Nitrogen Deposition :: Coastal saltmarsh

Effects and implications

• Increased graminoid (grasses) biomass, with potentially adverse effects on forbs.

Overview: evidence, processes and main impacts

 There are very few studies of N deposition effects on these systems, but work undertaken in the Netherlands suggest salt marsh vegetation is N limited (Mitsch & Gosselink, 2000), which would make it vulnerable to eutrophication effects from atmospheric N deposition. However, the N addition experiments that have been undertaken have neither used very realistic N doses nor input methods i.e. they have relied on a single large application more representative of agricultural discharge. These studies have shown that the age of the marsh will influence the N response. This is because as marshes age i.e. during succession, N availability changes, increases as organic matter that has accumulated in the sediments is released through mineralization. They also demonstrate that N eutrophication will accelerate successional change and the speed at which some forbs decline.

These systems are typically inter-tidal, ie subject to continual, daily, periodic flooding with saline water. The degree and frequency of flooding and the salinity vary, decline from the coast moving inland up the estuary and similarly species richness increases. They are considered to be among the most productive natural ecosystems because of the continuous flushing with nutrient rich waters. Also vegetation breakdown is quite rapid.

In the absence of experimental studies of N deposition effects no quantified effects of potential modifiers are available. 

Overall N deposition is likely to be of low importance for these systems as the inputs are probably significantly below the large nutrient loadings from river and tidal inputs. Recent review by Boorman & Hazelden (2012) suggests that the pioneer low – mid saltmarsh areas are more resilient to N deposition than the mature upper areas.  Any effects of N deposition are likely to be found in the tall vegetation of the closed  upper marsh communities where interspecific competition is greatest.  These more mature areas may also be subject to direct run-off from the surrounding catchment.  Biogeochemical cycling of nutrients through microbial activity is quite rapid in this open system and N losses via denitrification may be considerable (Barnes &Owen 1998).

There may be some localized effects of ammonia from wintering wildfowl, especially large geese flocks. Since P availability affects N responses in this habitat and wildfowl provide an additional source of P and K this factor should be considered. Most likely impacts would be loss of N sensitive species and increases in tall grass and graminoid biomass.

Pollutant type and risk

Indicators of N enrichment

• None available

Example evidence of species specific responses

• None available

What factors modify N deposition impacts?

• Age of the marsh, older ones are probably more N sensitive where interspecific competition is greatest.