The following selected water quality parameters were chosen to illustrate the trophic state of
the Roger Williams Park ponds system. Data for Mashapaug and Spectacle Ponds were also
included for comparison, as these ponds drain into the Park Ponds system. Trophic state
refers to the biomass, or the amount of algae, plants and animals in a pond. The greater the
amount of organisms in a pond, the higher the trophic state. The lowest trophic state is called
oligo and the highest is eutro. Oligotrophic ponds tend to have high transparency, low levels
of nutrients and low algal concentrations. Eutrophic ponds tend to have low transparency,
high levels of nutrients and high algal concentrations.
It is common to determine trophic state by looking at chlorophyll a, total phosphorus and
Secchi disk transparency measurements. Chlorophyll a is a proxy for the amount of algae in
a pond. Total phosphorus is a measure of dissolved and particulate phosphorus. Phosphorus
is generally the limiting nutrient in a freshwater system, meaning that it controls the growth
of algae and plants. In the ocean or a saline system, nitrogen controls this growth. Secchi
disk transparency is a visual measure of water clarity or transparency. The greater the Secchi
disk reading the more transparent the pond is. Transparency can be affected by sediments in
the water, algae growth or the color of the water itself. Some ponds are naturally brownish in
color due to tannins, a natural compound that is released during the decay of plants and
leaves.
A comparison of the average total phosphorus, chlorophyll a and Secchi disk measurements
in the Ponds with the trophic state indicators categorizes all the Ponds, including Mashapaug
and Spectacle ponds, as eutrophic.
Table 1 - Pond Water Quality Data Summary
Parameter
Chlorophyll a
Total
Phosphorus
SURFACE
Total
Phosphorus
DEEP
Secchi Depth
Average
Range
Date range
of data
Units
ug/L
ug/L
Average
Range
Date range
of data
ug/L
ug/L
Average
Range
Date range
of data
ug/L
ug/L
Average
Range
Date range
of data
m
m
All data from URI Watershed Watch
Mashapaug
Pond
28.0
3.8-65.3
1999-2001, 20032005; 2009-2010
Spectacle
Pond
41.3
4.6 - 241.2
37
12 - 74
1999-2004;20092010
52
26 - 97
1999-2010
1999-2010
58
28 - 185
1999-2001;20092010
2007-2010
1.03
0.45 - 2.48
1999-2004;20092010
1.00
0.40 - 4.56
1999-2007;
2009-2010
Pleasure Lake
29.3
0.6 - 87.5
1993-1994; 20012002; 2005
94
31 - 194
1993-1994; 20012002; 2005
Roosevelt
Lake
20.6
4.7 - 57.0
Cunliff Lake
54.1
11.8 - 100.7
Elm Lake
55.9
17.0 - 79.8
1993-1994
2003
2005
67
44 - 85
120
113 - 127
97
56 - 171
1993-1994
2003
2005
1.56
1.03 - 2.00
0.70
0.43 - 1.23
0.60
0.40 - 0.80
1993-1994
2003
2005
82
18 - 367
1.16
0.20 - 3.10
1993-1994; 20012002; 2005
Table 2 - Ranges of nutrients and corresponding lake trophic state (Carlson's Trophic State
Index (TSI))
Trophic State
Oligotrophic
Mesotrophic
Eutrophic
Description
Low productivity;
low algal growth;
clearer waters
Moderate
productivity
High productivity;
high algal growth;
poor water clairity;
often low dissolved
oxygen
Seechi Depth
m
Chlorophyll a
ug/L
Total Phosphorus
ug/L
>4
<2.6
<12
2-4
2.6 - 7.2
12 - 24
<2
>7.2
>24