Nutrients: introduction

Plants need certain nutrients to grow, particularly nitrogen and phosphorus. High concentrations of these nutrients in water bodies can cause excessive aquatic plant growth rates, and some nutrients are toxic to aquatic organisms. High plant growth rates can lead to algal blooms and nuisance weeds, which can alter river condition by changing the acidity or oxygen levels.

Such algal blooms can also reduce the recreational and aesthetic values of water bodies and block water intakes and pumping systems.

For the national nutrient picture, nitrate-nitrogen and phosphorus are considered in terms of their current state and recent trends.

Summary of national nutrient picture

There is a lot of variability in the national nutrient data collected, which often corresponds to land use in a river’s catchment. Rivers drawing water from catchments that have predominately indigenous land-cover have generally very high quality water. In comparison, rivers in lowlands surrounded by pasture and cities are often of poorer water quality.

Trends show that in some areas and for some nutrients, river water quality is decreasing, with pressure associated with land cover likely to be the driver for these changes.

International comparison

International comparisons of river water quality are difficult for a number of reasons – differences in site selection, differences in methodology, and differences in approaches to dealing with high concentrations. However, based on the most recent OECD data from 2002 until 2004 (available here as an Excel spreadsheet), the median concentrations for nitrate-nitrogen and total phosphorus in the three New Zealand rivers included in the study (Clutha, Waitaki and Waikato Rivers) compare well to other OECD countries (top third for total phosphorus and top 5% for nitrate-nitrogen). However, data from only three rivers provides a poor representation of water quality in New Zealand as a whole.

River Condition approach

In this section we discuss what each nutrient can tell us about stream health, followed by a discussion about the state and trends of our rivers as shown in the data. Please select a nutrient to view information on its current state and trend: