Examination of the relationship of river water to occurrences of bottom water with reduced oxygen concentrations in the northern Gulf of Mexico

Six years of comprehensive data sets collected over the northern continental shelf and upper slope of the Gulf of Mexico during the LATEX-A and NEGOM-COH programs showed that low-oxygen waters (<2.4 mL÷L-1) are found only in spring and summer and only in water depths between 10 and 60 m. Four regions in the northern Gulf show considerable differences in the occurrence of low-oxygen waters. Lowoxygen waters are observed almost exclusively in regions subject to large riverine influences: the Louisiana and Mississippi-Alabama shelves. Hypoxic waters (oxygen concentrations <1.4 mL÷L-1) are found only over the Louisiana shelf. No low-oxygen water is found over the Florida shelf which has minimum riverine influence. Lowoxygen water is found at only one station on the Texas shelf; this is during spring when the volume of low-salinity water is at maximum. The distributions of low-salinity water influenced the different distributions of low-oxygen and hypoxic waters in the four regions. Low-oxygen occurrences are clearly related to vertical stratification. Lowoxygen occurred only in stable water columns with maximum Brunt-Väisälä frequency (Nmax) greater than 40 cycles÷h-1. When Nmax exceeded 100 cycles÷h-1 in summer over the Louisiana shelf, oxygen concentrations dropped below 1.4 mL÷L-1, and the bottom waters became hypoxic. Salinity is more important than temperature in controlling vertical stratification. Locations where temperature influence was larger were found in summer in water depth greater than 20 m over the Louisiana shelf, along the near shore areas of the Mississippi-Alabama shelf west of 87úW, and in the inner shelf waters of the Texas shelf. The extent of oxygen removal at the bottom of these stable water columns is reflected in the amount of remineralized silicate. Silicate concentrations are highest closest to the Mississippi River Delta and decrease east and west of the Delta. EOF analyses show that more than 65% of the oxygen variance is explained by the first mode. The amplitude functions of the first EOF modes of bottom oxygen, water column Brunt- Väisälä maxima, and bottom silicate are well correlated, indicating that much of the variance in bottom oxygen is explained by water column stratification and bottom remineralization

Examination of the relationship of river water to occurrences of bottom water with reduced oxygen concentrations in the northern Gulf of Mexico

Six years of comprehensive data sets collected over the northern continental shelf and upper slope of the Gulf of Mexico during the LATEX-A and NEGOM-COH programs showed that low-oxygen waters (<2.4 mL÷L-1) are found only in spring and summer and only in water depths between 10 and 60 m. Four regions in the northern Gulf show considerable differences in the occurrence of low-oxygen waters. Lowoxygen waters are observed almost exclusively in regions subject to large riverine influences: the Louisiana and Mississippi-Alabama shelves. Hypoxic waters (oxygen concentrations <1.4 mL÷L-1) are found only over the Louisiana shelf. No low-oxygen water is found over the Florida shelf which has minimum riverine influence. Lowoxygen water is found at only one station on the Texas shelf; this is during spring when the volume of low-salinity water is at maximum. The distributions of low-salinity water influenced the different distributions of low-oxygen and hypoxic waters in the four regions. Low-oxygen occurrences are clearly related to vertical stratification. Lowoxygen occurred only in stable water columns with maximum Brunt-Väisälä frequency (Nmax) greater than 40 cycles÷h-1. When Nmax exceeded 100 cycles÷h-1 in summer over the Louisiana shelf, oxygen concentrations dropped below 1.4 mL÷L-1, and the bottom waters became hypoxic. Salinity is more important than temperature in controlling vertical stratification. Locations where temperature influence was larger were found in summer in water depth greater than 20 m over the Louisiana shelf, along the near shore areas of the Mississippi-Alabama shelf west of 87úW, and in the inner shelf waters of the Texas shelf. The extent of oxygen removal at the bottom of these stable water columns is reflected in the amount of remineralized silicate. Silicate concentrations are highest closest to the Mississippi River Delta and decrease east and west of the Delta. EOF analyses show that more than 65% of the oxygen variance is explained by the first mode. The amplitude functions of the first EOF modes of bottom oxygen, water column Brunt- Väisälä maxima, and bottom silicate are well correlated, indicating that much of the variance in bottom oxygen is explained by water column stratification and bottom remineralization

Summertime Nutrient Supply to Near-Surface Waters of the Northeastern Gulf of Mexico: 1998, 1999, and 2000

