The sensitivity of estuaries

Hydrology

Saltmarshes

Sand dunes

Estuaries can be defined as the downstream part of a river valley, subject to the tide and extending from the limit of brackish waters. There is a gradient of salinity from fresh water in the river to increasingly marine conditions towards the open sea. Inputs of sediment from the river, shelter from wave action and, often, low current flows lead to the presence of extensive sediment flats. In short, estuaries are complex ecosystems interlinking the terrestrial and aquatic environments and are composed of an interdependent mosaic of subtidal, intertidal and surrounding terrestrial habitats.

Of all the Annex I features in the mSAC programme, estuaries are perhaps the most diverse and dynamic. For analytical purposes, they are divided into core areas, covering the intertidal and subtidal areas, and surrounding areas of associated terrestrial maritime and sub-maritime habitats, such as sand dunes and grazing marshes, linked to the estuary by their physical, chemical or biological processes. Estuaries support a mosaic of habitat types, some of which are also independently designated features under the Habitats and Birds Directives:

• intertidal mud and sand flats
• reefs
• subtidal sandbanks

Other important non-marine features which fall within the estuarine designation are:

• birds
• saltmarshes
• sand dunes

Saltmarshes and sand dunes are briefly reviewed below in terms of their importance to the overall estuarine system and their possible sensitivities.

Estuarine Hydrology

The two most important variables within the estuarine system, in terms of hydrology, are the amount of mixing which occurs between fresh water and salt water and the rate at which this mixing occurs. Levels of salinity within the system can have a significant influence on vegetated habitats and the animal and bird communities which they may support. There are six major factors which influence this mixing:

• river inflow
• precipitation (rain)
• evaporation
• tidal variations
• wind strength
• estuarine topography (shape and level of the ground surface)

Recreation alone is unlikely to be a major influence on any of these factors, although large-scale land claim for recreational infrastructure development may adversely affect tidal processes. There are other equally important variables within estuarine systems, such as sedimentation arising from port and harbour operations, which should be considered as part of the management process. These issues are discussed in more detail in a further section of this report and in a related report of the UK Marine SACs project, which details the effects of port and harbour activities (ABP Research,1999).

The tables below summarise the potential threats to estuaries from land and waterborne processes.

Water-based processes

Land based processes

Saltmarshes

Saltmarshes often occur in estuaries adjacent to mudflats. Silt builds up at the top of the tidal range, and it is here that plant colonisation takes place. Salt marshes, with few exceptions, tend to support a moderately low diversity of vegetation (e.g. Spartina grasses). However, there are many benthic organisms living both on the surface and within the soils of the marsh. These macrofauna (larger than 1mm, e.g. polychaete worms), meiofauna (animals between 0.1mm and 1mm, e.g. small molluscs), and microfauna (organisms smaller than 0.1mm, e.g. protozoans and bacteria) are important sources of food for many shorebirds, seabirds and wildfowl.

Long and Mason (1983) provide detailed studies of the variety of European marshes which may be useful in determining the site level characteristics of this habitat. As with the overall estuarine system, saltmarsh processes are influenced by a number of external factors, including:

• exposure
• salinity
• temperature
• tidal range

Tidal range is perhaps the most prominent influence as all saltmarshes are within the intertidal zone and are, therefore, exposed to tidal immersion. The once daily (diurnal), or more commonly, twice daily (semi-diurnal) flooding of the saltmarsh and the associated impact of tidal waters, are likely to be the most important influence on its development. Ranwell (1972), for example, suggested that many species of Spartina are able to tolerate over three hours of submergence per tide. Marshes can also withstand severe buffeting, and can act as a buffer through the stilling of waves. It should be noted, however, that not all saltmarshes are inundated at least once a day. High saltmarsh may only be inundated a few times a year at the very highest tides.

The tables below summarise the potential threats to saltmarshes from land and waterborne processes.

Water based processes

Land based processes

Sand Dunes

Sand dunes are an essential buffer which provide protection for terrestrial areas from the effects of waves and wind. They also replenish near shore areas which may loose sediment following storms. The constant exchange between beach and dune is an important natural process and helps to maintain the stability of the system and the ecological diversity within the site. Dunes are also important systems as, in addition to absorbing wave stresses, they also shelter inland communities and help to protect the ‘fresh’ water-table from saline intrusion.

Dune based vegetation is generally considered to be fragile or vulnerable due to the nutritionally poor soils of dune systems, the hazards of being engulfed by sand and the possibility of drought. The dunes themselves are also extremely sensitive to disturbance.

Sand dunes have suffered significant damage from human activities. Many dune systems have been irreversibly changed by development, coastal defence work and by leisure activities. Indirect human impacts, such as the increasing rabbit population, have also had a profound effect on the habitat. However, it is only in the last 30 years that a scientific information base concerning these systems has begun to develop. For example, Carter (1995) suggests that dune systems may be more robust than originally thought, having shown the capacity to recover from repeated interference over the years.

The tables below summarise the potential threats to sand dunes from land and waterborne processes.

Water based processes

Land based processes

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