Mangrove wetlands, an example of an ecosystem at risk, exist in areas subject to periodic inundation by saline water, between 25°N and 25°S of the equator. Mangrove wetlands occur only within the limits of the tidal range, therefore they are confined to coastal areas. This ecosystem is characterised by anoxic soils and dominated by mangrove trees. Mangroves wetlands support extensive estuarine food webs, act as a nursery for fish and marine invertebrates, provide shoreline stability, and prevent erosion. Negative human impacts on the mangrove wetland ecosystem can be examined within a framework that focuses on the interaction between the four spheres of the biophysical environment.

Human modifications of the atmosphere in mangrove wetlands includes the changing wind patterns caused by the inappropriate location and design of buildings adjacent to the wetlands, and poorly designed walkways within them. The construction of boardwalks in mangrove wetlands can be detrimental if they are constructed in a linear fashion. This leads to wind tunnelling, which eventually results in mangrove dieback.

The characteristically high levels of humidity found in mangrove wetlands can be affected by any alterations to water flows. For example the straightening of Powell’s creek in the Bicentennial Park wetlands in the 1950s dramatically altered drainage patterns. The removal of mangrove wetlands, for example along the Parramatta River, has reduced the gases contributed by mangrove trees through transpiration.

Human impacts on the hydrosphere include altered salinity levels, increased magnitude of tidal inundation, and increased phosphate levels. The health of mangrove wetlands depends on the influx of both fresh water and saline water, however when dramatic changes in salinity levels or tidal inundation occur ecosystem functioning is affected. Large-scale changes in salinity may be due to runoff from areas of dryland salinity, imbalance of fresh water caused by flooding, or altered drainage patterns changing the ratio of salt water to fresh water. This often results from the construction of seawalls, bund walls, roads, and bridge embankments, or the emplacement of dams, reducing fresh water outflows.

Variations in salinity can kill mangrove trees, and have detrimental affects on other estuarine organisms such as air-breathing molluscs. If the magnitude or frequency of tidal inundation increases, as a result of to increased runoff following heavy rain, or altered drainage into the wetlands, the functioning of the ecosystem is put under increased stress. Pneumatophores cannot survive a long period of inundation, and this may lead to mangrove dieback.

Urban settlements within the catchment area of mangrove wetlands contribute excess amounts of phosphates, derived from detergents and fertilisers. Large amounts of fertilisers also enter the ecosystem via runoff from agricultural areas. Excess phosphate in the water is a cause of eutrophication, which may lead to algal proliferation, affecting the natural functioning of the ecosystem. Runoff from industrial sites may also introduce heavy metals and chemicals into the wetlands, affecting food webs.

Negative human impacts on the lithosphere include increased rates of sediment deposition and acidification of soils. Sediment deposition is a natural process in mangrove wetlands, however human impacts can increase the rate at which it occurs. If the rate of sediment deposition increases, for example due to land clearing in areas adjoining the wetlands, pneumatophores can become buried, leading to mangrove dieback.

Mangrove wetlands naturally contain acid sulfate soils. Acid sulfate soils only become a problem when exposed to air, as a result of excavation, drainage, or lowering of the water table as a result of other human activities. Once exposed to air, the iron sulfides in the soils react with oxygen to form sulfuric acid. After rain, the acid can be flushed into creeks where the water turns so acidic that fish may be killed. In addition, toxic metals such as aluminium and iron may be dissolved out of the soils by the acid, and leach into waterways, poisoning fish, crabs, and other estuarine organisms.

The introduction of feral animals into mangrove wetlands has impacted greatly on the biosphere. Carnivorous species are not commonly found in these wetlands, so the introduction of foxes and feral cats severely disrupts the food chains and nutrient cycles of the ecosystem by significantly affecting bird populations. Other human impacts on the biosphere include the accumulation of toxic chemicals, and oil spills.

In areas where industrial activity was common, such as the area surrounding Sydney’s Bicentennial Park, the accumulation of toxic chemicals poses a major threat to the health of the mangrove wetland ecosystem. These chemicals can have serious impacts on the bacteria and decomposers responsible for nitrogen fixing and nutrient recycling.

Because mangrove wetlands are found in coastal regions, oil spills from tanker ships are a constant threat. Oil entering the ecosystem from the ocean can smother mangrove pneumatophores, preventing the plant from obtaining oxygen. This leads to mangrove dieback. Oil spills also kill mangrove seedlings, and reduce leaf biomass in surviving trees. Other estuarine organisms such as mud crabs, fish, and worms can also be affected by oil spills. These organisms may be immediately killed, or be affected behaviourally.

Human impacts negatively affect all spheres of the biophysical environment of mangrove wetland ecosystems, making them an ecosystem at risk.