Transport and Cycling of Nutrients in Rivers and Catchments Implications for Eutrophication Risk, Resilience & Recovery Helen Jarvie, Centre for Ecology & Hydrology, U.K.
Decades of nutrient mitigation have not always delivered expected, lasting ecological improvements.. Gulf of Mexico Chesapeake Bay Great Lakes FL SW Coast Baltic Sea
Delayed responses: release of legacy P Loch Leven, Scotland Lake P µg/l P input 300 reduced 60% 250 200 150 100 50 0 Recovery phase Internal recycling of legacy P (10-15 y) Water quality target met 1990 1995 2000 2005 2010 Data from L. May & B. Spears, CEH Edinburgh Sharpley et al. (2013) JEQ Water quality target 40 µg/l
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...
The River Thames, London, in 2016
The River Thames, London, in the 1930s
Total P (mg L -1 ) Nitrate-N (mg L -1 ) Timeseries of N and P for the River Thames 12 10 8 6 WWII: ploughing grassland for crops Nitrate-N 4 2 Delayed groundwater response Lack of response to mitigation? Howden et al (2010) 0 1936 1941 1946 1951 1956 1961 1966 1971 1976 1981 1986 1991 1996 2001 2006 7 6 5 Total-P Population increases: Increasing sewage effluent discharges 1991: Urban Wastewater Treatment Directive: upgrades to sewage treatment 4 3 2 1 0 1936 1941 1946 1951 1956 1961 1966 1971 1976 1981 1986 1991 1996 2001 2006 Data from Nicholas Howden
Historical catchment P budgets, Thames, UK Cumulative P flux (kt/y) 1936 1942 1948 1954 1960 1966 1972 1978 1984 1990 1996 2002 2008 800 700 600 500 400 300 200 Cumulative P input Cumulative P output Cumulative P balance 100 0 Powers et al.(2016)
1936 1942 1948 1954 1960 1966 1972 1978 1984 1990 1996 2002 2008 Cumulative P or N balance (kt/y) 75-yr trajectories of P & N accumulation and depletion in the Thames catchment 1400 1200 1000 800 600 400 200 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 0-200 -400 Catchment Source: net N loss -600-800 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
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
River Soluble Phosphorus Loads entering Lake Erie 10 8 200 Clean Water Act; Great Lakes Water Quality Agreement 180?? 160 140 120 Water flux 6 100 80 4 60 SRP load 40 2 Cumulative water flux (ML ha-1) Cumulative SRP load (kg ha-1) 12 20 0 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 0 Sandusky River: 40-yr of daily P data collected by Heidelberg University, OH Jarvie et al (2017). JEQ
Quantifying the effects of increasing runoff on increased soluble P fluxes to Lake Erie after 2002 1985-2002 2003-2013 ~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.
Changing watershed P budgets: Cumulative watershed P balance (kg ha -1 ) 5 0-5 Sandusky River -10-15 -20-25 -30 Maumee River Raisin River -35-40 River SRP fluxes -45 increase! 1995 2000 2005 2010 2015 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...
Conservation Tillage % High residue management 90 80 70 60 50 40 30 20 10 0 Data from USDA-NRCS Jarvie et al. (2017) JEQ 1980 1985 1990 1995 2000 2005 2010 2015 Sandusky Raisin Maumee Soil stratification, broadcast fertilizer applications & accumulation of labile P fractions at the soil surface
Expansion of tile drainage Changing hydrology: increases in stream flashiness - accelerating the delivery of surface-accumulated labile P from land to river.
Converging cumulative impacts of gradually increasing SRP source availability & increased hydrological connectivity: contributing to a critical threshold in SRP delivery after 2002?
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, 123-132.
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.
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.
Merci ~ Thank you! hpj@ceh.ac.uk