Acute Toxicity Tests of Cassava and Rubber Effluents on the Ostracoda Strandesia prava Klie, 1935 (Crustacea, Ostracoda)
Acute toxicity of cassava mill and rubber processing
plant effluent to the ostracoda Strandesia prava were evaluated.
Cassava LD50 values for 24, 48 and 96 h was 0.4786, 0.311 and
0.2818% effluent concentrations, respectively, while LT50 were
169.82, 346.74, 446.68, 562.34 and 2754.23 min for 25, 12.5, 6.25, 3.125
and 1.5625% effluent concentrations, respectively. Rubber waste water
LD50 values for 24, 48 and 96 h was12.59, 11.89 and 11.22%
effluent concentrations, respectively. All were significant at p<0.05.
Rubber LT50 was 239.88, 794.33, 1584.89 and 3548.13 min for
100, 50, 25 and 12.5% effluent concentrations, respectively. Cassava mill
effluent were more toxic than rubber processing mill effluent and caused
ostracod mortality faster within short exposure time. Pretreatment of
cassava effluent in holding tanks before discharge into the aquatic ecosystem
would possibly reduce its toxic impact.
Industrial effluent discharges are worldwide sources of potential pollution
(Ajao, 1985). Acute toxicity tests give firsthand information
on the effects of such discharges on organisms and the ecosystem as a whole
and are valuable in creating awareness as to the potential harmful effects of
such industrial discharges to the environment. The continuous increase in supply
and demand for cassava (Manihot esculenta) in developing countries has
accentuated the negative impact of cassava processing on the environment and
biodiversity (Arimoro et al., 2008). Studies
on the effects of human activities and industrial effluents on Nigerian biota
have focused on various macrobenthic invertebrates and fish (Ajao,
1985; Arimoro et al., 2008; Edokpayi,
1993) but none has addressed the effects on freshwater ostracod crustaceans.
Published records of Nigerian freshwater ostracoda have also focused on their
ecology and taxonomy (Onyedineke, 2000; Victor,
1981). Garri, a Nigerian staple food and cassava starch used by the textile
and paper mill among other usage are end products from the cassava mill plants.
Effluent resulting from the cassava mill plants are often directly or indirectly
discharged into aquatic system without any prior treatment. Cassava wastewater
contain unextracted starch, cellulose carbohydrates, nitrogenous compounds and
cyanoglycosides. Cyanogens and glycosides are easily hydrolysed into hydrogen
cyanide which is toxic to aquatic animals and pose serious treat to the environment
(Abiona et al., 2005). Effluent from rubber processing
plants also find their way into the aquatic systems. Wastewater from rubber
processing plants contain organic and inorganic matter that originate from the
natural latex rubber and from chemicals used in processing, such as ammonia,
formic acid, sodium metabisulphite and sodium sulphite (Kantachote
et al., 2005). There are no available records of the toxicity of
rubber processing wastewater on crustaceans. Ostracods are important components
of the aquatic food chain. Knowledge of the toxic effects of these effluents
and response of crustaceans to them would be useful indicators and parameters
for protecting our freshwater bodies from pollution.
This study examines the response and toxic effects of cassava and rubber plant effluents on the freshwater ostracoda Strandesia prava.
MATERIALS AND METHODS
The experimental crustacean ostracod Strandesia prava were collected
into 18 L plastic buckets from a stagnant freshwater pond located at Ujoelen,
Ekpoma, Edo State, Nigeria., Nigeria (6 o19'N and 6o20'E)
with bolting silk nets (mesh 65 μm). Agile ostracoda were placed in pondwater
in Petri dishes to acclimatize for two days prior to use in bioassay. Groups
of ten agile ostracoda were thereafter exposed in 20 mL of test solution with
varying concentrations of effluent wastewater. Adequate levels of dissolved
oxygen were ensured in the test solutions according to APHA
Rubber and Cassava Effluent
Effluent were collected in 2 L plastic containers from a cassava (Garri)
processing plant at Ujoelen, Ekpoma and rubber processing plant at Tepoga, Benin
City, Nigeria. The physico-chemical parameters of the pondwater, cassava and
rubber effluents were determined prior to bioassay.
Test solutions containing various percentages of the effluent by volume based on a suitable logarithmic series were prepared using aerated dilution pondwater.
The study was conducted September, 2006. Glass dishes each containing 20
mL of test solutions were used. Rubber effluent concentrations by volume used
were 100, 50, 25, 12.5, 6.25, 3.125 and 0.00% (control). All ostracoda died
within 1 h in a preliminary bioassay with cassava concentrations >50% and
so lower concentrations were used for the experiment. The cassava concentrations
by volume used were 12.5, 6.25, 3.125, 1.5625, 0.78125 and 0.39062%. Behavioral
changes were recorded and mortality recorded every 2 h for 32 h and thereafter
every 6 h till 96 h.
Lethal dose (LD50) for ostracoda and lethal time (LT50)
for effluent were calculated using Probit Analysis (Finney,
1971). Best fit lines of expected probits were calculated by regression.
