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  Investigating Modeling Water Quality
 

Part I - The Vollenweider Model

Your client has nearly all the information she needs, but wants to know how Ice and Independence Lakes fit in with other typical lakes. Are they considered average, cleaner, or poorer quality than other lakes? Use Figure 3 to estimate the "trophic status" of each lake. What are they?

Background Information: A variety of relatively simple empirical models (i.e. based on relationships between measured data rather than derived from an understanding of all the processes) have been developed since the mid-1960's to predict eutrophication on the basis of phosphorus loadings. The P-loading concept assumes that algal growth is limited by the availability of phosphorus in the water and that increased P, which is derived from sewage discharges and from runoff into lakes and streams, has caused water quality degradation - but the sources are controllable. Typically, these models are used to relate the loading rates for P into the lake to summer concentrations of phosphorus in the lakewater. Then, other empirical relationships are used that link P to various measures of water quality, such as clarity (Secchi depth), algae (chlorophyll) and oxygen depletion in bottom waters.

The Vollenweider plot shown in Figure 3 was developed by Richard Vollenweider, a Canadian limnologist. He noted that deeper lakes were generally less susceptible to phosphorus pollution than shallower lakes. He compiled loading rates, mean depths and trophic states for a set of hundreds of temperate lakes around the world and then visually drew the lines separating the lakes into categories (oligo-, meso- and eutrophic). These plots could then be used by planners to predict how new developments (which cause varying amounts of "new" P-loading) would impact a lake, or by lake managers to determine how to best reduce P-loading to improve an already degraded lake.

Figure 3.
Vollenweider Loading Plot of Annual Phosphorus Loading versus mean depth. Dashed lines show boundaries of Eutrophic, Mesotrophic and Oligotrophic lakes. Values for US lakes taken from Horne, A.J. and C.R. Goldman.1994. Limnology & Welch, E.B. 1992. Ecological Effects of Wastewater

Lake Tahoe and Lake Superior are also shown as examples of extremely deep lakes with low loading rates.

graphic

 

Figure 3 shows a plot of annual phosphorus loading versus mean depth for selected U.S. lakes (data from Table 12). The dashed lines were based on a large data set of the world's lakes and separate them into Eutrophic (productive) and Oligotrophic (unproductive) lakes on the basis of their depth and phosphorus imputs from the surrounding watershed. These loading rates are simply the total weight of phosphorus entering the lake each year - mostly from streams, of course.

Table 13. Mean depth (meters) and annual phosphorus loading (gP/m2/yr) for selected U.S. lakes
LAKE
MEAN DEPTH
(meters)
ANNUAL P-LOADING
(gP/m2/yr)
Western L. Erie
~8
7.0
L. Mendota, WI
13
0.80
L. Ontario
92
0.20
L. Superior
150
0.037
L. TahoeCA-NV
303
0.024
L. Independence
5.5 
0.39 
Ice Lake
7.0 
0.070 

 

Thought Questions:
Does the plot suggest that Tahoe and Superior are extremely sensitive to relatively small increases in phosphorus loading ? Do you think they would be very sensitive?

Can you think of other factors that might be equally important in regulating the amount of algal growth in lakes?

Take a shot at drawing curves relating P-concentration in a lake to Secchi depth (clarity) and chlorophyll (algal biomass).


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date last updated: Friday October 08 2004