing and heterogeneous conditions. There was a big change in the composition of the macroinvertebrate community upstream of the weir, with taxa preferring lotic conditions replacing taxa associated with the lentic conditions artificially created by the weir. The findings suggest that removing weirs from urban rivers can promote a shift in macroinvertebrate communities towards a more natural composition.

Methods

Sampling and identification of the aquatic macroinvertebrate community was undertaken by a citizen science team composed of DCRT staff and trained volunteers. Six sites were sampled (see Figure 1); one was located downstream of the weir (denoted as D/S Impact), three were located upstream of the weir within the impounded river reach (denoted as U/S Impact 1-3), and two were located further upstream above the weir impoundment (i.e. above the hydraulic influence of the weir; denoted as Control 1-2). We expected weir removal to have a greater impact upstream than downstream, so we included more site replicates upstream.

Samples were collected on seven separate dates between 2018 and 2022; three pre-weir removal and four post-weir removal. Sampling occurred each autumn and spring, except autumn 2019 when a long period of heavy rain prevented sampling, and spring 2020 when COVID-19 travel restrictions delayed sampling until summer 2020.

Contextual data for each site was collected for each sampling event, including mean river depth (cm), wetted channel width (m) and estimates were made of the percentage cover of five riverbed substrate classes; ‘silt and clay’, ‘sand’, ‘pebbles and gravel’, ‘boulders and cobbles’, and ‘bedrock’.

All sampling was carried out using UK Environment Agency guidelines (BT001) (Murray-Bligh 1999). Using a standard 250 mm wide kick sampling net with a 1 mm mesh, three 1-minute kick samples were collected along a transect line across each of the six sites. The three samples were combined to make a single sample per site. Afterwards, a 1-minute stone search was carried out, with macroinvertebrates scraped by hand into the combined site sample. For the site immediately upstream of the weir (U/S Impact 1), where the river was too deep to kick sample before weir removal, a sweep sample was collected using a net on an extended pole.

Samples were stored in 80% ethanol for later identification. Specimens were identified to family-level, or, if possible, to genus or species level, with verification provided by a lead volunteer with expertise in invertebrate identification. Identification was aided by tuition and written materials including the Guide to Freshwater Invertebrates (Dobson et al., 2013 ).

Description of the data and file structure

The data is composed of two spreadsheets containing raw data collected by a team of staff and volunteers from the Don Catchment Rivers Trust from the study location

The study location (53°17’11” N, 1°21’06” W) is a low-gradient reach of the River Rother in England, UK. Slittingmill Weir was built in c. 1920 to divert water from the river to power a waterwheel.

IMPORTANT NOTE:

We use different site names in our Conservation Evidence paper to those used in the data:

Data US2 = Paper Control 1
Data US1 = Paper Control 2
Data SC1 = Paper Upstream 3
Data SC2 = Paper Upstream 2
Data DS1 = Paper Upstream 1
Data DS2 = Paper Downstream 1
Data DS3 = Not used in paper