U.S. DEPARTMENT OF COMMERCE
National Technical Information Service
PB-289 939
The Acute Toxicity of Zinc to
Rainbow and Brook Trout
Comparisons in Hard and Soft Water
(U.S.) Environmental Research Lab, Duluth, MN
Oct 78
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United States
Environmental Protection
Agency
Environmental Research
Laboratory
Duluth MN 55804
Research and Development
Brook Trout
Comparisons in
Hard and Soft
Water
EPA-600/3-78-094
October 1978
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This document is available to the public through the National Technical Informa-
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/3-78-094
2.
4. TITLE AND SUBTITLE
THE ACUTE TOXICITY OF ZINC TO RAINBOW AND BROOK TROUT
Comparisons in Hard and Soft Water
3.
5. R
October .1978 issuing date
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Gary W. Holcombe and Robert W. Andrew
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
SAME AS BELOW
10. PROGRAM ELEMENT NO.
1BA608
11. CONTRACT/GRANT NO.
In-house
12. SPONSORING AGENCY NAME AND ADDRESS
Environmental Research Laboratory - Duluth, MN
Office of Research and Development
U.S. Environmental Protection Agency
6201 Congdon, Blvd., Duluth, MN 55804
13. TYPE OF REPORT AND PERIOD COVERED
14. SPONSORING AGENCY CODE
EPA/600/03
15. SUPPLEMENTARY NOTES
16. ABSTRACT
The means and ranges of the 96-hour LCSO's derived from three tests were 0.55 (0.37 to
0.76) and 2.5 (1.9 to 3.0) mg Zn/liter for rainbow and 2.0 (1.6 to 2.4) and 6.0 (5.0 to
7.0) mg Zn/liter for brook trout in soft and hard water, respectively. Based on overall
means, brook trout were approximately 2.7 times more resistant than rainbow trout. Zinc
toxicity to both species increased with increasing pH, and decreased with increasing
hardness and alkalinity.
17.
a.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
Freshwater biology
Trout
Zinc sulfates
Bioassay
Toxicology
Fishes
18. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
I).IDENTIFIERS/OPEN ENDED TERMS
Z.IIK- poisoning
Brook trout
Rainbow trout
96-hour LC50
Acute toxicity
19. SECURITY CLASS (This Report)
UNCLASSIFIED
20. SECURITY CLASS (Thilpage)
UNCLASSIFIED
c. COSATl l-'icld/Croup
06/C.T
21. f
22. PRICE
EPA Form 2220-1 (R«v. 4-77) PREVIOUS EDI TION la OBSOLETE
«us cwttMnimmworna iin—ts?.060/IMl
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EPA-600/3-78-094
October 1978
THE ACUTE TOXICITY OF ZINC TO RAINBOW AND BROOK TROUT
Comparisons in Hard and Soft Water
by
Gary W. Holcombe and Robert W. Andrew
Environmental Research Laboratory
Duluth, Minnesota 55804
ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
DULUTH, MINNESOTA 55804
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DISCLAIMER
This report has been reviewed by the U.S. Environmental Protection
Agency, Environmental Research Laboratory-Duluth, and approved for
publication. Mention of trade names or commercial products does not
constitute endorsement or recommendation for use.
ii
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FOREWORD
Zinc is a ubiquitous pollutant and occurs in water from natural sources
as well. Regulatory agencies face increasing difficulty trying to establish
acceptable limits for such pollutants and at the same time not setting limits
far below ambient concentrations which seemingly cause little harm to the
aquatic environment.
Several important factors are known that make zinc biologically less
active and therefore less toxic. Complexes of zinc in mineral form or with
organic materials may reduce its availability. Inherent water quality
characteristics such as pH, hardness, and alkalinity also change the
biological activity of zinc.
The work described in this report further elucidates the subtleties
of these latter effects. Such information should make the task of
specifying acceptable limits easier for a variety of natural waters.
Donald I. Mount, Ph.D.
