Coastal and Intertidal Habitats

Coastal, intertidal and shallow nearshore ecosystems tend to be highly diverse and variable at local, regional and global scales. The impacts on these systems of anthropogenic activities and climate change are highly complex, cumulative and interlinked. Therefore, quantifying habitat-specific responses to changes in climate is extremely challenging. 

Increasing storm frequency and severity are altering tidal flows, exposing saltmarshes to stronger wave action and accelerating erosion. As sea levels rise, mudflats shift, low marsh areas flood more often, and habitats migrate landward. This drives changes in plant communities and the loss of flood‑sensitive species. Together, these pressures highlight how vulnerable saltmarsh habitats are to climate change.

Climate change may be facilitating the spread of established invasive non-native rocky shore species, with recent range extensions observed in the English Channel, southern North Sea, and the Celtic Sea.

As machair – a rare coastal grassland found only on the exposed western coasts of Scotland and Ireland – is limited to low-lying coastal areas with high winter water levels, it can be particularly vulnerable to changing weather patterns, relative sea-level rise (RSLR), and increased frequency of storm events.

Interannual climate variability fundamentally influences the ecological functioning of dune slacks. Reduced rainfall limiting water table recharge, erosive pressure from storm surges and RSLR may be capable of forcing more significant shifts in dune slack hydrology and ecology than seasonal variation.

More detailed confidence rationale and ratings are provided below, for each habitat type.
 
HIGH CONFIDENCE

• Rocky intertidal habitats (High evidence, High consensus): Spatial and temporal climate change impacts on the abundance and distribution of intertidal invertebrates and macroalgal species in the UK rocky shores have been monitored comprehensively.
• Saltmarsh (High evidence, High consensus): There is high certainty that relative sea-level rise and coastal hazards are affecting saltmarshes, despite some differences in habitat definitions and some uncertainties regarding erosion, accretion and infilling relationships between saltmarshes and adjacent mudflats. More climate impacts evidence has been collected for saltmarsh than for any other coastal habitat. 
• Machair (High evidence, High consensus): Almost all machair lies below the 10 m contour, with significant areas of machair below mean high-water springs (2.03 m). Even existing levels of storminess and extreme water levels involve a higher risk of flooding by sea water or fresh water (or both).

MEDIUM CONFIDENCE

• Intertidal and subtidal seagrass beds (Medium evidence, High consensus): There is comprehensive empirical evidence on the physiological responses of different species to climate change impacts including warming and ocean acidification from laboratory, mesocosm and field studies from across the world, but less so in UK waters, especially for cumulative effects.
• Intertidal soft sediments (Medium evidence, Medium consensus): Impacts of gradual long-term climate change are often much smaller than the short-term variability characteristic of these environments, which makes them difficult to detect. The coupled biophysical system has yet to be fully integrated and addressed holistically. 
• Sand dunes (Medium evidence, Medium consensus): Relative sea-level rise, strong winds, rainfall and seasonal drought all affect sand dunes and their ecology. There is currently a lack of knowledge on the interplay between these factors, and their impacts on dune dynamism and drying.

LOW CONFIDENCE

• Maritime cliff and slopes (Low evidence, Low consensus): The varied geophysical and climatic characteristics of coastal zones make them susceptible to a range of extreme natural events such as erosion, flooding and cliff instability, operating on different timescales and of different magnitudes, mean impacts are very site specific and difficult to ascribe to climate change.   

Rising sea levels and more frequent, intense storms will continue to alter coastal sediment transport, and – together with freshwater inputs and human activities – are reducing the extent of UK saltmarshes, with an estimated 11% loss projected by 2060 without putting effort into restoration. Shifts in temperature, rainfall patterns and the increasing occurrence of heatwaves also risk driving major ecological regime changes within saltmarsh habitats, altering species composition and overall ecosystem function. 

Anthropogenic and Marine Heat Wave (MHW) impacts are likely to be more intense in shallow and intertidal waters where the temperature stress is the greatest and exposed plants are likely to experience desiccation, suggesting that the intertidal seagrass Zostera noltii may be particularly vulnerable. 

Cumulative impacts of climate change and direct anthropogenic stressors including high suspended sediments and eutrophication are likely to increase the vulnerability of seagrass beds to climate change. 

Increased frequency and intensity of heatwave events may start to negatively affect the abundance of cold-affinity boreal species in rocky intertidal systems. Some cold-water boreal species, such as the barnacle species Semibalanus balanoides and Balanus crentaus may disappear from some rocky intertidal habitats in south-west England.

The coincidence of storms with large tides and sediment availability may be a key factor in long-term dune loss. Rates of dune erosion are expected to continue to increase due to climate change.

For maritime cliff and slope habitats, increased rainfall in the future may lead to increased slope failure, particularly affecting the movement of groundwater in softer lithologies. The role of RSLR in accelerating soft cliff retreat is shown in a modelling study that estimates future erosion rates three to seven times higher in North Devon and Yorkshire by 2100 based on the current predictions of RSLR. This is an increase much greater than previously predicted.

Although the focus of restoration programmes is often on the active habitat creation, the importance of conserving existing habitats through removal of pressures is likely to bring the largest benefits for climate change mitigation and adaptation. Restoration practices should also plan beyond single species and single habitats to a multi-habitat seascape.

