Living organisms alter environments through their presence and activity, with implications for geomorphic and biochemical processes. In terrestrial and marine systems, the impact of animals on the development, structure and stability of soils and substrates has been widely studied for over 100 years (see HYDRALAB tab). However, in rivers the impact of animals in the functioning of the physical environment has received little attention and is poorly understood. I have embarked on a number of studies, detailed below, to quantify the relative significance of invertebrate animals in altering the transport of material through river systems.

Signal crayfish
Signal crayfish (Pacifasticus leniusculus) are an internationally widespread invasive species with many, well-documented detrimental impacts on the ecology of rivers. A series of laboratory flume and field based studies, in collaboration with Prof. Stephen Rice, Prof. Ian Reid and the Environment Agency, demonstrate they can also have potentially series detrimental impacts on the physical environment in rivers by destabilising river beds and banks and promoting the transport of sediment. 

The impact of crayfish on river beds was quantified used repeat laser scanning of surfaces, to generate Digital Elevation Models (DEMs) before and after exposure to crayfish we found the difference between these surfaces demonstrated that a single crayfish could move 1.7 kg of gravel per m2 each day and were able to move material over six-times their own body weight. Their impact could be divided into two types. Firstly, crayfish constructed pits and heaped excavated material into a series of mounds. These biogenic bedforms altered the topography of surfaces and the protrusion of grains. Secondly, as crayfish moved they brushed past grains, altering their geometry, with substantial implications for surface structure and stability. The combination of these effects was found to double the amount of material transported from water-worked surfaces in comparison to control surfaces. This reduction in the stability of gravel beds could be of great significance to native communities of animals which will not only have to adapt to the increased competition and predation by crayfish, but also to a fundamentally altered habitat.

Crayfish also alter the transport of fine sediments by foraging and burrowing into bed and bank material. The fine sediment transport through a tributary of the River Nene in Northamptonshire was monitored for two-years and it was found that during periods of stable flow conditions, peaks in sediment transport occurred at night. These correspond to the nocturnal activity of signal crayfish in the rivers, which was quantified using shallow-depth sonar and radio-telemetry. In addition, laboratory aquaria experiments demonstrate the potential for crayfish to rapidly increase fine sediment when fighting. Modelling would suggest that during the summer of 2013, crayfish contributed over 20% of the fine sediment leaving this catchment. This is highly significant as fine sediment is considered a major polluter of waterways, especially through agricultural land where pesticides and other chemicals are attached to sediments.

Caddisfly larvae
Whereas crayfish can destabilise sediment, some animals have the potential to increase the stability of sediments. Caddisfly are a large and diverse group of insects, the larvae of which live in freshwaters. Caddisfly larvae spin silk threads that are used for a variety of purposes. In collaboration with Prof. Rice, Prof. Reid and Prof. Paul Wood, we focused on Hydropsychid caddisfly larvae that spin silk to construct nets that are used to filter organic matter from the flow. We found that these nets bind grains together, increasing the flow forces required to move gravels by 30%. Considering these animals have a global distribution, can occur at densities of many thousand per m2 and occur in over 80% of UK rivers, they could be of great significance to sediment transport in many river reaches.

Some caddisfly larvae use silk to construct cases that they carry around for protection from predators. These animals can occur in dense aggregations and potentially alter the dynamics of fine-gravel and sand transport by fixing numerous grains together and, eventually, firmly fixing them to coarser material. In the River Dove and in collaboration with Prof. Rice, Prof. Wood, Kate Mathers and Lydia Bailey, we estimated the number of caddisfly cases by field monitoring. We also used image analysis techniques to measure the number and size of grains that make up cases. We estimate that through a 2 km stretch of river there are 128 million cases, fixing 8 tonnes of gravel sized between 0.4 and 4 mm, to the river bed.
A Signal Crayfish (Pacifastacus leniusculus) digging into gravel
A Digital Elevation Model (DEM) of a gravel surface after 24 hours of crayfish activity. Red areas are pits and blue areas indicate mounded material. The surface was initially flat.
An originally planar gravel surface after 12 hours of crayfish activity.
A video showing increasing turbidity as two crayfish interact over 5 minutes
Rocks attached to the out-side edge of an experimental tray with caddisfly silk after removal from the River Soar after 21 days. Trays are 5 cm deep.
An experimental tray with gravel attached to the side.

A caddisfly net flourescing under UV light
Caddisfly cases attached to a larger rock from the R.Dove
Caddisfly case aggregations in the River Dove
Caddisfly case aggregations in the River Dove