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Organic matter leaching from mires and forests offers no benefit for lake food webs

29 Apr 2015

A new study funded by the Academy of Finland indicates that the fatty acid composition of phytoplankton is variable and, thus, their nutritional value to zooplankton and fish is either excellent, good or moderate. The biodegradable plant material and bacteria that enter water bodies from drainage basins are of poor quality and offer only minor added value to the food chains in lakes. The study results indicate that the vendace is an efficient accumulator of beneficial omega-3 fatty acids from its food, which makes its nutritional value to humans excellent.

The leaching of organic matter from mires and forests has increased in recent decades, as shown by the darkening of lakes, for example. Soil material has an impact on lake food webs, particularly on the nutritional quality of fish through phytoplankton and zooplankton.

“It’s important what types of materials end up in lakes through leaching and what materials are produced by the lakes’ own phytoplankton communities,” said Docent Paula Kankaala, from the University of Eastern Finland, who led the study.

Essential fatty acids and amino acids are compounds that zooplankton, fish and humans cannot produce themselves with sufficient efficiency. In lakes, phytoplankton use the energy of sunlight to produce organic compounds from carbon dioxide and inorganic nutrients. Such compounds include the important omega-3 and omega-6 fatty acids. The most crucial ones of these are polyunsaturated EPA and DHA, which zooplankton and fish need for growth and reproduction. Fish also need DHA for brain and eye development.

According to the study, green algae and some blue-green algae only synthesise short-chain polyunsaturated fatty acids, the most common of which is ALA. For this reason, their nutritional value to zooplankton is only moderate, and thus also to fish that eat zooplankton. Diatoms, cryptophytes and dinoflagellates, on the other hand, are also capable of synthesising long-chain polyunsaturated fatty acids such as EPA and DHA, which means that their nutritional value to zooplankton is excellent.

“We studied the nutritional value to zooplankton of particulate material originating from bacteria and terrestrial plants, such as the common reed and birch leaves. Because bacteria contain no essential fatty acids or sterols, zooplankton cannot grow by eating bacteria alone. The nutritional value of material originating from terrestrial plants is also poor, because it only has low concentrations of omega-3 fatty acids and sterols,” said Postdoctoral Researcher Sami Taipale from the University of Helsinki.

Fatty acids in the vendace makes it superfood

The fatty acid composition of different zooplankton groups varies significantly, which is also shown in the fatty acid composition of the fish that eat them. Water fleas contain high concentrations of EPA fatty acids, while copepods have a high content of DHA fatty acids. Water fleas are the main zooplankton group in small lakes, while the share of copepods is higher in large lakes.

“Fatty acids are efficiently accumulated from food by vendace, a species of planktivorous fish typically found in large boreal lakes. This particularly applies to DHA, which accounts for 20–40 per cent of the total fatty acid composition of vendace. The fatty acid composition of vendace makes them an excellent food for humans. It’s real superfood,” said Paula Kankaala.

The preliminary results also indicate that global warming and precipitation have an impact on phytoplankton composition, zooplankton growth and the EPA and DHA content in fish. One objective for future research is to study more specifically the effects of eutrophication and darkening on the production levels of lakes and the EPA and DHA content in fish.

More information

  • Docent Paula Kankaala, University of Eastern Finland, tel. +358 294 453 046, paula.kankaala(at)uef.fi
  • Postdoctoral Researcher Sami Taipale, University of Helsinki/Lammi Biological Station, tel. +358 50 351 6261, sami.taipale(at)jyu.fi

Academy of Finland Communications
Communications Manager Riitta Tirronen
tel. +358 295 335 118
firstname.lastname(at)aka.fi

Last modified 5 May 2015
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