Hypoxia in the Mississippi Basin, Gulf of Mexico, and Other Major Ecosystems
Presiding: C Kendall, U.S. Geological Survey; B Boyer, University of California, Berkeley
H14B-01 15:30h
Hypoxia Adjacent to the Mississippi River Plume
The northern Gulf of Mexico receives the freshwater and constituent flux from the Mississippi River, which integrates 40% of the lower 48 United States. In the last half of the 20th century, the flux of nitrogen tripled, phosphorus concentration appears to have increased, and silicate concentration decreased. These changes result from landscape alterations over two centuries with an intensification of human activities that increased the flux of nitrogen and phosphorus particularly in the 1960s to 1980s. Evidence for eutrophication in the coastal ecosystem includes an increase in algal biomass, carbon accumulation from nutrient-enhanced production, worsening oxygen deficiency in the lower water column, and shifts in food web structure. The extent of the oxygen deficiency reaches 20,000 km2 of the inner continental shelf over long periods in summer with the potential for affecting commercially important fisheries in the Gulf. There is daily, weekly and seasonal variability in currents and stratification on the shelf and, therefore, no simple description of the couplings between nutrient delivery, carbon production in surface waters and delivery to and cycling in bottom waters. There are, however, multiple lines of evidence to implicate changes in riverine nutrient loads with overall primary and secondary production, carbon accumulation at the seabed, and low oxygen conditions on the shelf. The change in nutrient loads and responses of the northern Gulf coastal ecosystem, including widespread, severe seasonal hypoxia, parallel similar conditions in the coastal ocean on a global scale.
H14B-02 15:45h
A Watershed History of Nutrient Loadings Leading to Hypoxia, GOM
Two centuries of land use in the Mississippi River watershed are reflected in the water quality of its streams and in the continental shelf ecosystem receiving its discharge. The most recent influence on nutrient loading, from intense and widespread farming and especially from fertilizer use, has had a more significant effect on water quality than the conversion of native vegetation to cropland and grazing pastures, or of land drainage. The 200-year record of nutrient loading to offshore water is reflected in the paleo-reconstructed record of plankton in dated sediments. Evidence for nitrogen control of the size of the hypoxic zone is demonstrated by statistical analyses of the variability of loading, bioassays and other results. The development of fair and sustained management of both inland and offshore ecosystems is thereby linked. The watershed is fully occupied and under the spell of the social policies that can be modified for better or worse, but which will probably change only gradually because of the strong buffering capacity of the soil and sediment ecosystems upstream and on the continental shelf.
H14B-03 16:00h
A 25-Year Retrospective Analysis of River Nitrogen Fluxes in the Atchafalaya
Nitrogen enrichment from the upper Mississippi River Basin has been attributed to be the major cause for the hypoxia in the Northern Gulf of Mexico. The hypoxia threatens not only the aquatic ecosystem health but Louisiana's fishery industry directly among other problems. Although fresh water diversion from the lower Mississippi River into the region's wetlands has been considered an alternative means for reducing nitrogen loading, it is largely uncertain how much nitrogen can actually be retained from the overflowing waters in these natural wetlands. Generally, there is a knowledge gap in what tools are available for accurate assessment of nitrogen inflow, outflow and removal potential for the complex and diverse coastal floodplain systems. This study is to seek answers to three critical questions: (1) Does the Atchafalaya River Swamp remove a significant amount of nitrogen from the overflowing water or release more nitrogen into the Gulf than removing it? (2) How seasonally and annually do the nitrogen removal or release rates fluctuate? (3) What are the relationships between the nitrogen removal capacity and the basin's hydrologic conditions such as river stage and discharge? By utilizing river's long-term discharge and water quality data (1978-2002), monthly and annual nitrogen fluxes were quantified, and their relationships with the basin's hydrologic conditions were investigated. A total Kjeldahl nitrogen (TKN) mass input-output balance between the upstream (Simmesport) and downstream (Morgan City and Wax Lake Outlet) locations was established to examine the organic nitrogen removal potential for this largest freshwater swamp basin in North America. The results showed that on average, TKN input into the Atchafalaya was 200,323 Mg yr-1 and TKN output leaving the basin was 145,917 Mg yr-1, resulting in a 27% removal rate of nitrogen. Monthly nitrogen input and output in the basin were highest from March to June (input vs. output: 25,000 vs. 18,000 Mg mon-1) and lowest from August to November (8,000 vs. 6,000 Mg mon-1). There was a large variation in both annual and inter-annual nitrogen removals, and the variability was positively correlated with the amount of inflow water at Simmesport. However, no close relationship between the river inflow and percentage nitrogen removal rate was found. The results gained from this study suggest that regulating the river's inflow will help reduce nitrogen loading of the Mississippi River to the Gulf of Mexico. The in-stream loss of nitrogen indicates that previous studies may have overestimated nitrogen discharge from the Mississippi-Atchafalaya River system. Furthermore, the study found that knowledge on spatial hydrological conditions in the basin is needed to understand nitrogen dynamics in the Atchafalaya River Swamp.
