Organic sediment pulses impact rivers across multiple levels of ecological organization

Abstract

Sedimentation is a pervasive environmental pressure affecting rivers globally. Headwaters draining catchments rich in organic soils (i.e., peat) are particularly vulnerable to enhanced sedimentation caused by land management and environmental change, yet many of the ecological consequences of peat deposition are poorly understood. We conducted a before‐after‐control‐impact experiment in two rivers draining blanket peatland in Northern England to test the effect of sediment inputs on water quality, macroinvertebrate drift, macroinvertebrate community structure, and ecosystem metabolism. Sediment addition increased concentrations of dissolved organic carbon, total oxidised nitrogen and suspended sediment concentration in rivers, and intensified the total drift of macroinvertebrates particularly at night. By contrast, the abundance and richness of benthic macroinvertebrates were unaffected, except for declines in Coleoptera abundance in one river. The gross primary production of both rivers was strongly suppressed as the benthos was smothered by sediment. Community respiration also declined, albeit by different extents in the two rivers. Our experiment revealed that short‐term pulses of organic sediment in rivers can have broad effects on water quality and biota, from influences on the dispersal of individual organisms to the modification of ecosystem processes. Organic sediments therefore warrant further examination, to include longer observation periods and more sites. It is particularly important to clarify the extent to which impacts extend from peatland streams into larger rivers downstream. Such studies are necessary to inform global management efforts to restore the integrity of river ecosystems under a range of water and biodiversity policy mechanisms.

Sediment deposition from eroding peatlands alters headwater invertebrate biodiversity

“…Nitrogen dynamics have long been considered to be influenced heavily by sediment influx in peatland catchments, with Crisp () suggesting 80% of nitrogen output in an upland headwater river was a consequence of peat erosion, and Daniels et al () showed that NH 4 released from eroded peat was nitrified rapidly. These nutrient subsidies might drive alterations to river metabolic processes in otherwise low‐productivity peatland river systems (Aspray et al, ). This points towards a need for more peatland river studies to understand the nature of interacting multiple stressors, in a manner similar to experimental manipulations that have uncovered biodiversity responses to sediment interactions with nutrients, flow and temperature alterations in lowland agricultural settings (Matthaei, Piggott, & Townsend, ; Piggott et al, ).…”

“…Our study illustrates that more complete understanding of the mechanistic basis of invertebrate biodiversity responses to organic sedimentation requires refinement of trait databases, similar to studies focused primarily on inorganic sediments (Wilkes et al, ). While we focused on invertebrate communities in our experimental study spanning week‐ to month‐long sediment deposition events, previous work in the UK uplands showed that even short‐term (1–2 day) pulses of sediment cause effects throughout the whole aquatic ecosystem, including water quality, invertebrate drift, invertebrate community structure and ecosystem metabolism (Aspray et al, ). Such pulses are likely to occur particularly with rainfall after summer desiccation events and after needle ice weathering in winter/spring (Li et al, ; Li, Holden, & Grayson, ).…”

“…Water temperature, electrical conductivity (EC), dissolved oxygen (DO) and pH were measured weekly in each of the channels and the source river using a Hach HQ40d portable multi‐parameter meter. Water was collected from each channel and the river in the final week of the experiment to examine effects of sedimentation on suspended sediment concentrations (SSC), dissolved organic carbon (DOC) and total oxidized nitrogen (TON) which have been shown to increase in response to peat inputs to rivers in other studies (Aspray et al, ; Daniels, Evans, Agnew, & Allott, ). Water samples (500 ml) were filtered, dried and weighed to determine SSC, while water samples for DOC and TON were passed through 0.45‐μm Whatman cellulose filters prior to analysis with a Thermalox 8000 total carbon analyser and a Skalar SAN++ continuous flow analyser, respectively.…”

“…Particulates have been shown to constitute up to 75% of the organic load of some temperate‐zone blanket peatland rivers (Evans & Warburton, ), while permafrost–slump sediment inputs can dominate particulate organic fluxes in high Arctic rivers (Lamoureux & Lafrenière, ). Severe erosion of organic soils presents the potential for major changes to the biodiversity of receiving headwaters via modifications to river habitat, smothering of the benthos, and modification of functional processes such as primary production which provide energy to aquatic food webs (Aspray, Holden, Ledger, Mainstone, & Brown, ; Chin, Lento, Culp, Lacelle, & Kokelj, ). However, knowledge of aquatic biodiversity and trait responses to organic soil deposition in rivers is lacking when compared to the effects of inorganic sand and silt (Jones et al, ; Larsen & Ormerod, ; Mustonen et al, ).…”

Limited impacts of experimental flow releases on water quality and macroinvertebrate community composition in an upland regulated river

“…Chironomidae [Robinson et al, 2004b] Durance & Ormerod, 2007;Gillespie et al, 2015a;White et al, 2017). These upland systems are characterized by relatively low pH, flashy flows, and low productivity (Ramchunder et al, 2009;Aspray et al, 2017); thus, biological communities are necessarily adapted to these conditions. Combined with the fact that flow releases greater than those experienced due to reservoir overspill could not be achieved, these factors can explain the minor response in macroinvertebrates.…”