Ecosystem services and threats of rare species: the isoetid environment

[Departement_IRSTEA]Eaux [TR1_IRSTEA]BELCA

International audience

English. Isoetid macrophytes such as Lobelia dortmanna and Littorella uniflora are endangered species presenting an extensive below-ground system, which controls many biogeochemical cycles through its elevated radial oxygen loss. Despite the number of laboratory studies, the metabolism of those valuable macrophytes has been scarcely considered within in situ conditions. Moreover, given the regression of the meadows all along Europe, it has become necessary to assess the threats for their habitat. We here present an overview of different approaches we are carrying out in the four biggest French Atlantic Lakes (Carcans-Hourtin, Lacanau, Cazaux-Sanguinet and Parentis-Biscarrosse) on isoetid lawns. Since ‘80s, isoetids occurrence is periodically monitored on 100-m long shore-sections, in order to obtain a temporal population dynamic. During 2013-2014, light-dark incubations were performed by benthic chambers on vegetated sediments for the measurement of oxygen and carbon benthic fluxes. In 2015-2016, a sedimentary characterization was performed to define habitat preferences based on granulometry and organic matter content; quadrats technique was used to assess co-occurrence with other macrophytes. In 2017, multivariate modelling and simulations were carried out to evaluate the relationship between natural/anthropic hydromorphological factors and the occurrence of different macrophytes species within the lake. Our results show that isoetids population is on a stable dynamic, with no significant regression in Aquitaine lakes. Those small plants occur within wind-exposed sandy shores, generally in concomitance with charophytes. The presence of isoetids and the complex associated benthic community reduces up to 100% of the methane emitted at the sediment-water interface, by significantly mitigating greenhouse gas emissions from the lake. When compared to other macrophytes, isoetid occurrence appears not to be affected by anthropic modifications of the shore; nevertheless, the simulated scenario shows that part of the lake habitats which are suitable for isoetids development might be occupied by exotic macrophytes in the future.