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 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)
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