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II. Characterization of Nutrient Fate, Transport and Sources: Nutrient loads, concentrations, speciation, seasonality and biogeochemical recycling processes have been suggested as important causal factors in the development and persistence of hypoxia in the Gulf. The Integrated Assessment (CENR 2000) presented information on the geographic locations of nutrient loads to the Gulf and the human and natural activities that contribute nutrient loadings.
A. Given the available literature and information (especially since 2000), data and models on the loads, fate and transport and effects of nutrients, evaluate the importance of various processes in nutrient delivery and effects. These may include:
i. The pertinent temporal (annual and seasonal) characteristics of nutrient loads/fluxes throughout the Mississippi River basin and, ultimately, to the Gulf of Mexico.
Total annual N flux discharged to the Gulf of Mexico, primarily nitrate-N and particulate/organic N, has decreased during the past 25 years, as has the spring (April-June) flux. Neither total P nor SRP fluxes show major annual or seasonal trends during the same period.
As discussed in Section 3.1, the upper Mississippi and Ohio-Tennessee River subbasins contribute about 82% of the annual nitrate-N flux, 69% of the TKN flux, and 58% of the total P flux to the Gulf of Mexico while representing only 31% of the drainage area of the MARB. When the upper Mississippi River basin is further divided, the subbasin contributing to the upper Mississippi River between Clinton, IA and Grafton, IL (only 7% of the drainage area) contributes about 29% of the total annual nitrate-N flux to the Gulf. Perhaps more importantly, the upper Mississippi and Ohio-Tennessee River subbasins currently contribute nearly all the spring N flux to the Gulf. These subbasins represent the tile-drained, corn-soybean landscape of Iowa, Illinois, Indiana, and Ohio and illustrate that corn-soybean agriculture with tile drainage leaks considerable N under the current management system. The source of riverine P is more diffuse, although these subbasins are also the largest sources of P.
ii. The ability to determine an accurate mass balance of the nutrient loads throughout the basin.
Estimates of mass balances for nutrient inputs during the period since the Integrated Assessment have been recalculated and are discussed in Section 3.2, but the research needs described in the Integrated Assessment remain unresolved. Therefore, the Panel’s ability to determine an accurate mass balance of nutrient inputs to the MARB is limited by the available information and understanding. For example, some components of the N mass balance (e.g., denitrification, N2 fixation, manure N, soil N pool processes such as mineralization and immobilization) are not measured each year. N2 fixation and manure N are the only two of these components that can be estimated. There are too few data available for the remaining processes to allow calculations. There also is still a disconnect between estimates of inputs to the land (i.e. fertilizer and manure use) and estimates of the proportion of N and P from those inputs that reach the riverine system and contribute to the nutrient flux. Point sources discharge N and P directly to rivers, and are estimated by this Panel to contribute about 22% and 34% of the annual riverine N and P flux respectively, yet their contributions continue to be estimated from permit limits and are not actually measured. Better point-source data are needed to improve mass balance estimates of nutrient loads.
iii. Nutrient transport processes (fate/transport, sources/sinks, transformations, etc.) through the basin, the deltaic zone, and into the Gulf.
As discussed in Section 3.3, the percentage of annual N and P inputs removed by in-stream processes varies by MARB subbasin and ranges from 20 to 55% for N and 20 to 75% for P based on model estimates. Denitrification can be a significant pathway for N removal in small streams during low flow, warm periods, thereby enhancing local water quality. However, most nitrate-N is exported to the Gulf during high flows in the period from January to June, when denitrification is not an effective removal process. Although current estimates of denitrification rates in coastal wetlands are higher than the estimates used in the Integrated Assessment, current studies still conclude that river diversions to coastal wetlands would remove only small amounts of nutrients relative to the total fluxes. However, better estimates of nutrient and organic matter loss rates (denitrification; long-term burial of C, N, and P; and plant uptake) are needed to better understand observed differences between wetland inputs and outputs in coastal areas.
B. Given the available literature and information (especially since 2000) on nutrient sources and delivery within and from the basin, evaluate capabilities to:
i. Predict nutrient delivery to the Gulf, using currently available scientific tools and models; and
ii. route nutrients from their various sources and account for the transport processes throughout the basin and deltaic zone, using currently available scientific tools and models.
In Section 3.4, the SAB Panel singled out three models for discussion: SPARROW, SWAT, and IBIS/THMB. Each is capable of N and P load estimation on the scale of the MARB, yet each has strengths and weaknesses requiring further development. The uncertainty of results from each model reflects the uncertainty of the model structure and algorithms, as well as that propagated by the input data, user parameterization, the calibration process, other user-defined conditions, and the skill of the model user. Even though the capability to predict and route nutrients throughout the MARB has improved since the Integrated Assessment, future adaptive management will require a smooth interface between watershed, economic, and Gulf of Mexico hypoxia models that will allow resource managers the capability to assess the effects of policy decisions and management practices on the sources, fate, and transport of nutrients from the MARB to the Gulf of Mexico.
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