Abstract
Land use and climate change are driving widespread modifications to the biodiverse and functionally unique headwaters of rivers. In temperate and boreal regions, many headwaters drain peatlands where land management and climate change can cause significant soil erosion and peat deposition in rivers. However, effects of peat deposition in river ecosystems remain poorly understood. We provide two lines of evidence—derived from sediment deposition gradients in experimental mesocosms (0–7.5 g/m2) and headwaters (0.82–9.67 g/m2)—for the adverse impact of peat deposition on invertebrate community biodiversity. We found a consistent negative effect of sediment deposition across both the experiment and survey; at the community level, decreases in density (1956 to 56 individuals per m2 in headwaters; mean 823 ± 129 (SE) to 288 ± 115 individuals per m2 in mesocosms) and richness (mean 12 ± 1 to 6 ± 2 taxa in mesocosms) were observed. Sedimentation increased beta diversity amongst experimental replicates and headwaters, reflecting increasing stochasticity amongst tolerant groups in sedimented habitats. With increasing sedimentation, the density of the most common species, Leuctra inermis, declined from 290 ± 60 to 70 ± 30 individuals/m2 on average in mesocosms and >800 individuals/m2 to 0 in the field survey. Traits analysis of mesocosm assemblages suggested biodiversity loss was driven by decreasing abundance of invertebrates with trait combinations sensitive to sedimentation (longer life cycles, active aquatic dispersal of larvae, fixed aquatic eggs, shredding feeding habit). Functional diversity metrics reinforced the idea of more stochastic community assembly under higher sedimentation rates. While mesocosm assemblages showed some compositional differences to surveyed headwaters, ecological responses were consistent across these spatial scales. Our results suggest short‐term, small‐scale stressor experiments can inform understanding of “real‐world” peatland river ecosystems. As climate change and land‐use change are expected to enhance peatland erosion, significant alterations to invertebrate biodiversity can be expected where these eroded soils are deposited in rivers.
Highlights
The headwaters of river systems make a major contribution to global aquatic biodiversity
The headwaters of many major river systems originate from these peatlands, where a water surplus leads to slow rates of decomposition and the build‐up of low‐density organic soil cover (Charman, 2002)
We developed new insights into the effects of peatland erosion and sedimentation on river invertebrate communities via comparative evaluations of aquatic biodiversity responses in: (a) a fully controlled and replicated riverside mesocosm experiment examining the impacts of benthic organic sedimentation on aquatic invertebrates, and (b) surveys of peatland rivers across the Pennine region of northern England with different levels of fine organic matter deposits on the bed
Summary
The headwaters of river systems make a major contribution to global aquatic biodiversity. Northern peatlands have historically been subject to intensive drainage to lower the water table in an attempt to make the land more suitable for animals, arable agriculture, forestry and/or gun‐sports, but in places cause up to 200‐fold increases in river sediment loads (Ahtiainen & Huttunen, 1999; Prévost, Plamondon, & Belleau, 1999; Ramchunder, Brown, & Holden, 2009, 2012). In some areas, this enhanced erosion has been due to decay of the peatland through subsurface evacuation of sediment from large cavities (peat pipes)
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