Director
Environmental Research Laboratory-
Duluth
iii
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ABSTRACT
The means and ranges of the 96-hour LCSO's derived from three tests were
0.55 (0.37 to 0.76) and 2.5 (1.9 to 3.0) mg Zn/liter for rainbow and 2.0 (1.6
to 2.4) and 6.0 (5.0 to 7.0) mg Zn/liter for brook trout in soft and hard
water, respectively. Based on overall means, brook trout were approximately
2.7 times more resistant than rainbow trout. Zinc toxicity to both species
increased with increasing pH, and decreased with increasing hardness and
alkalinity.
iv
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CONTENTS
Foreword iii
Abstract iv
Tables vi
Acknowledgments vii
1. Introduction 1
2. Conclusions 2
3. Recommendations 3
A. Materials and Methods A
Exposure system A
Water characteristics . A
Toxicant solutions A
Biological methods 6
Statistics 6
5. Results 9
6. Discussion 11
References 1A
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TABLES
Number
Exposure Dates, Water Quality Characteristics, and Test
Conditions of the Three Acute Zinc Exposures
Measured Zinc Concentrations in the Soft and Hard Water
Exposures During Each of the Three Tests, and Percent
Mortality of Rainbow and Brook Trout Occurring at Each
Concentration in 96-Hours
Zinc Toxicity (LC50) and Slope of the Probit Mortality
Curve in Soft and Hard Water in the Three Acute Tests 8
Correlation Coefficients of Experimental Variables, LC50,
and Slope of Mortality Curves for Rainbow (Upper Right)
and Brook Trout (Lower Left), Respectively (N=6) 10
vi
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ACKNOWLEDGMENTS
We thank E. N. Leonard and J. Poldoski for water quality analyses and
zinc measurements; and other members of the Environmental Research Laboratory-
Dulutli staff for advice and critical review of this manuscript.
vii
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SECTION 1
INTRODUCTION
Zinc is widely used in industry and is a common water pollutant; thus,
knowledge of its toxicity to aquatic organisms is important. Numerous studies
have documented the acute toxicity of zinc to various species cf fish tested
under conditions of stable water quality. Several of these studies include
work on salmonid species (1-5). Another study by Nehring and Goett.1 (6)
compared the acute toxicity of zinc to four different salmonid species. Acute
tests comparing the toxicity of zinc to rainbow trout under conditions where
water quality parameters were varied have also been conducted (7-9). A study
has been conducted comparing the effects of varying pH and hardness on the
toxicity of zinc to fathead minnows (Pimephales promelas) (10). Cairns et al.
(11) studied the effects of variable pH, temperature and zinc solubility in a
test on the bluegill sunfish (Lepomis macrochirus). Both Skidraore (12) and
Pagenkopf (13) have reviewed the current state of knowledge of the toxicity
of zinc to fish and the relationship to water quality.
This study included three separate exposures of rainbow trout (Salmo
gairdneri) and brook trout .(Salveli.""5 fontinalis) to zinc (as the sulfate) in
hard and soft water to determine acute lethality, to compare toxicity differ-
ences between the two species, and to determine the effects of water quality
changes on the toxicity of zinc to each species. The concentration of zinc
which was lethal to 50 percent'of the fish exposed at that concentration in
96 hours (96 hour LC50) was calculated for each species of fish in both hard
and soft water for each of the three tests. Tests were conducted using the
same nominal zinc levels and with the same stock of fish at different ages
to yield information on the effects of size of the fish on susceptibility to
zinc. In addition, two methods of artificially hardening water were used to
compare resulting changes in zinc toxicity. Information gained from this
study was used to compare and supplement data obtained by other investigators,
and to estimate the effect of differing water quality conditions on the
determination of an application factor (14) for zinc.
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SECTION 2
CONCLUSIONS
Results of these acute tests indicated that rainbow trout were approximately
3.7 and 2.4 times more sensitive to zinc than were brook trout in soft and hard
water, respectively. Based on overall means, rainbow trout were approximately
2.7 times more sensitive to zinc than brook trout under the same experimental
conditions.
Increasing the mean water hardness from approximately 44 to 170 mg/1 (as
CaC03) using Ca(N03)2*4H20 did not change alkalinity, but did decrease the
toxicity of zinc by a factor of 2.5 and 2.1 for rainbow and brook trout,
respectively.
Increasing both hardness (from 47 to 179 mg/1 as CaC03) and alkalinity
(from 42 to 170 mg/1 as CaCQ-$) using a water hardener, decreased zinc toxicity
by a factor of 5.7 to 6.8 and 3.3 to 3.9 for rainbow and brook trout,
respectively.
Both the LC50's and the slopes of the probit mortality curves were
significantly correlated with hardness, alkalinity and pH.
