Toxicity or community ecology: which is the better measure of nitrate pollution?

I think you’d all agree that protecting our valuable natural resources while enabling social and economic growth is challenging. The recently recommended limit for nitrate in water of 0.8 mg/L proposed for the Tukituki River is a good example of the challenges that we face. The Board of Inquiry into the proposed plan change for the river (which included building and operating Ruataniwha Dam) was presented with two different sets of numbers proposed as targets for managing the river – one based on nitrate toxicity and the other on the relationship between nitrate concentrations and the health of the macroinvertebrate (or stream bug) community (MCI or Macroinvertebrate Community Index). The Board of Inquiry decided that MCI better represented ecological health and, as such, was more likely to give effect to the National Policy Statement on Freshwater Management (NPSFWM), which sets the agenda for how we manage freshwaters in New Zealand.

But is this really the case? Is a number based on toxicity any less relevant as a measure of ecological health than one based on a single component of the aquatic ecological community (i.e macroinvertebrates)? And what does this mean for the currently proposed limits (or bottom lines) set within the National Objectives Framework (NOF) which give effect to the NPSFWM and which are based on nitrate toxicity?

Nitrate toxicity

Toxicity test setup

Setting up for toxicity testing. Photo: N Phillips

Nitrate toxicity effects data were recently updated as part of the process for developing limits under the NOF. Data from 22 species (9 fish, 8 invertebrate, 4 amphibians and 1 algae) were used to derive these limits, and included new data on two native New Zealand species, as well as covering different life stages (eggs, larvae, juveniles, adults). Only data that reported effects on growth, development and reproduction as the experimental endpoints (rather than death) were used to derive a nitrate toxicity ‘limit’. A statistically-based procedure, similar to that used to derive the ANZECC (2000) guidelines, was used to calculate the lowest concentration at which there was no observable effect (NOEC), as well as the threshold effect concentration (TEC), above which effects were observed. This analysis pointed to greater sensitivity of fish to nitrate when compared with a number of macroinvertebrate species.

The proposed nitrate limits for the Tukituki were based on chronic (long-term) toxicity and ranged between 2.5 mg/L and 5.6 mg/L. A two-number system was proposed, representing a “Grading” limit (based on the NOEC) and a “Surveillance” limit (based on the TEC). NOEC values provide ecosystem protection from exposure to average long-term nitrate concentrations, while the TEC allows management of seasonal maximum concentrations. In addition, the limit also varied depending on the management aim, with lower limits (and hence higher levels of protection) in less degraded regions of the catchment.

An advantage of toxicity testing is that you’re able to control for potentially confounding factors and, as such, establishing a direct causal relationship is feasible. The major disadvantage is that contaminants generally are not the only factor affecting the growth, development and survival of aquatic organisms. Toxicity tests are therefore not necessarily representative of real world environments.

Macroinvertebrate Community Index (MCI)

Intolerance

Image: Nick Kim

MCI is a numeric indicator of ecological health based on the relative sensitivity of macroinvertebrates to contaminants. It was originally developed with nutrient pollution in mind. Sensitivity values assigned range from 1 (very tolerant) through to 10 (very sensitive). The idea is that a macroinvertebrate community comprised of mostly tolerant species is likely to indicate poor water quality. Conversely mostly sensitive species would indicate good water quality. As macroinvertebrates play many important functional roles in streams (e.g. breakdown organic material, provide a source of food for fish), they reflect how well the stream is functioning and hence are considered a good indicator of overall ecological health. Using a correlation-based model of the relationship between MCI values and nitrate concentrations in streams, a nitrate limit of 0.8 mg/L was derived. The strength of this relationship can be measured statistically. While no measure of the strength of the model was presented for the Tukituki case, similar studies estimate correlation coefficients of 0.44, which means that 44% of the variation in mean MCI values are explained by nitrate.

An advantage of using macroinvertebrate communities is that they reflect the real world complexity of stream environments. The disadvantage of this approach is that direct causal relationships are very difficult to determine, because of the presence of confounding factors. For example, on an arable field where nitrogen fertilizer is added and soil is eroding, the major factor impacting on the stream macroinvertebrates could well be the physical impact of the sediment. In this example the nitrate would be an incidental co-variate.

Which is the better measure of ecological health?

It’s important to remember that toxicity isn’t just about measuring the concentration that causes death. Sub-lethal responses, such as growth, development and reproduction, as well as behavioural responses such as avoidance indicate more subtle effects that are likely to become evident over time. And by using a range of different organisms at different life stages, there’s a good chance that the derived concentration will be highly protective of most species. MCI also reflects changes over time and integrates the effects of multiple factors. It can also detect the loss of entire species from an aquatic community, as only those species most tolerant will survive. Clearly both approaches have advantages and disadvantages. In my opinion, both approaches provide a measure of ecological health, but they apply different lenses.

So what does this mean for the NPSFM?

