EU network of mesocosms facilities for research on marine and freshwater ecosystems open for global collaboration.

Tracking the effects of reduced nitrogen loading

A mesocosm experiment at LMWE Silkeborg Denmark 2018

Whilst we know for certain that eutrophication in fresh waters is caused by increased nutrient levels, the relative influence of the two most important nutrients, phosphorus and nitrogen, is the subject of much debate. Recent work has asserted that eutrophication cannot be controlled by reduction in nitrogen, and indeed there have even been suggestions that the addition of nitrate may be an effective way of countering blue-green algal blooms. Other work has presented a more balanced approach and empirical studies have found nitrogen to be related to higher algal biomass and reduced submerged plant richness.

At the LMWE in 2018 we will address this issue by altering the nutrient input to the long-running experiment. The current experimental set-up consists of 12 mesocosms at low nutrient levels and 12 where both phosphorus and nitrogen are added to increase nutrient loading. In addition to this, there are 3 temperature treatments: 1) Ref – a reference temperature which follows the ‘normal’ seasonal variation, 2) A2 treatment which is 2-3 C above reference and 3) A2+50% which is 5-6 C above reference. The core of this year’s experiment is ceasing the addition of nitrogen to the 12 high nutrient treatment mesocosms on June the 15th.

The LWME experiment provides an excellent set up with which to test the importance of nitrogen in driving eutrophication and how the ecological response might vary with temperature. By reducing the nitrogen load to the high nutrient mesocosms for a year we can investigate how reduced N will influence a range of factors related to community composition, biodiversity and various ecosystem, processes.

During the experiment scientists from a team led by Mark McCarthy from Wright State University, Ohio will investigate the nitrogen cycle, across the different temperature before and after the N addition is removed. Specifically, they will measure water column ammonium (NH4) regeneration, potential uptake, nitrification, and N fixation rates in the water columns of selected mesocosms. They will also elucidate changes in the bacterioplankton community related to the nitrogen cycle using molecular tools.

A second team led by Dr. Natasha Barteneva from Nazarbayev, Kazakhstan will visit the facility to study structural changes in phytoplankton community within mesocosms with low and high nutrient loading at three different temperature regimes based on climate warming scenarios. Specifically, they will assess phytoplankton composition and abundance using flow imaging cytometry FlowCAM and light microscopy in the mesocosms.