Susceptibility to zinc appeared to decrease slightly with an increase in
the size of exposed fish but there was no significant correlation of LC50's
or slopes of the mortality curves with weight of the fish.
Zinc was slightly more toxic to rainbow and brook trout in water hardened
with Ca(N03)2*4H20 than in water hardened to the same degree with a water
hardener (18). It appeared that this difference was correlated with changes
in alkalinity and possibly with zinc complexation or precipitation by
carbonate.
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SECTION 3
RECOMMENDATIONS
It appears that if a source of hard water is unavailable, a water
hardener (18) should be used to artificially harden water for use in toxicity
tests rather than Ca(N03)2*4H20. The latter method does not correspondingly
increase the alkalinity with increasing hardness as usually occurs in natural
waters.
When fish that are being held in soft water are used in a hard water
toxicity test, the fish should be acclimated to the hard water conditions for
at least one week or until they are actively feeding to prevent placing a
double stress on the fish.
Better control and more accurate measurement of variables such as pH,
hardness, and alkalinity, coupled with measurements of complexation or
precipitation, are needed to adequately interpret such metal toxicity tests
in the future.
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SECTION 4
MATERIALS AND METHODS
EXPOSURE SYSTEM
The exposure systems for these tests consisted of two, two liter/cycle,
proportional diluters (15) with dilution factors of 0.65 and adjusted to cycle
almost simultaneously. Each diluter had a series of flow splitting chambers
(16) which divided each of the six concentrations equally into two duplicate
stainless steel tanks. One diluter operated with soft water and one with
artificially hardened water. Both rainbow and brook trout were exposed during
each test, each species in one set of duplicate tanks in each system so that
exposure conditions would be identical between species for each type of
water. The tanks measured 92 by 36 by 41 cm deep with a 31 cm standpipe which
yielded a Lank volume of 100 liters. Flow rates for both diluters averaged
650 ml/minute with a 90 percent replacement time of six hours (17). Tanks
were covered with fiberglass mesh screens and were surrounded by curtains to
minimize disturbance of the fish. A constant 12 hour photoperiod was
used during all tests and a combination of Sylvania Gro-Lux and Duro-Test
Vita Lite fluorescent lamps was the source of illumination.
WATER CHARACTERISTICS
Water for all tests was obtained directly from Lake Superior, passed
through an ultraviolet sterilizer to kill pathogens, and heated to 15^1°C
(Table 1) by a hot water heat exchange system. Water used in the three soft
water exposures was unaltered Lake Superior water. The hard water used in
tests //I and #2 (Table 1) was produced by passing Lake Superior water through
a water hardener (18). For test #3, Lake Superior water was hardened by
dripping calcium nitrate (Ca(N03)2'4H20) dissolved in distilled water into
the diluter headbox from a Mariotte bottle. In this way, calcium hardness was
increased without increasing the alkalinity. Hardness, alkalinity, specific
conductance, and pll measurements were made daily on the test water in one or
more tanks of each diluter system and in most cases measurements were made on
samples from nil tanks on one day during each test. Water chemistry information
from all tests Is contained in Table 1. Dissolved oxygen measurements were
made twice during each test in each diluter and ranged from 9.4 to 10.2 mg/
liter. All water characteristics were measured using methods described by the
American Public Health Association (19).
TOXICANT SOLUTIONS
Zinc sulfate (granular, hydrated ZnSOl|-7H20, reagent grade), manufactured
by Mallinckrodt Chemical Company, was dissolved in distilled water in a
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TABLE 1. EXPOSURE DATES, WATER QUALITY CHARACTERISTICS, AND TEST CONDITIONS Or THE THREE ACUTE ZIHC EXPOSURES
Ui
Test number
and Water
exposure dates type
#1
6-28-73 Soft
through
7-2-73
Hard I
\n
8-1-73 Soft
through
8-5-73
HardS
#3
12-17-73 Soft
through
12-21-73
Hard//
pH
7.63 (7.53-7.69)*
(14) t
7.41 (7.32-7.46)
(14)
7.58 (7.39-7.70)
(14)
7.17 (6.9A-7.A5)
(14)
7.38 (7.19-7.50)
(15)
7.31 (7. 14-7. A3)
(15)
Hardness
(ing/liter as CaC03)
46.8+l«4t
(12)1
177.6+4.6
(2A)
47.0+1.5
(12)
179.0+5.0)
(12)
44.4+0,5
(A)
169.7+4.0
(9)
Alkalinity
(mg/liter as CaC03)
41.8+0.6t
(12)?
