Comparison of Microtox® and Amphipod Bioassays for
Assessment of Sediment Toxicity
Jennifer Abbott and Jackie Zakarian
COSEE Florida REPT program
Introduction
The Indian River Lagoon (IRL) is the largest estuary in the United States
and encompasses 156 miles of Florida’s east coast. The lagoon is
uniquely located in a biogeographic transition zone which assists in
supporting the greatest diversity of species in North America (Sime,
2005). Taylor Creek is a component of the IRL watershed, and includes
discharge from a series of canals that originate near Lake Okeechobee
and connect with the IRL in Fort Pierce, Florida. The Fort Pierce Farms
Water Control District Canal Number 1 and the C-25 Canal are
examples of these canals which have shown to be significant
contributors of agricultural and urban runoff and associated
contaminants (USACOE and SFWMD, 2002). Contaminants may
become immersed within the sediment of the IRL and present
considerable risk to the estuary’s sustained ecological health.
The purpose of this study was to determine the presence of toxins in
the sediment where Taylor Creek and the IRL intersect (Fig.10). Two
methods commonly used to determine sediment toxicity were
conducted and compared in this study. The Microtox® Basic SolidPhase Bioassay utilizes the luminescent properties of the bacteria
Vibrio fischeri, to determine if toxins are present in the sediment. The
EPA 10-Day Amphipod Survival Bioassay was also completed to
ascertain which assay, if either, was more sensitive to possible
toxicants in sediment from Taylor Creek. Survival rates of the
amphipod, Leptocheirus plumulosus, were determined following
exposure to sediment from each of the collection sites.
Materials and Methods
Microtox® Bioassay
Figure 5. Vibrio fischeri
Results
Conclusions
Microtox® results of Taylor Creek sediment indicate Toxicity Units (TU)
ranging from 82.34 to 94.48 on a TU scale of 0-100. Reference
sediment resulted in an unexpectedly high TU of 96.76. Amphipod
survival rates exhibit site averages from 64% to 93% amphipods. Total
organic carbon averages for the sample sites range from 1.03% to
2.09% with the reference sample average at 3.11%. Positive control
amphipod toxicant test was not accepted due to variability of survival
rates among NH4Cl concentrations.
Figure 6. Decreased luminescence
as a result of increased toxicity
Figure 10. Toxicity levels for sample sites as indicated by Microtox
bioassay results.
Literature Cited
Microtox® Bioassay Results
Materials and Methods
Using a grab sampler, five sediment samples were collected from each
of the sites shown in Figure 10. The samples from each collection site
were then combined into one homogenized sediment sample per site
for total organic carbon and particle size analyses, and subsequently
used for both the Microtox® and amphipod bioassays. Two 100g
aliquots of each sediment sample were used to determine particle size
and dry weight. Two 20g aliquots of each sediment sample were used to
determine total organic carbon. A 96-hour positive control amphipod
bioassay was conducted using reference toxicant ammonium chloride
(NH4Cl) at increasing concentrations from 0 to 250 mg/L per EPA
standard protocol (EPA, 1994).
Materials and Methods
Amphipod Survival Bioassay
6
5
Sample Site
(6= Reference
Sediment)
4
3
2
1
0
0
50
100
Microtox® (TU)
Figure 11. Microtox® bioassay toxicity levels for Taylor
Creek sample sites and reference sediment of amphipod
collection site.
Figure 7. Leptocheirus plumulosus
No correlation was observed between Microtox® toxicity units and
survival rates of the 10-Day Amphipod Survival Bioassay. The
reference toxicant positive control test of the amphipod bioassay
exhibited variable survival rates, including increased survival rates
among higher concentrations of reference toxicant. High toxicity
levels present in the reference sediment (Figure 11), may have
affected sensitivity to toxicants among amphipods used in the
bioassays.
Based on these findings, Microtox® appears to be a more sensitive
indicator of sediment toxicity than the Amphipod 10-Day Survival
Bioassay. ORCA has previously standardized and verified the accuracy
of the Microtox® assay to measure sediment toxicity. However, it is
unclear why the amphipods were not more sensitive to the toxic
sediment from Taylor Creek. Further testing is needed to determine
whether this is a real difference in sensitivity of the bioassays tested.
Figure 4. SDIX Microtox®
The Microtox® Basic Solid-Phase Assay was based on the reduction of
luminescence by the bacteria Vibrio fischeri when exposed to toxins.
The exposure of toxicant disrupted respiration in the bacteria which, in
turn, inhibited luminescence. The reduction of the light output was
measured by the Model SDI M500 temperature controlled photometer
in accordance with Azur Environmental methods (Azur 1998, and ORCA
SOP, 2009).
Microtox® Solid-Phase Assay results indicated high toxicity among
sediment samples from the Taylor Creek sites.
AZUR Environmental. (1998). Microtox Acute Toxicity Test. Carlsbad,
CA.
Florida Department of Environmental Protection; Southeast District.
(2003). Evidence of Impairment: Fort Pierce Farms C-1 Canal (WBID
3163). Ambient Water Quality Section , Port Saint Lucie, FL.
Sime, P. (2005). St. Lucie Estuary and Indian River Lagoon Conceptual
Ecological Model. Wetlands , 25 (4), 898-907.
United States Environmental Protection Agency. (1994). Methods for
Assessing the Toxicity of Sediment-associated Contaminants with
Estuarine and Marine Amphipods. Washington, D.C.: Office of
Research and Development.
Amphipod Bioassay Results
6
5
Figure 8. Amphipod bioassay
Figure 1. Sediment
collection
Figure 3.
Measurement of
sediment aliquots
Figure 2. Particle
analyses
Figure 9. Sediment sieved
to quantify amphipods
The Amphipod 10-Day Survival Bioassay measured the survival rate of the
burrowing estuarine amphipod, Leptocheirus plumulosus, following a 10day exposure to sediment under static conditions. Amphipods were
placed in glass chambers containing 175 ml (~2cm) of sediment and 800
ml of filtered seawater. The percentage of surviving amphipods was
determined following the completion of the 10-day test. The whole
sediment tests were conducted in accordance with USEPA amphipod
testing methods (USEPA 1994).
Sample Site
(6= Reference
Sediment)
4
3
2
Acknowledgments
1
0
0%
50%
100%
Amphipod Survival Rate
Figure 12. Amphipod bioassay survival rates for Taylor
Creek sample sites and reference sediment.
The financial support of COSEE Florida through National Science
Foundation grant OCE#1039107 to Indian River State College is
gratefully acknowledged. Special thanks to COSEE Florida, ORCA, and
Indian River State College for making this research experience
possible.