Legacy nitrogen may prevent achievement of water quality goals in the Gulf of Mexico

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Science  27 Apr 2018:
Vol. 360, Issue 6387, pp. 427-430
DOI: 10.1126/science.aar4462
  • Fig. 1 N loading trajectories at the MRB outlet compared to sediment core pigment concentrations.

    The ELEMeNT model accurately predicts long-term trends in Mississippi nitrate loading, as demonstrated by the close relationship between the modeled annual loads (green line) and depth-varying chloropigment concentrations (purple and orange) within a sediment core obtained from the northern Gulf of Mexico (20). The panels represent two different periods of watershed land use: (A) European settlement and large-scale conversion of prairie lands to row crop (1820–1930) (zeaxanthin, coefficient of determination R2 = 0.66, P < 0.001; β-carotene, R2 = 0.57, P < 0.001) and (B) rapid 20th century increases in N fertilizer use (1960–2000) (zeaxanthin, R2 = 0.0.86, P < 0.001; β-carotene, R2 = 0.84, P < 0.001). Both chloropigments shown here (zeaxanthin and β-carotene) are known proxies of high phytoplankton biomass, with β-carotene being common to many types of algae and zeaxanthin being specifically associated with cyanobacteria. ktons, kilotons.

  • Fig. 2 Model-predicted NO3-N export from the MRB under future scenarios.

    Scenarios include (A) business as usual (BAU) and then (B) 25%, (C) 75%, and (D) 100% decreases in the agricultural N surplus. The green dotted lines represent mean N loading for the period 1980–1996, and the red dotted lines represent target N loading to achieve water quality goals for the Gulf of Mexico. For these scenarios, reductions in N loading ranging from 11 to 55% will be achieved. Note that under all scenarios, it takes ~30 years to reach new steady-state loading levels after the 2017 shift in watershed management.

  • Fig. 3 Combined effects of watershed N surplus reductions and time on achieving N loading goals for the MRB.

    (A) Predicted reductions in N loading by 2025 as a function of the reduction in the agricultural N surplus imposed in 2017 (note: 2025 is the interim WNTF target year for achieving a 20% reduction in N loading). (B) Cost-time trade-offs in achieving reductions in MRB N loads. The contour lines represent fractional reductions in N loading as a function of both percent reductions in the agricultural N surplus and the time required to see changes in loading. The red and gray arrows, respectively, demonstrate that it may take between 7 and 35 years to achieve a 20% reduction in N loading, depending upon the extent to which N surplus values are decreased.

  • Fig. 4 Changing age distribution of MRB nitrate loadings to the Gulf of Mexico from 1940 to 2050 for the BAU scenario.

    Note that in 2010, close to 50% of the catchment NO3-N load is made up of N that has resided within the watershed for more than 30 years. In contrast, by 2050, the magnitude of N in this age range has shrunk to only 21%. As demonstrated by the changing proportions of legacy N, decreases in N loading under the BAU scenario (dark brown line) are driven almost entirely by reductions in sources of legacy N over the simulation period.

Supplementary Materials

  • Legacy nitrogen may prevent achievement of water quality goals in the Gulf of Mexico

    K. J. Van Meter, P. Van Cappellen, N. B. Basu

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