. Water masses can become undersaturated with oxygen when natural processes alone or in combination with anthropogenic processes produce enough organic carbon that is aerobically decomposed faster than the rate of oxygen re-aeration. The dominant natural processes usually involved are photosynthetic carbon production and microbial respiration. The re-supply rate is indirectly related to its isolation from the surface layer. Hypoxic water masses ( Hypoxia development and continuation in many areas of the world's coastal ocean is accelerated by human activities, especially where nutrient loading increased in the Anthropocene. This higher loading set in motion a cascading set of events related to eutrophication. The formation of hypoxic areas has been exacerbated by any combination of interactions that increase primary production and accumulation of organic carbon leading to increased respiratory demand for oxygen below a seasonal or permanent pycnocline. Nutrient loading is likely to increase further as population growth and resource intensification rises, especially with increased dependency on crops using fertilizers, burning of fossil fuels, urbanization, and waste water generation. It is likely that the occurrence and persistence of hypoxia will be even more widespread and have more impacts than presently observed. Global climate change will further complicate the causative factors in both natural and human-caused hypoxia. The likelihood of strengthened stratification alone, from increased surface water temperature as the global climate warms, is sufficient to worsen hypoxia where it currently exists and facilitate its formation in additional waters. Increased precipitation that increases freshwater discharge and flux of nutrients will result in increased primary production in the receiving waters up to a point. The interplay of increased nutrients and stratification where they occur will aggravate and accelerate hypoxia. Changes in wind fields may expand oxygen minimum zones onto more continental shelf areas. On the other hand, not all regions will experience increased precipitation, some oceanic water temperatures may decrease as currents shift, and frequency and severity of tropical storms may increase and temporarily disrupt hypoxia more often. The consequences of global warming and climate change are effectively uncontrollable at least in the near term. On the other hand, the consequences of eutrophication-induced hypoxia can be reversed if long-term, broad-scale, and persistent efforts to reduce substantial nutrient loads are developed and implemented. In the face of globally expanding hypoxia, there is a need for water and resource managers to act now to reduce nutrient loads to maintain, at least, the current status.

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Dynamics and distribution of natural and human-caused hypoxia

Current status of agricultural and rural non-point source Pollution assessment in China.

Mitigating cyanobacterial harmful algal blooms in aquatic ecosystems impacted by climate change and anthropogenic nutrients.

Riverine nitrate N in the Mississippi River leads to hypoxia in the Gulf of Mexico. Several recent modeling studies estimated major N inputs and suggested source areas that could be targeted for conservation programs. We conducted a similar analysis with more recent and extensive data that demonstrates the importance of hydrology in controlling the percentage of net N inputs (NNI) exported by rivers. The average fraction of annual riverine nitrate N export/NNI ranged from 0.05 for the lower Mississippi subbasin to 0.3 for the upper Mississippi River basin and as high as 1.4 (4.2 in a wet year) for the Embarras River watershed, a mostly tile-drained basin. Intensive corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] watersheds on Mollisols had low NNI values and when combined with riverine N losses suggest a net depletion of soil organic N. We used county-level data to develop a nonlinear model of N inputs and landscape factors that were related to winter-spring riverine nitrate yields for 153 watersheds within the basin. We found that river runoff times fertilizer N input was the major predictive term, explaining 76% of the variation in the model. Fertilizer inputs were highly correlated with fraction of land area in row crops. Tile drainage explained 17% of the spatial variation in winter-spring nitrate yield, whereas human consumption of N (i.e., sewage effluent) accounted for 7%. Net N inputs were not a good predictor of riverine nitrate N yields, nor were other N balances. We used this model to predict the expected nitrate N yield from each county in the Mississippi River basin; the greatest nitrate N yields corresponded to the highly productive, tile-drained cornbelt from southwest Minnesota across Iowa, Illinois, Indiana, and Ohio. This analysis can be used to guide decisions about where efforts to reduce nitrate N losses can be most effectively targeted to improve local water quality and reduce export to the Gulf of Mexico.

http://www.epa.state.il.us/water/nutrient/presentations/sources_of_nitrate_yields.pdf

Sources of nitrate yields in the Mississippi River Basin.

