vertical temperature stratification may be pronounced in the upper 100 

 m, particularly during summer (Etter and Cochrane 1975, Figure 12). 



Dissolved Qxygea 



Dissolved oxygen levels in shelf waters vary with seasonal trends 

 in temperature, mixing, salinity, organic carbon load, respiration, and 

 photosynthesis. High dissolved oxygen concentrations are promoted by 

 low temperature, high wave turbulence, high rate of photosynthesis and, 

 in general, low salinity. Dissolved oxygen levels are generally higher 

 in surface than in bottom waters due to atmospheric input, 

 photosynthesis, and lower salinities of surface waters; and higher 

 during winter than during summer periods, primarily as a function of 

 temperature and mixing. 



The occurrence of hypoxic (£2 mg/0 bottom waters has been noted for 

 that area of the shelf lying between the Mississippi River Delta and 

 about Grand Isle, Louisiana, since about the midthirties (Bedinger et 

 al. 1980). Such conditions are pronounced during spring and summer 

 months and have been associated with (1) high loadings of organic 

 materials from the previous spring flooding of the Mississippi and 

 Atchafalaya Rivers; (2) the resulting isolation of near-bottom water 

 having a high oxygen demand from a surface layer of fresh water during 

 periods when temperatures are high, and waves and other sources of 

 energy are insufficient to mix the two water masses; and (3) the long 

 residence time of water in the area due to the eddy current. 



The extent of the area affected by hypoxia and the duration of the 

 event appears related to flooding. Following the 100-yr maximum flood 

 of the Mississippi River in 1973, Ragan et al. (1978) determined that 

 between May 1973 and March 1974, an average of 52 % of bottom shelf 

 waters fronting Grand Isle out to the 110-m depth contour were hypoxic 

 (with over 50 % of the area being anoxic). The condition was pronounced 

 between depth contours of 6 and 33 m. The size of the area affected 

 within the sampling zone varied by month, ranging from 27 % in December 

 to 93 % in July. 



Ragan et al. (1978) continued their investigations from May 197^ 

 through August 1976, a period accompanied by a decline in the volume of 

 river discharges (outflows diminished from the record level in 1973 to a 

 point well below the long-term mean in 1976). The hypoxic zone within 

 the investigated area decreased to an average of 39 $ in 197^-75, and 12 

 % in 1975-76. Based upon an expanded sampling regime in 1975-76, Ragan 

 et al. (1978) reported that oxygen-deficient water covered about 7 % of 

 the Louisiana shelf. 



During the summer of 1978, Bedinger et al. (1980) reported an 

 estimated 10- to 15-km wide band of hypoxic (or nearly so) bottom waters 



21 