170.2+0.7
(24)
42.8+0.2
(12)
170.1+1.3
(14)
42.5+0.7
(A)
43.0+0.3
(9)
Specific conductance
(micromhos)
72.4 (64.9-80.5)*
(14)?
261.8 (248.0-269.0).
(14)
74.3 (65.4-93.0)
(14)
256. A (227.0-280.0)
(14)
68.8 (65.5-73.1)
(15)
274.5 (256.0-312.0)
(15)
Temperature
(°C)
14.9 (14.7-15.0)*
(5)1
1A.9 (14.4-15.4)
(5)
15.2 (14.8-15.5)
(5)
15.7 (15.3-16.5)
(5)
15.0 (14.9-15.2)
(5)
14. 8 (14.6-14.9)
(5)
- Mean and range.
- Mean and standard deviation.
— Number of measurements.
§
— Water hardened with a water hardener (Lezike IB) .
* - Water hardened using Ca(H03)2i4H20.
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Mariotte bottle and introduced to the diluters by a toxicant metering device
(20). Nominal zinc concentrations were different for the soft and hard water
diluters, but were kept the same throughout the three tests for each water
type. Measured zinc concentrations (Table 2) varied somewhat due to slight
changes in the toxicant metering device. At the start of each test, after
fish had been placed in them, the tanks were spiked with a calculated volume
of solution from the Mariotte bottle to bring the zinc concentrations up to
approximate nominal levels. Daily water samples were taken from one set of
duplicate tanks in both diluters and analyzed for zinc daily during each test.
Zinc concentrations in the test solutions were measured by direct aspiration
of acidified samples (10~?-K HCIO^) into a Perkin-Elmer 403 Atomic Absorption
Spectrophotometer-with an air-acetylene flame (21). On several occasions zinc
concentrations were also measured by differential pulse polarography using a
PAR Model 174 Polarographic analyzer with a dropping mercury electrode to
determine an approximation of "dissolved" zinc. Calibration curves constructed
by the method of known additions of zinc to control water were used in both
methods of zinc analysis. Comparisons of results from the two methods of
analysis showed that between 90 to 100 percent of the total zinc present in
the test water was dissolved.
BIOLOGICAL METHODS
The juvenile rainbow and brook trout used in the three tests were
obtained from Cedar Bend Trout Farm in Scandia, Minnesota. The species were
kept in separate fiberglass holding tanks in unaltered Lake Superior water
maintained at 15°C prior to being transferred to the test tanks. Fifty trout
of each species were placed by random assignment in the respective duplicate
tanks of both diluter systems for tests //I and #2. In test #3, only 30 brook
trout were used per tank in one set of duplicates in both diluters to avoid
overcrowding; only 11 -rainbows were used per tank in the other sets of
duplicates because of an inadequate supply of fish. Exposure dates and the
approximate size of each species of trout at the time of exposure are given
in Tables 1 and 3, respectively. For all tests, the fish were allowed to
acclimate to test water conditions before zinc exposure was started. Acclimation
was considered adequate when all fish were actively feeding in the test tanks,
which required one week in test //I and three weeks in tests #2 and //3. Feeding
was discontinued 48 hours prior to zinc exposure and fish were not fed during
the tests. Mortality was the effect used throughout this series of exposures.
Procedures used regarding aeration, tank cleaning, and checking for and
removing dead fish followed the basic flow-through acute toxicity test method
described by the Committee on Methods for Toxicity Tests with Aquatic Organisms
(22).
STATISTICS
The 96 hour LCSO's and the slope of the percent mortality vs. concentra-
tion curves for each species in each test were determined using the probit
mtsthod (23). The computer program used (24) fits a linear curve to the probit
of the percent mortality vs. log zinc concentration by iteration of maximum
likelihood. Stepwise multiple regression (25) of LC50 and the slope of the
mortality curves vs. the experimental variables: species, weight of fish, pH,
hardness and alkalinity were also performed. For comparative purposes similar
calculations were performed using data from Mount (10).
6
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TABLE 2. MEASURED ZINC CONCENTRATIONS IN THE SOFT AND HARD WATEE EXPOSURES DURING EACH OF THE THREE TESTS, AND PERCENT MORTALITY OF RAINBOW AND BROOK TROUT OCCURRING AT EACH CONCENTRATION IN 96-HOURS
Test
Soft
Ztnc cone. 96-hr 96-hr
in ralnbov brook
(Z) (!)
