N and P in B. Everett Jordan Reservoir

The figure below is from a post-impoundment survey of Jordan Lake by U. N. C. - Chapel Hill scientists.
The station was located near the Vista Point park area between US 64 and the dam on the New Hope River arm of the lake.

Background Information about the Data

• These are graphs of oxygen and nutrient chemicals at a single station in Jordan Lake over time.
• Notice that the time course (X-axis) is three years, and that
• only alternate months are labeled.
• These three years were the first three after the new dam was closed and the reservoir filled completely.
• Each chemical is on a separate graph.
• The two lines on each graph represent data from the surface water and from water near bottom.
• At "Conservation Pool" (usual sustained water level of the lake), the station was about 5 m deep.
• At this depth, the bottom sample was usually below a summer thermocline.
• However, cool, stormy weather or high runoff could mix the lake as deep as 5 m, even in summer.
• Data were taken approximately once a month.
• At this interval, short-term events such as temporary summer mixing may not have shown up in the data.

Nutrient Behaviors

• Oxygen and ammonia graphs show when the lake was thermally stratified
• Shallow and deep water concentrations diverged.
• Why was oxygen lower near the bottom?
• Why was oxygen higher in winter months than in summer months near the surface?
• What do the oxygen data reveal about the process of fall overturn in Jordan Lake?
• How do you explain the relationships between oxygen and ammonia concentrations through seasonal cycles?
• Why was ammonia / ammonium ion higher near the bottom?
• What processes might tie low oxygen to high ammonia?
• Nitrite + nitrate did not differ between surface and near-bottom levels.
• However, they oscillated strongly with season, dropping to very low concentrations during summer.
• Were the same processes causing the oscillations at both shallow and deep levels, or did different processes produce the similar patterns independently?
• Total phosphorus concentrations appeared to be relatively low most of the time.
• Remember that total P will include all inorganic and organic, all dissolved and all particulate, all living and all non-living forms of phosphorus.  Contrast this with the nitrogen data, which cover only two of the dissolved, inorganic forms.
• Sustained TP concentrations were actually fairly high for a lake, between 20 and 50 mg L^-1 at the surface in mid-summer.
• They look low because of the large "spike" of TP near bottom in July 1983.
• Nutrient concentrations are often very high in new reservoirs, but then decline over the next few years as the water and inundated soils reach a new equilibrium.
• Why do you think TP was generally at its highest levels for the year near bottom in late summer?
• What is the relationship between dissolved oxygen and TP concentrations near bottom?
• How do you explain the January 1984 increase in TP at both surface and deep levels?
• N:P ratios can indicate which of these two nutrients is most likely to be limiting phytoplankton primary production at any given place and time.
• You do not have information in these graphs about organic N, but compare the (nitrite+nitrate+ammonia)/TP ratio.
• During which time intervals do you think N was probably an important limiting factor for algal primary production?

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Last revised October 6, 2000