Using acoustic monitoring to detect oxygen gas

The optodes system is therefore one of the most robust methods for measuring oxygen in an ecosystem. The data this method has gathered has contributed to the establishment of a new method of measuring oxygen, resulting from the meeting of several research groups at the STARESO station. Due to the fact that they could compare their results to those of such a reliable system that Portuguese researchers were able to conclude their research. This time it was not optics that were involved but acoustics. And this method, even though it is not yet calibrated, could be used in unison with optodes yielding complementary observations in a multidisciplinary context.
 
The system is quite easy to understand. The theoretical speed of sound propagating in water is calculated while also taking account of measurements carried out simultaneously. They take account of a series of data relative to this environment which influence the speed of sound (density, water temperature, salinity, pressure which depends on depth and the surface wind). In parallel with this, a transmitter, as well as three receptors (hydrophones) is placed under the water at a distance of 122 meters from each other. The hydrophones are grouped together at different depths. The transmitter emits sounds at different frequencies and the hydrophones record them.  

By consolidating what they measured with the help of their system of theoretical data, the researchers observed that sound actually travelled slower in practice and there were variations in its speed. “These variations coincided with our measurements of the concentration of oxygen; explain Alberto Borges and Willy Champenois. “The more we observed a high level of oxygen, the slower the speed of sound was. However, the oxygen dissolved in the water does not influence the speed of sound. On the other hand, what can influence its speed is the presence of oxygen gas bubbles in the water column”. This is a physical property linked to the question of the density of matter. The more space there is during the movement of the speed of sound, the slower it gets. Therefore, the less dense an environment is, the slower the speed of sound. The speed of sound is slower in air than in liquid, and even slower in a liquid than in a solid. The appearance of oxygen bubbles therefore constitutes a barrier which slows the speed of sound more, in relation to its concentration in the water column. “We were surprised by these results. Our system does not enable us to detect oxygen in the form of bubbles, just as the acoustic system cannot detect oxygen dissolved in the water. But it was possible for us to demonstrate a phenomenon that we had not anticipated. We did not imagine that there could be such a high quantity of oxygen bubbles. Nonetheless, we thought we would be able to gauge a relatively complete estimate of the quantity of oxygen produced and therefore the primary production of this ecosystem. This new study showed that we had underestimated our values”. Even more surprising was the fact that, at sunrise, the acoustic apparatus recorded an increase in the formation of oxygen bubbles where the optodes had not yet recorded the increase in activity linked to the day/night cycle. It was more precise than the optodes for determining the moments when photosynthesis began. 

A method that is still at the embryonic stage

The synergy created during the meeting at Calvi made it possible to compare the acoustic data and the data obtained by the optodes and to notice a correlation between the variations in the two phenomena studied. This synergy made it possible to demonstrate that the production of the ecosystem is more important than was initially imagined. But the added-value of the acoustic system still presents many limitations. In particular, it allows to observe the relatively high or lower presence of oxygen bubbles but not to calculate the volumetric quantity from a qualitative point of view, whereas optodes are quite precise when it comes to measuring the level of dissolved oxygen. It must also be noted that the system is difficult to put in place because it requires the permanent presence of several physicists and divers to put the microphones in place and record the data which presents a lot of constraints with regard to the independence of the optodes. In conclusion, the method encountered several difficulties linked to the presence of many parasitic noises which needed to be factored into the analysis of the recordings. These sounds were linked to currents or were of biological origin. Several noises from fish were recorded for example. The relatively noisy environment therefore did not help the identification of the evolution of the sound emitted and to establish the contribution made by the oxygen bubbles to this variation to. Nonetheless, the acoustic apparatus, though still at the embryonic stage, is providing complementary information on the photosynthetic activity of the posidonia meadows. “At the moment, conclude the researchers, “this technology cannot be applied as routinely as that of the optodes. It would be necessary to avoid the problems linked to its autonomy and the necessity to quantify the observed phenomena. But it does enable us to listen to the posidonia and therefore come up with new approaches”.