More detailed confidence rationale and ratings are provided below, for each habitat type.

HIGH CONFIDENCE

• Saltmarsh (High evidence, High consensus): There is a high level of agreement regarding a future overall reduction in the aerial extent of UK saltmarsh and evidence supporting adaptation is more comprehensive than for other coastal habitats, although there is some uncertainty about the impact of coastal hazards on community ecology over short and long timescales.

MEDIUM CONFIDENCE

• Rocky intertidal habitats (Medium evidence, High consensus): Forecasting future impacts of extreme events, such as Marine Heatwaves and storm events, is difficult due to their stochastic, site-specific nature. When combined with interacting anthropogenic impacts, future predictions on climate change impacts on rocky shores become less certain, although the use of species community temperature indices (TCI) analyses is showing some promise in addressing this uncertainty.
• Machair (Medium evidence, High consensus): In addition to increasing risks from RSLR, winter rainfall and reduced wave attenuation, any increase in storminess will increase risk of flooding. The dynamic nature of machair habitats means there is some inherent resilience, and the main threat to biodiversity relates to adverse impacts on crofting.
• Intertidal and subtidal seagrass beds (Medium evidence, Medium consensus): Predictive models of organism- and population-level responses of seagrasses to future climate change impacts imply medium to high confidence from global studies. However, for UK species in UK waters, there is less evidence, including for the adaptive response of seagrass ecosystems to climate change and potential areas for seagrass restoration.
• Sand dunes (Medium evidence, Medium consensus): Landward migration of the dune ridge with RSLR and stronger winds, and a shift towards dryer dune slack communities with a lowering of the summer water table are anticipated, but there is uncertainty on the latter, due to the interplay of inter-annual and long term change and the effects on plant species.

LOW CONFIDENCE

• Intertidal soft sediments (Low evidence, Medium consensus): The current lack of knowledge on the coupled biophysical system is especially important for future assessments where a number of reinforcing or competing effects combine. The importance of local context makes transferability and generalisation of case studies difficult.
• Maritime cliff and slopes (Low evidence, Low consensus): Given the site-specific nature of impacts, any predictions of general coastal response due to climate change have a low confidence. In the absence of a clear understanding of the coastal-change processes, and a reliable predictive tool, the default position is to assume that present-day coastal change will persist; however, it is very likely that currently eroding stretches of coast will experience increased erosion rates due to sea-level rise. 

NATURE-BASED SOLUTIONS

• Nature-based solutions (NBS) have become the focus of climate change adaptation and mitigation discussions in the UK. Within coastal ecosystems, NBS tend to centre around utilising natural elements and processes to enhance holistic flood and coastal erosion risk management for the benefit of local communities. 
• In the UK, the increasingly popular NBS for coastal, intertidal and shallow nearshore ecosystems include restoration and creation of coastal wetlands including saltmarsh habitats, dune management, beach nourishment, and restoration and creation of intertidal and shallow nearshore habitats such as seagrass beds. In addition to flood risk and coastal erosion alleviation, NBS promotes biodiversity protection and enhancement, increased carbon sequestration and storage potential, provision of fish nursery habitat, and improved nutrient and turbidity levels. 
• The Restoring Meadow, Marsh and Reef (ReMeMaRe) initiative has enabled a growing community of practitioners, researchers and government agencies to come together in a common cause to restore at least 15% of our coastal and estuarine habitats in England for nature and people by 2043. Understanding how to 'future proof' restoration efforts to secure naturally functioning ecosystems that are resilient to changing climate is fundamental for unlocking the natural capital benefits associated with coastal and marine habitat restoration. Conserving existing habitats through removal of pressures is likely to bring the largest benefits though for climate change mitigation and adaptation.

SEASCAPE APPROACH TO RESTORATION

• Restoration practice is currently dominated by single habitat approaches underpinned by single species monocultures, potentially limiting the range of benefits that restoration can provide. There is increasing evidence to suggest that for ecosystem restoration to meet its full potential in delivering resilience to climate driven environmental change, restoration practices should plan beyond single species and single habitats to a multi-habitat seascape, where positive interactions can help stabilise and even accelerate ecosystem recovery of connected habitats. 
• Addressing the key local manageable pressures common to multiple habitats, such as nutrient enrichment, coastal development, and recreational mooring, is critical for practical conversation, restoration and management actions and will improve ecosystem resilience to climate change. Restoration efforts should also consider locations where future habitats will exist.

THE ROLE OF GENETICS IN FUTURE PROOFING MARINE CONSERVATION AND RECOVERY

• Better understanding of species traits and thermal tolerances are needed to make more robust predictions of future impacts of climate-driven changes. This includes sustaining time-series that are tracking climate-driven changes in intertidal biodiversity. In the UK, genomic sequencing of species, including those associated with vulnerable coastal and intertidal habitats is undertaken by the Darwin Tree of Life Project. Ecological genomics studies will allow a greater understanding of the genetic mechanisms underpinning of species responses to climate change.
• The role of genetic diversity may be important in determining whether protected and/or restored habitats such as intertidal and subtidal seagrass beds will survive and thrive.