A one way Analysis of Variance (ANOVA) was used to determine the significance
of the regression coefficient of probit on dosage. All levels of statistical
significance were determined at p<0.05 (Zar, 1998).
Physico-chemical characteristics of dilution pondwater and typical effluent discharges from the cassava and rubber plant on any working day are shown in Table 1.
|| Physico-chemical characteristics of pondwater and wastewater
effluents from the cassava and rubber plant
|nd: Not determine
The cassava and rubber waster water were more acidic than the pondwater. Turbidity,
conductivity, total dissolved solids, nitrate, sulphate, phosphate, potassium
and total iron content were higher in the cassava than in the rubber wastewater.
Colour and Behavioral Changes
The ostracoda exhibited degrees of restlessness, random motion, overturning
(loss of balancing), sluggishness and eventual death with the degrees of this
behavior increasing as the effluent concentration increased. There were no color
change, the greenish color of the animals were retained even after death in
the rubber and cassava effluents.
All test animals died after 1 h in the 100 and 50% concentrations of the
cassava effluent. There were no deaths in the control (0% effluent concentration).
Figure 1-3 show the results of probit analysis
for 24, 48 and 96 h exposure time, respectively. LD50 values for
24, 48 and 96 h was 0.4786, 0.311 and 0.2818% effluent concentrations, respectively.
Cassava effluent was highly toxic to the ostracods even at very low concentrations.
All were significant at p<0.05. LT50 were 169.82, 346.74, 446.68,
562.34 and 2754.23 min for 25, 12.5, 6.25, 3.125 and 1.5625% effluent concentrations,
respectively. Survival time increased only at very low concentration (1.5625%).
Seventy ostracods were used for the bioassay. There were no deaths in the
control (0% effluent concentration) however; there was 100% mortality after
24 h in the 100% effluent concentration.
Figure 4-6 show the results of probit analysis
for 24, 48 and 96 h exposure time, respectively. LD50 values for
24, 48 and 96 h was 12.59, 11.89 and 11.22% effluent concentrations, respectively.
All were significant at p<0.05. The LT50 were 239.88, 794.33,
1584.89 and 3548.13 min for 100, 50, 25 and 12.5% effluent concentrations, respectively.
|| Twenty four hour probit analysis for cassava wastewater
|| Forty eight hour probit analysis for cassava wastewater
|| Ninty six hour probit analysis for cassava wastewater
|| Twenty four hour probit analysis for rubber wastewater
|| Forty eight hour probit analysis for rubber wastewater
|| Ninty six hour probit analysis for rubber wastewater
Ostracods could survive even at 100% effluent concentration, however, survival
time increased as effluent concentration decreased.
Strandesia prava survived in the control experiment throughout the 96 h of the bioassay indicating that mortality during the bioassay was not caused by time nor container induced stress rather it was toxicant induced. Death of test organisms during bioassay could be as a result of time or container induced stress other than those arising from the toxicant.
Rubber effluent showed higher LD50 values than that of cassava,
24 LD50 of rubber was 12.59% while that of cassava was 0.4786% at
the same time interval. In addition, 1.5625% concentration by volume of cassava
effluent caused mortality at a shorter time (LT50 = 2754.23 min)
than a 12.5% concentration by volume of rubber effluent (LT50 = 3548.13
min). These indicate that cassava effluent was more toxic than rubber effluent
and produced a high level of mortality rate at low concentrations within a short
exposure time. Low values of pH of effluent test water compare well with other
waterbodies receiving untreated wastes. Water bodies receiving untreated cassava
water have been reported to be highly acidic sometimes with pH as low as 2.6
(Zvauya and Muzondo, 1994; Arimoro
et al., 2008). The toxic effects of cassava effluent at low concentrations
could be due to dissolved hydrocyanic acids indicatng that under field conditions
where dilution is possible, ostracod mortality rate would still be high when
exposed to low concentrations of cassava effluent even for short periods of
time. The acute toxicity of cassava effluent on ostracod crustaceans agrees
with the findings of Arimoro et al. (2008), who
reported that cassava effluent had decimating impact on macrobenthc invertebrates.
High values of physico-chemical parameters of cassava effluent as well as the
low pH might have caused physiological stress on ostracoda thereby causing high
mortality. Arimoro et al. (2008) also found out
that untreated cassava water caused the absence of crustaceans and mollusks.
Rubber effluent mortality on ostracods could also have resulted from physiological
stress imposed by low pH. The lower toxicity of rubber on the test animals could
be due to the absence of hydrocyanic acids. The results obtained here show that
ostracods could survive in rubber polluted waters for a period of time. Ostracods
play an important role in the aquatic food chain. Exposure to untreated cassava
waters could lead to decimation and extinction. Ostracods could be used as indicator
organisms for cassava polluted environments just as harpacticoid crustaceans
are used as indicators of endocrine disrupting chemicals (Wollenberger,
2005). The absence of pretreatment measures before these effluents are released
into the environment are a cause for concern. Cassava mills are often small
scale industries that discharge their effluents directly into the environment,
pretreatment in holding tanks before discharge into the environment would be
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