CO2 being an acid pollutant, its absorption in sea water leads to the process called. The expected increase of [CO2] could promote the fixation of CO2 during photosynthesis. 

It has been shown in the last decade that the increase of [CO2] would have a positive effect on some species of marine phanerogams. But the epibionts of the leaves of P. oceanica being mainly calcareous organisms, they can potentially see their calcification rates decreased in contact with acidified sea water. 

The composition of the P. oceanica meadows could therefore undergo important modifications in the current century by direct or indirect impact of the anthropic acidification of the oceans. 

Global warming has caused sea levels and temperatures to rise over the past few decades. This is one of the many causes of the gradual disappearance of posidonia in the Mediterranean.

The change in temperature modifies marine biodiversity, favoring the introduction of native species. Some species of fish or shellfish feed on Posidonia leaves and invasive varieties of algae can also prevent them from thriving.

Furthermore, the increase in sea temperature associated with climate change can lead to changes in the phenology (timing of life cycle events) of Posidonia, such as flowering and fruiting, which can affect the reproductive success of the plant. 

The impacts of sea level rise on Posidonia and its associated ecosystems can have cascading effects on the entire marine ecosystem, including changes in the abundance and distribution of fish and other organisms that rely on the seagrass meadows for food and shelter. Therefore, it is crucial to take action to mitigate climate change and protect these vital ecosystems. 

Global warming has also induced extreme weather events such as hurricanes, floods, fires ... Storms have an impact on marine life because they modify the turbidity of the water, thus altering photosynthesis and consequently biodiversity.

Posidonia, like many other seagrass species, is adapted to live in specific salinity conditions. Therefore, any significant modifications to the salinity of the water can have a significant impact on the growth and survival of Posidonia.

There are several ways in which salinity modifications can affect Posidonia:

1. Reduced salinity: A decrease in salinity levels can lead to a reduction in Posidonia growth and biomass. This is because low salinity levels can impair the plant's ability to take up nutrients and water from the surrounding environment, which can lead to a decrease in photosynthesis and overall growth.

2. Increased salinity: Similarly, an increase in salinity levels can also negatively impact Posidonia growth and biomass. High salinity levels can lead to water stress in the plant, which can cause leaf shedding, reduced photosynthesis, and a decrease in biomass.

3. Changes in salinity levels: Fluctuations in salinity levels can also impact Posidonia. For example, sudden changes in salinity can cause osmotic stress, which can impair the plant's ability to regulate water uptake and lead to a reduction in growth and biomass.

It is worth noting that the impacts of salinity modifications on Posidonia can also have cascading effects on the entire marine ecosystem. For example, changes in Posidonia biomass can affect the abundance and distribution of other species that rely on seagrass meadows for food and shelter.

Overall, maintaining stable salinity levels in the marine environment is crucial to ensuring the health and survival of Posidonia and the ecosystems it supports.