By Alison Ballance
"What we see causing the greatest harm to aquatic invertebrate communities in these streams is deposited fine sediment."
Jeremy ‘Jay’ Piggott, University of Otago freshwater scientist
An ambitious experimental set-up is allowing freshwater scientists to tease apart the impact that environmental stresses, such as increased sediment and nutrient run-off from intensive farming, are having on the declining health of streams and rivers.
“We’re interested in how multiple stresses affect freshwater biodiversity," says Jeremy 'Jay' Piggott. "So when we think about agriculture, which is one of the major drivers of change in New Zealand freshwaters, we think about nutrient enrichment from run-off, we think about sedimentation coming in and smothering the [stream] bed. We can also think about herbicides, pesticides, and we can think about deforestation upstream which changes the riparian cover which in turn changes the water temperature. We can also think about changes in water quantity, with water abstraction for irrigation.”
Jay says that all of these stresses are happening simultaneously, so it’s often difficult to work out which is causing the effects or harm that’s being observed.
“These studies we’re conducting – which are quite pioneering – are trying to disentangle all those effects.”
What their results have shown so far is that deposited fine sediment is a key stressor in streams and rivers, as it smothers habitat. Invertebrates tend to respond by drifting downstream away from the affected area. The researchers have observed a complex interplay between the effects of sediment, increased temperatures and increased nutrients.
"Fine sediment tends to be really bad on its own – but other stresses tend to make its effects worse.”
The most effective mitigation is to plant riparian vegetation along the stream banks, as this shades the water and prevents nutrient and sediment run-off. Jay says it is particularly important to plant riparian strips along small streams that feed into larger rivers.
The ExStream, or Experimental Stream Mesocosm System, comprises 128 miniature circular streams. It has a 20 metre long, 5 metre high and 5 metre wide scaffold structure, and about 10,000 individual pieces of piping and other bits of plumbing paraphernalia that connect to a pump submerged in the nearby Kauru River, a relatively pristine river running through farmland in North Otago. Jay developed the system during his PhD studies, and laughingly says that “on successful completion of my PhD I informed everyone that I am now officially a plumbing and hose doctor.”
The University of Otago researcher and his team are using the system to tease out how freshwater ecosystems respond to multiple agricultural stresses such as nutrient and sediment run-off as well as climate change and rising water temperatures.
"Climate change is the big elephant in the room when we think about the impact of human activities,” says Jay. Rather than simply making matters better or worse, he “would argue that it’s just going to make things more complicated.”
As well as increasing water temperatures, climate change is predicted to being changes to precipitation patterns that might result in more frequent droughts as well as more frequent storm and flood events. Increasing carbon dioxide levels in freshwater are not well understood, but are believed to impact on the availability of food in the food web.
The system currently sits on the edge of a field, and it takes water directly from the nearby Kauru River.
The water and the aquatic life it contains are piped to the mesocosms, which are actually ring-type cake tins with a central hole that allows the water to flow through. The system is exposed to the same weather and light conditions that the river experiences, which means it is a much more realistic experimental set-up than one situated in a lab. Prior to an experiment the mesocosms are naturally colonized by periphyton and invertebrates from the river.
“The substrates in the mesocosms reflects the natural substrates that we see in rivers running through sheep and beef farms here in North Otago, and we’ve got a natural community in there that you would expect to see in a relatively pristine stream.”
The researchers can manipulate a range of experimental variables. For example they can decrease or increase the rate of water flow, add different grades and amounts of fine sediment, increase temperature by heating the incoming water using gas califonts, and drip in either nutrients such as nitrogen or phosphorus, or commonly used agricultural chemicals such as nitrogen inhibitor, and glyphosate or Roundup.
“We allow the organisms coming in here to do what they actually do in their natural environment, which is either to decide to stay or leave, or they can develop there and emerge as adult insects. So we can look at immigration, emigration and development processes [in the invertebrate populations].”
In the latest experiment the research team is experimenting with three different grades of fine sediment and four different water speeds. Master’s student Lisa Carlin will be measuring the effect on motile algae, while student Matthew Ward will be measuring the impact on the invertebrate community.
The ExStream system is also being used in Germany. Jay and colleague Christoph Matthaei have created a company to market the system and they are hopeful it will soon be in use in other countries such as Japan.