2.84+0.17* lOOt 78t
1.90+0.14 98 66
1.25+0.10 96 44
0.84+0.10 80 16
0.56+0.12 74 2
<0.01 2 0
(control)
«1
Hard
Zinc cone. 96-hr 96-hr
in rainbow brook
CO (I)
7.39+0.42" lOOt 78t
4.80+0.24 100 12
3.21+0.30 90 10
2.21+0.17 16 0
1.47+0.15 6 -2
<0.01 0 2
(control)
Test ti
Soft
Zinc cone. 96-hr 96-hr
in rainbow brook
nig/liter mortality mortality
(I) (I)
2.45+0.14* 96t 52t
1.55+0.11 90 46
1.00+0.05 82 6
0.64+0.03 68 8
0.41+0.02 32 0
£0.01 0 0
(control)
Hard
Zinc cone. 96-hr 96-hr
in rainbow brook
m (i)
9.64+0.07* lOOt 96t
6.58+0.18 100 28
3.83+0.08 94 4
2.34+0.15 8 0
1.31+0.03 0 0
<0.01 0 0
(control)
Test
Soft
Zinc cone. 96-hr 96-hr
in rainbou brook
m m
2.93+0.10* lOOr 73. 3i
1.83+0.10 100 20
1.32+0.09 90.9 0
0.86+0.16 36.4 0
0.53+0.04 36.4 0
<0.01 0 0
(control)
«
Hard
Zinc cone. 96-hr 96-hr
»•> (Z)
7.86+0.28* 10oT 96. 7S
4.84+0.44 100 36.6
3.31+0.26 100 10
2.19+0.14 54.5 3.3
1.49+0.15 27.3 0
<0.01 0 0
(control)
Percent mortality of 50 fish exposed.
^ Percent mortality of 11 fish exposed.
Percent mortality of 30 fish exposed.
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TABLE 3. ZINC TOXICITY (LC50) AMD SLOPE OF THE PROBIT MORTALITY CURVE IN SOFT
AND HARD WATER IN THE THREE ACUTE TESTS '
Species Teat
tested no.
Rainbow trout 1
Rainbow trout 2
Rainbow trout 3
Brook trout 1
Brook trout 2
Brook trout 3
Rainbow trout 1
Rainbow trout 2
Rainbow trout 3r
Brook trout 1
Brook trout 2
Brook trout 3?
Mean
Fish
wt.*
(g.)
3.9
4.9
28.4
3.0
3.9
19.0
3.9
4.9
28.4
3.0
3.9
19.0
LC50
(mg/D
Soft water
0.370
(0.195-0.494)+
0.517
(0.410-0.610)
0.756
(0.538-0.944)
1.55
(1.38-1.75)
2.12
(1.41-14.4)
2.42
(2.20-2.72)
Hard water
2.51
(1.65-3.71)
2.96
(2.77-3.15)
1,91
(1.53-2.28)
6.14
6,98
(3.87-9.97)
4.98
(4.49-5,58)
Slope
(probit/log cone.)
2.98
2.87
4.66
3.68
3.22
8.15
9.42
13.84
7.37
5.21
9.08
6.90
* Mean live weight of 50 fish weighed in aggregate.
t 95 percent confidence (fiducial) limits.
T Water hardened using Ca(N03)2-4H20.
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SECTION 5
RESULTS
The percent mortality resulting at all zinc concentrations in 96 hours
for both species of trout in the three tests is shown in Table 2. A sumnary
of the 96-hr LCSO's calculated from the mortality data, the slope of the
mortality curves (probit vs. log cone.), and size of test fish is shown in
Table 3. The means and ranges of the 96-hr LCSO's derived from the three
tests were 0.55 (0.37 to 0.76) and 2.5 (1.9 to 3.0) mg Zn/liter for rainbow and
2.0 (1.6 to 2.4) and 6.0 (5.0 to 7.0) mg Zn/liter for brook trout in soft and
hard water, respectively.
Based on overall means, brook trout were approximately 2.7 times more
resistant to zinc toxicity (higher LC50) than rainbow trout, under the
same experimental conditions. Linear correlations of LC50 and the slope
of mortality curves with the experimental variables (Table 4), showed
significant correlations with the covariants; pH, hardness, and alkalinity.
