Autumn leaves feed flowing waters

Did you know that the autumn leaves falling into rivers and streams from trees are an important source of nutrition for aquatic organisms? The leaves are attacked by different kinds on zoobenthos, which shred and grind them for nutrition. Climate change is even expected to change the volume of the mass of leaves remaining in flowing waters: when the autumn precipitation still increases from the current level, the flow rates will increase and the leaves falling into rivers will be carried farther away than they used to be. This will change the water ecosystem as well.

The temporal and local variation of the benthic fauna and microbes in natural rivers and rivers transformed by human activities are studied by the research group of flowing water ecology at the University of Oulu. “The benthic fauna and microbes are a good indication of the status of a water system, of how a stream breathes and pulsates, and of when it is suffering from arrhythmia,” explains Professor Timo Muotka, who leads the group.

“One area we are studying is the river Riisijoki in the Riisitunturi National Park, flowing into the lake Kitkajärvi. In Finland, it is in about as natural state as can be. The head streams of Riisijoki are very representative examples of head waters sensitive to disturbances.”

An object of study from the other extreme is the river Isojoki, meandering across the fields in South Ostrobothnia, surrounded entirely by agriculture and habitation.

“We aim to establish as long time series as possible, because the situation varies a lot from year to year. Since we have a time series we have been collecting since the early 2000s, we already dare to draw some conclusions,” Timo Muotka says.

“Climate change is one of the key areas of research. What will happen to northern flowing water ecosystems, when the climate and waters get warmer, and precipitation and flow rates change between seasons?”

“An important ecological question is whether communities with an abundance of species are more stable and resistant to environmental changes than those with less varied species?”

A flowing water ecologist collects zoobenthos and biofilm

Let us dive into the bottom of a stream to see what kind of life we can find there. The underwater microfauna is described by Postdoctoral Researcher Kaisa-Leena Huttunen.

“The zoobenthos are a large group of different invertebrate animals that spend at least part of their lives at the bottom of the streams, either attached to vegetation or rock surfaces, floating around in the water or dug into mud and sand,” Huttunen explains.

“Many Chironomid larvae, nematodes and oligochaetes live in the bottom mud. The biting midge and blackfly larvae live on top of the bottom sediment and mosquito larvae in the air-water interface. The larvae of caddis flies, stoneflies and mayflies, on the other hand, move around among the stones and vegetation in the bottom.”

Oulu has its own dedicated species among the zoobenthos: the stonefly Febryary Red (potnapekka) rises on top of snow from the river as early as in April, thus announcing that the spring has arrived.

“The zoobenthos can also be classified according to what they eat. Water slater is an ecologically important species that specifically chops tree leaves into smaller chaff, i.e., they belong to shredders. The filter feeders eat fine suspended organic material; e.g., blackfly larvae are filter feeders. The grazers, such as gastropods, eat the biofilm found on plant, tree trunk and stone surfaces.”

There are also predators among the zoobenthos. “The dragonfly larvae are big enough to eat even fry,” Huttunen says.

In addition to the zoobenthos, flowing water ecologists are interested in microbes. Planktonic microbes float freely in the water, and the slippery surface layer accumulating on rocks is biofilm composed of bacteria, fungi and algae.

“The groups of and relationships between the species of microbes, mainly bacteria, and changes in them are a more rapid indication of the variations in the status of the waterbody than the zoobenthos, as their life cycle is extremely fast.”

How do you obtain samples of the benthic fauna and microbes in flowing waters? "When collecting zoobenthos samples, you agitate the riverbed by kicking it, and the material detached from the bottom with its fauna is transported into a net held under the disturbed material. Samples of microbes are collected by wiping some microbial biofilm from the surface of stones with a sponge. I also use ceramic tiles as a substrate for biofilm.”

“I examine the temporal variation of the benthic fauna and microbes from one season and year to the next both in watercourses in their natural state and in ones that have been changed by humans. I am particularly interested in the occurrence of species and changes in their relative abundance, as well as different environmental factors behind the changes,” Kaisa-Leena Huttunen explains.

Phases in the work carried out by Postdoctoral researcher Kaisa-Leena Huttunen, who examines the underwater microfauna in streams (from up left to down right): using the underwater viewer, examining the samples caught in the net, taking water samples and a sample of tree leaves picked up from the bottom of a stream. Photos: Maija Murtoperä

Experimental research in artificial streams

Postdoctoral researcher Jussi Jyväsjärvi is especially interested in the human impact on aquatic ecosystems.

"The forestry industry has had a dramatic impact on flowing waters,” he points out. “Due to log floating, thousands of river channels were cleared, which wreaked havoc in the river bottoms and disturbed the operation of the ecosystems. Ditching of forests and bogs has increased the volumes of nutrients, solids and humus in water bodies, which has had dramatic effects on the organisms and water quality.”

Agriculture has also had similar impacts on the waterways. “When the turmoil and changes in the environment are sufficiently dramatic, the waterways will be permanently changed. In ecology, we talk about the tipping point, which means reaching the point from where the ecosystem will no longer return to its former state.”

The fish farming facility of the Natural Resources Institute Finland located in Paltamo has undergone a transformation, and now serves as a location for, for example, flowing water research. “In the Paltamo centre we have artificial streams, where we perform experimental research. In these streams, we can adjust the flow rate, volumes of nutrients and humus, and the darkening effect,” Jyväsjärvi describes.

The darkening of waters is a global phenomenon, which is due to the increased run-off of humus.

The darkening of waters is a global phenomenon, which is due to the increased run-off of humus. Land use changes, global warming, changes in the rainy seasons and drought periods, and the reduced sulphur and nitrogen deposition are the reasons behind the waters turning browner.

“I examine the effects of the darkening on the aquatic environment in Paltamo by adding humus to the artificial water channel. It darkens and silts up watercourses, but it also acts as fuel for microbes and may accelerate the metabolism of streams. This, in turn, increses the volume of carbon dioxide released from water into the atmosphere,” Jussi Jyväsjärvi says.

In other words, a vicious feedback loop in operation: human activity and climate change darken the waters, and the darkening further increased the release of greenhouse gases into the atmosphere.

Text: Satu Räsänen

Stream Ecology Research Group, University of Oulu

Main photo: A stream in Kilpisjärvi by Soili Jussila / Rodeo




Last updated: 10.10.2018