RESUMEN
We report on the evolution and accuracy of a model used to predict the mid-summer area of hypoxia (oxygen ≤2 mg l(-1)) in the northern Gulf of Mexico, use it to test for impacts from the Deepwater Horizon oil spill (2010), and estimate the N loading that would meet a management goal. The prediction since 2000 were 100%±6% (µ±1 SE) of the actual value. The predicted in 2010 was 99% of that actual value, suggesting that the net effect of the 2010 oil spill on the hypoxic zone size was negligible. A tropical storm, however, may have reduced the potential size of the hypoxic zone. Lowering the May nitrogen load to about 70,000 mton N nitrate+nitrite would bring the model's predicted hypoxic zone size down to the management goal of 5000 km(2) and restore hypoxic waters to normoxic conditions.
Asunto(s)
Modelos Teóricos , Oxígeno/análisis , Estaciones del Año , Agua de Mar/química , Monitoreo del Ambiente , Golfo de México , Nitrógeno/análisis , Reproducibilidad de los ResultadosRESUMEN
We conducted a statistical analysis to discern the relative strengths of the loading of various forms of nitrogen, phosphorus, dissolved silicate and their molar ratios on the variance in the size of the summertime low oxygen zone found off the Mississippi River, northern Gulf of Mexico. A stable statistical model that included Year and riverine nitrate+nitrite loading for the 2 months prior to the measurement of hypoxic zone size described 82% of its variation in size from 1978 to 2004. The usefulness of the term Year is consistent with the documented increase in carbon stored in sediments after the 1970s, which is when the hypoxic zone is predicted to have become a regular feature on the shelf and to have expanded westward. The increased carbon storage is anticipated to cause a sedimentary respiratory demand influencing the size of the zone, and whose temporal influence is cumulative and transcends the annual variations in nitrogen loading. The variable Year is negatively correlated with the TN:TP ratio in a way that suggests N, not P, has become more important as a factor limiting phytoplankton growth in the last 20 years. Nitrogen, in particular nitrate+nitrite, and not phosphorus, dissolved silicate, or their molar ratios, appears to be the major driving factor influencing the size of the hypoxic zone on this shelf. This conclusion is consistent with cross-system analyses that conclude that the TN:TP ratio in the Mississippi River, currently fluctuating around 20:1, is indicative of nitrogen, not phosphorus, limitation of phytoplankton growth. Nutrient management that places stronger emphasis on reducing nitrogen loading as compared to phosphorus loading, is justified.
Asunto(s)
Ambiente , Nitrógeno/química , Oxígeno/metabolismo , Fósforo/química , Ríos/química , Silicio/química , Monitoreo del Ambiente , Predicción , Modelos Químicos , Modelos Estadísticos , Análisis de Regresión , Estadística como Asunto , Factores de TiempoRESUMEN
An 18-year monitoring record (1978-1995) of dissolved oxygen within a region having hypoxia (dissolved oxygen less than 2 mgl(-1)) in the bottom layer was examined to describe seasonal and annual trends. The monitoring location was near or within a well-described summer hypoxic zone whose size has been up to 20,000 km(2). The monitoring data were used to hindcast the size of the hypoxic zone for before consistent shelfwide surveys started, and to predict it for 1989, when a complete shelfwide survey was not made. The concentration of total Kjeldahl nitrogen (TKN) in surface waters and concentration of bottom water oxygen were directly related, as anticipated if organic loading from surface to bottom was from in situ processes. The TKN data were used to develop a predictive relationship that suggested there was no substantial hypoxia before the 1970s, which was before nitrate flux from the Mississippi River to the Gulf of Mexico began to rise. The peak frequency in monthly hypoxic events is two to three months after both the spring maximum in discharge and nitrate loading of the Mississippi River. These results support the conclusion that persistent, large-sized summer hypoxia is a recently-developed phenomenon that began in the 1970s or early 1980s.
Asunto(s)
Modelos Teóricos , Oxígeno/análisis , Agua de Mar/química , Monitoreo del Ambiente , Predicción , Louisiana , Mississippi , Estudios Retrospectivos , Estaciones del AñoRESUMEN
Seasonally severe and persistent hypoxia, or low dissolved oxygen concentration, occurs on the inner- to mid-Louisiana continental shelf to the west of the Mississippi River and Atchafalaya River deltas. The estimated areal extent of bottom dissolved oxygen concentration less than 2 mg L-1 during mid-summer surveys of 1993-2000 reached as high as 16,000 to 20,000 km2. The distribution for a similar mapping grid for 1985 to 1992 averaged 8000 to 9000 km2. Hypoxia occurs below the pycnocline from as early as late February through early October, but is most widespread, persistent, and severe in June, July, and August. Spatial and temporal variability in the distribution of hypoxia exists and is, at least partially, related to the amplitude and phasing of the Mississippi and Atchafalaya discharges and their nutrient flux. Mississippi River nutrient concentrations and loadings to the adjacent continental shelf have changed dramatically this century, with an acceleration of these changes since the 1950s to 1960s. An analysis of diatoms, foraminiferans, and carbon accumulation in the sedimentary record provides evidence of increased eutrophication and hypoxia in the Mississippi River delta bight coincident with changes in nitrogen loading.