Long term diatom community dynamics of a lowland UK river from a 30 year biomonitoring record. Impacts of hydrology climate and pollution

Anthropogenic impacts on surface waters have resulted in a deterioration in water quality, habitat, and ecosystem functions. Improvements to wastewater practices, however, have not yielded the desired effect of improved ecological status, which leads to the deferral of targets by policy makers. In this regard, lowland rivers in industrialised regions present a challenging area for research, representing a dynamic system with multiple interconnected physical and chemical processes. As such, the main drivers of changes to this ecosystem are poorly understood despite the main principles of biological responses being well documented. Macrophytes and microphytobenthos have been identified as a useful component of the river ecosystem to monitor ecosystem health and integrity; and adopted for the routine monitoring of the ecological status of inland waters. This study presents a unique, 29-year biological record of periphytic diatom communities from artificial substrates, paired with secondary environmental and hydrological data derived from a routine water quality monitoring time series at Darley Abbey on the River Derwent, upstream of the city of Derby. Multivariate analysis is implemented to identify the main drivers of diatom community dynamics within antecedent environmental conditions, over a period of shifts in water quality management strategies, climate, and flow regime. Results are compared to common diatom indices (metrics designed to assess ecological status) and their effectiveness in characterising the underlying processes in the context of a large lowland UK river is discussed. The study also presents a novel method (using artificial substrates; SterlininTM tubes) for the sampling of diatom communities in the routine monitoring of larger water bodies, and its performance relative to other popular methods is assessed. Sterilin tubes are presented as a suitable artificial substrate for community analysis of diatoms, replicates demonstrated limited variability between samples and adequately represented communities from natural substrates, there was little evidence of substrate specific selection pressure. The site and substrate specific colonisation processes are investigated. The Derwent remains under a state of nutrient enrichment; the main response of diatom communities is along a nitrogen and temperature gradient, phosphorus concentrations proved important between 1998 and 2007 but not in the context of the entire record. Flow too is important, notably the variability in high flow events and the duration of low flow periods, and there is evidence that metal pollution is still significant within the catchment. The performance of diatom indices raises questions on their implementation in post-industrial lowland rivers.