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Nutrient-enhanced productivity in the northern Gulf of Mexico: past,present and future 总被引:2,自引:0,他引:2
Rabalais Nancy N. Turner R. Eugene Dortch Quay Justic Dubravko Bierman Victor J. Wiseman William J. 《Hydrobiologia》2002,(1):39-63
Nutrient over-enrichment in many areas around the world is having pervasive ecological effects on coastal ecosystems. These effects include reduced dissolved oxygen in aquatic systems and subsequent impacts on living resources. The largest zone of oxygen-depleted coastal waters in the United States, and the entire western Atlantic Ocean, is found in the northern Gulf of Mexico on the Louisiana/Texas continental shelf influenced by the freshwater discharge and nutrient load of the Mississippi River system. The mid-summer bottom areal extent of hypoxic waters (<2 mg l–1 O2) in 1985–1992 averaged 8000 to 9000 km2 but increased to up to 16000 to 20700 km2 in 1993–2001. The Mississippi River system is the dominant source of fresh water and nutrients to the northern Gulf of Mexico. Mississippi River nutrient concentrations and loading to the adjacent continental shelf have changed in the last half of the 20th century. The average annual nitrate concentration doubled, and the mean silicate concentration was reduced by 50%. There is no doubt that the average concentration and flux of nitrogen (per unit volume discharge) increased from the 1950s to 1980s, especially in the spring. There is considerable evidence that nutrient-enhanced primary production in the northern Gulf of Mexico is causally related to the oxygen depletion in the lower water column. Evidence from long-term data sets and the sedimentary record demonstrate that historic increases in riverine dissolved inorganic nitrogen concentration and loads over the last 50 years are highly correlated with indicators of increased productivity in the overlying water column, i.e. eutrophication of the continental shelf waters, and subsequent worsening of oxygen stress in the bottom waters. Evidence associates increased coastal ocean productivity and worsening oxygen depletion with changes in landscape use and nutrient management that resulted in nutrient enrichment of receiving waters. A steady-state model, calibrated to different observed summer conditions, was used to assess the response of the system to reductions in nutrient inputs. A reduction in surface layer chlorophyll and an increase in lower layer dissolved oxygen resulted from a reduction of either nitrogen or phosphorus loading, with the response being greater for nitrogen reductions. 相似文献
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Brian C. Perez John W. DayJr. Dubravko Justic Robert R. Lane Robert R. Twilley 《Hydrobiologia》2011,658(1):41-54
A 3-month field study was conducted to examine the effects of Atchafalaya River discharge on nitrogen and phosphorus concentrations
in the Fourleague Bay system, to document patterns with salinity variation, to evaluate stoichiometric nutrient ratios of
nitrogen and phosphorus in the river and bay, and to examine the relationship between estuarine freshwater residence time
and export of total nitrogen (TN) and total phosphorus (TP) to the Gulf of Mexico. During spring peak discharge of the Atchafalaya
River, nutrient ratios in lower Fourleague Bay indicate potential phosphorus limitation with an average dissolved inorganic
nitrogen (DIN) to dissolved inorganic phosphorus (DIP) ratio of 32:1, primarily a result of high concentrations of nitrogen
entering the northern bay from the Atchafalaya River and of fairly stable phosphorus concentrations. Ratios of DIN to phosphorus
in the river were much higher (54:1), indicating a significant loss of nitrogen within the Fourleague Bay system. Freshwater
residence time averaged approximately 7 days during the study and ranged from 2 to 100 days. TN export averaged 57% over the
study and ranged from less than 3% at long residence times to greater than 80% at short residence times. TN export to the
coastal ocean with respect to residence time is considerably less than has been shown in other studies. Nitrate + nitrite
export averaged 49% for the 3-month study. Percentages of TP export were greater than TN, averaging 82% for the study period.
By examining the Atchafalaya River delta as a natural analog for controlled river diversions, which are currently being used
as coastal restoration tools, this study shows that discharging river water into highly productive shallow coastal estuarine
and wetland systems can significantly reduce the amount of nitrogen exported to the Gulf of Mexico. 相似文献
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Emily C.G. Hyfield John W. Day Jaye E. Cable Dubravko Justic 《Ecological Engineering》2008,32(4):347-359
Most wetlands of the Mississippi deltaic plain are isolated from riverine input due to flood control levees along the Mississippi River. These levees have altered hydrology and ecology and are a primary cause of massive wetland loss in the delta. River water is being re-introduced into coastal basins as part of a large-scale ecological engineering effort to restore the delta. We quantified freshwater, nitrogen, and phosphorus inputs to the Breton Sound Estuary for three climatically different years (2000, 2001, and 2002). Water budgets included precipitation, potential evapotranspiration, the diversion, stormwater pumps, and groundwater. Precipitation contributed 48–57% of freshwater input, while the diversion accounted for 33–48%. Net groundwater input accounted for less than 0.05% of freshwater inputs. Inputs of ammonium (NH4-N), nitrate (NO3-N), total nitrogen (TN), and total phosphorus (TP) were determined for each of the water sources. Atmospheric deposition was the most important input of NH4-N (57–62% or 1.44 × 105–2.32 × 105 kg yr−1) followed by the diversion. The diversion was the greatest source of NO3-N (67–83%, 7.78 × 105–1.64 × 106 kg yr−1) and TN (60–71%). The diversion contributed 41–60% of TP input (1.17 × 105–2.32 × 105 kg yr−1). Annual loading rates of NH4-N and NO3-N were 0.17–0.27 and 1.2–2.3 g N m−2 yr−1, respectively, for the total basin indicating strong retention of nitrogen in the basin. Nitrogen retention through denitrification and burial was estimated for the upper basin. 相似文献
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Population dynamics of mangrove Avicennia bicolor on the Pacific coast of Costa Rica 总被引:2,自引:0,他引:2
Deterministic and stochastic class structured population models were used to simulate the life cycle of Avicennia bicolor of the Pacific coast of Costa Rica. The models were based on an extensive data set collected during a 6 year period in a 0.52 ha plot of monospecific A. bicolor. This data set included density, growth, mortality and transition rates of seedlings, saplings and trees of eight different diameter classes, as well as propagule production for the reproductive tree classes. Model simulations carried out over a 100 year period indicated a stable size class structure of the forest. Sensitivity analysis showed a significantly greater sensitivity of the model population to simulated changes in the mortality of seedlings, in comparison with the mortality of saplings and trees. An increase of 1% in the mortality of seedlings, for example, was sufficient to cause significant changes in the density of individual size classes. In contrast, neither a 10% increase in the mortality of saplings and trees nor a 20% decrease in the propagule production of fecund trees significantly affected the overall forest structure. 相似文献
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