Time-Depth Diagram of Nitrogen in Lawrence Lake

The figure below is from Wetzel 1983 (Figure 12-6).

Lawrence Lake is a small (about 5 ha), oligotrophic lake, part of the Kellogg Biological Station in Battle Creek, Michigan.  It has been the site of a great deal of fundamental work on the chemical limnology of north-temperate zone lakes by R. G. Wetzel and his students at Michigan State University.

The data for these time-depth diagrams were taken approximately weekly for a year-and-a-half at 1-m depth intervals at a station near the deepest point in the lake by Bruce Manny and Robert Wetzel (unpublished except in the textbook).  Units are micrograms per liter.  Concentration isopleths were derived by interpolating between data points to find the location of selected concentrations.  Ice cover is shown to correct depth scale by solid black bands.

This is a simple situation, in which loading of nitrate into the lake by streams and springs was high, combined nitrogen was not limiting for phytoplankton primary production most of the time, and nitrogen-fixation was negligible.  The most important processes affecting concentrations were nitrification, releases by bacterial decomposition, denitrification, and phytoplankton uptake.

• The top panel shows the concentrations of nitrite (NO₂^-) plus nitrate (NO₃^-).  Nitrite contributed very little to the totals, so I will simply refer to nitrate below.
• The bottom panel shows the concentrations of ammonia (NH₃) plus ammonium ion (NH₄⁺).  I will represent both as "ammonia" below.

1. a decrease in nitrate near bottom in September and October 1971, generally matched by an increase in ammonia-ammonium.  The trend was not as strong in 1972.
• Algal blooms would have deposited a layer of easily decomposed, nitrogen-rich organic matter on the sediment surface earlier in the summer.  The nitrogen-rich detritus would support considerable bacterial decomposition.  Since thermal stratification was preventing vertical mixing, and the ratio of mud surface area to the volume of the adjacent water layer was greatest in the deepest part of the lake, that is where ammonia released by decomposing bacteria built up.
• Although classified as mesotrophic, Lawrence is a relatively shallow lake and oxygen is often depleted in the deepest parts of the hypolimnion.  When oxygen concentrations (probably, or evidently) dropped to 0 in September, nitrate-nitrite decreased due to denitrification plus ammonification.
2. two large, temporary increases in nitrate within the thermocline in June and August, 1971.
• The thermocline extended from about 3 m to about 8 m, judging from 1968 data presented elsewhere in Wetzel's text.
• Approximately the same times and depths had decreases in ammonia.
• This indicates that nitrification was occurring most intensively in the thermocline.
• Probably, short algal blooms died back suddenly and the algae settled toward the bottom.  As water density increased and turbulence decreased in the thermocline, settling detritus from the die-backs accumulated enough to support a concentrated layer of decomposing bacteria.
• Also, ammonia diffusing upward from the hypolimnion encountered good environmental conditions for nitrifying bacteria - warmer temperature and higher oxygen concentrations.
3. a large increase in nitrite-nitrate near bottom in February 1972 after the lake froze over.  This trend was not matched by any such large changes in ammonia.
• Reverse thermal stratification under ice prevents most mixing.  However, as the sun heats the lake bottom through the ice in shallow areas, water near bottom warms toward 4 C.  Now heavier than the water at greater depths, it flows down-slope and pools in the deepest part of the lake, carrying nitrate released from the sediments with it.
• Nitrate near the sediments comes from nitrification of ammonia released by bacterial decomposition.  Nitrification is more important near bottom in winter because oxygen concentrations are higher.
4. a brief period of nitrate and ammonia depletion just under the ice in spring (March) 1972.
• Sunlight penetrating the ice was supporting the beginnings of an algal bloom.  Reverse stratification prevented the replenishment from deeper layers of the combined nitrogen that algae were taking up.

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Last revised October 7, 1999.