MARS Experiments: Low Flows in Nordic Rivers

The experimental channels. Image: Aarhus University

Over the last few weeks we’ve been introducing the experiments that the MARS research team have been carrying out across Europe to understand the effects of multiple stresses on freshwater ecosystems. This week we turn our focus to Denmark, where a team led by researchers led by Annette Baattrup-Petersen and Daniel Graeber from Aarhus University are investigating the effects of low water flows on freshwater streams.

Low stream flows, the build up of fine sedimentation, and eutrophication – caused by dissolved nitrogen and phosphorus – are the main stressors of stream ecosystems in Nordic agricultural landscapes.  In these experiments, the MARS researchers seek to understand how these stressors affect small stream ecosystems in these landscapes, particularly focusing on how the stressors might interact. Research in this area is particularly important given that predictions of future climate change suggest that many rivers in northern Europe will experience longer periods of low flows, due to changing precipitation patterns and climate warming.

Experimental channel diagram. Image: Aarhus University

The MARS team are currently conducting a series of measurements and experiments, seeking to couple stream ecosystem structure to ecosystem function, from which the potential effects of stress on ecosystem services can be derived. The research team have constructed 12 experimental stream channels, 12 m long, 0.6 m wide and 0.3 m deep, which are designed to simulate natural small stream ecosystems and the effect of multiple stressors. The stream channels consist of a series of runs and riffles with sediment typical for such habitats.  They are fed by stream water, which results in natural water chemistry and also brings new algae, water plants and invertebrates into the experiment (as would happen in a real stream). The flow of water down the channels can be controlled to simulate normal and low stream flows.

Runs and riffles on the experimental channel. Image: Aarhus University

The MARS team are focusing their attention on two key components of small stream ecosystems: benthic invertebrates and benthic algae, with a focus on primary production and nutrient uptake as ecosystem functions. Benthic algae are the main primary producers in small stream ecosystems and provide food for grazers, such as benthic invertebrates, as well as oxygen for all animals and fungi. Benthic invertebrates are the main food source for fish in Danish small streams and can control the growth of benthic algae by feeding on them. Therefore, the combination of all elements of a stream ecosystem defines its capability to take up and therefore retain nutrients.

To measure primary production in the sediments and on stones, the team use specially made experimental chambers, and take samples for algal biomass and composition and benthic invertebrate density and composition. To measure nutrient uptake, the team initiate short-term nutrient releases for ammonium, nitrate and phosphorus, and measure the ecosystem response.

A stone covered in sediment in an experimental channel. Image: Aarhus University

Three stressors in the experiment:

The three chosen stressors are likely to have several effects on the interaction of benthic invertebrates and algae in the experiment.

Low flow is likely to result in a reduction of densities of typical stream benthic invertebrates (mayflies, stoneflies) many of which are grazers. Similarly, due to less physical abrasion on the streambed, low flows are likely to result in higher benthic algae growth. Combined with the reduction of stream benthic invertebrates, a higher primary production can be expected and due to the longer residence time (i.e. the amount of time a particle spends in a system), nutrient uptake should also increase.

Eutrophication is known to increase the biomass of benthic algae due to higher nutrient availability and will likely result in higher primary production and higher nutrient uptake. Based on existing scientific literature, eutrophication is likely to have minimal effects on benthic invertebrates.

Fine sedimentation severely affects benthic algae simply by covering them and by removing suitable habitats (coarser sediment), which are buried by fine sediment. Fine sediment also affects benthic invertebrates by clogging the room between coarser sediment particles and removing the access of scraper invertebrates (see the FSC guide to invertebrate feeding methods) to their food source (benthic algae). Finally, fine sediment delivers a large amount of organic matter to the stream, which is expected to strongly increase benthic respiration – which removes oxygen – and thus may counterbalance the effects of algal primary production.

We will keep you updated with the results of this experiment, and the others that the MARS team are carrying out across Europe. You can find links to all the MARS experiment blogs here.