Water is a huge part of the New Zealand lifestyle. We drink it freely straight from the tap, we eagerly swim and boat in the rivers and lakes, and we benefit as a country from our tourism industry which capitalises on environmental sustainability. We seem to take for granted how lucky we are in New Zealand to have access to this seemingly endless resource.

Recently in New Zealand, and especially in Canterbury due to the recent rise in dairy conversions, the subject of our fresh water resource is a hot topic. Questions develop due to increasing evidence of environmental degradation. Why is the quality of our water declining? Will there be enough water to support our growth in 20 years? Who even has rights to the water? What can we do to improve our current water quality situation? These questions, and more, are being tackled by groups of scientists and policy experts with the common aim of developing an effective method of water resource management.

Looking into conservation, one area that has been receiving considerable interest is riparian restoration. The riparian zone is usually the vegetated strip of land along the banks of rivers, streams, lakes, and wetlands. The aim of riparian restoration is to provide a shield for aquatic systems from surrounding land uses. Theoretically, this technique of planting vegetation seems to be infallible – increasing vegetation results in a ‘nature helping nature’ scenario. In reality, this may not be the best solution, which is why it’s important to investigate its effectiveness.

Riparian buffer zone surrounding a river flowing through farmland, Iowa.

Image source: Merrill College of Journalism on Flickr (creative commons)

Kathryn Collins and colleagues, in a 2013 study, conducted a research project investigating the effectiveness of riparian restoration on water quality in lowland streams around Lake Ellesmere, Canterbury. They used four vegetated sites, and four corresponding non-vegetated control sites. Water quality and macro invertebrate community measurements were made over a period of 5 months. Just how much was the riparian restoration helping the water quality?

The results showed some positive water quality attributes at the riparian restoration buffered sites, with an increase in dissolved oxygen, which supports plant and animal life, and a decrease in turbidity, or how much sediment was in the water. These attributes were opposed by an increase in conductivity, likely caused by an increase in temperature. High conductivity readings suggest that the water is likely to contain high levels of dissolved minerals, which are not necessarily beneficial for stream health. The nutrient and bacteria levels remained the same at non-buffered and buffered sites. Some reasons for the varied results could be due to some limitations within the study. The sites used in this research were relatively young at 2, 4, 5 and 20 years. The immaturity of the restoration sites meant that the plants were still small with limited canopy. Canopy aids in water quality by supplying shade, which lowers temperatures and reduces light. Another limitation was having the control and buffered sites spaced out along the same stretch of stream; additional nutrients and suspended sediments were able to enter the buffered zone in the gap between the two sites.

Future research on improving water quality through vegetation use should include the influence of stream shade. Shade reduces plant and algae growth and improves dissolved oxygen and aquatic life by reducing temperature extremes. However, shade could also increase nutrients by preventing plant nutrient absorption. Streambed types may also effect water quality, as invertebrate habitats differ; silty beds are not suitable for planting due to the likelihood of flushing events with significant rainfalls, which would remove organic matter and sediments. An introduction of river objects like stones and woody debris could improve prospective invertebrate habitats. It would also be beneficial to see an assessment of implementation costs of various buffer widths vs. the likely water quality benefits. Buffer strip width is an important factor in effectiveness and it has been found that a 10-meter buffer is ideal. A buffer less than 5-meters results in non self-sustaining vegetation zones with weed issues. The authors note that the wider and longer the planting is, the better the filtration ability of the buffer zone, and the more positive the effect on water quality.

Riparian buffer strip (mainly grasses) surrounding a stream on a farm in Lancaster County.

Image source: Chesapeake Bay Program on Flickr (creative commons)

So all up, what does this study tell us about the effectiveness of riparian restoration, and should we all rush out to plant the river and stream banks? While there definitely are positive water quality effects from planting, this article has shown it could come at a cost, both financially and potentially even environmentally.

For more information regarding this case study, you can gain access to the original article here:

http://dx.doi.org/10.1111/j.1526-100X.2011.00859.x

The author Emma Barr is a postgraduate student at Lincoln University. She wrote this article as part of her assessment for ECOL 608 Research Methods in Ecology.

References

Collins, K. E., Doscher, C., Rennie, H. G. and Ross, J. G. (2013), The Effectiveness of Riparian ‘Restoration’ on Water Quality—A Case Study of Lowland Streams in Canterbury, New Zealand. Restoration Ecology, 21: 40–48. doi: 10.1111/j.1526-100X.2011.00859.x