Seagrasses are flowering plants that form large meadows in shallow vegetative habitats. These meadows have been impacted by climate change, by warming and heatwaves, more varied storm patterns and rising sea levels.
Seagrasses have generally responded to long-term increases in water temperature by shifting their habitat ranges poleward. Some species are better adapted to these shifts than others; faster-growing species keep pace with temperature changes more easily, altering abundance and distribution patterns.
Different responses to temperature changes by different species will also alter the structure and function of the entire community.
Seagrasses generally reproduce clonally, but to keep up with changing temperatures, sexual reproduction is needed to achieve long-distance dispersal goals of multiple kilometres per year.
Rising water temperatures also affect nutrient cycling in seagrass meadows, which can lead to net losses of carbon dioxide and methane.
Mortality of intertidal seagrasses seems to be highest when higher than average air and water temperatures coincide with low spring tides, as they have less protection from heat and solar radiation.
Losses of large meadows have been reported following more extreme heatwaves, especially in shallow regions that experience minimal wave movement.
In 2010, Shark Bay, Australia, experienced sea temperatures that were 3C higher than normal. The resulting loss of seagrass impacted a range of animals including turtles and dugongs.
Few studies have examined how ocean acidification impacts seagrasses, but evidence suggests minimal overall effect, though associated species and ecosystem functions might be more affected.
Seagrasses might benefit from increased CO2 with higher photosynthesis and growth rates, while sea level rise could create new colonisation spaces or push seagrasses into shallower, high-temperature areas depending on local conditions.
Climate change impacts perennial persistent and ephemeral colonising seagrasses differently, with persistent species recovering slowly from disturbances and colonising species recovering quickly. This can lead to a shift toward fast-growing species after heatwaves, reduced genetic diversity, and significant changes in ecosystem functions like carbon sequestration.
Changes in fauna living within seagrass meadows have been noted. This will lead to changes in grazing pressure on the seagrasses themselves, as well as the algae that live on them.
There are also growing concerns about the potential changes to host-pathogen interactions, due to an increased prevalence of seagrass wasting diseases.
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Salt marshes
Salt marsh – image credit: Unsplash
Salt marshes are predominantly inhabited by salt-tolerant herbaceous plants and small shrubs living along sedimentary shorelines. In the tropics their ranges are limited by competition with mangroves and by the presence of ice scour in the polar regions.
The foundation species are mainly grasses, sedges, rushes and succulents, the abundance of these differs depending on location.
A growing number of studies have investigated the impacts of climate change on salt marsh foundation species, reporting positive and negative responses.
The effects of rising CO2 levels are predicted to be positive, as the growth and functions of the plants will likely be enhanced, particularly for C3 plants.
The impacts of warming can be mixed. An experiment in Maine found that increased temperatures resulted in increased dominance of a few grass species but also resulted in reduced species diversity.
In colder regions, warming can boost the growth of salt marsh plants. In tropical regions, warming seems to be reducing growth or having no effect, whilst enabling the spread and encroachment of nearby mangroves.
Climate change is increasing the rate of sea level rise, putting salt marshes under increasing pressure from changes in salinity, larger flooding depths and more. In theory, salt marshes can mitigate against these pressures by expanding their ranges landward, but often steep terrain and man-made structures prevent this.
Without enough space for marshes to migrate inland, coastal squeeze could cause significant global marsh loss, though this risk can be reduced if landward areas remain accessible for marsh expansion.
Salt marshes are often exposed to a combination of climate-related stressors. These factors weaken marsh plants, making them more susceptible to herbivores and leading to die-offs, as seen in China and the U.S. where drought and overgrazing interact to degrade marsh ecosystems.
Although controlled fire has been used for marsh management, rising wildfire frequency and intensity now pose significant risks by damaging plant health, soil structure, and invertebrate communities.
The impacts of climate change on salt marshes are complex and often involve interactions between multiple drivers, like drought and El Niño, or local human activities. These interactions can have synergistic, additive, or antagonistic effects, which remain poorly understood.
For example, while some marsh species thrive under warming temperatures and elevated CO2, others suffer from reduced belowground biomass and increased vulnerability to sea level rise, with some even altering their growth patterns to enhance resilience.
Both seagrass meadows and salt marshes are vital coastal ecosystems increasingly impacted by climate change. As sea levels rise, temperatures warm, and environmental conditions shift, these habitats face challenges that could alter their distribution, biodiversity, and ecological functions.