Saddleback Journal of Biology - Saddleback College

Fall 2009 Biology 3B Paper
environments to determine if urban runoff can
significantly increase the degree of eutrophication. We
hypothesized that rural lakes would have a lower
concentration of soluble phosphorous than lakes in
urban environments.
Methods and Materials
Ten 3.5 L water samples were taken from
lakes in California. Five of the lakes were chosen from
the Inyo National Forest, near Bishop, CA. These lakes
included Lake Sabrina, Echo Lake, North Lake, TJ
Lake, and Intake #1. The other five samples were
taken from lakes in Orange County, Riverside County,
and San Diego County. These included Laguna Niguel
Lake, Irvine Lake, Lake Elsinore, Mission Viejo Lake,
and Dixon Lake.
Because the relative concentration of
phosphorous measured was so minute and the risk of
contamination was so high, care was taken to wash all
glassware with phosphorous free soap. All chemicals
and facilities were provided by the Chemistry
Department at Saddleback College. The samples were
decanted to remove any solid particles, and then
standardized to 3.00 L. To make the testing and
precipitating of phosphate more manageable, all of our
samples were boiled down to approximately 40 mL, to
be able to fit into a test tube. A 0.0010 M solution of
aluminum chloride was prepared and 5 mL were added
to each of the samples to form a precipitate. To remove
the possible interference of aluminum hydroxide, one
drop of 0.100 M nitric acid solution was added to each
of the samples. The samples were then centrifuged for
15 minutes and the precipitates were decanted and
washed. The washing and decanting process was
repeated an additional three times to eliminate any
extraneous dissolved ions. The resulting solutions were
then placed into beakers of known weight and the
excess water was boiled off. The beakers were
weighed again using an analytical balance, accurate to
10 -4 grams, and the mass of the aluminum phosphate
precipitate was determined. Accounting for the
aluminum chloride added and considering that our
original samples were 3.00 L, we converted the mass of
the precipitate into parts per billion of phosphorous.
Results
A one-tailed unpaired t-test was performed
comparing the mean soluble phosphorous
concentration for both urban and rural lake samples.
The test yielded a p-value of 1.42x10 -4 , which shows
that the urban lakes sampled had significantly higher
levels of soluble phosphorous than the rural lakes
sampled (Figure 1).
Soluble Phosphorous (ppb)
10
90
8
7
6
5
4
3
2
1
0
Urban Lake Location Rural
Lake Location
Figure 1. Mean soluble phosphorous in water samples
from urban and rural lakes. The mean phosphorous
concentration in urban lakes was 82.1589 ± 5.27461
ppb (± SEM, n=5). The mean phosphorous
concentration in rural lakes was 34.6547± 4.62362 ppb
(± SEM, n=5). A one-tailed unpaired t-test revealed a
significant difference between the phosphorous
concentration in rural and urban lakes (P=1.42x10 -4 ).
Figure 2. Carlson’s Trophic State Index which
describes the degree of eutrophication in a lake based
on the amount of total phosphorous in parts per billion.
Discussion
For further analysis of the eutrophication
problem, the trophic state of the lakes was compared
using The Carlson Trophic State Index (Figure 2). The
mean phosphorous concentration of urban lakes, 82.16
ppb, fell into the hypereutrophic zone. This rating
describes a lake with excessive algal blooms, which
has reduced oxygen content at lower depths and dead
zones beneath the surface (Boesch et al., 2001). The
rural lakes’ mean phosphorous concentration was 34.65
ppb, falling into the mesotrophic to slightly eutrophic
zone. Lakes in this classification are productive and
support a regularly functioning ecosystem with a
healthy balance between the primary producers and
consumers.
Our experiment also points to the possible
efficacy of using aluminum to remove phosphate from
a body of water. Our precipitate was obtained using
very minute amounts of reagents and a slightly acidic
solution, which mimics the natural condition of most
lakes (Smith et al. 2001) Because aluminum phosphate
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Saddleback Journal of Biology
Spring 2010