The nitrate toxicity limits proposed in the National Objectives Framework (NOF) are aimed at protecting the majority of organisms to a level which reflects the current state of a waterway, as well as to provide a target for enhancement of current values. This in no way negates how nitrate should be managed for other issues e.g. periphyton growth. It simply allows for targeted management. Interestingly, while the NOF sets limits for periphyton growth, it doesn’t set limits for nitrate or phosphorus, key drivers of periphyton growth. Surely we should be managing for causes rather than effects?

The real challenge, I believe, is ensuring that the most relevant measure(s) are being used. We need to recognise that aquatic environments are complex, dynamic, integrative systems. In my opinion, we should not be trying to simplify management by thinking we can manage for single attributes.

By Ngaire Phillips

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Comments (8)

  1. Mike Joy :

    Ngaire great stuff but I don’t think I’m clear from this what level will protect ecosystem health? If I look at the Manawatu example at Hopelands road around 1.2mg/l DIN we had huge O2 fluctuations through algal proliferation that means the fish long dead or gone before nitrate toxicity becomes a problem, so to me toxicity is beside the point and so we have to protect the Ecosystem first as that is first hit on life in river?

  2. Brett Stansfield :

    Hi Naire, In my view the 0.8 mg/l limit specified to protect macroinvertebrate community health is obviously more conservative than the toxicity limit of between 2.5 to 5.6 mg/l. I am more in favour of the 0.8 mg/l limit that provides for the protection of all aquatic ecosystem processes rather than a toxicity threshold that determines whether a handful of species live or die. This doesn’t discount that the toxicity thresholds should not be used for other purposes such as setting limits for point source discharges. The toxicity limits have their place but they shouldn’t be used to set limits for an entire river catchment.

  3. Ngaire Phillips :

    Hi Mike The situation you describe is a great example of what my last paragraph was trying to get at – that it’s important to ensure that the most appropriate measure (s) are being used. Managing only for nitrate toxicity would be inappropriate in your example. The point is that managers need to be aware of the major drivers of ecosystem health in their catchments, and manage accordingly. And that is why I believe that managing for multiple attributes is likely to result in better environmental outcomes.

  4. Ngaire Phillips :

    Hi Brett As I said in my article, the nitrate toxicity limits were not based on mortality but on sub-lethal responses. While my initial question was about “which one was best” the answer is in fact “both”, depending on the question.

  5. Vernon Brown :

    Hi Mike,

    As eluded to, the appropriateness of different DIN limits comes down to the objective you are trying to manage to. My understanding is that oxygen levels in rivers are driven by diurnal temperature fluctuations, uptake by plant-life and decomposition and not DIN itself. I don’t know this site you are talking about. However, if the objective is to protect fish and the amount of algae is causing oxygen stress at night and DIN is the limiting factor then having a DIN threshold much lower than what is proposed by N toxicity thresholds would be appropriate. However, if you have a river you are trying to manage native fish in and has all stock excluded and best practice riparian management (therefore high levels of shade), good habitat availability and high MCI scores the system may be able to sustain high DIN levels. Ultimately, you identify the critical factors causing the stress and manage them.

    Brett- my understanding of the N toxicity thresholds in NOF is that the levels of protection to a community is to avoid impairment of growth. Mortality doesn’t occur until much higher concentrations that even what D band suggests.

    Ngaire- fully support both your points.

  6. Mike Joy :

    Vernon, surely the idea of limits to protect ecosystems are what we should aim for? The idea of the NPS is a “fresh start for freshwater” and having National Standards should be precautionary which is why we had the N guidelines in the past. This N toxicity approach is the result of the single nutrient approach to freshwater management which is to my knowledge never been shown to be effective in New Zealand or in hundreds of other studies worldwide.

  7. Moritz Lehmann :

    Ngaire,
    thank you for this very informative posting. My gut response is to applaud the Board of Inquiry’s decision for a lower nitrate target, especially since regulation of this kind all too often goes the other way. But your conclusion is a well-presented reminder that as scientists, even those serving the environment, we must not forfeit our objectivity and apply critical analysis to balance environmental and developmental objectives correctly.

    Still, I must say that I am impressed by the low target for nitrate as well as the move to the two-nutrient management approach. In Canada, the nitrate limit is 13 mg/l for long term averages, based on toxicity. Moreover, recent changes to the federal environmental assessment act pretty much remove all federal oversight and leave permitting and enforcement to the provinces. The problem is that the provinces keep using the economic argument to avoid progressive water quality management: they claim to be too poor to enforce/monitor or too greedy, for example, in the case of a province with an abundant and dirty fossil fuel resource. Correct me if I’m wrong, but it seems that in this case, New Zealand’s federal oversight over regional issues worked well in the favour of our most precious natural resource.

  8. Robin Connor :

    Ngaire
    Good discussion. I agree with your horses for courses approach to indicators. It seems to me there is room in the system for both a general integrative indicator of ecosystem health (or “water quality”) such as MCI, along with measures and thresholds on contaminants such as nitrate that promote management of the balance.
    In your last section on the NOF: the place of periphyton in the NOF is (while nitrate levels are subject to the toxicity tables) as an outcome indicator – a bit like MCI. It integrates across a number of factors (shade, nitrate, water flow etc.). Where its effect is undesirable these other things need to be managed to bring it under control, but it may not always be the same cause.

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