"Good practice is widely used already but getting all landowners on board will make even more difference," Buchanan says. "Getting the basics right across all farmers and land use types will take us a long way towards where we need to go."
The report said the most pronounced recent change to pressures facing the gulf was Auckland's population (it increased by 18 per cent between 2006 and 2016). But, it found, there was also "uncertainty about the ability of the sea to assimilate current nitrogen loads".
Much of the nitrogen load delivered to the Gulf (via the Firth) is converted to atmospheric nitrogen by the natural process of denitrification. Nutrients are also 'exported' from the system via the annual harvesting of around 30,000 tonnes of shellfish from mussel and oyster aquaculture operations.
Nitrogen comes from a range of sources, including human wastewater overflows, storm water and from all types of farming, horticulture or cattle farming. The Firth of Thames is adjacent to dairy farming areas and farmers are actively involved in reducing nitrogen run-off from their land.
The report said pollution of waterways was one of the "unmet costs" of rural and urban development. Climate change was also a factor as warming seas, rising sea levels and storms were increasing flooding, erosion and sediment loads in the ocean.
Buchanan, who runs an 800-head herd on his property, says work on the plan (known as the Sea Change process) clearly showed the biggest issue facing water quality in the gulf is sediment. Its impact dates back to land clearance and gold mining in the late 1800s to early 1900s.
According to Craig Depree, Principal Water Quality scientist at DairyNZ, historic surveys indicated that around 44 million cubic metres of sediment was deposited within the lower Waihou River and in the Southern Firth of Thames in the 40 years prior to 1918. This massive amount of sediment equates to around 300 years of suspended sediment loads delivered by the Waihou and Piako Rivers under current land use.
Despite the massive historic impacts of sedimentation, Depree, citing a Waikato Regional Council report, says current land use (from draining swamps and flood protection stop banks) has resulted in rates of sediment deposition in the southern Firth of Thames that are higher than many other North Island estuaries.
Buchanan says the plan also identified a need to ensure nutrient run-off - some of which comes from dairy farms and can cause reductions in dissolved oxygen concentrations in sea water - does not increase further.
"The waters of the gulf are one of our most productive marine environments because it's naturally a high nutrient environment but, like anything, too much of a good thing can go too far," he says.
"Sediment run-off is clearly the major water quality issue in the gulf. However, there are still enough question marks around nutrient levels to stop further increases and put a real focus on science to understand the situation."
Buchanan estimates there are just under 2000 dairy farms in the catchment and believes there are a number of ways farmers can reduce the amount of nitrogen getting into the gulf. The farms bring approximately $2 billion worth of milk solids to the region every year and farmers take their environmental responsibility seriously.
They include fencing off waterways, ensuring bankside planting so stream banks and waterways are managed to minimise erosion during times of high water flow or flooding, implementing appropriate drainage design and cleaning practices and planting trees and shrubs alongside drains to reduce run-off. Wetlands will play an increasing role in the future, he adds.
"A lot of these are measures already being followed by farmers because they are good farming practice," he says. "Sediment run-off is actually the loss of good soil and no farmer wants that.
"We've planted plenty of trees on our farm. The shade and shelter for the cows is great, the increase in biodiversity is a big plus and the aesthetic and land value they add is a bonus.
"I think I'm no different from most farmers – we want to make sure the land we are looking after is as good, or better, as when we started."
The Sea Change group included representatives of a range of interested parties including recreational and commercial fishing, environmental, aquaculture, iwi, infrastructure and community groups. Buchanan was involved as a voice not just for the dairy sector, but for agricultural land users in general.
Although not a statutory or legally binding document, the plan provides recommendations for each of the stakeholders and partner agencies who manage the gulf and its resources.
Buchanan says one of the benefits of the group was being able to get interested parties around one table: "I found the collaborative approach valuable and I was impressed with people from other organisations who were all genuinely trying to achieve a good outcome.
