Such measurements of primary production by monitoring oxygen concentrations already exist: they are taken from various seagrass meadows in no less than 123 sites worldwide, with two sites in the Mediterranean as well as the Stareso one, ULg’s oceanographic research station in Calvi. In general, these measurements are carried out manually thanks to a closed bell placed in the seagrass meadow, in which the oxygen variations are measured. However, this approach requires considerable human and financial means and as a result, data can’t be collected that often. It is collected every few weeks at best, and during periods that are too short to provide researchers with information on variations in oxygen concentrations from one year to the next or in the long term. “In addition, it’s intrusive to place a bell in the seagrass meadow – it causes damage - and inevitably biases the results owing to the confinement of the seagrass meadow in a determined volume of water. In the final analysis, these classic yet costly methods tend to underestimate the values and, ultimately, only provide researchers with an overall view of the dynamics of the seagrass meadows”, Alberto Borges warns.

Continuous data collection

However, since the beginning of the 2000s, the existence of sophisticated sensors (optodes) mean that it is now possible to reliably measure the concentrations of oxygen in water columns over a very long period of time. “Instead of the classic methods, we chose an approach consisting of deploying a certain number of sensors (one at a depth of 9 metres, another at a depth of 7.5 metres and a third at 5 metres) on a 10-metre mooring line – i.e. a line continuously kept taught by a weight and floaters, between the surface and the seabed". For more than three years, between August 2006 and October 2009, and at a comparatively low cost, the sensors in place allowed primary production and ecosystem respiration values to be continuously collected all year round. “We were the first to carry out such a long study, with reliable non-intrusive material, gather such a vast amount of data, and compare it with the data we obtained using the more conventional bell method. We currently have the best seasonal resolution ever achieved in a seagrass meadow”. Furthermore, the long observation period has allowed the researchers to observe extreme and rare phenomena such as “exceptional years” – a particularly hot year, for instance – thus giving them an insight into how an ecosystem may behave in the future. “In a century, for instance, when the average temperature will have increased as a result of global warming. And it just so happens that we’ve had this opportunity”, Alberto Borges explains. “We deployed our mooring line in August 2006, and the winter that followed was the mildest winter recorded in Europe in 500 years. As a result, while the warmer winter didn’t really affect our measurements (the reason being that the posidonia meadow is dormant during this season, i.e. its growth cycle is interrupted), the fact that the Mediterranean had very few storms during this period is, on the other hand, very interesting".