Fine sedimentation affects CPOM availability and shredder abundance in Alpine streams

Abstract

Fine sedimentation was quantified in the upper stretch of the Pellice river basin (Italian Alps), in an area partially disturbed by mining activities. Sediment traps were placed in 52 sites and benthic samples were collected to assess the amount of coarse particulate organic matter and the structure of macroinvertebrate community. The results indicated that the increase of fine sediment in the river reduced the amount of coarse particulate organic matter, affecting the abundance of invertebrate shredders. In small alpine streams, where allochthonous organic matter is the main energy source, fine sedimentation reduces the availability of trophic resources, affecting the abundance of benthic communities.

Keywords:

• coarse particulate organic matter
• macroinvertebrates
• sedimentation
• mining
• allochtonous energetic inputs

The increase of fine sediments has become a key problem in many rivers (Weigelhofer & Waringer 2003), particularly in mountain lotic systems which are characterised by coarse substrata because of high flow velocities and great tractive forces (Bona et al. 2008). Fine sedimentation affects benthic organisms directly (i.e., damaging soft body parts or decreasing habitat availability; Bo et al. 2007) and indirectly (i.e., altering trophic resources), with detrimental effects on invertebrate filterers (Jones et al. 2012) and scrapers (Kreutzweiser et al. 2005). In mountainous, low-order lotic systems, the main energy source is represented by allochthonous organic matter, mainly terrestrial leaves (Tank et al. 2010; Bo et al. 2014). In such environments, storage and availability of benthic coarse particulate organic matter (CPOM) is influenced by a wide range of processes. Recent studies emphasised the importance of typology and abundance of in-stream structures in trapping and retaining allochthonous materials (Jones 1997; Small et al. 2008).

Little information is available about the impact of fine sediments on the retention of allochthonous organic materials in Alpine streams and its ecological effect on shredders (i.e., benthic invertebrates that feed on this resource). In this study, fine sedimentation was quantified using sediment traps, designed according to Bond (2002). The traps were randomly deployed in 52 spots in the upper part of the Pellice river basin (Italian Alps, Figure 1), in an area partially disturbed by mining activities (Sandrone et al. 2004). The traps were placed on 28 June 2013 and removed 17 days later. Sediment that was collected in the traps was returned to the laboratory, dried and sieved. Several studies have examined the effect of sediment grain size (Wood & Armitage 1997; Kreutzweiser et al. 2005); in this study, only the <0.105 mm fraction, belonging to ‘very fine sand’ in the Wentworth grain size classification, was considered (hereinafter, fine sediment). We selected this fraction because its occurrence is expected to be more ‘unnatural’ and related to mining activity in Alpine environments. On the removal date, one benthic sample was collected in each point to quantify CPOM and macroinvertebrate community, using a Surber sampler (250 μm mesh size; 0.062 m² area). The sampler was placed in the section of streambed immediately after the removal of sediment traps, close (laterally) to trap sites. The traps collected a wide- ranging amount of fine sediment, ranging from a minimum of 1.64 g to a maximum of 64.7 g. Mean mass of the trapped fine sediment was 16.9 g (±2.27 SE). Also, benthic CPOM showed a high variability, ranging from 0.27 to 19.5 g/m² with a mean of 3.86 g/m² (±0.46 SE). In total, we collected 13,528 macroinvertebrates. Mean abundance in the substratum was 4196 organisms/m² (±451.5 SE). Regarding the functional composition of the invertebrate assemblages, collectors–gatherers were the most represented functional feeding group (75.1%), followed by shredders (10.3%), predators (8.4%), scrapers (4.9%) and filterers (1.3%). The relative proportion of shredders showed a high variance, ranging from a maximum of 18.5% to total absence. Three species of Plecoptera:Euholognatha, Protonemura sp., Leuctra sp. and Nemoura sp., followed by Diptera: Tipulidae Prionocera sp. constituted the most abundant shredder taxa. Analysis of the relationship between fine sediment and CPOM amounts and shredder abundance was performed with log-transformed data. Trapped fine sediment was significantly related to CPOM amount (t = −5.099, r = −0.58, p < 0.001) and to shredder abundance (t = −6.685, I = −0.69, p < 0.001, Figure 2).

Figure 1. River Pellice basin and study area; black lines indicate sampled reaches and the grey zone indicates mining area.

Figure 2. Relationship between fine sediment amounts and CPOM availability (above) and invertebrate shredder abundance (below).

The presence of large mining areas and related intense road traffic in the sampled watershed resulted in an increase in fine sediment accumulation into the streambed. This massive amount of fine sediment homogenised the substrate, occluding the gaps, and reducing morphological diversity and presence of elements that can trap and retain coarse organic particles. In the most impacted sites, we observed that stream boulders, small woody debris and riverbanks were covered with a thick patina which probably diminished their effectiveness as retention structures. The results suggested that the increase of fine sediments in the river bottom directly reduced the amounts of CPOM, affecting the abundance of invertebrate shredders. Alpine lotic systems are facing increasing challenges because of local increases in human activities (Wohl 2006) and global effects of climate change (Fochetti 2012). In these environments, where allochthonous organic matter represents the main energetic source, fine sedimentation constitutes an important factor that reduces biodiversity and abundance of benthic communities.

Acknowledgements

This work was supported by the University of Piemonte Orientale Parco del Po-S. Fenoglio and University of Torino “Ricerca Locale 60% 2014 - F. Bona grants

Disclosure statement

No potential conflict of interest was reported by the authors.