T.C. Ballard, A.M. Michalak, G.F. McIsaac, N.N. Rabalais and R.E. Turner
A study by Van Meter et al. concluded that it would take decades to reach nitrogen reduction goals in the Gulf of Mexico because of legacy nitrogen effects. To reach this conclusion, Van Meter et al. developed a model to estimate nitrate loading from the Mississippi River Basin, which drains to the Gulf of Mexico. We highlight issues with their model validation approach and with the assumption that all excess nitrogen cycles through soil organic matter over several years. Using a simple regression model between precipitation and cumulative nitrogen inputs at a range of legacy effect time scales, we also show that such a simple model captures the same or more variability in observed nitrogen as the Van Meter et al. model, indicating that a range of legacy effects are consisitent with ovservations. Our results therefore indicate that reaching nitrogen reduction goals by the target year 2035 is still achievable.
Figure: Modeled nitrate loads from Van Meter et al. (green, R2 = 0.67) and from a two-variable regression based on 8-year cumulative N inputs and 2-year cumulative basin-wide precipitation (yellow, R2 = 0.72) both represent the observed loading (black) well. Note that modeled and observed loading are presented here on an annual basis, whereas it is unclear what multiyear smoothing was applied in Van Meter et al. The zeaxanthin (purple) and β-carotene (orange) chloropigments selected in Van Meter et al. do not consistently scale with the Van Meter et al. modeled loads, nor are they good indicators of productivity resulting from nitrate loading. Furthermore, the selected core (D50) is outside of the area of the Gulf of Mexico most sensitive to variations in loading. A more appropriate indicator, biogenic silica, from a more relevant location in the Gulf (core E30; cyan) does not provide evidence in support of multidecadal legacy effects. For visualization purposes, the range of the secondary vertical axis (Concentration) has been selected to maximize the correspondence between the zeaxanthin data and observed loads during the overlapping (1955–1997) period. The two-variable regression was hindcast to 1900, corresponding with the temporal extent of available precipitation data.
Van Meter et al. (Reports, 27 April 2018, p. 427) warn that achieving nitrogen reduction goals in the Gulf of Mexico will take decades as a result of legacy nitrogen effects. We discuss limitations of the modeling approach and demonstrate that legacy effects ranging from a few years to decades are equally consistent with observations. The presented time scales for system recovery are therefore highly uncertain.
Ballard, T.C., A.M. Michalak, G.F. McIsaac, N.N. Rabalais, R.E. Turner (2019) "Comment on "Legacy nitrogen may prevent achievement of water quality goals in the Gulf of Mexico"," Science, 365 (6455), eaau8401, doi:10.1126/science.aau840.