[For Acid Deposition processes see overview link]
Effects and implications
- Effects are likely to be small as these habitats are inter-tidal and experience large influxes of nutrients.
Overview: evidence, processes and main impacts
Salt marshes are coastal and tidal (repeatedly flushed with saline, brackish water), occurring in the upper coastal intertidal zone between land and open salt water. They are dominated by dense stands of salt-tolerant plants (halophytes) such as herbs, grasses, or low shrubs. These plants stabilise the salt marsh by trapping and binding sediments, providing coastal protection. They play an important role in the aquatic food web and the delivery of nutrients to coastal waters.
These systems are typically inter-tidal, i.e. subject to continual, daily, periodic flooding with saline water. The degree and frequency of flooding and the salinity vary, declining from the coast inland up the estuary; similarly species richness increases. Because of the large influx of nutrients, but also the frequent flushing, it was believed these systems had an inexhaustible capacity to tolerate nutrient enrichment and likely pH changes. However, recent work in the US (Deegan et al., 2012) has shown that continuous enrichment is damaging these systems; with the main species producing fewer roots, restricting their ability to bind the sediments leading to a breakdown of the marsh structure. (see N deposition and Coastal Saltmarsh)
The likely contribution of acidification to this breakdown is not understood but the risks from acid deposition compared with eutrophication are probably small, based on available evidence.
Interventions that lead to the stopping of tidal flooding will cause a drop in the water table triggering pyrite oxidization processes, leaching of cations and salts and soil acidification. In many cases, these transformations have given rise to environmental problems such as the development of acid sulphate soils, a resulting decrease in pH, and the leaching of carbonates (Fernández et al. 2010)
Pollutant type and risk
|
Type of acid deposition |
Pollutant |
Risk areas |
|
Dry deposition Gaseous |
SO2 |
Significant reductions in sulphur emissions have successfully addressed by International control measures. Areas where exceedances could still occur are around industrial zones and port areas (due to shipping emissions). |
|
Dry deposition Gaseous |
NOx |
Salt marshes close to power stations and road networks |
|
Wet deposition precipitation and occult (cloud, mist) |
H+, NO3- SO42- |
Coastal salt marsh that may be subject to high concentration episodes. |
Indicators
None available
Evidence of species specific responses
None available
| Critical Load/ Level |
|---|
|
No estimate available |