Periodic Reporting for period 1 - HEALSEA (Innovative indicators of resilience to protect the health status of seagrass meadows: from ecological theory to conservation)
Período documentado: 2018-04-09 hasta 2020-04-08
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.
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.
The HEALSEA project and its objectives were completed by combining: a systematic review, a large manipulative field experiment and advanced statistical models. The field experiment and the analysis of its results represented the main focus of the project.
The first part consisted in a systematic review of the existing literature describing experiments and observations that focused on understanding the response of seagrass biological and functional traits to different types of stress and disturbance. A systematic search was performed on ISI Web of Science. The results of this review showed that the majority of the references did not use gradients but rather extreme values between a control and a single stress or disturbance level. Results from this review were used to design and discuss the outcome of the field experiment.
The second and main part of the project was the design and implementation of a large manipulative field experiment to test for abrupt changes and indicators of resilience in seagrass meadows. It was implemented in a shallow lagoon on the Mediterranean coast that shelter healthy Zostera noltei meadows. Zostera noltei is a common seagrass species found in European shallow areas. Its small size and fast growth rate make it a good model species for experimental studies. We created a nutrient-enrichment gradient to expose 0.5 x 0.5 m portions of the seagrass meadow to different stress intensities and measure the changes in plants, from physiological to morphological and functional traits through time. Benthic chambers were also used to estimate in situ primary production and community respiration along the gradient, thus allowing the evaluation of the functioning of a seagrass community under threat (eutrophication due to the nutrient enrichment gradient).
When the gradient of stress was stable, we tested experimentally the critical slowing down phenomenom, by creating a pulse disturbance and measuring for recovery capacity. This additional pulsed disturbance consisted of removing the leaves by clipping them while leaving the roots and rhizomes in place. The field experiment was initiated in February 2019 and lasted until the end of September 2019.
The third part of the project was the use of advanced statistical models to analyse the outcome of the field experiment. Generalised additive models (GAM) were used to identify thresholds in the response patterns of seagrass traits. I used generalised linear mixed models (GLMM) to identify the descriptors of seagrass health that changed following the intensity of the stress and through time and whether there was a decrease in recovery rate according to the intensity of the stress.
Our results show a switch from a seagrass to an algae dominated system along the nutrient enrichment gradient. We showed that this switch may be abrupt, particularly after a pulsed disturbance and that it could not recover easily after clipping.
Two scientific articles are currently being prepared for publication in peer-review journals:
The first publication analyses the results from the field experiment. It describes the response patterns of seagrasses along a nutrient enrichment gradient and how a pulsed disturbance can change the resilience of the system and lead to a state shift. Indicators of resilience based on the experimental data are evaluated.
The second publication analyses the results from the benthic chamber experiments. It describes the changes in a seagrass community when nutrient enrichment increases and how these changes evolve along the growing season.
The first part consisted in a systematic review of the existing literature describing experiments and observations that focused on understanding the response of seagrass biological and functional traits to different types of stress and disturbance. A systematic search was performed on ISI Web of Science. The results of this review showed that the majority of the references did not use gradients but rather extreme values between a control and a single stress or disturbance level. Results from this review were used to design and discuss the outcome of the field experiment.
The second and main part of the project was the design and implementation of a large manipulative field experiment to test for abrupt changes and indicators of resilience in seagrass meadows. It was implemented in a shallow lagoon on the Mediterranean coast that shelter healthy Zostera noltei meadows. Zostera noltei is a common seagrass species found in European shallow areas. Its small size and fast growth rate make it a good model species for experimental studies. We created a nutrient-enrichment gradient to expose 0.5 x 0.5 m portions of the seagrass meadow to different stress intensities and measure the changes in plants, from physiological to morphological and functional traits through time. Benthic chambers were also used to estimate in situ primary production and community respiration along the gradient, thus allowing the evaluation of the functioning of a seagrass community under threat (eutrophication due to the nutrient enrichment gradient).
When the gradient of stress was stable, we tested experimentally the critical slowing down phenomenom, by creating a pulse disturbance and measuring for recovery capacity. This additional pulsed disturbance consisted of removing the leaves by clipping them while leaving the roots and rhizomes in place. The field experiment was initiated in February 2019 and lasted until the end of September 2019.
The third part of the project was the use of advanced statistical models to analyse the outcome of the field experiment. Generalised additive models (GAM) were used to identify thresholds in the response patterns of seagrass traits. I used generalised linear mixed models (GLMM) to identify the descriptors of seagrass health that changed following the intensity of the stress and through time and whether there was a decrease in recovery rate according to the intensity of the stress.
Our results show a switch from a seagrass to an algae dominated system along the nutrient enrichment gradient. We showed that this switch may be abrupt, particularly after a pulsed disturbance and that it could not recover easily after clipping.
Two scientific articles are currently being prepared for publication in peer-review journals:
The first publication analyses the results from the field experiment. It describes the response patterns of seagrasses along a nutrient enrichment gradient and how a pulsed disturbance can change the resilience of the system and lead to a state shift. Indicators of resilience based on the experimental data are evaluated.
The second publication analyses the results from the benthic chamber experiments. It describes the changes in a seagrass community when nutrient enrichment increases and how these changes evolve along the growing season.
The HEALSEA project provides a better scientific understanding of the response patterns and resilience of seagrass meadows. We used a field experiment to verify ecological concepts that were mostly used in mathematical models. Abrupt state changes exist in seagrass meadows and indicators of nearness to collapse can be measured. The creation of a gradient allowed to identify at which thresholds some traits may change and induce a shift.
Our results can be used by practitioners and stakeholders as tools to anticipate and understand changes in seagrass meadows and community. They may apply in conservation as well as in restoration to better understand the processes in place.
Our results can be used by practitioners and stakeholders as tools to anticipate and understand changes in seagrass meadows and community. They may apply in conservation as well as in restoration to better understand the processes in place.