No correlation was found with fish weight. Zinc toxicity in both species
increased (decreasing LC50) with increasing pH, and decreased with increasing
hardness and alkalinity. Slope of the mortality curve for both species
decreased with increasing pH, and for rainbow trout increased with increasing
hardness and alkalinity. With brook trout the relationship of slope to
hardness and alkalinity was non-significant. On the basis of these correlations,
multiple regression techniques (least squares) were used to fit equations for
each species and the two species combined, relating LC50 to the significant
variables pH, hardness, and alkalinity:
Rainbow Trout LC50 (mg Zn/1) = 12.39 - 1.65 (pH) + 0.0081
(hardness) + 0.0056 (alkalinity)
Brook Trout LC50 (mg Zn/1) = 26.50 - 3.42 (pH) + 0.018
(hardness) + 0.010 (alkalinity)
Combined LC50* (mR Zn/1) = 11.16 - 1.47 (pH) -I- 0.0074
(hardness) -I- 0.0047 (alkalinity)
* In order to combine LCSO's from both species into a single regression
equation, brook trout LCSO's were divided by 2.7 which is the ratio of
the mean LCSO's for the two species.
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TABLE 4. CORRELATION COEFFICIENTS OF EXPERIMENTAL VARIABLES, LC50, AND SLOPE
OF MORTALITY CURVES FOR RAINBOW (UPPER RIGHT) AND BROOK TROUT (LOWER
LEFT), RESPECTIVELY (N»6)
LC50 Slope
Fish
pH Hardness Alkalinity wt.
LC50 - 0.97 -0.83 0.96
Slope N.S. - -0.87 0.87
pH -0.83 -0.93 - N.S.
Total hardness 0.96 N.S. N.S.
Total alkalinity 0.85 N.S. N.S. N.S.
Fish wt. N.S, N.S. N.S. N.S.
Brook
trout
0.86 N.S.* Rainbow
trout
0.87 N.S.
N.S. N.S.
N.S. N.S.
N.S.
N.S.
* Not significant, P=0.05, to be significant at P=0.05 with N=6, r must be
greater than 0.73.
10
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SECTION 6
DISCUSSION
Previously published zinc LC50's for rainbow trout exposed in soft water
agree closely, even in static exposures, with the range of LCSO's observed
for rainbow trout (0.37 to 0.76 mg Zn/liter) in the present study. In a series
of soft water (hardness = 25 mg/liter as CaC03) exposures, Goettl et al. (9)
observed 96-hr LCSO's for rainbow trout ranging from 0.24 to 0.83 mg Zn/liter
depending on size of fisli and test conditions. Chapman (5) reported 96-hr
LCSO's for steelhead trout in soft water (hardness * 25 mg/liter as CaC03)
ranging from 0.093 to 0.82 mg Zn/liter depending on which life stage was
exposed. Nehring and Goettl (6) reported a 14-day LC50 of 0.41 mg Zn/liter
for rainbow trout in soft water (hardness = 40 mg/liter as CaC03). Herbert
and Shurben (8) found the 48-hr LC50 for rainbow trout in soft water (hardness
= 44 mg/liter as CaCO^) to be 0.91 mg Zn/liter in a static bioassay. Soft
water LC50 values obtained from zinc exposures of salmonid species other
than rainbow trout were also within the range of those obtained for rainbows
in the present study. Sprague (4) obtained a 24-hr LC50 of 0.65 mg Zn/liter
exposing Atlantic salmon (Salmo salar), Rabe and Sappington (3) derived a
24-hr LC50 of 0.42 mg Zn/liter using cutthroat trout (Salmo clarki), Chapman
(26) found a 96-hr LC50 of 0.7 mg Zn/liter with sockeye salmon (Oncorhynchus
nerka), and Nehring and Goettl (6) calculated 14-day LC50s of 0.67 and 0.64
mg Zn/liter for cutthroat and brown trout (Salmo trutta), respectively.
The hard water LCSO's for rainbow trout (1.9 to 3.0 mg Zn/liter) in the
present study were gc'iuirjlly lower than those derived by other investigators.
