The Firth of Thames Water Quality and Ecosystem Health report written for DairyNZ and Waikato Regional Council by NIWA shows dissolved inorganic nitrogen (DIN) levels have increased by 5.1% a year, from 1998-2013.
In a follow up DairyNZ report using Waikato Regional Council and NIWA data to analyse the state and 10-year trend of the six largest rivers leading into the Firth of Thames, the rivers are not carrying any more nutrients into the Firth than they did back in 2004.
The Hauraki Rivers report shows nitrogen loads from the rivers have either remained stable or have slightly decreased since 2004, depending on the trend analysis used. Phosphorus loads reduced by 3.5% a year with both techniques.
This evidence doesn’t seem to suggest the 5% increase in DIN in the Firth of Thames is coming from the rivers or is driven by land use, DairyNZ water quality scientist Tom Stephens said.
There were two sources carrying nutrients into the Firth of Thames, the rivers and the coastal upwelling from the continental shelf. Deep ocean water was very rich in nitrogen and in strong El Nino years, that water could be pushed up into the Firth of Thames.
This or changes in the supply from other catchments, even as far away as Auckland, could be a reason behind the rise in DIN levels out to sea, he said.
The stable and decreased levels of nutrients into the rivers in the past 10 years was in response to better effluent management, livestock exclusion, nutrient budgets and riparian planting by farmers as well as urban users like meat processors and township wastewater plants, Stephens said.
“It didn’t surprise me when I looked at the water quality trends, because the improvements are probably good practice effects.”
Unlike other newer dairying areas in New Zealand, the Hauraki catchment was historically dairy and pastoral-focused, for more than 100 years. In the last 10 years there had been little change to dairy farm, herd or milksolids production numbers across the big three districts of Thames-Coromandel, Hauraki and Matamata-Piako.
But the way they farm appears to have improved, he said.
To get a 3.5% decrease in the tonnes of phosphorus carried down to the Firth year-on-year for 10 years was a great result.
“Combined, that’s a reduction of nearly a third overall between 2004 and 2013.”
While nitrogen and phosphorus loads in the major rivers, Waihou, Piako, Waitoa, Ohinemuri, Kauaeranga and Hikutaia, looked to be steady or improving, other water quality issues remained.
There were issues with oxygen levels in the western rivers, which could affect fish and some insects. Water clarity was also an issue, with the Piako and Waitoa having two to three-times murkier water, although some of this was natural and because of the tidal effects of the Firth on those rivers.
There was more work needed in the rivers themselves to see if nutrients were having adverse effects regardless of their moderate to slight improvement in the tonnes being carried overall. Little was known about the actual amount of algae in the rivers as well.
That meant dairy farmers could still need to make changes, he said.
“It might feel completely removed from you – the Firth of Thames if you’re a farmer in Putaruru – but it isn’t. Every land user, whether urban or rural, is impacting on it courtesy of its connection to the Hauraki rivers.”
The Hauraki Rivers report was written by DairyNZ to provide science to support the limit-setting process for the Healthy Rivers plan change, which will set limits for farmers in the various catchments around the Waikato region. The draft plan is due later this year.
The earlier Firth of Thames Water Quality and Ecosystem Health report was written for DairyNZ and Waikato Regional Council to provide science for Sea Change – Tai Timu Tai Pari – the planning process being finalised for water quality in the Hauraki Gulf, which could ultimately influence the freshwater limits in the Healthy Rivers plan change.
The Sea Change final plan was originally due by September 2015.
A big driver in that limit-setting would come from community and council expectations for water quality and what they determined was acceptable, Stephens said.
The Firth of Thames report used five indicators to study the ecosystem health – sediment, nutrients, oxygen, acidity and algae, with sediment standing out as the largest, pervasive and historic issue.
Understanding the triggers in this body of water was a lot more complex than rivers, Stephens said.
Even though the Firth of Thames is a coastal body, water sits there for about 30 days and in that time nutrients can be recycled between three or four times before being lost to the deeper sediments, the open coast or the atmosphere.
That means any input of nutrients into that system, its effect is multiplied.
“At this early stage, we think it’s going to be more sensitive to nutrients than the Hauraki rivers themselves because of that reason – nutrients accumulate within it,” Stephens said.
There is still a lot unknown about what is causing changes to the water quality in the Firth of Thames. The next step was an understanding of how those processes interlink, he said.
DairyNZ had invested in the reports because there was a huge proportion of dairying in those catchments, with nearly half the 2500ha of land in dairy.
“Undoubtedly we will have a role to play in problem-fixing, it’s which problems and what part do we play and what we can do that counts.
“That’s the only interest we have and making sure the grounding of the answers for those questions is accurate, which comes back to doing decent science to start with and putting it into a decent model.
“If either of those is crap, your plan is going to be pretty rubbish.”
Limit-setting model – water quality vs economics
DairyNZ is investing in a cutting-edge, world-class catchment model to predict water quality of nutrient limits to be set by the Waikato Regional Council on the Hauraki rivers.
The model will be able to run different land practice and use scenarios to predict water quality outcomes, but will have financial information used to inform those scenarios and ensure they are viable for dairy farms.
The model is being developed by DairyNZ and Deltares, a leading international research group based in the Netherlands. It is expected to be available for limit-setting collaborative groups to use as part of the Healthy Rivers plan change, by May this year.
“It is world leading – there is no model in NZ now that has the power of this thing,” DairyNZ water quality scientist Tom Stephens said.
“Because most models to date have assumed all dairy farms leach the same amount and can only reduce the same amount, patently not true.”
DairyNZ’s previous economic modelling focused on 12 farms spread across the different soil types, climate and systems present in the Hauraki rivers, to look at the nitrogen and phosphorus losses of each.
The new model would be able to use this information to focus on nutrient losses from individual farm blocks and the economic flow-on effect to the whole farm.
“We can do individual budgets on farms and then scale those up to each catchment reflecting the mix of different farms across catchments.”
Understanding the state and trends of water quality in those catchments would be critical to the model.
“This is essential before we can make forecasts about how we could farm based on the DairyNZ economic modelling with AgResearch.
“Where NZ is going with limit-setting, it needs to set those limits on both water quality effect and economic cost.”
The plan was to use the model in other catchments.
“That is why DairyNZ has invested, because if you are going to project forward and guess what the world is going to look like you may as well do it with the aid of a good computer.”
To read more about Sea Change – Tai Timu Tai Pari process or to view the Hauraki Report visit DairyNZ.
To view the Firth of Thames Water Quality and Ecosystem Health Report visit Waikato Regional Council.
