measurements with deionised water
Results of four reference measurement series with deionised water (see section 3.1) at 18°C showed that despite a consistent water temperature as well as apparent consistency for all other tank parameters including duration of acoustic measurement phase, quiescent phase, water volume injection and duration of the jet, initial BRT at the start of
measurement as well as mean BRT values differ significantly, ranging from a mean value of 200 seconds (18°C-1 experiment on Figure 3.1 c) to 140 seconds (18°C-4 experiment, refer to Figure 3.1 c in results section). The mean oxygen saturations (Table 3.1) indicate that this parameter was not the main cause for the observed differences for the reference measurements, as the highest mean BRT (202 seconds) occurred when mean oxygen saturation was lowest (54.8%). However, oxygen saturation has some influence on BRT as it is shown by the lower BRT of the first 8 measurement indices (Figure 3.1 c.) for the 18°C-4 measurement series and its rapid equilibration that occurred simultaneously with an increase in oxygen saturation (Figure 3.1 d). The differences in oxygen saturation for the 18°C measurement series can be mainly explained by the different residence times of the deionised water in the tank system prior to the beginning of measurements. The higher oxygen saturation for 18°C-4 and the increase in saturation during the first 8 measurements can be accounted for by a saturation increase resulting from warming of the water, which had previously been kept in the tank system at a temperature of 12°C. Similarly, the intermediate oxygen saturation of 62% to 65% of series 18°C-2 can be accounted for by the longer residence time of the water in the tank system (2 weeks prior to the beginning of measurements, see section 2.6.2 for preparation of experiments). For series 18°C-1 and 18°C-3 BRT measurements were started soon after the filling of the tank system. Despite similar oxygen saturations for 18°C-1 and 18°C-3 series, mean BRT values were very different. It is possible that nitrogen saturation was higher for 18°C-1 than for 18°C-3, causing the higher BRT. However, as nitrogen saturation was not measured and the methodological preparation of the two reference experiments were similar, certain
conclusions about nitrogen saturation causing the higher BRT for 18°C-1 cannot be made.
Reference measurements with deionised water at a temperature of 12°C show much
smaller differences in mean BRT between the three different measurement series, however, differences still exist and these differences also do not correlate with oxygen saturation. However, it can be noted that the use of newly filled water (series 12°C-1, refer back to Figures 3 a and b) resulted in a slightly higher mean BRT whereas two measurement series using the same water (series 12°C-1 and 12°C-3) showed nearly identical mean BRT and oxygen saturation. This suggests that the differences in BRT are not a result of changing factors of the tank system but can rather be attributed to different characteristics of the water used for the various reference experiments. One possibility is that the differences in mean BRT of reference water experiments resulted from some sort of contamination (see
section 4.2) of the water, which is likely to have occurred as the water in the tank system is susceptible to contamination by bacterial and planktonic plaque from tube and valve linings, as discussed in section 4.2. As measurements could not be carried out under sterile conditions, bacteria were present in the water, converting particulate organic carbon (POC) to DOC. This may also explain the smaller differences in mean BRT for reference
measurements at 12°C, when bacterial activity was presumably lower than at 18°C. As deionised water is not buffered as well as seawater, it reacts more sensitively to
contamination. Thus, it is possible that different degrees of contamination could have led to the differences in BRT between different reference water experiments. This assumption is supported by the findings of Detwiler (1979), who states that bubble rise speed is directly affected by the concentration and adsorption dynamics of surface active
contaminants onto the bubble surface and that the contaminant concentrations only need to be on the order of a few parts per million or less to alter bubble rise speed.
