WSP08741

• File Number: 8170
• Description: Arkansas Basin Water Quality Issues
• State: CO
• Basin: Arkansas
• Water Division: 2
• Date: 1/1/1993
• Author: USGS
• Title: Reconnaissance of Water Quality of Lake Henry and Lake Meredith Reservior - Crowley County - Southeastern Colorado - April-October 1987
• Water Supply Pro - Doc Type: Report/Study

005G

Trophic Status

In calculating Carlson's trophic-state index (TSI)
for Lake Henry. Secchi-disk depth (SD), average sur-
face concentrations of total phosphorus (TP), and aver-
age surface concentralions of chlorophyll a (ChI) were
used from samples collected at site HEW2 on June 29
and August 19. The calculated TSI values for each
lrophic variable are:
TSI (SD) = 87
TSI (TP) = 62
TSI (Chl)= 50

According to Carlson (1979), a TSI value of 41 is con-
sidered the upper limit for oligotrophy, and a TSI
value of 5 I is considered the lower limit for eutrophy.
Based on the TSI values calculated using all three
trophic variables, Lake Henry would be classified as
eutrophic using Secchi-disk depth and total phospho-
rus and would be borderline eutrophic using chloro-
phyll a.

In applying the lrophic variables to the fixed-
boundary system, data were used from samples

collected at site HEW2 on May I, June 29, August ]9,
and October 6. The trophic classification for Lake
Henry using this system uses mean annual values for
the trophic variables and the minimum Secchi-disk
measurement and the peak chlorophyll a concenlration.
The boundary values for trophic categories and the val-
ues for the trophic variables for Lake Henry are listed
in table 7. Because there were only four sampling dates
during the sampling period. the trophic classification
based on the mean annual values may be biased. How-
ever, the trophic classification for the minimum
Secchi-depth measurement and peak chlorophyll a
concentration are the same as the trophic classification
using the mean values. The resulting trophic classifi-
cation using the fixed-boundary system agrees closely
with the calculations for the trophic-state index. The
trophic variables, where applied to the fixed-boundary
system, indicate Lake Henry is hypereutrophic with
respect to Secchi-disk depth and eutrophic with respect
to total-phosphorus concentration and chlorophyll a
concentration.

Tabte 7. Comparison of fixed-boundary-system values to trophic-variable values for Lake Henry and lake Meredith (modified
from Organisation for Economic Co-operation and Development,1982)

[ii:;, greater than or equal to; ::;:. less than or equal 10]

Trophic variables

Trophic category

Ultra~oligotrophic
Oligotrophic
Mesolrophic
Eutrophic
Hypereutrophic
Lake Henry (sile HEW2)'
Lake MerediLh (sites M28,
M I B. M4B)'
IData were used from samples collected on May I, June 29. August 19, and October 6.
20aL3. were used from samples collected at siles M2B. MIB. and M4B on May I. June 30. and Augusl20-21, and at siLe M28 on October 6.

Minimum
Secchl-dlsk
deplh
(feel)

<:: t9.7
<::9.8
9.8 -4.9
4.9 - 2.3

:02.3
.5
.5

Mean
Secchl-dlsk
deplh
(feel)

<::39.4
<::19.7
19.7 - 9.8
9.8 - 4.9
:>4.9
.8
.8

Mean total.
phosphorus
concentration
(mllllgrems
per Iller)
:>0.004
:0.010
.010 - .035
.035 -.10
<::.10
.066
.106

Peak
chlorophyll 8
concentration
(micrograms
per Iller)

:02.5
:08.0
8.0 - 25
25 -75
<::75
26.t
37.8

Mean
chlorophyll 8
concentration
(micrograms
per liter)

;S1.0
:02.5
2.5 - 8.0
8.0 -25
<::25
10.0
11.7

WATER-QUALITY CHARACTERISTICS OF LAKE HENRY AND COMPARISON TO WATER.QUALlTY STANDARDS 19