H14B-04 16:15h
Simulated Changes in the Ratios of Nutrients Delivered to the Gulf of Mexico in Response to Changes in the Nutrient Sources of Inland Watersheds of the Mississippi River Basin
Increases in riverine nitrogen loads to the northern Gulf of Mexico have contributed to increased hypoxia in the coastal waters of the Louisiana shelf during the past several decades. Nitrogen is the most limiting nutrient for algal production in these waters, however, Mississippi River nutrient loads entering the Gulf are closely balanced with seasonally shifting Redfield ratios. Moreover, state concerns over phosphorus, which is generally more limiting to primary production in inland waters, may contribute to an increased emphasis on future phosphorus reductions to meet designated use requirements in state waters. Currently, knowledge is lacking about how nutrient ratios in Mississippi River loads are likely to respond to future changes in nutrient sources in inland watersheds. An improved understanding is initially needed of the major sources and processes in the Mississippi River Basin (MRB) that control both phosphorus and nitrogen delivery to the Gulf. Earlier modeling studies of nutrients in the MRB have focused primarily on nitrogen with little attention to phosphorus. Here, we develop a Spatially Referenced Regression on Watershed Attributes (SPARROW) model of mean-annual total phosphorus (TP) loads for streams in the MRB. The SPARROW model links measurements of TP loads in streams with geographic data on phosphorus sources (e.g., fertilizer, livestock wastes, urban sources) and properties of the landscape that influence transport (e.g., climate, topography, vegetation, soils, water routing). The model employs mechanistic components and mass balance constraints within a formal parameter-estimation structure to empirically quantify the sources, attenuation rates, and transport of phosphorus in the terrestrial and aquatic ecosystems of the MRB. The model was used to quantify the interior watersheds and nutrient sources that contribute to phosphorus delivery to the Gulf. Using a previously estimated SPARROW nitrogen model for the MRB, we computed Redfield ratios of the predicted nutrient loads near the outlet of the Mississippi River. We evaluated the change in these nutrient ratios in response to simulated changes in agricultural and urban sources of phosphorus and nitrogen in the MRB. The results indicate that changes in nutrient ratios are sensitive to changes in both the type of source and the location of inland watersheds of the MRB that are targeted for reduction.
H14B-05 INVITED 16:30h
Nutrient Enrichment of Coastal Receiving Waters from Catchments Across the USA
Though the abundant supply of reactive nutrients to the landscape provides many benefits to society in terms of food and energy production, the environmental consequences of nutrient over-enrichment are severe, particularly in the coastal zone. We assess eutrophication of surface waters, considered to be the most widespread water quality problem in the USA. We highlight hot spots of mass loadings of nutrients to coastal receiving waters based on results from several spatially referenced regression models applied at the national scale. We explore inter-annual variability and long-term trends of nutrient delivery from several key catchments to sensitive estuaries based on long-term monitoring data. We assess the coastal response and ecological effects resulting from these nutrient loads, considering differences such as the physicochemical characteristics and hydrological residence times of estuaries. Further, we discuss the need to understand precursor source of nitrogen to receiving waters. For example, recent research on algal blooms in both the east and west coasts of the US shows that the growth of toxic and harmful algae is stimulated specifically by urea, an organic nitrogen compound dominant in nitrogen inputs from agricultural and urban runoff, over inorganic nitrogen sources such as ammonium and nitrate that are dominant in nitrogen inputs from atmospheric deposition.
