Climate Change Will Affect Nutrient Dispersal In UK Estuaries

Abstract:

It is still largely unclear how nutrients that travel through the catchment-river system are distributed within estuaries. How long will nutrients remain in the estuary, and what proportion will disperse offshore into the oceans? In the UK, where many catchments are relatively small and steep, estuaries react rapidly to rainfall events, which crucially control the mixing process, even though tidal stirring is generally large. Seasonal and short-term variability in estuarine functioning is therefore greater than variabilities over semi-diurnal timescales linked to tidal cycling.

We present both published and on-going research that is emerging from an interdisciplinary pan-UK NERC Macronutrient Cycles Programme (macronutrient-cycles.ouce.ox.ac.uk). We pull together intensive field campaigns (Howlett et al. 2015) and model simulations (Robins et al. 2015), and present for the first time coupled simulations of catchment-river-estuary nutrient transport, using a variety of hydrological and hydrodynamic models. We investigate the response of the hydrodynamics and nutrients to extreme flows and storm surge events, and the response to climate change by simulating the IPCC 5th Assessment projections for 2100. On-going research will extend this integrated approach into the macronutrient controls on atmospheric-land exchange.

Emerging research from our UK case study suggests that simulating the hourly river hydrograph, rather than daily-averaged, is important for estuarine response and recovery; daily-averaged flowrates, which are commonly used, under-predict the offshore transport of nutrients. Moreover, biogeochemical processing, whilst detected over estuarine residence times, did not measurably alter the estuarine concentrations, due to the much stronger advective fluxes. By simulating past mean and extreme events, using time-series analysis of river flow and tidal level data collected over the past 50 years, we are able to characterise the future estuarine nutrient response due to climate change in terms of sea-level rise, and future rainfall and storm surge climatologies.