Aquatic ecosystems need nutrients to survive, but excess nutrients can be a big problem – they can lead to blooms of algae in lakes, ponds, and bays (I’ve written about algal blooms before, here and here). Algal blooms are a natural phenomenon often exacerbated and made more frequent by human activities, primarily through the addition of nutrients to a watershed – a glut of nutrients adds fuel to the fire of a bloom.
Wastewater is one of the many sources of human-added nutrients in aquatic systems. Treatment plants collect wastewater from households (and sometimes industrial customers), process the water in some way, and release the treated water, or effluent, back into the local watershed. Depending on the type of facility, there may still be large amounts of nitrogen and phosphorous – the main nutrients in aquatic ecosystems – present in the effluent. Those nutrients will be washed downstream, where they can harm humans and natural systems by, among other things, contributing to algal blooms.
As older wastewater treatment plants are replaced by more efficient facilities, they need to be evaluated. The first step is deciding which metrics to use – what do you measure to see if an ecosystem is responding to a reduction in nutrients?
A research team working in France recently had the opportunity to answer that question when the city of Nîmes, in southwestern France, updated their wastewater treatment plant. The scientists measured a suite of metrics to assess water quality, before and after the new wastewater treatment plant opened; the results of their study were reported recently in the journal Freshwater Science.
Macroinvertebrate activity – the way aquatic insects are behaving – as well as their numbers and diversity can be a kind of barometer of stream health. In this study, the researchers found that the way the macroinvertebrates were functioning in the stream (the way they behaved) told a different story than the way they were structured in the stream (the number of different species, and the number of individuals of each species).
By the numbers, the sites below the wastewater treatment plant had begun to recover – they began to resemble a reference site above the treatment plant outfall, and sites in other, more pristine streams – within three months of the improved system coming online. The functional metrics, however, suggested that the health of the stream still had room to improve – by the end of the study, three years after the new treatment plant was built, the sites below the treatment plant still had not recovered according to many of those measures.
“Taxonomy-based metrics detected the first signs of river reach recovery rapidly,” the scientists write, “but combinations of trait-based metrics and taxonomic abundance-based metrics are more likely to identify functional recovery” of macroinvertebrate communities following nutrient reductions. In other words, in order to figure out if we’re cleaning up our act as much as we think we are when we make improvements to our wastewater treatment plants, we probably need to measure several different metrics of ecosystem response.