Mean BRT was also found to have been lower for reference measurements at 12°C than for 18°C reference measurements. This would agree with the findings of Leifer et al. (2000) and Patro et al. (2000), who detected reduced rise velocities of small air bubbles at lower temperature due to decreases in bulk water viscosity. However, as discussed in section 4.1, it could not be distinguished between increases in BRT resulting from decelerated rise velocity and increases in BRT resulting from the production of a greater number of bubbles with the acoustic measurement method applied. Thus, the slightly higher mean BRT values for measurements at 18°C could be accounted for by enhanced bubble production, which was observed by Hwang et al. (1991) to occur for increasing temperatures between 11 and 17°C. Retrospectively, the use of artificial seawater (deionised water with artificial sea salt) as reference water would have been more advisable as the carbonate system of seawater acts as a buffer. However, comparison of the initial values of BRT of natural filtered seawater (without nutrient medium and algal cultures), that was used for the three growth experiments with monocultures (Figure 4.1 a) also show differences in BRT ranging from mean values of 157 seconds, 184 seconds and 228 seconds respectively. For seawater reference measurements, oxygen saturation covaried with BRT (Figure 4.1 b). Thus, if contamination influence was reduced as a result of the buffering capacity of seawater, the changes in BRT could be accounted for by changes in oxygen saturation. These results lead to several possible conclusions. It is likely that BRT is extremely
the findings of Detwiler (1979), Thorpe (1992) and Patro et al. (2000), who state that the bubble’s surface is initially clean but becomes dirty within a few seconds after formation. They found that the accumulation of colloidal substances on the bubbles’ surfaces led to surface tension gradients and thus to reduced rise velocity. It is also possible that a combination of contamination effects as well as gas saturation effects of the deionised water may have occurred.
4.3.1 Equilibration of seawater in the tank system
The high initial mfBRT values followed by a decline in BRT that occurred during the second and third Kiel Firth water growth experiments and during the growth experiment with Chaetoceros muelleri further demonstrate the problem of standardisation of the tank system. For these experiments, BRT measurements were started directly after filling the water into the tank system. For experiment 4 (Chaetoceros muelleri), the initial decrease in mfBRT covaried with the decrease in oxygen saturation (Figure 4.3 d) whereas for
experiment 2 (second Kiel Firth water growth experiment), the initial decrease in mfBRT did not show much covariation with oxygen saturation (Figure 4.3 b).For experiment 3 (third Kiel Firth water experiment), oxygen saturation was not measured during the first 3.5 days, therefore it cannot be stated that the mfBRT decline covaried with oxygen saturation. For experiment 6, oxygen saturation covaried with the small initial decline in mfBRT (Figure 4.3 f). However, the decline in initial mfBRT for experiment 6 was much weaker than for experiment 2 despite the same water type used (aged North Sea water). The constant initial mfBRT values for experiment 5 (Phaeocystis, Figure 4.3 e) and the initial BRT of the first Kiel Firth water growth experiment (experiment 1, Figure 4.3 a), where the water was left to equilibrate within the system for several days prior to the beginning of BRT measurements (refer to sections 2.7.1 and 2.7.5) further demonstrate that the water needed some time to equilibrate within the tank system. These results suggest that equilibration and mixing effects of the water with the tank system play an important role and may take several days. However these effects seem to be specific to the various types of water used in this study and cannot be generalised. Freshly filled Kiel Firth water showed much higher initial BRT and more time was needed for the water to equilibrate with the tank system compared to aged and filtered North Sea water used for the
monoculture growth experiments. Reasons for the differences may be varying degrees of gas saturation, differences in dissolved and particulate substances present in the
water and different lengths of storage of aged North Sea water used in monoculture growth experiments.
Figure 4.1 Changes in BRT (a) and oxygen saturation (b) with increasing measurement number for filtered seawater at 18°C. M e d ia n f ilt e re d b u b b le r e s id e n c e t im e [ s e c ] 100 120 140 160 180 200 220 240 260 18°C-1 18°C-2 18°C-3 Measurement number 0 2 4 6 8 10 12 14 16 18 20 22 24 26 O x yg e n % S a tu ra ti o n 30 40 50 60 70 80 90 18°C-1 18°C-2 18°C-3 a b