Figure $2\mathrm{.}7$ shows the $DO$ and temperature profiles in Upper Mystic Lake on the Aberjona Watershed in eastern Massachusetts on September $30,1991.$

a$.$ Estimate the minimum possible flux density of downward Fickian oxygen transport through the thermocline at a depth of approximately $7\mathrm{.}5m-9$m$.$

b$.$ How could the actual flux density be greater than this estimate, assuming the gradient data are accurate?

c$.$ If oxygen is being transported downward, why is there essentially no oxygen in the hypolimnion?

Summer

Spring stratification $($DO ${?}^{{?}^{?}}$ Temp$)$

Temperature $(C)$

Temperature $(C)$

Dissolved oxygen $($mg$/$liter$)$

Dissolved oxygen $($mg$/$liter$)$