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Listening to posidonia seagrasses

3/25/15

By studying variations in oxygen production within posidonia seagrasses it is possible to assess how healthy they are. This is extremely important research because these ecosystems along the Mediterranean coast contain a large biodiversity, play a role in preventing erosion of the seabed and beaches, produce oxygen and store large quantities of CO2. These seagrasses are also excellent indicators of the health of the environment. A new method of detecting oxygen production has been tested at the STARESO station at Calvi in Corsica. This involves an acoustic system which makes it possible to analyse the variations in the speed of sound in these seagrasses and to correlate these variations to a high or low level of oxygen gas present in the form of bubbles. This system is allied to an already well-established set of devices which can make a contribution to an understanding of these ancient and fragile ecosystems.  

The study of sound propagation in aquatic environments is important for very different scientific interests. Some of these are quite original to say the least. This is certainly the case with regard to a recent study(1) during which physicists and oceanographers recorded and “listened” to the production of oxygen in posidonia seagrass meadows. The objective of this operation was to apply a multidisciplinary approach to new techniques for analyzing variations in the production of oxygen. These variations in oxygen production depend directly on photosynthetic activity and the health of the ecosystem. While this approach may appear surprising, it is contributing to the development of new technologies which backs up the data gathered by means of the usual techniques.

“The article was written in the context of a European networking project (ESF COST Action 0906), organised in 2011 at the STARESO station in Calvi Bay in Corsica”, explains Sylvie Gobert, a lecturer and head of the Oceanography Laboratory of the University of Liege. “Some twenty researchers from several countries gathered there to study a posidonia seagrass meadow in all its forms, from a cellular level to the entire ecosystem”. These few days of all-out research created a great synergy, resulting in the acquisition of new knowledge. The choice of the Bay of Calvi in Corsica was no accident. The area is barely affected by pollution and does not suffer any damage due to high frequentation by humans. The water there is very clear and stable and light can penetrate deeply into it (2). These conditions are suitable for the development of healthy posidonia seagrasses and for optimal photosynthetic activity.  

An Environmental challenge

Today, posidonia seagrass meadows are the subject of various research programs, rooted in a politico-scientific movement based on the problem of blue carbon. These initiatives are aimed at preserving oceanic systems that produce a high level of photosynthesis. This preservation can even take the form of attempts to recolonize and redevelop ecosystems that have been damaged by human activity.

Herbier posidonie (c) A.Abadie 

experimental setup posidoniaThe problem has become urgent. Posidonia only develops in the Mediterranean between the surface and 40 metres in depth. It is therefore a coastal plant and this is the area worst affected by human activity in our seas and oceans. Several causes of the decline of this plant can be listed. For example, the development of ports and marinas. An increase in the nutrients that are consumed by epiphytic algae that develop more easily. These multiply, cover the posidonia and deprive them of the light required for photosynthesis. Another destructive factor is the increase in the turbidity of the water. This prevents the light from passing through it and limits the depth at which posidonia can develop. Very often, they do not venture beyond 15 to 25 meters below the surface. This is a pity from an ecological point of view. “These are veritable forests under the sea”, explains Sylvie Gobert. “They enable other plants and animals to live and lead to increased biodiversity. But they also have a slowing effect on erosion and stabilize sediment, their long leaves slow down currents and protect beaches. They produce oxygen and constitute an extremely important carbon sink”. (See also Posidonia under surveillance and The vigils of the coastal environment)

By way of comparison, a forest stores 8 grams of carbon per square meter per year. For a similar surface and over the same period, a posidonia meadow can absorb 200 to 300 grams. Its storage capacity is 20 to 30 times greater than that of a forest. “These ecosystems have surfaces that are not comparable to European, African or Amazonian forests”, explains Alberto Borges, senior research associate at the FRS-FNRS and head of the Chemical Oceanography Unit of ULg. He goes on, “But they also constitute non-negligible carbon sinks. We can also see the problem from the opposite viewpoint. If these systems disappear, not only will we lose a sink of CO2, but the stored carbon will be remobilized and rereleased into the atmosphere”.

Sediment as a means of storage

The mechanism that transforms these meadows into CO2 sinks also transforms their disappearance into a veritable environmental sword of Damocles. “The posidonia leaves can reach one meter in height”, explains Willy Champenois, who is a chemist and a doctoral student of oceanography. “Their large size and the way there are organised into clumps has the effect of slowing down the currents. These same currents are laden with sediment. When they slow down, the sediment falls and covers the seabed. If the posidonia had no way to counteract this drawback, they would end up being completely buried”.  These leaves grow from a network of rhizomes, horizontal underground rods. In order to prevent themselves from being buried, these rods can also grow vertically. Thanks to this mechanism, the bottom of the meadow is raised by 1 milimeter per year on average. “In one thousand years the meadow will have risen by one meter. Within this meter of sediment known as the mat, the old rhizomes, roots and other forms of waste are captured at an almost fossilized state. They do not decompose, and the organic carbon of which they are partially composed, remains trapped”.  

