Waterose Lab Discussion: Pulp Mill Effluent and Mytilus edulis


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The Effects of Pulp Mill Effluent on

Byssal Thread Production by the

Marine Blue Mussel Mytilus edulis

by Waterose

Section 4. Discussion:

4.3 Water Quality Abiotic Parameters:

The first trial of the experiment was discarded because all of the test organisms expired. The organisms were subjected to extreme shock in temperature changes when they were placed into the solutions and spawned immediately. The solutions were neither changed nor aerated and the organisms may have expired due to lack of oxygen or due to the high concentrations of the reproductive ejaculations with no natural mechanism to dilute the material. Furthermore, the organisms used significant energy reserves to reproduce.

4.2 Trial 2:

Due to the observations in trial 1, significant modifications were made for the procedures in trial 2 to: mitigate significant temperature stress, provide adequate oxygen, and to provide fresh solutions that were not contaminated by the organisms themselves. The results of trial 2 were more successful; however, they were still inadequate to complete the appropriate statistical analysis which is discussed in more detail in the concluding section.

4.2.1 Byssal Regeneration:

The test organisms in the control regenerated 1.76 mg of byssal thread growth. Two individuals spawned and no individuals expired. This is an acceptable control with which to make comparisons to the results observed in the remaining solutions.

The mean average of the byssal thread regeneration in the control was plotted in Figure 1 as a separate axis to illustrate the comparisons with the regeneration in the dilution’s of the untreated effluent. The dilution’s are logarithmic; however the scale on the graph is not a ratio scale for the dilution factors.

Initially, at low concentrations of untreated effluent, the amount of regeneration was moderate, with 0.898 mg of growth, at the 0.00001 dilution factor. The amount of regeneration peaked, with 1.95 mg of growth, at the 0.0001 dilution factor, exceeding the regeneration of the control group of organisms. This may be attributed to the factor that pulp mill effluent contains high concentrations of residual organics that could be a food source to the test organisms (Teng, 1988). Then, with increasing concentrations of untreated pulp mill effluent, the amount of regeneration begins to decline: 1.24 mg, 0.828 mg, 0.717 mg, and 0.275 mg. This trend may be attributed to the effects of the untreated effluent on the ability of the organism to regenerate new byssal thread growth. The underlying assumption is that the organisms ability to regenerate new byssal thread growth is impaired by chemical stressors in the environment.

It is important to recognise that there was zero regeneration in the solution of treated effluent. Furthermore, there was no spawning behaviour, they all survived, but the organisms were very tightly closed which indicated significant environmental stressors which are discussed in the next section.

4.3 Water Quality Abiotic Parameters:

There are other chemical factors that must be taken into consideration. The key factors are illustrated in Figure 2 which plots the mean regeneration of byssal threads and the abiotic factors of salinity and pH. This graph compares the regeneration and abiotic parameters of the control, the treated effluent, and the untreated effluent dilution solutions. The parameters summarised in Table 2 are also referred to for this discussion.

It is the chemical properties of the effluent that are the chief stressors on the regeneration of the byssal threads by the groups of Mytilus edilus. The abiotic parameters of the original home site of the Blue Mussels were: water temperature of 20 degrees Celsius, pH of 7.84, and salinity of 25 ppt. The control (Solution A) had abiotic parameters of pH at 8.15 to 8.30, and salinity of 25 to 26 ppt. The temperature factor is discussed separately below.

The group in the treated effluent (Solution B) appeared to be the most stressed and had the lowest regeneration. This may be attributed to the salinity readings of 1 to 2 ppt and the acidic pH of 6.73 to 6.87; both of these parameters are well below the norm for the test organism.

The group in the pure untreated effluent (Solution C) had the second lowest regeneration. This may be attributed to the low salinity of 10 ppt. The pH did not appear to be a factor.

The group in the untreated effluent dilution of 0.1 (Solution D) had slightly higher regeneration than those in Solution C. This solution had an anomalous pH reading of 7.65 to 7.72 for all three trial sets of solutions but did have a reasonable measurement of salinity at 22 ppt.

The group in the untreated effluent dilution’s of 0.01 and 0.001 (Solutions E and F) show increasing trends in regeneration, pH and salinity.

The group in the untreated effluent dilution of 0.0001 (Solution G) peaked with regeneration exceeding that of the control as discussed above. The abiotic parameters of Solution G are optimal when compared to natural conditions, with the salinity at 25, and the pH at 8.62 to 8.76. The combination of optimal abiotic factors and increased nutrients from the effluent (this solution may have more nutrients than the salt water control from the effluent) may account for the peak in byssal thread regeneration by this group.

The group in the untreated effluent dilution of 0.00001 (Solution H) also had optimal abiotic water parameters, but the regeneration declined dramatically. This may be compared to the group in Solution G which had a higher concentration of effluent and possibly organic food.

The temperature of the environment merits discussion. Recall from the introduction that a previous experiment indicated that byssal thread regeneration increased with increased water temperatures. Maximum regeneration occurred at the maximum temperature of one degree below the fatal temperature of 30°C (Clarke, 1996).

The mussels were moved from the home environment with an ambient water temperature of 20 degrees Celsius and placed into a synthetic aquarium environment of 6 degrees Celsius. They were then moved into the trial 1 solutions with a mean temperature of 18 degrees Celsius which triggered spawning behaviour. The trial 2 mussels were freshly harvested from the home environment and place immediately into trial 2 solutions with a mean temperature of 17 degrees Celsius and spawning behaviour was minimised. The test organisms in the first set of static trial 2 solutions (2a) were placed into the cold room with an ambient air temperature of 8-12 degrees Celsius. The second set of trial 2 solutions (2b) were also placed into the cold room at the same time. It is reasonable to assume that both trial 2 solution sets cooled down at a reasonable rate to the mean temperature of 13 degrees Celsius. Limited spawning behaviour was observed, so the temperature factor that triggered spawning in trial 1 was mitigated.

Based on the results of the Clarke temperature experiment, it is possible that the total byssal thread regeneration that occurred in trial 2 was inhibited by the low temperature mean of 13 degrees Celsius.