Transport and Cycling of Nutrients in Rivers and Catchments Implications for Eutrophication Risk, Resilience & Recovery

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1 Transport and Cycling of Nutrients in Rivers and Catchments Implications for Eutrophication Risk, Resilience & Recovery Helen Jarvie, Centre for Ecology & Hydrology, U.K.

2 Decades of nutrient mitigation have not always delivered expected, lasting ecological improvements.. Gulf of Mexico Chesapeake Bay Great Lakes FL SW Coast Baltic Sea

3 Delayed responses: release of legacy P Loch Leven, Scotland Lake P µg/l P input 300 reduced 60% Recovery phase Internal recycling of legacy P (10-15 y) Water quality target met Data from L. May & B. Spears, CEH Edinburgh Sharpley et al. (2013) JEQ Water quality target 40 µg/l

4 Storage & Remobilisation of Legacy P Legacy P storage timescales Jarvie et al. (2013) ES&T Continued chronic release of nutrients may mean a long wait for downstream water-quality improvements...

5 The River Thames, London, in 2016

6 The River Thames, London, in the 1930s

7 Total P (mg L -1 ) Nitrate-N (mg L -1 ) Timeseries of N and P for the River Thames WWII: ploughing grassland for crops Nitrate-N 4 2 Delayed groundwater response Lack of response to mitigation? Howden et al (2010) Total-P Population increases: Increasing sewage effluent discharges 1991: Urban Wastewater Treatment Directive: upgrades to sewage treatment Data from Nicholas Howden

8 Historical catchment P budgets, Thames, UK Cumulative P flux (kt/y) Cumulative P input Cumulative P output Cumulative P balance Powers et al.(2016)

9 Cumulative P or N balance (kt/y) 75-yr trajectories of P & N accumulation and depletion in the Thames catchment Cumulative P balance Cumulative N balance Shift to net N accumulation, within subsoils and into groundwater matrix Catchment Sink: building P and N legacy stores Catchment Source: net N loss Ploughing of permanent grassland: mineralisation & release of soil N Historical decoupling of P and N has led to shifting patterns in accumulation and depletion of legacy nutrient stores

10 Re-eutrophication of Lake Erie: linked to increasing riverine fluxes of Soluble Reactive P Aug 4 th 2014 MICHIGAN Lake Erie Aug 7 th 2015 Maumee River watershed Sandusky River watershed OHIO

11 River Soluble Phosphorus Loads entering Lake Erie Clean Water Act; Great Lakes Water Quality Agreement 180?? Water flux SRP load 40 2 Cumulative water flux (ML ha-1) Cumulative SRP load (kg ha-1) Sandusky River: 40-yr of daily P data collected by Heidelberg University, OH Jarvie et al (2017). JEQ

12 Quantifying the effects of increasing runoff on increased soluble P fluxes to Lake Erie after ~35 % of the increase in soluble P loads after 2002 can be attributed to higher runoff volumes, with ~65% arising from enhanced delivery of labile P fractions.

13 Changing watershed P budgets: Cumulative watershed P balance (kg ha -1 ) Sandusky River Maumee River Raisin River River SRP fluxes -45 increase! Jarvie et al (2017) JEQ A net drawdown in watershed P stores since the late 1990s Changes in watershed functioning: sustained increases in Soluble P delivery, despite a declining P balance...

14 Conservation Tillage % High residue management Data from USDA-NRCS Jarvie et al. (2017) JEQ Sandusky Raisin Maumee Soil stratification, broadcast fertilizer applications & accumulation of labile P fractions at the soil surface

15 Expansion of tile drainage Changing hydrology: increases in stream flashiness - accelerating the delivery of surface-accumulated labile P from land to river.

16 Converging cumulative impacts of gradually increasing SRP source availability & increased hydrological connectivity: contributing to a critical threshold in SRP delivery after 2002?

17 Conservation measures successfully reduced erosion and transport of particulate P..but may have unintentionally contributed to the rise in ecologically-damaging SRP fluxes? Jarvie H, Johnson L, Sharpley A, Smith, D, Baker D, Bruulsema, T, Confesor, R (2017) Increased Soluble Phosphorus Loads to Lake Erie: Unintended Consequences of Conservation Practices? Journal of Environmental Quality, 46,

18 Transport and Cycling of Nutrients in River Catchments. Rivers are reactive conduits for nutrient transport; Accumulation of P and N in catchments results in nutrient legacies with varying residence times; Chronic release of P and N from these legacy stores may continue to impair water quality for years and decades.

19 Eutrophication Risk, Resilience & Recovery What is needed going forward? Better quantification of nutrient sources and sinks in catchments; Evaluation of the implications of buffering of acute nutrient loads vs chronic release of nutrients, on downstream water quality and ecology; Realistic expectations about adoption of mitigation strategies and timescales for recovery; Recognition that there can be unintended consequences of mitigation measures and that adaptive management is essential.

20 Merci ~ Thank you! hpj@ceh.ac.uk