Understanding the natural history of New Zealand’s nutrient fluxes

By Waiology 06/11/2014 6


By Emily Diack and Sarah Mager

2014IconWater 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).

Figure 1: An example of a low disturbance catchment with native forest as the dominant vegetation cover. Pristine catchments like the Haast River, shown above, typically have forest cover right to the waters edge that keep water temperatures cool and provide shady habitats for native species.
Figure 1: An example of a low disturbance catchment with native forest as the dominant vegetation cover. Pristine catchments like the Haast River, shown above, typically have forest cover right to the waters edge that keep water temperatures cool and provide shady habitats for native species.
Figure 2: An example of a low disturbance catchment with tussock land as the dominant vegetation cover. The Cameron River, South Island, New Zealand.
Figure 2: An example of a low disturbance catchment with tussock land as the dominant vegetation cover. The Cameron River, South Island, New Zealand.

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.

Figure 3: The Fox River is a low disturbance alpine catchment on the West Coast of the South Island of New Zealand, that has lots of scree and slip slopes that release new rock minerals, like silica, into waterways.
Figure 3: The Fox River is a low disturbance alpine catchment on the West Coast of the South Island of New Zealand, that has lots of scree and slip slopes that release new rock minerals, like silica, into waterways.

References:
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.


6 Responses to “Understanding the natural history of New Zealand’s nutrient fluxes”

  • ” Eutrophication problems in coastal marine ecosystems are intensifying because of the increased delivery of nutrients from connecting freshwater ecosystems.”

    Are you sure nutrients are increasing? This seems like a predictive statement.

    Regardless of whether or not the statement was predictive, evidence exists to suggest phosphate has been declining in rivers.

    http://www.mfe.govt.nz/environmental-reporting/fresh-water/river-condition-indicator/summary-key-findings.html

    http://www.lawa.org.nz/explore-data/national-picture/nutrients/dissolved-reactive-phosphorus/

    It is also fair to say that the MFE findings raise questions if Nitrogen has significantly increased on a National scale.

  • A recent paper by Rich McDowell and co, is another good reference to try and understand what the baseline or reference conditions might have been like in NZ rivers and how anthropogenic inputs have changed things.
    Establishment of reference or baseline conditions
    of chemical indicators in New Zealand streams
    and rivers relative to present conditions.
    Marine and Freshwater Research, 2013, 64, 387–400
    http://dx.doi.org/10.1071/MF12153

  • Mr E,

    Thank you for your comment. The analysis of nutrient trends is generated from data collected since 1989 by NIWA and local body authorities and comprises of a network of 77 different rivers over the country, and from this data set there is a complex set of responses across the country that depends on the range and scale of landscape development in the respective catchments. As you point out, some rivers have improved over that time, however, there remains other catchments where key indicators of nutrient status continue to decline (although a minority). The approach of my research is to investigate nutrient fluxes from upland catchments in New Zealand, where anthropogenic influence is modest, and where the bulk of the catchment land use is indigenous cover. In this way, the level of nutrients are likely to reflect the conditions prior to large land clearance. The assumption in my work is that the level of phosphates and nitrates are likely higher than they once were before land use change and other anthropogenic factors that influenced the water quality of New Zealand rivers; but it is not the focus of the work to test this assertion. Preliminary analysis of the samples collected to date shows that the phosphate and nitrate levels are either undetectable or on the cusp of the detection limit in my selected low impact catchments.

  • Chris,

    Thank you for the reference, I appreciate it and will definitely use it as my research continues.

  • Emily,
    I have wanted to see this type of work for a long time. I have stated on this blog before that we need a measure of pollution that relates to the pre – human state of water quality as a significant number of native waterways fail historical water quality standards.

    Larned et al. 2004 Water quality in low-elevation streams and rivers of New Zealand: recent state and trends in contrasting land-cover classes
    New Zealand Journal of Marine and Freshwater Research, 2004, Vol. 38: 347–366.

    That aside, I regularly read broad statements from water quality scientists about water quality declining in New Zealand due to nutrients. However, I struggle to find data that corroborates this view, and frankly the reports I read – like the links above, suggest the opposite. It seems the statement – “nutrients are either stable or improving in NZ rivers” is more supported by data than ” problems in coastal marine ecosystems are intensifying because of the increased delivery of nutrients from connecting freshwater ecosystems.”

    In Southland water quality declines in Waituna Lagoon were largely blamed on the dairy industry, with the word intensification, thrust forward as the cause. However water quality in the surround catchments streams were reported as largely stable or improving. Farmers were bish bosh bashed in the media, dairy became the scourge of water quality. The federated farmers stood up, employed a scientist to review the issue and subsequently took issue. It seems bank stability was a significant causal factor, and identification of it lead to a major bank management programme.

    Recently Dairy NZ, Council and NIWA have teamed up to investigate the source of sediment in 2 estuaries in Southland. One being the Waituna. Preliminary findings have suggested that 95% of the sediment in the Estuary is from bank erosion. Not farming intensification as originally mooted by the Council.
    There has been very little recognition of this work, and I suspect because of embarrassing errors of the past.

    http://www.dairyatwork.co.nz/land-water/sediment-fingerprinting-in-southland-waterways/

    From your answer I am still not sure if you believe the statement you presented “” Eutrophication problems in coastal marine ecosystems are intensifying because of the increased delivery of nutrients from connecting freshwater ecosystems.” I do hope that me questioning it will have another Water Quality Scientist contemplating if improvements have been made, whether these are recognised in media, and if a fair view of nutrients in NZ water is promoted.

  • Very interesting and relevant work.

    We look forward to reading the research publications.

    In the meantime, can you watch out for the spelling of phosphorus, i.e., it is not phosphorous. 🙂