Seagrasses are marine flowering plants that form structurally complex habitats, called seagrass meadows. They represent one of the most valuable resources in the coastal environment as their presence can help fight climate change, protect from erosion, and provide a shelter and food for many species, thus improving biodiversity. Unfortunately, seagrasses are nowadays under serious threat all around the globe, affecting their health status. Threats come from many different sources, from climate change to human activities. The documented and worldwide decline of seagrass meadows is leading to the loss of the services they provide. This raises the need to undertake effective conservation measures to protect them. Our capacity to succeed in this task largely depends on our aptitude to anticipate their potential decline.
Ecological theory suggests that complex systems, such as seagrass meadows, may respond abruptly to increasing threats. That is, when conditions change under a threat, they first are able to resist and maintain themselves, until a certain threshold at which the system suddenly collapses. Ecological theory also suggests the existence of indicators of nearness to collapse. One of these indicators is called a critical slowing down. A critical slowing down (CSD) is a phenomenon during which the system becomes more variable, and takes longer to recover if any other disturbance would happen. It occurs when a system gets close to the threshold of collapse. Those theories have proven right for some complex systems but remain mostly based on mathematical studies. We still do not know whether these theories apply to seagrass meadows and how to identify them in real conditions.
The HEALSEA project aimed at testing whether these theories on abrupt changes and indicators of resilience applied to seagrasses in real conditions. During the project, we followed the response of different descriptors of seagrass health through a deliberately created gradient of increasing intensity of a threat (nutrient enrichment) in the field, and tested the CSD phenomenon. The final goal was to use our results to improve conservation measures and contribute to the preservation of seagrass meadows health status.
Our work showed that seagrasses are able to resist under a certain level of threat, until the system becomes more variable, slow in recovery and collapses. This experiment showed that some of the descriptors of seagrass health presented clear abrupt changes to increasing threats and provided a good estimate of a threshold at which the system collapses. This was the case for several of these descriptors at different levels of biological complexity, from physiological to ecosystem functioning.
Since these descriptors are important for sustaining the communities of organisms living within the meadows and for determining services that seagrasses provide, they can be considered as good indicators. We can thus conclude that decline of ecosystem functioning and services could be predicted if such indicators of resilience were used in seagrass monitoring programs.
