Figure 2.7 (pg. 95) shows the dissolved oxygen (DO) and temperature profiles in

Question

Figure 2.7 (pg. 95) shows the dissolved oxygen (DO) and temperature profiles in Upper Mystic Lake on the Aberjona Watershed in eastern Massachusetts on September 30, 1991. Estimate the minimum possible flux density of downward Fickian oxygen transport through the thermocline at a depth of approximately 7.5m.

ISPURT IN SURFACE WATERS 95 Spring Summer stratification (+ D0+Temp) Temperature ('C) 0 6 12 18 24 Temperature (C) 06 12 18 24 0 5 10 15 E 10 a 15 20 25 20 25 0 2 4 6 8 10 12 14 0 2 46 8 10 Dissolved oxygen (mg/liter) Dissolved oxygen (mg/liter) Thermocline Hypolimniorn Well mixed Stratified FIGURE 2.7 Measured temperature and oxygen profiles from the Upper Mystic Lake in eastern Massachusetts, USA, on April 1, 1991 and September 30, 1991. The left graph shows that the lake is unstratified and well mixed during turnover, which occurs in spring and fall The right graph shows that during summer, this eutrophic (biologically productive) lake becomes depleted in oxygen in its hypolimnion, while the epilimnion remains well mixed by the wind and oxygenated by photosynthesis and by contact with the atmosphere. An oli- ßotrophic lake, which has lower biological productivity, may retain its high springtime con- gen in the hypolimnion throughout the summer. Data from Aurilio (1992)

Explanation / Answer

Answer- The dissolved Oxygen (DO) and temperature profile of spring and summer in the upper mystic lake on Abberjona Watershed is given. Left profile (the DO vs temperature profile of spring) is unstratified representing spring and fall season. temperature remains the same in all depth (6 degrees centigrade throughout all depth) and also the dissolved oxygen within the lake is 12mg/L all the depth. there is no thermocline in the spring and fall season. On the right side, the profile is well stratified, there is a presence of well-established thermocline and a steady decrease in the concentration of dissolved oxygen on increasing depth.

according to fickian diffusion, high concentration flow will spread to low concentration under the diffusion process.

diffusion coefficient for oxygen 10C is 0.15cm2/s

the depth for the determination of minimum flux density of downward oxygen transport is 7.5m

DO concentration at 7.5m depth during the spring season is 12mg/L

DO concentration at 7.5 depth in well stratified lake during summer season when thermocline layer is formed is 4mg/L

So minimum possible flux density is qz = D* dC/dz = 0.15*(12mg/L-8mg/L)/7.5 = -0.15*4/7.5 = 0.08

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