Hypolimnetic Chemistry in an Eutrophic Lake
The figure below is from Wetzel 1983 (Figure 13-4),
and he took it from a paper by C. H. Mortimer published in 1971.
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In studying the figure, begin with the events marked on the
top X-axis (time).
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Then notice the way in which carbon dioxide increases and
oxygen decreases over time, especially:
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where oxygen reaches the minimu level, and
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where it appears again at fall overturn.
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Next, compare the time course of nitrate and ammonia concentrations,
and see that
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ammonia only increases after nitrate has decreased to low
levels.
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Although they are not directly illustrated, you can infer
from data on ammonia, nitrate, and nitrite when nitrification, ammonification,
and denitrification (see textbook) are occurring.
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To reconstruct the processes leading to phosphorus increases
in the water, trace the Eh (reduction-oxidation, or redox potential)
as it falls due to the reducing metabolic activities of aerobic, then anaerobic
bacteria in the mud.
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Eventually, it is low enough that Fe+++ [not shown]
is reduced to Fe++ [shown on graph].
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The phosphate, which had formed an insoluble precipitate
with Fe+++ (FePO4), becomes soluble as
Fe++ + PO4--- and begins to diffuse from
the mud into the water.
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As redox falls even lower, sulfate begins to be reduced to
sulfide,
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precipitating some of the ferrous iron Fe++ as
insoluble FeS.
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When oxygen is re-introduced into the hypolimnion by the
partial overturn in mid-September,
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Eh jumps up.
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Fe++ and PO4--- decrease
rapidly. As the Fe++ is oxidized back to Fe+++,
iron and phosphorus are precipiated back down to the mud as FePO4.
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The data show that only after complete mixing in mid-October
does Fe++ completely disappear.
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Phosphate also declines more in October.
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This partly due to uptake by bacteria and algae in the lake
into organic phosphorus compounds.
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PO4--- lasts a little longer in solution
than Fe++, so some of it must have been mixed upward into (returned
to) the euphotic zone at overturn.
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This increase in euphotic-zone phosphorus in early October
probably stimulated a short algal bloom, a form of "self-fertilization"
(by recycling) in lakes that have anaerobic hypolimnia.
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Finally, notice the changes in pH, alkalinity, and conductivity.
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Changes in pH can be related to changes in dissolved carbon
dioxide, some of which reacts with water to form carbonic acid (H2CO3)
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Lower pH dissolves several chemical precipitates from lake
sediments, especially CaCO3 (which becomes Ca++ and
HCO3-) , and reductions and solubilization of several
metal ions, which are insoluble when oxidized, contribute to the increases
in alkalinity and conductivity.
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The most important buffering chemical in most lakes is HCO3-.
Maintained by Sam Mozley, s_mozley@ncsu.edu
Last revised August 21,2001