Stream water quality

 
 
Image/logo for showing indicator 'Steady'
 
Macroinvertebrate Community Index (MCI) score
 
 

Purpose of indicator

A macroinvertebrate is described as an animal large enough to be seen with the naked eye and having no backbone. An example might be a koura, snail or insects like a May Fly. The number and types of macroinvertebrates found in a stream is telling of water quality. While some species are sensitive to changes to their habitat or pollutants in water, others are hardy and can withstand more extreme changes. Macroinvertebrates are an important part of the food chain and are a food supply mainly for fish and birds.

Macroinvertebrate Community Index (MCI) is a score given to what is found at a site based on the type of macroinvertebrates and the number found. From this the extent of pollution in a stream can be determined. Pollution sensitive macroinvertebrates are given higher scores than those that are more pollution tolerant. This means the higher the MCI score, the less polluted the stream. Land use in the catchment of a stream and extent of riparian vegetation are important factors affecting the health and quality of water.

Current information and trend

In general sites with MCI scores greater than 100 are considered to be in good condition and sites with scores above 120 are deemed pristine. Sites with scores of less than 100 are considered polluted.

In Rotorua, streams monitored include those running through horticultural, forestry, farm and urban environments as well as a reference site. Reference sites are streams and catchments that have little or no modification. The reference site, Waiiti Stream, is included in figure 1.

Pastoral, urban and forestry sites all show pronounced changes in scores when compared to the more gradual changes seen in the reference site. This suggests that high value native forest riparian environments buffer streams from abrupt change.

Compared with the reference (Waiiti) stream site, the pastoral streams’ MCI seldom reaches scores exceeding 100, with the exception of Ngongotaha Stream (Paradise Valley Road. Streams in plantation forestry can have MCI scores comparable to native forest sites. However when forests are harvested, light, sediment and nutrient levels can abruptly affect the stream habitat and fauna including macroinvertebrates. Retaining permanent, intact riparian buffers protects streams from events like this, and climatic events such as storms and flooding.

Figure 1 shows that the trend for all sites has decreased in MCI scores between 2006/07 to 2008/09, except for Ngongotaha Stream. However, this trend is also apparent in the reference site suggesting that land use is not the cause. The decreasing score may be climate or weather related. Both Paradise Valley Road and Hamurana Road sites of the Ngongotaha Stream show a state of recovery likely due to planting of riparian vegetation in 2006.

Both sites of the Ngongotaha Stream and the Pongakawa Tributary show increasing MCI scores from 2004/05. The Okaro tributary shows the same trend from 2005/06. The Puarenga Stream shows marked variability in yearly scores but the overall trend from 2001/02 shows it is steady.

 
Line graph showing the stream water quality
 
Figure 1
Source: Bay of Plenty Regional Council, 2011
 

In Summary 

  • Ngongotaha Stream (Hamurana Road and Paradise Valley sites) is in a period of recovery following community riparian planting in 2006.
  • All sites irrespective of surrounding land use show a decline in MCI scores at 2005/06, including the reference site, suggesting land use is not the driver of the decline in MCI.
  • All sites, except the reference site show marked variations each year, suggesting vegetated riparian zone and surrounding land use protects the stream from abrupt changes.
Page reviewed: 14 Jan 2016 4:02pm