If the plant dies and decays it can no longer act as a protective barrier to the seabed and does not therefore slow the current down. The hydrodynamics will progressively erode the surface of the mat and release the organic matter that was trapped there. This matter will then decompose. The carbon which was stocked there up to that point will then come into contact with an aerobic environment and released in the form of CO2. “We recorded a mat depth of six to seven meters in places. This represents carbon-stocking of six or seven thousand years which goes back to a period just after the last ice age. This will give some idea as to the concentration of carbon that would be released into the atmosphere if these meadows were to disappear completely”. “And if the vertical growth of the posidonia communities is one milimeter per year, continues Alberto Borges, “their horizontal growth over a similar period is one or two centimeters. They take a considerable amount of time to spread out. And when, for example, the anchor of a sailboat is raised tearing off a square meter of seagrass, one century of growth has been destroyed”. 

The emission of oxygen is an indicator of the health of the ecosystem

The production of organic matter by a seagrass meadow is linked to its photosynthetic activity. The intensity of this activity is dependent on its state of health. The more matter it produces, the more oxygen it releases. We can therefore conclude that the more it emits oxygen, the healthier it is. One of the challenges in relation to posidonia-based research is to succeed in measuring this oxygen production. For around ten years, such a measurement has been facilitated by the marketing of optodes. These are small probes which by means of an optical system can measure and record the quantity of oxygen dissolved in the water and by extension the quantity of oxygen produced by an ecosystem. “We have been using this technology at the Stareso station for almost eight years”, explain the oceanographers. “Since then, we have been obtaining data on oxygen levels in Calvi Bay on a daily basis. This data is unprecedented and enables us to measure the variations in oxygen and understand how the meadow can evolve. It is an incredibly valuable tool, especially if we want to observe the health of the endangered meadows”.  

Before the arrival of optodes, the production of a meadow was calculated according to the variation in its biomass. This method had several disadvantages. “Observing the evolution of the primary production of a meadow is a relatively laborious process”, explains Alberto Borges. “To give a simpler example, if we want to measure the primary production of our lawn, we will need to cut it every week, weigh the volume of grass cut and the job is done. In water, things are more complicated. We obviously cannot cut posidonia grass like we do our lawn. Up to a short time ago, it was necessary to dive every day and develop complex sampling techniques”. In addition, the study of biomass does not make it possible to take account of the entire production of the meadow. “A posidonia meadow produces up to 600 grams of dry matter per year per square meter”, explains Willy Champenois. “This is what we used to measure up to recently. But if we confine ourselves to this, we miss an entire set of organic matter also produced during photosynthesis, but directly dissolved in the water and which is not factored into the biomass. By contrast, oxygen production also depends on this dissolved matter. By studying the oxygen, we can therefore quantify this matter which is not taken into account if we study dry matter alone”. Finally, optodes take account of the activity and production of the entire ecosystem. If the meadow is healthy, it is autotrophic which means that it produces more than it consumes. “These oxygen surpluses and organic matter enable an entire heterotrophic fauna community to survive. This community also has an impact on the ecosystem, an impact which is also calculated by means of optodes”.

Several cycles of variation in oxygen emissions

The study of variations in oxygen production is a determining factor in order to understand the evolutions of the ecosystem and more broadly, our environment. The data gathered over the last 8 years has made it possible to measure these cycles over several timescales. The first variation measured was during daytime. During the day, light enables photosynthesis to take place and the production of oxygen is higher than during the night. The second cycle observed was seasonal. “The ecosystem is less exposed to the sun in winter than in summer, and therefore produces less oxygen. Its lowest level, in the month of February, production reached slightly more than 5 grames per day per square meter. In summer, the production exceeded 25 grames. This was three times more than the production of a forest. In autumn, the leaves fall, the photosynthetic activity diminishes and the cycle starts again”. Lastly, there are the inter-annual cycles. The variation in production from year to year is extremely marked. These cycles are placed in perspective with other data including meteorological data.

Evidently, these scales are still too small to underline general trends upon which human activity has an impact. “We are still far from generating the kind of data available to climatologists who have been gathering information about temperature variations for nearly two centuries. Litter posidonia (c) Arnaud AbadieAt such large scales, even if the temperature rises and falls from year to year, it is possible to establish a trendwhich points to climate warming. This is the objective of our research, to be able to establish series of measurements that are sufficiently long to describe natural variations and also identify the trends relating to human impact. We will maintain such an observation for as long as possible in order to measure long-term trends. For the moment, the inter-annual variations are already very interesting. We are trying to understand these variations and perhaps find anthropic causes”.

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”.

(1) Paulo Felisberto, Sérgio M. Jesus, Friedrich Zabel, Rui Santos, João Silva, Sylvie Gobert, Sven Beer, Mats Björk, Silvia Mazzuca, Gabriele Procaccini, John W. Runcie, Willy Champenois, Alberto V. Borges, Acoustic monitoring of O2 production of a seagrass meadow, Journal of Experimental Marine Biology and Ecology, vol.464, Mars 2015 http://hdl.handle.net/2268/176515

2() Mazzuca, S., Bjork, M., Beer, S., Felisberto, P., Gobert, S., Procaccini, G., Runcie, J., Silva J, Borges, A., Brunet, C., Buapet, P., Champenois, W., Costa, M., D'esposito, D., Gullstrom, M., Lejeune, P., Lepoint, G., Olivé, I., Rasmusson, L., Richir, J., Ruocco, M., Serra, I., Spadafora, A., & Santos, R. (2013). Establishing research strategies, methodologies and technologies to link genomics and proteomics to seagrass productivity, community metabolism, and ecosystem carbon fluxes. Frontiers in Plant Science, 4(38), 1-19.  http://hdl.handle.net/2268/145672


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