Seagrass meadows under climate change

Seagrasses are the only flowering plants that are found in the marine environment, covering extensive areas in temperate and tropical coastal areas. Their meadows are among the most valuable ecosystems on Earth. Apart from their cultural services, seagrasses provide key ecological functions for maintaining healthy estuarine and coastal ecosystems. Accordingly, these plants are regarded as a useful indicator of water quality due to their sensitivity to anthropogenic pressures, being highly relevant as one of the biological quality elements required for the assessment of the ecological quality status in coastal and transitional water bodies within the European Water Framework Directive (2000/60/EC). The high ecological importance of their habitats has also promoted their inclusion in the list of Natural habitats of Community interest of the European Habitats Directive (92/43/EEC, Annex I).

However, over the last two decades, up to 18% of the world’s documented seagrass area has been lost, with rates of decline accelerating in recent years. This has placed seagrass meadows among the most threatened ecosystems on earth, with loss-rates comparable to those reported for mangroves, coral reefs, and tropical rainforests. These valuable ecosystems and the services they provide are subjected to many pressures, both anthropogenic and natural. Moreover, the present situation of declining seagrasses may be exacerbated by global change drivers. Recent climate change has already impacted marine environments with documented effects on the phenology of organisms, the distributional range of species, and the composition and dynamics of communities. According to the latest IPCC report (AR5), in the coming decades, coastal systems and low-lying areas will increasingly experience adverse climate-related impacts intensified by increasing human-induced pressures. Particularly, the vulnerability of the Basque coast and its marine environment to potential change in present climate regimes is expected to be increased by the demographic pressure, the overexploitation of resources and the high human use of the marine space.

Within this context habitat suitability models, habitat restoration and habitat classification approaches represent highly promising tools for seagrass conservation. On one hand, through the application of niche models, habitat requirements of the species can be determined and the suitable areas can be estimated under present conditions and projected to future climate scenarios. On the other hand, small-scale transplant experiments of seagrass meadows can provide valuable information for planning major restoration efforts, which might contribute to the improvement of the ecological status in coastal areas. Lastly, extensive and high-resolution habitat maps can support the assessment of the ecosystem status and the influence of natural and anthropogenic impacts over time. Therefore, these tools can provide relevant information not only to implement conservation, restoration and management plans of natural resources, but also to establish adaptation plans to face climate change impacts. This PhD thesis primarily emerged from the need to conserve Zostera noltii (Hornemann, 1832), although research has also been performed for the semi-annual flexible type of Zostera marina L. Z. noltii is the unique seagrass species occurring in the Basque estuaries which is presently under extinction risk and is found to be highly vulnerable to climate change due to its intertidal distribution. The semi-annual flexible type of Zostera marina L. is an intertidal habitat forming seagrass species that shows a high inter annual variability in extent and location, leading to be particularly vulnerable to local threats.