H14B-06 16:45h
Potential Contribution of Particulate Organic Matter in the Mississippi River to Hypoxia in the Gulf of Mexico
Episodic hypoxia in the Gulf of Mexico has drawn attention to high levels of nitrate in the Mississippi Basin that may be fueling algal productivity in the Gulf and resulting in low levels of dissolved oxygen. However, about half the N being transported in the Mississippi River is in the form of dissolved organic matter, and if much of it is bioavailable, it may be an important contributor to hypoxia in the Gulf. Since autochthonous organic matter is probably more bioavailable than terrestrial detritus, determination of (1) the relative contributions of different sources of particulate organic matter (POM) and (2) how controls on POM sources, characteristics, and lability vary both temporally and spatially, may provide critical information about other factors contributing to hypoxia. Therefore, riverine POM samples were collected bi-weekly to monthly from 30 USGS NASQAN sites in the Mississippi River Basin from 1996-2004, and analyzed for d13C, d15N, and C:N. These samples demonstrate that there is considerable temporal and spatial variation in the composition of fine-sized POM. A four-source mixing model (plankton, fresh terrestrial plant material, macrophytes, and soil organic material) is used to differentiate general sources of POM using d13C, d15N, and C:N ratios. Average values of d13C and C:N indicate that plankton and heterotrophic bacteria account for approximately half of the POM in the Basin, with higher percentages of plankton downstream of reservoirs and lower percentages in the mainstem of the river. Temporal patterns of d13C are complex but low d13C and C:N values in spring and summer suggest the occurrence of plankton blooms, whereas relatively elevated values in fall and winter are consistent with greater proportions of decaying macrophytes and/or terrestrial material. Many sites also show seasonal trends in d15N. Although soil and macrophyte detritus have overlapping d13C and C:N values, their relative contributions can be distinguished at many sites by (1) differences in d15N values and (2) strong correlations of d13C and C:N values with discharge and discharge-related parameters consistent with soil-derived materials in runoff. Periodic inputs of fresh terrestrial plant detritus at a few sites, principally on the Ohio River sites, are suggested by C:N ratios greater than 15. The d15N and d13C of POM also reflect the importance of internal and external sources of dissolved carbon and nitrogen, and the degree of in-stream processing.
H14B-07 16:55h
Eutrophication Links between the Watershed and Estuary in the Neuse River Basin, NC
The Neuse River drains into the Neuse River Estuary and Pamlico Sound, which is part of the second largest estuarine ecosystem in the United States and a key nursery for Mid and Southeast Atlantic fisheries. RiverNet, ModMon, and now FerryMon have monitored nutrient fluxes in the watershed and ecosystem responses in the estuary. Poor water quality in the 1980's led to a phosphorus ban in the basin that decreased P inputs to the watershed and improved water quality in the freshwater portions of the basin. High temporal resolution nutrient monitoring in the river indicates that significant flux variations are associated with point sources, and that N fluxes have been underestimated by previous monitoring efforts. N loss in the watershed is associated with hydric soils that are primarily located in the lower coastal plain. The 17O composition of nitrate suggests that Amospherically Deposited Nitrogen (A.D.N.) is event driven and is controlled by land use in the sub-basin. New regulations imposed by the State of NC are decreasing N fluxes in the watershed, but these fluxes are highly variable and controlled to some extent by extreme rainfall events that result from direct hurricane strikes and droughts. The greater decrease in P flux to the estuary compared to N flux (which has decreased slightly or remained the same) has reduced P-limited primary production in the freshwater upper portion of the estuary. This limits the N assimilation in this region, and allows more efficient N transport to N-sensitive coastal waters in the lower portion of the estuary. Chl a and phytoplankton pigment monitoring in the estuary indicate that site of the maximum primary productivity has moved form the upper estuary in the 1970's and 1980's to the lower estuary today. This displacement of the eutrophication gradient may explain the reduction of Cyanobacteria algae blooms in the upper estuary, and the increase in harmful algae blooms, hypoxia, and declines in fisheries habitats in the lower estuary. These findings underscore the need for basin scale nutrient reduction strategies that consider the entire freshwater-marine continuum, and the need for long term monitoring that take in account climatic and hydrological variability (i.e. hurricanes and droughts).