The long-term dynamics of invasive signal crayfish forcing of fluvial sediment supply via riverbank burrowing

Abstract

Animals are important drivers of sediment dynamics. Invasive signal crayfish (Pacifastacus leniusculus) have been shown to supply sediment to rivers by burrowing into riverbanks. Burrowing directly transfers excavated sediment into the river and also has an additional indirect affect by promoting riverbank failure. While previous research has isolated burrow densities at a point in time, rates of burrow construction and of burrow loss due to erosion are unknown, which introduces uncertainty into estimates of how much sediment burrows contribute to rivers. Here we report results from a 5-year study that evaluated the temporal dynamics of crayfish populations, rates of burrow loss due to erosion, and the mass of sediment directly supplied to rivers by burrow excavation. At ten reaches across five lowland streams in England, we estimated the mass of sediment displaced by 1861 new and previously constructed burrows. Both crayfish and burrow densities were variable over time, suggesting that burrows contribute temporally variable amounts of fine sediment to riverine systems. 42 % of observed burrows were constructed within the previous 365 days, and individual burrows lasted on average 461 days. Applying this to comparable historical data, an average of 2.0 t km−1 a−1 of sediment was excavated to construct burrows in the study reaches, which is eight times more than estimated in previous studies. Whilst total burrow densities in each year were not consistently correlated with contemporary crayfish densities, the mass of sediment excavated over the prior year was strongly correlated with contemporary crayfish densities. Current fine sediment management practices are largely aimed at controlling fine sediment delivery, predominately from agricultural activities, but biotic burrowing into riverbanks may represent an important and overlooked source of fine sediment supply. Incorporation of biotic processes in sediment dynamics would improve the accuracy of fluvial sediment budgets and enhance the knowledge base underpinning effective fine-sediment management practices.