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Anything New in the Story of Nitrogen Moving into the Gulf of Mexico?

by December 14, 2011

Dialogue Earth has roots in a broad, stakeholder-based national report on the condition and use of U.S. ecosystems published by the Heinz Center in Washington, D.C. (the report is available in hard-copy from Island Press). Curiosity about the ongoing relevance of these indicators has led me to dive in and see what, if anything, can be said about trends since the release of the report in 2008.

The first indicator in the spotlight is one that describes the movement of nitrogen (N) into major waterways, through run-off as well as point discharges such as sewage treatment facilities (here’s a link to download a pdf of the indicator). Why is this an important indicator of the state of U.S. ecosystems? Our explanation in the report does a good job of answering this question:

Nitrogen is an important plant nutrient and is essential to all life. Nitrogen is an abundant component of the earth’s atmosphere, but it is unavailable to most life in gaseous form. In order to be used by plants and other organisms, nitrogen gas must be “fixed,” or converted to a “reactive” form, that plants can use, such as nitrate. Nitrogen is fixed and accumulates in ecosystems through natural processes, such as the growth of nitrogen-fixing plants like clover and soybeans. However, human activity has greatly increased the amount of reactive nitrogen added to ecosystems. The largest human-caused input of nitrogen to ecosystems comes from the conversion of atmospheric nitrogen gas into fertilizers. Additional reactive nitrogen gas is produced by the combustion of fossil fuels. Reactive nitrogen from all these sources can ultimately enter streams and rivers. Excess nitrogen transported to coastal waters by rivers can lead to low oxygen conditions, threaten fish and animal life, and degrade coastal water quality.

Below is the main graphic from the indicator. The lower panel is a map of the lower-48 states, with major watersheds colored based on the amount of nitrogen that enters waterways from runoff, discharges or other sources. The watersheds in black had the highest amount of nitrogen per area entering waterways; those in light blue had the least (there were no data available for those watersheds shown in gray).

The graph in the upper panel rolls up the mapped data, showing the percent of land area in the different N delivery categories. The main take-home message from the graph is that the 600-1500 pounds per square mile per year category shrunk, with those acres moving into the lower category (10-600 pounds per square mile per year). While it is easy to get tongue tied explaining that an increase of land in a lower N delivery category is good news, that’s a reasonable conclusion from the data.


So, can we tell anything about how things have changed since the 2001 — 2005 period? It doesn’t look like the data shown in the graphic above have been updated, but we can tell a good bit from the third graph in the indicator: how much nitrogen is flowing from the Mississippi to the Gulf of Mexico.

In terms of flow of nitrogen to the Gulf of Mexico via the Mississippi-Atchafalaya river basin, there does not seem to be any improvement, and possibly an uptick in discharges. The graph below compares three U.S. Geological Survey (USGS) data sets: (1) the blue line is from the 2008 report from the Heinz Center and is only for nitrate discharges and is through 2004 ; (2) the orange line is for both nitrate + nitrate, which is typically a minor component, and is from a predictive model called LOADEST; (3) the yellow line, which tracks the orange line closely, is a composite of samples, and is not available from every year (note there is no data point during the spike witnessed in 1993. Note that data from the Heinz Center report can be accessed here, and the USGS data for the orange and yellow lines are available here.


So, not great news on the flow of nitrogen into the Gulf. What about the low-oxygen zone in the Gulf of Mexico? This is, after all, the main motivator for studying the movement of nutrients downstream. Here’s a map from the Hypoxia Research Team at the Louisiana Universities Marine Consortium showing the extent of low-oxygen bottom waters during their 2011 summer cruise (see their press release here):


The graph below, also from the Consortium, shows that the size of the zone with low oxygen levels (bottom waters below 2 mg/L dissolved oxygen) was somewhat smaller than in 2010, yet well above the goal set by the Hypoxia Action Plan—see the second graph below that reports data through 2010. Note that Consortium researchers describe how a recent tropical storm likely mixed waters before their survey during the summer of 2011, most likely causing their area estimate to be lower than it might have been days earlier.

2011_updated_data_hypoxia_zone_si midsummer_hypoxic_zone_size

Stepping back, it looks like discharge of nitrate into the Gulf of Mexico from the Mississippi-Atchafalaya basin has been more-or-less unchanged since the data reported in the 2008 Heinz Center ecosystem report. Plus, while there’s been fluctuation, the driving reason for concern—they zone of hypoxia in the Gulf of Mexico—remains well above the management target.

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