McDowell, who heads a National Science Challenge looking at water quality, concluded the most likely reason for the improvement is the strategy to mitigate phosphorus loss from land with guidelines directing where to best use it and policy for its management.
“These findings support the development and implementation of mitigations, supported by voluntary guidelines and regulation.”
Other possible reasons for the improvement are changing land-use decreasing erosion, more nitrogen fertiliser use assimilating soil phosphorus and a greater awareness of it as an environmental issue.
The work found little evidence the improvement was caused by a decrease in soil Olsen P concentrations or imported phosphorus such as fertiliser, a change to low water-soluble phosphorus fertilisers or greater nitrate loads assimilating phosphorus from groundwater or sediments.
Analysis of data from 1994-2013 showed that for 145 monitoring sites in catchments dominated by intensively grazed pasture, median filterable reactive phosphorus (that immediately available to algae) concentrations decreased at 46% of locations, increased at 21% and had no change elsewhere.
However, when examined at 277 sites between 2004 and 2013 median FRP concentrations fell at 57% of locations and increased at 15% while 29% were unchanged.
For the 159 sites dominated by intensively grazed pasture monitored for total phosphorus (slowly available to algae in streams but fully available in lakes and reservoirs because it has time to dissolve) between 1994-2013, 41% improved and 21% were worse while for 304 sites monitored between 2004-2013, 65% improved.
The increase in sites exhibiting an improvement in water quality occurred despite a 26% increase in national dairy cow numbers and the continued expansion of dairying into new areas commonly used for sheep farming. Sheep numbers decreased by 22% over the same period, Statistics New Zealand figures for 2018 show.
Despite the challenges an ever-increasing amount of data was available.
“We are now at a stage where we can provide some guidance on the strength of factors such as land use practices put forward in the scientific and public literature as likely causes of phosphorus trends in stream flow.”
Such factors include but are not limited to land use change, a decrease in imported phosphorus as fertilisers and feed, a change in phosphorus fertiliser form, a decrease in soil Olsen P concentration, greater assimilation of phosphorus in soils, greater assimilation of phosphorus in groundwater and in-stream, an increase in the use of mitigations, better awareness and education of rural professionals and the use of policy instruments.
Previous study of water quality data showed most sites examined by regional authorities and science providers had phosphorus concentrations likely to limit periphyton growth.
Chlorophyll-a is a measure of periphyton growth, which is in turn driven by the concentration of the limiting or co-limiting nutrient, nitrogen or phosphorus.
Noting previous work by others, McDowell’s team concluded that while reasons for increases or decreases in phosphorus concentrations can be implied, the strength of any reasoning is dependent on factors such as flow-paths, land use practices and the quality and amount of information. For example, at a small or sub-catchment scale, stream flow might be dominated by surface flows that move quickly from the land into the stream.
However, at a large catchment or regional scale, more stream flow is sourced from deeper and potentially older water. That means that while regular sampling for phosphorus at a small scale might help pinpoint the effect of certain land use practices on stream flow phosphorus concentrations, at a larger scale the ability to ascribe phosphorus concentration trends to land use practices is compromised by the ability to monitor those practices at enough sites and water that is sourced from a range of flow-paths and ages.