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. At 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”.