Coffs coast waterways are bearing the brunt of a nitrogen double-whammy from fertilisers and recycled sewage, but in dry periods, the waterways can protect downstream habitats by removing much of the nitrogen naturally, Southern Cross University has found.
The Southern Cross researchers performed water quality investigations in 11 coastal catchments experiencing rapid land-use change, from Corindi in the north to Pine Creek in the south.
The work was guided by community concerns over the impacts of growing intensive horticulture on water quality in the creeks draining to the Solitary Islands Marine Park. It follows earlier work in Hearnes Lake and reveals some good and not so good news.
"The creeks can be extremely efficient at filtering out the nitrogen pollution from the upper catchment," said Shane White, a PhD researcher at the University's National Marine Science Centre, and lead investigator for one of the reports.
"These waterways remove nearly all the nitrogen during dry conditions, but lose the ability during rain events when large amounts of nitrogen in creeks can escape to the coast."
High nitrogen levels entering downstream waterways can cause impacts to fragile ecosystems. It is common across the world for excess nutrients to trigger algae blooms and fish kills in enclosed waterways near intensive horticulture.
Hearnes Lake, between Coffs Harbour and Woolgoolga to the north, is the last naturally-occurring filtration defence before catchment run-off enters the iconic Solitary Islands Marine Park.
Collaborating with Monash University, the NMSC researchers used novel techniques to fingerprint the sources of nitrogen.
"About 50 per cent of the nitrogen is sourced from fertilisers, and the rest from treated sewage in the Hearnes Lake catchment. Management of both sources is necessary," Mr White said.
"Treated sewage is released to the farms to irrigate the crops and is relatively easier to manage but fertilisers are more widespread and complicated to manage."
Praktan Wadnekar, also a PhD researcher at the NMSC, was the lead author of one of the latest reports.
"Nearly all creeks with agriculture and urban land use had water quality issues. Hearnes Lake, Woolgooga Creek and Coffs Creek are the main areas of concern," said Mr Wadnerkar.
Based at Coffs Harbour, the NMSC team is working in partnership with a number of stakeholders to consider innovative solutions to improve creek water quality.
Professor Isaac Santos, part of the research team and co-author, said: "We are working with Coffs Harbour City Council and North Coast Local Land Services. Together, we are trialling bioreactors deployed just downstream of farms to filter out the nitrogen before it escapes to the creeks."
Woodchip bioreactors utilise the natural denitrification process performed by bacteria in soils.
"The bacteria use the carbon from the woodchips and the nitrogen in the water as food sources. If we can create the perfect environment for bacteria to thrive, with all the food they need, they can remove some of the nitrogen for us," said Mr Wadnerkar.
Professor Santos said he remained optimistic.
"While there is an obvious water quality problem in our creeks, the solutions are achievable. There seems to be the political will and community support for measures that will retain nitrogen and phosphorus on the farms, preventing downstream issues," said Professor Santos.
The Southern Cross researchers are also working in conjunction with the NSW government's Clean Coastal Catchments initiative.
"Collaborating with NSW Department of Primary Industries as a part of the Clean Coastal Catchments project, we are investigating on-farm practices and fertiliser loss-pathways in blueberry and macadamia farms," said Mr White.
"We are working with farmers, industry and government to improve on-farm practices and provide a framework to protect the waterways from harm."
Key recommendations include:
- Plant native vegetation on both sides of creeks to reduce the amount of nitrogen entering the creek.
- Install tailwater catchment ponds on farms to catch the high water flows and slowly release this water to the creek.
- Reassess the use and suitability of nitrogen-rich recycled sewage on farms in coastal catchments.
The research project was funded by the Coffs Harbour City Council's Environmental Levy Grants program.