Can the carbon cycle be complete in a cultivation method that combines aquaculture of fish and aquaponic cultivation of plants? This is now being studied at the University of Eastern Finland for the first time.
In aquaponic cultivation, plants utilise nutrients produced by fish directly from water, and no nutrients are released into the environment. The method has been used for a long time both in Finland and abroad, and it is easily scalable from at-home solutions to industrial halls, as long as there is sufficient growth area for plants. Food could be grown this way in, for example, restaurants and care facilities.
Although there’s been plenty of interest in the “plant factory”, the cultivation method is still awaiting its breakthrough in Finland. Finding the right balance has been a problem, because many things affect the nutrient balance of water and, consequently, the growth and health of fish and plants.
“Previously, we have studied the nitrogen cycle in co-cultivation of fish and plants. Now, for the first time, we are also studying the carbon cycle,” Researcher Harri Kokko from the Department of Environmental and Biological Sciences says. He has been studying vertical farming in urban settings for many years.
Indeed, the University of Eastern Finland is home to plenty of expertise in the plant sector, covering different plant species and their coexistence, nutrient needs, and optimisation of growth conditions. The cycle of various compounds is also modelled by using stable isotope analysis.
“It is possible to achieve a fully closed carbon cycle if conditions can be optimised.”
“We already have an idea of how to make the cycle work,” Research Manager Japo Jussila says. His responsibility in the study pertains to fish.
With a new continuous system, data is collected six times per minute. The database contains information on the amount and ratio of nitrates and ammonium, water temperature, and acidity. The fish are migratory whitefish, and the plants are nettle and basil.
In the aquaponic cultivation experiment, food containing a C13-isotope label is fed to the fish. It can be used to monitor the circulation of carbon in the system.
The fish eat their feed, metabolise it, and produce carbon dioxide and nitrogen compounds that move to a biofilter.
In the biofilter, microbes convert the nutrients into ones that are suitable for the plants. The nutrients are transferred from the biofilter to the fishpond and the plant bed.
In the plant bed, the plants’ roots are in water, in their own pool. The plants remove nutrients and compounds from the water, making the aquaponic system more comfortable for the fish. This creates a balance for the entire system.
Thanks to a simple overall cycle, clean production of fish and plants is possible.
One of the most important compounds we study is ammoniacal nitrogen re-cycling.
Harri Kokko
Researcher, Department of Environmental and Biological Sciences
A balanced function of the system is also affected by the number of fish and plants, the content of the fish feed, and the feed rate. In mere hydroponic cultivation, there are no soil bacteria at all. In addition to carbon dioxide, cultivation produces water vapour and organic compounds.
“The carbon cycle continues as the carbon dioxide exhaled by the fish gets passed to the plants and microbes. This, too, can be modelled in the future,” Jussila adds.
Already now, it is evident that the plants remove phosphorus from the water very effectively, and they produce a lot of plant biomass.
“However, more nitrogen remains circulating in the system, but this can be influenced by plant choices,” the researchers say.
“The next step is a more complex system in which cultivation can be fine-tuned even more.
Sanni Semberg, an environmental science student and research assistant, is writing her Master’s thesis on the circulation of carbon in the aquaponic system. By using carbon isotopes, the goal is to study how carbon produced by the fish gets passed to the plants. Semberg also examines whether some plants can use their roots to absorb dissolved inorganic carbon directly from the water.
“I have focused on water-related issues in my studies, and when I was looking for a Master’s thesis project, Harri mentioned this opportunity and I got interested in it. It is interesting to see whether the plants use their roots to absorb carbon from the water, because the studies I have read on the subject have reported varying results,” Semberg says.
“I don't have any more detailed plans for after graduation – we’ll see what happens. However, I will continue in the field of environmental science.”
The research environment for aquaponic cultivation has been built with EU investment funding and is part of a large water cluster project.
UEF Water is a water research community at the University of Eastern Finland whose activities aim at the sustainable use of water resources and the aquatic environment.