Current Issues in Rural Water Quality
How clean and green are our rural streams? This question is being highlighted in the current debate about the effects of agricultural intensification and the link between the marketing imagery and the reality of New Zealand agriculture. In this article John Quinn overviews the current issues and management options around agricultural intensification and stream water quality and biodiversity.
The Issues
The rural stream water quality debate has focused in the past on levels of contaminants. These include sediment (that can make streams murky and smother their beds), disease-causing pathogens from animal dung, pesticides, and nutrients that cause unsightly algal slimes in streams and algal blooms in lakes. Over the past decade the effects of riparian shade removal on stream temperature have been recognised as important in promoting nuisance algal growths and restricting heat-sensitive stream organisms. More recently, concerns about biodiversity (i.e. the diversity of plants and animals and the places they live) effects have been raised. Indeed, the Ministry for the Environment considers that loss of biodiversity is the number one environmental issue facing New Zealand.
The effects of many of these changes in stream habitat and increases in contaminant input vary with their magnitude (Fig. 1). Modest increases in stream nutrient loading or temperature usually have minor impacts on stream life and often enhance productivity without causing problems (referred to as subsidy effects). However, larger increases may result in undesirable changes, such as temperatures above the tolerance thresholds of sensitive stream insects and fish and nutrient concentrations that support development of nuisance blooms of algal slimes.
Figure 1: Conceptual model of the subsidy-stress effects of pastoral agriculture development on stream biota (adapted from Quinn 2000).
Potential Effects of Agricultural Intensification
Intensification of agriculture, such as conversion of dry stock farms to dairying, has the potential to increase the level of perturbation experienced by rural streams, although management can reduce these stresses (Table 1). Good examples of environmental management systems have been highlighted by events like the Waikato Farm Environment Awards.
Table 1: Summary of stream contaminant issues associated with dairy conversions/intensification and examples of management measures
Environmental stress |
In-stream issues (reference to more |
Management measures |
Increase stocking rate |
Fertiliser and contaminant leaching and
run-off (www.niwa.cri.nz/No8/nutrientrun-off.html ; |
Manage fertiliser application for pasture
needs and run-off risk. Filter strips for run-off. |
Heavier animals |
Soil compaction; streambank trampling |
Avoid pugging; fenceoff streams |
Twice daily use of raceways |
Contaminant run-off |
Divert run-off to pasture |
| Stand-off pads | Contaminant run-off | Treat run-off/divert to pasture |
Stream crossings |
Contaminant input; fish passage |
Good culvert and crossing design |
Irrigation Drainage |
Contaminant leaching |
Filter strips for run-off. |
| Dairy shed effluent | Contaminants in underfield drains Contaminants (www.niwa.cri.nz/No8/dairywaste1.html) |
Treatment wetlands on main drain lines |
Biodiversity
So what are the biodiversity effects of agricultural land use and intensification and what can be done about them? Agricultural land use typically results in loss of the native forest that occurred along the banks (the riparian area) of most New Zealand streams. This has resulted in a homogenising of stream habitat conditions and consequent loss of biodiversity.
In forest streams, a core of generalist species plus species adapted to cool temperatures and leaf litter based foodwebs (e.g. stoneflies) occur from the shaded headwaters downstream to where the stream opens out as the channel width increases above about 10 m (Fig. 2). Further downstream, the riparian vegetation no longer shades the wider stream channel effectively, and the headwater specialists are replaced by species adapted to warmer temperatures and in-stream plant-based food supplies (e.g. snails, grazing caddisflies and some midges). Forest removal with farming conversion removes the headwater habitat conditions and makes universal the unshaded conditions and associated species that previously occurred only in wide downstream reaches.
Pasture streams
Figure 2: Typical Waikato pasture and native forest streams showing that forest streams are well shaded up to moderately large catchment areas whereas pasture streams are unshaded from the headwater downstream
This effect of pastoral land use can be reversed by restoring shading riparian vegetation in the headwaters of pasture streams. Based on extensive aquatic invertebrate monitoring across a variety of stream types at Whatawhata (Collier et al. 2000), we estimate that restoring native forest along the headwaters of pasture streams will increase farm-scale biodiversity of stream invertebrate species by 33 percent (as well as increasing indigenous terrestrial biodiversity enormously).
References
Re-establishing riparian forest along pasture streams also helps to address several other water quality concerns in rural streams (Table 1). More advice is contained in Ministry for the Environment (1991) Guidelines for Managing Waterways on Farms.
Agricultural land use plays a key role in supporting human populations in New Zealand, but we need to manage its impacts on our vital water resources, indigenous biodiversity, and recreation/tourism. Many examples exist of good on-farm environmental management to maintain healthy and diverse rural waterways and new approaches are evolving constantly. Intensification of agriculture continues to raise challenges for farmers to adopt practices that will maintain healthy rural waterways and add substance to our clean, green marketing image.
Collier, K. J.; Smith, B. J.; Quinn, J. M.; Scarsbrook, M. R.; Halliday, N. J.; Croker, G. F.; Parkyn, S. M. 2000: Biodiversity of stream invertebrate faunas in a Waikato hill-country catchment with native forest and pastoral land use. New Zealand Entomologist 23: 9-22.
MFE. 2001: Managing waterways on farms: A guide to sustainable water and riparian management in rural New Zealand. Ministry for the Environment, Wellington.
Quinn, J. M. 2000: Effects of pastoral development. In: Collier, K. J.; Winterbourn, M. J., (eds.) New Zealand stream invertebrates: ecology and implications for management. Caxton Press, Christchurch. pp. 208-229.
 |
Dr John Quinn Principal Scientist – River Ecosystems NIWA Hamilton John is a graduate of Otago and Massey Universities who has been working in water management and research for 20 years. His research over the past decade has been mainly on effects on rivers of forestry and agricultural land use and the role of riparian management in managing these effects. He fronts the "Environment Watch" segment in TV3’s farming programme "No. 8 Wired". |
Contact for Enquiries
Amber Duncalfe
Editor - RM Update
Ministry of Agriculture and Forestry
PO Box 2526
Wellington
NEW ZEALAND
Tel: +64 4 894 0710
Fax: +64 4 894 0745
Contact this person
