Multiple stress response of lowland stream benthic macroinvertebrates depends on habitat type
Graphical abstract
Introduction
Worldwide, stream benthic macroinvertebrates are facing a plethora of anthropogenic environmental stressors. Altered precipitation patterns (Arnell, 1999) and water abstraction (Vanneuville and Uhel, 2012) create periods with critical low flow that result in loss of macroinvertebrate species typical for stream ecosystems (Graeber et al., 2013, Hille et al., 2014, Lorenz et al., 2016). At the same time, increased loads of fine sediments and elevated nutrient concentrations (Kronvang et al., 2005, Pacheco et al., 2014, Townsend et al., 2008) adversely impact the structure of stream macroinvertebrate communities (Piggott et al., 2015, Townsend et al., 2008, Wagenhoff et al., 2013). Although the interactive effect of these stressors is difficult to predict, stream mesocosm experiments consistently document that the effects of flow reduction on macroinvertebrate community structure are stronger when combined with fine sedimentation than with nutrient enrichment (Piggott et al., 2012, Townsend et al., 2008). Furthermore, these experiments have shown that subsidy effects of nutrient enrichment commonly appear in macroinvertebrates feeding on benthic biofilms (i.e. scrapers; Gruner et al., 2008) and that the effects of nutrient enrichment may be counteracted by fine sedimentation (Wagenhoff et al., 2012).
Field-based studies further document that the susceptibility of stream macroinvertebrate communities to different environmental stressors depends on physical habitat characteristics (Dewson et al., 2007, Rasmussen et al., 2012). For example, the effects of low flow on macroinvertebrate communities depend on the availability of suitable habitats acting as refugia, and this refugial capacity is intrinsically linked to other habitat-specific characteristics such as fine sediment cover and availability of food resources (Lancaster and Hildrew, 1993). Conversely, the effects of fine sediment cover on macroinvertebrate community composition depend on habitat type (Roy et al., 2003). Therefore, evaluating the effects of different stressor combinations in different habitat types is a fundamental prerequisite for robust quantification of the summed impacts of typical anthropogenic stress scenarios on stream macroinvertebrate communities.
In the present study, we explored how habitat-specific characteristics shape the response of macroinvertebrates to flow reduction, nutrient enrichment, and increased coverage of fine sediment by applying multiple combinations of these three stressors in a controlled experimental setup. We used twelve 12-m long outdoor flumes to assess the effects of the stressor combinations on the benthic macroinvertebrate community in contrasting habitat types typical for lowland streams (riffle and run habitats, Pedersen, 2003). In more detail, we assessed the effects of nutrient enrichment during a normal-flow phase followed by a low-flow phase with a reduction in discharge representative of summer time low-flow periods in lowland streams (Graeber et al., 2015). Within the low-flow phase, fine sediment collected from a nearby stream was added to half of the outdoor flumes. During both the normal- and low-flow phases, macroinvertebrate samples were collected with weekly intervals in both habitat types to assess the temporal development of the benthic macroinvertebrate community induced by the selected multiple stressors. Based on these samples, we tested the following hypotheses with specific focus on the temporality of responses:
- 1.
The combined effects of low flow and fine sediment addition on macroinvertebrate composition depend on habitat characteristics, with the strongest influence in run habitats compared with riffle habitats as fine sediment accumulation in run habitats is more pronounced than in riffle habitats.
- 2.
Nutrient enrichment changes the structure of the benthic macroinvertebrate community in both run and riffle habitats towards increasing abundance of grazers due to a stimulating effect of nutrients on the biomass of epibenthic algae. This effect is reduced by addition of fine sediment, which diminishes the algae biomass available to grazers.
Section snippets
Experimental setup
We conducted the experiment in twelve outdoor flumes during summer 2015 in Denmark (56°4′ N, 9°31′ E). The flumes consisted of rectangular 12 m long, 60 cm wide, and 30 cm deep channels. In each flume, four run-riffle sequences were created, resembling natural habitat conditions in lowland streams (Pedersen, 2003). Each of the four riffle and run habitats covered 1.5 m of the stream flume length with an average sediment depth of 5.5 cm (0.5–2, 2–4, 4–8, and 8–16 mm grain-sized sediment at a volume
Effects of low flow and fine sediment
During the low-flow phase, we found a significant mitigating effect of fine sedimentation in the riffle habitat; hence the flumes with FS treatment were not as strongly altered in their macroinvertebrate composition relative to the control as flumes without FS treatment (Fig.1a, Table 2). Due to the mitigating effect of fine sedimentation in the riffle habitat, the change from normal to low flow was not significant in combination with the FS treatment (Table 3). Without application of the FS
Habitat-specific effects of low flow and fine sediment
We found that the combined effects of low flow and fine sediment on the macroinvertebrate community were habitat dependent. This is in accordance with empirical evidence from other field studies also reporting habitat-specific effects of these two stressors (Lancaster and Hildrew, 1993, Roy et al., 2003). Furthermore, we observed that the response of the macroinvertebrate community to fine sedimentation and low flow occurred rapidly, being detectable after just one week with no or only little
Acknowledgements
This study was supported by the MARS project (Managing Aquatic ecosystems and water Resources under multiple Stress) funded under the EU 7th Framework Programme, Theme 6 (Environment including Climate Change), Contract No.: 603378 (http://www.mars-project.edu). We thank Johnny Nielsen, Marlene Venø Skjærbæk and Dorte Nedergaard, who conducted the nutrient measurements in the laboratory, and Anne Mette Poulsen for linguistic assistance.
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