Wetlands

Importance of wetlands to Adelaide’s coastal water quality

Introduction

Wetlands have long played an important role in the Adelaide area. Prior to European settlement much of the Adelaide area was covered by wetlands, swamps and marshes. These wetlands, swamps and marshes trapped and filtered runoff from the Adelaide plains and surrounding hills, before draining to Gulf St Vincent through the Patawalonga Creek and Port River system.

Since European settlement almost all of these wetlands, swamps and marshes have been lost due to drainage, grazing, sand mining and peat extraction. Runoff from the Adelaide plains and surrounding hills is now transported to the coast through a highly modified environment which is less efficient in trapping and filtering stormwater. As Adelaide developed, activities associated with urbanisation have been found to contribute various pollutants to stormwater as it flows to the coast which impact on Adelaide’s coastal water quality.

The reduced quality of Adelaide’s stormwater highlights the importance of wetlands. In recent years there have been a number of constructed wetlands established throughout the Adelaide metropolitan area. While these wetlands provide recreational benefits for the community and habitat for a number of species, their primary function is to improve stormwater quality. These wetlands remove pollutants from stormwater such as nutrients and suspended solids. Such pollutants from stormwater, wastewater treatment plants and industry were found by the Adelaide Coastal Waters Study to be the main causes of seagrass loss along Adelaide’s metropolitan coastline.

Nutrients in wetlands

Nutrients sourced from fertilisers, animal faeces, detergents and organic matter are transported into wetlands by stormwater. The wetland slows down the flow of stormwater allowing plants and microorganisms growing in and around the wetland to absorb these nutrients.

High volumes of nutrients in Adelaide’s coastal waters can cause the rapid growth of epiphytes, plants which grow on other plants, in a process know as eutrophication. These epiphytes grow upon, or attach themselves to, the seagrass leaves, causing them to become weak, brittle and break off. Rapid growth of epiphytes can also impact seagrass by reducing oxygen levels in the water and preventing adequate light from reaching the seagrasses.

Suspended solids in wetlands

As stormwater runoff moves through Adelaide’s catchments it collects soil, or sediment, from the ground in a process known as erosion. Once mobilised in the stormwater these sediments become suspended solids. The wetland slows down the flow of stormwater allowing these suspended solids to sink to the bottom of the water column in a process known as sedimentation.

High volumes of suspended solids in Adelaide’s coastal waters prevent adequate light from reaching the seagrass and can quite simply smother them.

Wetlands and coastal water quality improvement

Wetlands will play a vital role in improving coastal water quality in the future by improving stormwater quality.  These improvements will be achieved using existing wetlands as well as those planned for construction in the future. These wetlands will act as natural water treatment areas to remove pollutants from stormwater and consequently help to halt seagrass loss and provide the conditions suitable for seagrass recovery.