By Emily Diack and Sarah Mager
Water quality in New Zealand has been a hot topic of late, especially when it comes to the growing impact that agriculture and land use changes are having on our waterways. Maintaining good water quality is fundamental for sustaining our indigenous ecosystems, but how do we define what that ‘good’ level of water quality is?
The transformation of New Zealand’s vegetation cover and land use has had a significant impact on the functioning of freshwater ecosystems and water quality, with local waterways becoming increasingly subject to pollution and nutrient overloading. Over the past century agriculture in New Zealand has intensified from low density grazing to large-scale dairy and crop farming, and population increases have caused extensive urbanization. These developments and alterations of original land uses, to a new state, are dramatically disrupting freshwater systems from their pristine states (Moss 2008).
New Zealand freshwater and near shore marine environments developed and evolved in the absence of nuisance species and radical shifts in vegetation cover (Cooper and Cooper 1995). Native forest (Figure 1) and tussock grasslands (Figure 2) were once the predominant vegetation types in New Zealand. Research has identified that catchments with native forest cover export smaller concentrations of nitrogen and phosphorous compared to pastures and plantation, and native forests produce higher concentrations of dissolved organic matter, which is important for ecosystem health. Similarly, research has identified that the conversion from tussock land to plantations has had the effect of reducing the runoff and peak flows, ultimately resulting in a further increase in the concentration of nuisance nutrients like nitrogen and phosphorous. Although recent work has highlighted the transformations in water chemistry associated with land use change, few studies have focused on the natural nutrient levels of pristine river catchment areas and the impacts of water quality and nutrient exports on the aquatic ecosystem function. Shifts in the nutrient content of freshwater ecosystems can cause detrimental impacts to the ecosystem functionality and can have major impacts on the associated marine and estuary ecosystems. The relationship between terrestrial freshwater and marine environments is strong in New Zealand, with more than 300 significant estuaries around New Zealand’s coastline being directly feed nutrients from freshwater systems.
Two naturally occurring, vital nutrients for ecosystem function, which have received little attention in the spectrum of water quality research are dissolved organic carbon and dissolved silica. Dissolved organic carbon contributes to the acidity of natural waters and organic acids. Depending on the nature of the drainage catchment (e.g. forest versus wetlands) the concentration of dissolved organic carbon will vary from naturally high to naturally low concentrations. Low concentrations are observed in oceans and groundwater, whereas high concentrations are frequently found in freshwater where runoff is low, such as native forest cover in New Zealand (Evans et al. 2005). Silica is a widespread variable, always present in surface and groundwater, and is derived from the erosion of materials and is vital for primary production and phytoplankton community structure. Silica is seldom of concern in New Zealand, but is of great interest in terms of fluxes to the ocean and marine environments as silica is important for both marine invertebrates and shell formation (Davies-Colley and Wilcock 2004). Eutrophication problems in coastal marine ecosystems are intensifying because of the increased delivery of nutrients from connecting freshwater ecosystems. Silica is not affected by anthropogenic activities, excluding dam construction and intense abstraction, but becomes limited for biological uptake when in the presence of excess nitrogen and phosphorous. Discharge of these nutrients in excess (relative to silica) in coastal marine environments, therefore, limits the requirements of the present diatoms allowing nondiatoms (undesirable algae species) to thrive.
In order to bridge the gap in knowledge surrounding what that ‘good’ level of water quality is, a study at the University of Otago is investigating pristine (and low human disturbance) alpine catchments (such as the catchment in Figure 3) and the associated fluxes of dissolved organic carbon, dissolved silica, nitrogen and phosphorous. It is planned that research into this field will help characterize the natural nutrient status of pristine catchments that typically reflected the water quality conditions under which our indigenous species evolved.
Cooper, A. and Cooper, R.A. (1995). The Oligocene bottleneck and New Zealand biota: genetic record of a past environmental crisis. Proceedings of the Royal Society 261, 293-302.
Davies-Colley, R. and Wilcock, B. (2004). Water Quality and Chemistry in Running Waters, in J. Harding, P. Mosley, C. Pearson and B. Sorrell (eds) Freshwaters of New Zealand, Christchurch: Caxton Press, 11.1-11.18.
Evans, C.D., Monteith, D.T. and Cooper, D.M. (2005). Long-term increases in surface water dissolved organic carbon: Observations, possible causes and environmental impacts. Environmental Pollution 137, 55-71.
Garnier, J., Beusen, A., Thieu, V., Billen, G. and Bouwmann, L. (2010). N:P:Si nutrient export ratios and ecological consequences in coastal seas evaluated by the ICEP approach. Global Biogeochemical Cycles 24, 1-12.
Moss, B. (2008). Water Pollution by Agriculture. Philosophical transactions of the Royal Society 363, 659- 666.
Emily Diack is a MSc Student at the University of Otago, and Dr Sarah Mager is a Senior Lecturer in the Department of Geography at the University of Otago.