Macroinvertebrates, or stream bugs, have been the focus of much debate amongst New Zealand’s freshwater science fraternity in recent times. To address declining water quality, the New Zealand government has proposed a National Objectives Framework (NOF), which includes a set of national bottom line values (or limits) for key indicators of freshwater ecosystem health. Exclusion of what is a universally accepted indicator of aquatic health (macroinvertebrates) has generated considerable criticism. Indeed, New Zealand’s stance is inconsistent with international approaches, such as the UK’s implementation of the EU Water Directive. One reason put forward for this exclusion is uncertainty about the reliability of established measures such as the Macroinvertebrate Community Index (MCI). However, I believe a more fundamental issue is about whether the indicators proposed (mainly water quality) are sufficient to ensure that the health of our freshwaters is maintained.
So what’s all the fuss?
There’s plenty of evidence that demonstrates clear links between the health of macroinvertebrate communities and water quality. Why, then, do we need to monitor bugs AND water quality? Aren’t water quality measures adequate stand-ins (or surrogates)? Surely they provide an adequate picture of ecosystem health?
To answer this, we need to turn the question around. Why measure water quality? The answer is complex. We live in a world with competing demands for limited resources. Water quality guidelines reflect these competing demands and attempt to satisfy all users (including ecosystems).
Ecosystems provide services
Underpinning all of these demands is the need for healthy, functioning ecosystems that provide essential ecosystem services, such as whitebait fishing (food), removal of contaminants (water purification) and stabilization of river banks (erosion control). By monitoring water quality, we’re really monitoring the health of these ecosystem services.
Macroinvertebrates provide us with services too. Ask any trout fisherman about stream bugs and he’ll pull out his favourite fly! They’re not just fish food though. They have various roles within streams. They break down leaves, releasing nutrients for other organisms to use. They’re mostly around for quite some time, integrating responses to water quality over time. In contrast, water quality measures usually only tells us what’s happening at one point in time.
Do surrogates tell the whole story?
For water quality measures to be useful surrogates of ecosystem health, we need to have a good understanding of how they are related. Let’s consider two examples. Imagine a rural stream with excessive algal growth. Macroinvertebrates will be smothered, flow reduced and only certain types of macroinvertebrates will live there. We know that under stable stream flow conditions, algal growth is controlled largely by how much phosphorus and/or nitrogen is present. Therefore, measuring phosphorus and nitrogen in the water may be enough to gauge stream health.
What if there were lots of different factors that could be affecting the macroinvertebrates? In urban streams, they’re are typically exposed to a combination of factors, like low oxygen levels, high heavy metals concentrations, high temperatures and lots of sediment, caused by a multitude of activities within a catchment. In this case, using any single water quality measure is unlikely to reflect these complex relationships.
What’s the answer?
So, do we need to measure aquatic ecosystem health in its entirety? Is it even feasible to do so? And how does this help ecosystem managers do a better job? Recently developed methods, such as the Stream Ecological Valuation (SEV) method, attempt to do this. Using this method, ecosystem health is defined by a number of stream functions (which contribute to ecosystem services), which are determined using a mix of water quality, physical habitat and biotic (including macroinvertebrates) indicators. The limitation of course is that the world is not necessarily just the sum of its parts.
Do we need macroinvertebrates in the National Objectives Framework? In my view the answer is yes. The recent draft decision (Section 332) by the Board of Inquiry into the Ruataniwha Irrigation scheme accepted that “an approach based on ecological health [in this case indicated by macroinvertebrates] rather than toxicity is more likely to give effect to the NPSFM [National Policy Statement on Freshwater]”. But is measuring macroinvertebrates enough? The intention of the NPSFM is “to safeguard the life supporting capacity, ecosystem processes and indigenous species including their associated ecosystems of fresh water, in sustainably managing the use and development of land, and the discharges of contaminants”. Does this not suggest a holistic approach to how we measure ecosystem health is required?
Whilst water quality indicators are relatively easy to measure, they are not without problems when it comes to interpreting ecological significance (the subject of a future post). So while they may ‘appear’ to represent a pragmatic solution that relate the effects of land management to the health of our aquatic ecosystems, in my view we should not shirk from developing more direct measures. Surely the long-term goal should be to have a more ecological approach to setting national bottom lines for our freshwater resources.
By Ngaire Phillips