Herbert and Sliurben (8) nnd Brown (2) reported 48-hr LCSO's of 3.86 and 3.5
mg Zn/liter, respectively in water with a total hardness of 320 mg/liter as
CaC03. Solbe (1) found a 48-hr LC50 of 4.76 mg Zn/liter in very hard water
(hardness = 500 mg/liter as CaC03), and Ball (27) obtained a 120-hr LC50 of
4.6 mg Zn/liter in hard water. Goettl et al. (9) derived 96-hr LCSO's
between 1.19 and 7.21 mg Zn/liter in water with a total hardness of 350
mg/liter as CaC03 depending on size of fish and test conditions. Differences
in toxicity of zinc to rainbow trout between the present study and those found
by other investigators are believed due mainly to differences in total hardness
and slight variations in pH between exposures.
The range of 96-hr LCSO's for brook trout from the present study was
1.6 to 2.4 mg Zn/liter in soft water, while Nehring and Goettl (6) observed a
14-day LC50 of 0.96 mg Zn/liter for brook trout in soft water (hardness =
40 mg/liter as CaC03). Considering the longer exposure in the earlier work,
these results are in reasonable agreement.
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The relationship of zinc toxicity to carbonate alkalinity lias not been
reported previously, probably because this parameter has not been varied
independently of calcium hardness. Use of the water hardener (18) and
additions of calcium (as the nitrate) indicate that calcium hardness and
carbonate alkalinity are equally important factors governing the toxicity
of zinc to fish. Except for the work of Mount (10) and Cairns et al. (11),
pH likewise has not been widely recognized as having such a large effect
on zinc toxicity.
These tests present an ideal situation for comparison of toxicity
differences between species since both species were exposed in the same
d.lluter system under identical experimental conditions for both hard and
soft water. Rainbow were found to be more sensitive to zinc than brook trout.
in both hard and soft water. The mean ratio of LCSO's for the two species
was 2.7. A series of acute tests comparing zinc toxicity differences
between species was also conducted by Nehring and Goettl (6). They found
rainbows to be 2.5 times more sensitive to zinc in soft water than were
brook trout. The inverse relationship of decreased zinc toxicity with
increased water hardness found for both species of fish in the present
study agreed with previous reports of Cairns and Scheier (28) and Pickering
and Henderson (29) on bluegills (Lepomis macrochirus), Sinley et al. (30)
on rainbow trout, and Mount (10) using fathead minnows. Zitko and Carson
(31) also observed decreased zinc toxicity with increased water hardness on
Atlantic salmon and concluded this was caused by a competition for active
sites between magnesium and zinc.
When comparing the 96-hr LC50 values with the size of fish exposed
(Table 3) for both species in both soft and hard water (disregarding test
//3 in hard water), it appears that zinc toxicity decreased as size increased.
Although not statistically significant (p >_ 0.05), there appeared to be a
slight trend toward decreased zinc_toxicity with increased fish size. This
trend, however, more closely correlated with a decrease in pH, probably as
a result of increased C02 excretion from the larger fish. Coodman (32),
Goettl et al. (9), and Chapman (5) have also found that, in general, smaller
rainbow trout are more susceptible to zinc than larger ones.
Mount (10) also found that zinc was more toxic to fathead minnows with
increasing pH and decreasing hardness. Using Mount's data, a significant
correlation of LCSO's with hardness and pH were found. The relationship
with alkalinity was non-significant, since alkalinity was not varied
independently in his tests. The multiple regression equation calculated from
Mount's data was: Fathead minnow LC50 (mg Zn/1) = 47.87 - 6.04 (pH) + 0.072
(hardness). This indicated that the toxicity of zinc to fathead minnows
(LC50) fluctuated more widely with pH or hardness changes than it did with
either of the salmonid species tested in the present study. The LCSO's for
fathead minnows however, were much higher because a higher proportion of the
zinc was in a precipitated or other non-available form (13).
Because of the effects of pH, hardness, and alkalinity on LC50 values for
zinc, considerable caution should be exercised when comparing the sensitivities
of species tested in different waters. Slight differences in experimental
conditions, especially pH, result in changes in LC50 values that make direct
12
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comparisons of relntivo acute sensitivities impractical, unless the effect
of water quality on the LC50 has been previously determined. For example, at
a constant hardness and alkalinity the LC50 for trout varies by a factor of
approximately two for each unit change in pH. This also indicates that when
an application factor is determined for zinc, the water quality must be the
same in the acute and chronic tests.
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26. Chapman, G. A. Effects of Continuous Zinc Exposure on Sockeye Salmon
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