b'Strong and recurring seasonality revealed within stream diatom assemblagesSnell MA1,2, Barker PA1, B. Surridge WJ1, Benskin C McW H1, Barber N3, Reaney SM3, Tych W1, Mindham D1, Large ARG4, Burke S5 and Haygarth PM11Lancaster Environment Centre, Lancaster University, Lancaster LA1 4YQ, UK2Agri-Food and Biosciences Institute, Newforge Lane, Belfast BT9 5PQ, UK3Department of Geography, Durham University, Durham DH1 3LE, UK4School of Geography, Politics and Sociology, Newcastle University, Newcastle upon Tyne NE1 7RU, UK5British Geological Survey, Environmental Science Centre, Nicker Hill, Keyworth, Nottingham NG12 5GG, UKThe benthic biodiversity of freshwater streams is known to respond to multipleanthropogenic stressors, including excess sediment and nutrient delivery that is oftenmodulated by climate-dependent flow, water temperature and event-driven transfers from catchments. Short-duration temporal studies have shown that benthic diatoms, single-celled algae that form a ubiquitous component of stream benthos, are impacted at daily timescales to variables including light, temperature, nutrients and flow conditions. However, knowledge of their seasonal resilience over multiple years is less well known, even in catchments actively managed to reduce pressures. Here, six years of monthly diatom samples from three independent streams, each receiving differing levels of diffuse agricultural pollution, reveal robust and repeatedseasonal variation. Critically we highlight the importance of climate as an indirect variable that alters primary drivers such as nutrient concentrations in the water column and streamconditions. We recommend that future assessments of stream water quality must thereforecapture the inherent seasonal variability of ecological communities and predicated seasonal changes in climate to understand the apparent resistance of stream ecosystems to pollution mitigation measures.26'