In the summers of 1998, 1999, and 2000, deep water eddies induced strong anticyclonic currents along the upper slope and outer shelf from the Mississippi River delta to the west Florida shelf. Those currents transported Mississippi River discharge eastward along the outer shelf and slope, reversing the normal offshore increase in salinity, with the exception of a few regions very near the coast that were influenced by the discharges from other rivers or bays. The entrainment of low-salinity river water resulted in anomalously high chlorophyll a concentrations in the upper 15 m over the outer shelf and upper slope, in contrast to the concentrations that typically occur over deep water in the subtropics in summer. Nitrate concentrations in this surface water were quite low except near the mouths of rivers, which act as point sources for nutrients; presumably, this was because of the rapid utilization of nitrate by phytoplankton. A significant supply of nutrients to the euphotic zone at regions quite removed from these point sources resulted from eddies intruding onto or formed over the slope. These caused mid-depth water rich in nutrients to be uplifted to within the euphotic zone, the uplift depending on the location and intensity of the eddies. Based on measurements at approximately 100 stations on each cruise, estimates were made of the quantity of nitrate and silicate in the upper 15 m of the water column and in the depth interval from 15m to 60 m, the nominal depth of the euphotic zone. Study results suggest that the nitrate and silicate in the near-surface interval of 0-15 m largely resulted from riverine discharge and subsequent advection, while the nutrients between 15 and 60 m resulted from uplift of waters by circulation features. The euphotic zone occupied at least the upper 60 m of the water column, but standing stocks of nitrate and silicate in the 15- to 60-m layer were between two and six times those in the upper 15 m on all three cruises and appeared to depend on the strength and relative proximity to the shelf break of local anticyclonic features. The effects of these circulation features were potentially significant in supplying nutrients to the euphotic zone during these summers

Monsoon-driven seasonal hypoxia along the northern coast of Oman

Dissolved oxygen and current observations from a cabled ocean observatory in the Sea of Oman show that the annual recurrence of coastal hypoxia, defined as dissolved oxygen concentrations ≤63 μM, is associated with the seasonal cycle of local monsoon winds. The observations represent the first long-term (5+ years) continuous moored observations off the northern Omani coast. During the summer/fall southwest (SW) monsoon season (Jun-Nov), winds in the Sea of Oman generate ocean currents that result in coastal upwelling of subsurface waters with low dissolved oxygen concentrations. The source of the poorly oxygenated water is the oxygen minimum zone (OMZ) in the Arabian Sea, a layer approximately 1000-m thick within the 100 to 1200 m depth range, where dissolved oxygen values approach anoxia. During the winter monsoon season (Dec-Feb), the Sea of Oman winds are from the northwest, forcing strong and persistent southeast currents. These winds generate oceanic downwelling conditions along the coastal ocean that ventilate waters at depth. Possible impacts of the monsoon-driven seasonal hypoxia on local fisheries and implications due to climate change are also discussed in this study

Importance of physical processes on near-surface nutrient distributions in summer in the northeastern Gulf of Mexico

Due to the character of the original source materials and the nature of batch digitization, quality control issues may be present in this document. Please report any quality issues you encounter to [email protected], referencing the URI of the item.Includes bibliographical references (leaves 70-74).Issued also on microfiche from Lange Micrographics.As part of the northeastern Gulf of Mexico chemical oceanography and hydrography study, data on salinity, nutrients, and surface chlorophyll were collected three times per year over the northeastern Gulf of Mexico along 11 cross-shelf (normal to bathymetry) transects from the Mississippi River Delta to Tampa Bay. The transects covered the shelf from the 10-m to the 1000-m isobath. In all three summers of 1998, 1999, and 2000, eddies over the continental margin induced strong, eastward anticyclonic flows over the outer shelf and slope. This flow strongly affected the distribution of coastal water on the shelf in all three years, drawing Mississippi River water into the circulation along the 1000-m isobath and reversing the normal offshore salinity gradient. The entrainment of low salinity river water resulted in anomalously high nutrient and chlorophyll a concentrations in the upper 10-20 m over the outer shelf and upper slope. This is in contrast to the typically low standing stocks of phytoplankton that occur over deep water in subtropical summertime conditions. Vertical nutrient distributions indicated that there was uplift of mid-depth water to shallower depth caused by the presence of either cyclones, regions of diverging flow at eddy peripheries, or bottom Ekman layer transport. Comparison of near-surface nutrient distributions in the low salinity entrainment features with those in the region of uplifted mid-depth water showed that the intrusion of an anticyclonic secondary eddy onto this subtropical continental margin caused deep water rich in nutrients to rise to depths as shallow as 35 or 10 m depending on the location and the intensity of the anticyclonic and cyclonic eddies. Thus, the impact of both Mississippi River water and anticyclonic and cyclonic eddies on nutrient distribution may be important in enhancing productivity over the study region during summer. On average, during each summer, the amount of nitrate reaching the outer shelf and slope in the nutrient-depleted, low-salinity river water transported by eddy circulation, provided 15 to 32 % as much nitrate as came from uplifted mid-depth water from either cyclones, diverging flow at eddy peripheries, or bottom Ekman layer transport