Contribution of seagrass productivity to waste treatment in a highly oligotrophic urbanised coast

[Display omitted] •Seagrass treat waste through assimilating nitrogen, often discharged by land-based sources.•Using a replacement cost model, treatment can be valued demonstrating a previously unaccounted benefit to society.•Seagrass waste treatment has been valued at $729 – $1784 per hectare per year.•When seagrass is lost, the waste treatment value is also lost.•Realization of this value can only come with sustainable use, promoting seagrass conservation. Seagrass inhabit a large proportion of the world’s coastal waters, but these waters are used as a sink for vast loads of nitrogen pollution from many sources. Seagrass assimilate nitrogen, an essential nutrient, for growth, some is stored in sinks, while a proportion is transformed and lost from the system. When anthropogenic nitrogen is discharged into coastal waters, seagrass (and other vegetated coastal ecosystems) will assimilate nitrogen and treat wastes providing a critical ecosystem service. However, nutrient over enrichment will result in degradation and eventually loss of seagrass habitats and loss of services. The monetary value of the waste treatment ecosystem service provided by Posidonia seagrass has been valued using Adelaide, South Australia as a case study site. Posidonia leaf nitrogen content was used to estimate the nitrogen assimilation capacity across the Adelaide coastal waters, a highly oligotrophic system. By establishing a Posidonia baseline measured nutrient requirement for the region, coupled with estimates of nitrogen retention and export, the study demonstrates that nitrogen assimilation above the baseline, can be considered waste treatment ecosystem service. This waste treatment has been valued with a replacement cost model using monetary values determined from nitrogen reduction technology from wastewater treatment plants, stormwater and industry. The Posidonia seagrass along Adelaide’s coast has the potential waste treatment value of $729 – 1784 per hectare per year for Posidonia seagrass depending on three different treatment scenarios tested. Importantly, this work shows that when seagrass area or density declines, the waste treatment value also declines. This method of calculating waste treatment service for seagrass can be applied for seagrass meadows throughout the world, but consideration of waste treatment needs to be closely coupled to a long term and comprehensive monitoring program to ensure that any discharges are sustainable for the seagrass meadows and do not