Cladocera occupy an intermediate link in the trophic web between primary producers (algae) and consumers (planktivorous fish). Cladocera include such a favorite food for fish as daphnia, a filter feeder. In addition to the fact that they purify water from organic suspensions and serve as valuable food for fish, they participate in the transfer of physiologically important substances and energy from algae to fish. Daphnia are able to transform the biochemical composition of algae, unsuitable for fish food, into a composition that is easily absorbed by animals. One of the most important elements that is transferred from algae up the trophic web is phosphorus. Phosphorus is a vital element for animals. It is responsible for the synthesis of lipids, proteins and nucleic acids. A lack of phosphorus leads to a decrease in the rates of individual growth, reproduction and survival.
The aim of the work was to establish how the lack of phosphorus in algae changes the ratio between large and small representatives of daphnia in competition for food. Large and small species of daphnia differ not only in their survival strategies, but also in the phosphorus content in their bodies, namely, a higher phosphorus content is observed in large daphnia. Therefore, the quality of potential food for fish depends on the ratio between small and large species in cladoceran communities, based on the fact that the higher the phosphorus content per unit of carbon in the body of crustaceans, the higher the quality of food for higher trophic levels.
“We conducted a series of experiments on competition between small Daphnia longispina and large Daphnia magna, which confirmed that the phosphorus content in the green alga Chlamydomonas klinobasis changes the outcome of competition. “Algae enriched in phosphorus had a carbon to phosphorus ratio (C:P) of ≈100, while the same species of algae depleted in phosphorus had a C:P of ≈800. Thus, an 8-fold difference in phosphorus content can change the outcome of competition and ensure the transition of dominance from one species to another,” said I.Yu. Feneva, Doctor of Biological Sciences and leading researcher at the Laboratory of Lower Vertebrate Behavior at the A.N. Severtsov Institute of Ecology and Evolution of the Russian Academy of Sciences (IEE RAS).
Figure 2 shows that at low phosphorus levels the large Daphnia magna dominates, while at high levels the small Daphnia longispina dominates. The different outcomes of competition are explained by the change in the competitive ability of the species when the phosphorus content in the algae decreases. The competitive ability is determined by the threshold concentration of the resource, i.e. it is equal to the concentration of the resource at which the mortality rate is equal to the birth rate of crustaceans.
The results of this work are an important step forward in biomanipulation. By changing the phosphorus content in algae, which is done by changing the concentration of inorganic phosphorus forms in the environment, it is possible to change the ratio between cladoceran species in order to improve the quality of biological resources for fish. In particular, such a method can be applied in aquaculture to grow higher quality fish.
The work can be found at the link: Feniova, I., Dzialowski, A.R., Bednarska, A., Brzeziński T., Zilitinkevicz N., Dawidowicz P., 2025. Shifts in competition outcomes between two Daphnia species in response to algal phosphorus content. Oecologia 207, 32.