Dissolved oxygen concentrations strongly influence the distributions and growth of most pond organisms. Oxygen is exchanged between the water column and atmosphere, such that a well mixed water column, if it contained no living organisms, would be expected to be 100% saturated (in equilibrium with the atmosphere). Oxygen concentrations under such circumstances are determined solely by temperature (cold water holds more oxygen) and thus vary predictably with season, declining as temperatures increase during the spring and summer.
       Living organisms are a part of the pond, however, and their photosynthesis and respiration greatly modify oxygen concentrations (Equation 1).

       In equation 1 photosynthesis by plants and algae uses the energy in sunlight to take up carbon dioxide (CO2) and water (H2O) on the left side of the double-headed arrow, and produces glucose (C6H12O6) needed for growth, releasing oxygen (O2) as a byproduct to the water column (on the right side of the double-headed arrow). Algae and aquatic plants thus elevate oxygen levels near the pond surface during daylight hours. Respiration may be thought of as the reverse process, using up glucose and oxygen, and producing carbon dioxide and water. All organisms respire, and oxygen levels thus drop at night, particularly in highly productive ponds with high densities of organisms. Bacterial decomposition of dead organic material in particular is a major cause of high respiration rates. Because light is rapidly depleted with depth in some ponds, photosynthesis is less important than respiration in deeper water, causing a decline in oxygen near the bottom (Fig. 22). Organisms living on or in the bottom sediments are thus exposed to very low oxygen levels. Many species may find deeper areas of the pond uninhabitable under these conditions.

Fig. 22. Changes in dissolved oxygen with depth in a stratified pond (HW) in July. Dissolved oxygen near the surface was most influenced by high rates of photosynthesis and exchange with the atmosphere. High respiration relative to photosynthesis caused the sharp drop in dissolved oxygen below 1.0 m.

       Staying in the well-lit upper waters of a pond can be a life-and-death challenge to members of the phytoplankton. Phytoplankton cells are typically slightly heavier than water, and therefore depend on wind-driven mixing to remain suspended in the water column. Those cells that settle below the mixed layer are typically in the slow process of sinking to eventual demise at the bottom of the pond. Once they sink below the compensation depth, their respiration exceeds their ability to photosynthesize and they are likely to die, decompose and thus contribute to the net consumption of oxygen in the bottom waters (e.g., Brönmark and Hansson, 1998).
       Mean oxygen levels (averaged for the entire water column) in the 13 ponds studied in summer 2002 are shown in Figure 23. As mentioned above, cold water holds more oxygen at 100% saturation (the amounts of oxygen predicted solely by equilibrium with oxygen in the atmosphere above the pond) than warm water (the “pluses” in the figure how much oxygen (as mg/L) is expected at 100% saturation as water temperature increases toward the right); in effect, a pond in early spring with water just above freezing is expected to hold about 14 mg/L dissolved oxygen, while the same pond in mid-summer might be expected to hold just half that amount (about 7 mg/L).
       However, photosynthesis elevates, and respiration reduces, the amounts of oxygen predicted in the water column solely on the basis of water temperature. In Figure 23, photosynthesis elevated dissolved oxygen levels above 100% saturation in most of 13 ponds sampled during March, whereas respiration associated with the decomposition of organic material caused declines in oxygen below 100% saturation in most of those same ponds during July. In effect, even though the ponds appeared greenest during July, this was actually a time when many primary producers were already dying and decomposing in deeper water.

Fig. 23. Mean dissolved oxygen concentrations in the water column relative to mean water temperature during March (red triangles), May (open green squares) and July (solid blue squares). Expected oxygen concentrations at 100% saturation (+) were calculated assuming an atmospheric pressure at sea level of 760 mm Hg and an average elevation of the ponds equal to 100 m.