severn estuary

Estuarine Environments - Severn Estuary:

Overview:

● The Severn Estuary is the largest coastal estuary in the UK and one of the largest in Europe.

● The Severn has a macro-tidal range, 13m in some places.

○ The main reason for these very high tides is the variation in funnel shape of the estuary from wide to narrow, which forces the incoming tide higher as it travels further inland.

● This has helped create a unique estuarine environment consisting of a variety of habitats.

Estuarine habitats of the Severn Estuary:

● Mudflats are deposits of mud, silt and clay found in the sheltered intertidal areas of the estuary.

○ Formed by fine silt deposited by the tide and alluvium deposited by the River Severn.

○ Soft muds in the most sheltered inner areas of the estuary.

○ Firm sands in areas exposed to strong waves.

○ Submerged at high tide and fully exposed at low tide.

○ Provide area of immense nutrition for permanent roosting and wading birds.

○ Large invertebrate population provides food supply for migratory birds.

● Saltmarshes occurred where vegetation colonised the mudflats that were protected from the tide.

○ Saltmarshes contribute 4% of the UK’s total wildlife habitat.

○ They offer refuge above tidal level for many species.

○ There are many freshwater streams which intersect the marsh area.

● Rocky intertidal habitats are found along the shores of the estuary.

○ Composed of rocks, boulders, shingle and mussel scars.

○ Extreme habitat due to exposure to the tidal range, wave action and strong wind.

○ Large number of species can survive here, including winkles, limpets and algae.

■ Also provides a feeding ground for many wading and migratory birds, e.g. Dunlin.

● Subtidal sandbanks are permanently submerged sand deposits at a depth of less than 20m.

○ Invertebrate communities exist on the sandbank.

■ It is an important food source for many fish species.

● Biogenic reef systems are also found in both intertidal and subtidal zones of the estuary.

○ No other estuaries in south west UK have reef systems.

○ Intertidal reefs in the Severn Estuary include those at:

■ Redcliff Bay near Portishead

■ South of Severn Beach near Avonmouth

■ Goldcliff near Newport

○ Subtidal reefs occur in large groups throughout the mouth of the estuary.

Threats to the habitats of the Severn Estuary:

● Climate change will likely result in increased storm activity, meaning more storm surges, therefore more erosion.

○ It will also lead to sea level rise, with some habitats facing complete submergence and extinction.

○ Reduced salinity levels of the sea may also occur due to freshwater ice melt.

○ However, the reef system may benefit from increased temperatures due to climate change.

● Human intervention to prevent coastal erosion is having a detrimental effect on these habitats.

○ Sea walls and other hard engineering structures can lead to the accumulation of sediments in the estuary, damaging the ecosystems found there.