This study presents a method for estimating daily rainfall on a 0.05° latitude/longitude grid covering all of New Zealand for the period 1960–2004 using a second order derivative trivariate thin plate smoothing spline spatial interpolation model. Use of a hand-drawn (and subsequently digitised) mean annual rainfall surface as an independent variable in the interpolation is shown to reduce the interpolation error compared with using an elevation surface. This result is confirmed when long-term average annual rainfall data, derived from the daily interpolations, are validated using long-term river flow data. Copyright © 2006 Royal Meteorological Society

Thin plate smoothing spline interpolation of daily rainfall for New Zealand using a climatological rainfall surface

Seasonal hypoxia in the northern Gulf of Mexico has been linked to increased nitrogen fluxes from the Mississippi and Atchafalaya River Basins, though recent evidence shows that phosphorus also influences productivity in the Gulf. We developed a spatially explicit and structurally detailed SPARROW water-quality model that reveals important differences in the sources and transport processes that control nitrogen (N) and phosphorus (P) delivery to the Gulf. Our model simulations indicate that agricultural sources in the watersheds contribute more than 70% of the delivered N and P. However, corn and soybean cultivation is the largest contributor of N (52%), followed by atmospheric deposition sources (16%); whereas P originates primarily from animal manure on pasture and rangelands (37%), followed by corn and soybeans (25%), other crops (18%), and urban sources (12%). The fraction of in-stream P and N load delivered to the Gulf increases with stream size, but reservoir trapping of P causes large local- and re...

Differences in phosphorus and nitrogen delivery to the Gulf of Mexico from the Mississippi River Basin.

/pdf/pesticides-in-the-nation-s-rivers-1975-1980-and-implications-35v8r3mle2.pdf

Pesticides in the nation's rivers, 1975-1980, and implications for future monitoring

1 Introduction 1 Methods and scope 2 Trend analysis and interpretation 5 Chloride 5 Sulfate 8 Nitrate 9 Alkalinity and base cations 10 pH 12 Suspended sediment 13 Total phosphorus 13 Dissolved-oxygen deficit 14 Fecal coliform and fecal streptococcus bacteria 16 Lead 17 Arsenic 19 Cadmium 20 Other trace metals 20 Conclusions 21 References cited 22 FIGURES 1-7. Maps showing: 1. Locations of sampling stations in the National Stream Quality Accounting Network and the National Water Quality Surveillance System in the conterminous United States 3 2. Change in surface-coal production in NASQAN drainage basins, 1974-81 7 3. Trends in flow-adjusted concentrations of chloride at NASQAN stations, 1974-81 8 4. Trends in flow-adjusted concentrations of sulfate at NASQAN stations, 1974-81 9 5. Trends in flow-adjusted concentrations of nitrate at NASQAN and NWQSS stations, 1974-81 10 6. Trends in flow-adjusted concentrations of alkalinity at NASQAN stations, 1974-81 11 7. Trends in flow-adjusted concentrations of suspended sediment at NASQAN stations, 1974-81 12 8. Graph showing comparison of the distribution of point-source biochemical oxygen demand loads for river reaches in cataloging units containing combined network stations with the similar distribution for all cataloging units 16 9, 10. Maps showing: 9. Trends in flow-adjusted concentrations of fecal streptococcus bacteria at NASQAN and NWQSS stations,

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Analysis and interpretation of water-quality trends in major U.S. rivers, 1974-81

The US Geological Survey Operates two nationwide networks to monitor water quality, the National Hydrologic Bench-Mark Network and the National Stream Quality Accounting Network (NASQAN) The Bench-Mark network is composed of 51 stations in small drainage basins which are as close as possible to their natural state, with no human influence and little likelihood of future development Stations in the NASQAN program are located to monitor flow from accounting units which collectively encompass the entire land surface of the nation Data collected at both networks include streamflow, concentrations of major inorganic constituents, nutrients, and trace metals The goals of the two water quality sampling programs include the determination of mean constituent concentrations and transport rates as well as the analysis of long-term trends in those variables This report presents a station-by-station statistical summary of data from the two networks for the period 1974 through 1981 12 refs, 3 tabs

/pdf/a-statistical-summary-of-data-from-the-u-s-geological-survey-1gc90b1v6k.pdf

A statistical summary of data from the u.s. geological survey's national water quality networks

Suspended sediment has long been recognized as an important contaminant affecting water resources. Besides its direct role in determining water clarity, bridge scour and reservoir storage, sediment serves as a vehicle for the transport of many binding contaminants, including nutrients, trace metals, semi-volatile organic compounds, and numerous pesticides (U.S. Environmental Protection Agency, 2000a). Recent efforts to address water-quality concerns through the Total Maximum Daily Load (TMDL) process have identified sediment as the single most prevalent cause of impairment in the Nation’s streams and rivers (U.S. Environmental Protection Agency, 2000b). Moreover, sediment has been identified as a medium for the transport and sequestration of organic carbon, playing a potentially important role in understanding sources and sinks in the global carbon budget (Stallard, 1998).

Richard A. Smith

Papers

Differences in phosphorus and nitrogen delivery to the Gulf of Mexico from the Mississippi River Basin.

Pesticides in the nation's rivers, 1975-1980, and implications for future monitoring

Analysis and interpretation of water-quality trends in major U.S. rivers, 1974-81

A statistical summary of data from the u.s. geological survey's national water quality networks

A spatially referenced regression model (SPARROW) for suspended sediment in streams of the conterminous U.S.