Plant communities of coastal waters – such as mangrove forests or seagrass beds – contribute significantly to the water quality – they keep it clear and healthy. Mangroves function as natural filtration system for nutrients, pollutants and sediments from the hinterland Satheeshkumar & Khan, 2011; World Ocean Review, 2017).
Worldwide, the intensification of agriculture and aquaculture – with the aim of generating ever higher yields – results in the use of more and more nutrients and chemicals which are entering surrounding waters.
An oversupply of nutrients in water bodies leads to over-fertilization (eutrophication). This stimulates the growth of algae, which deteriorate the water quality. Strong algal blooms damage e.g. adjacent seagrass meadows by overgrowing them and taking the light they need. Mangroves provide help as long as they are healthy. Due to the often nutrient-poor sediment and their dense root system, they take up nutrients from the water very efficiently (Hogarth, 2015).
Mangroves also trap the fine-grained riverine sediment along the mangrove roots. This prevents an entry into ocean, where it can damage e.g. adjacent coral reefs by covering them. Pollutants / heavy metals in the water bind to the surface of fine sediment grains (adsorption), due to sedimentation they are removed from the water column. Mangrove sediments are a sink for pollutants.
Thus, mangroves are natural wastewater treatment plants and improve the water quality of coastal and estuarine waters.