Piotr Dawidowicz

We studied competitive interactions between Daphnia longispina and D. magna competing for a single algal resource of varying quality in treatments with low and enhanced amounts of bacteria. We proposed that the outcome of competition is influenced by the concentration of bacteria. We used three types of food including high-quality green alga Chlamydomonas klinobasis, phosphorus-poor C. klinobasis and the cyanobacterium Synechococcus elongatus, which is deficient in essential lipids. The medium in the low-bacteria experiment was replaced daily, while the medium in the enhanced-bacteria experiment was changed every other day, thus allowing for prolonged bacterial growth as determined by an additional short-term experiment. Furthermore, we found significantly higher abundances of both Daphnia species in monocultures with enhanced bacteria compared to those in the low-bacteria experiment, indicating that they were feeding on bacteria. In the experiment involving low bacterial densities, the dominance of both Daphnia species depended on the quality of the algae. However, D. magna was the superior competitor across all food quality treatments in the enhanced bacteria experiment. We conclude that enhanced bacteria are a significant factor that shapes competitive interactions in cladoceran communities.

Most alpine-type lakes in the Tatra Mountains were stocked with salmonid fish during the 19th and 20th centuries, triggering profound ecological shifts. In the Five Lakes Valley, trout introductions into Przedni Staw Polski, Czarny Staw Polski and Wielki Staw Polski had contrasting effects on pelagic ecosystems. In the smaller and shallower Przedni Staw Polski and Czarny Staw Polski, fish introduction was associated with the loss of large-bodied zooplankton, increased trophic status and a rapid decline in water transparency—from 13–18 m to 5–7 m. Between 2021 and 2023, partial fish removal in Przedni Staw Polski resulted in a marked increase in water transparency (from 7 to 17 m), coinciding with reduced surface phytoplankton biomass. This improvement likely reflected decreased phosphorus input from fish excretion rather than enhanced zooplankton grazing. Large-bodied cladocerans did not recolonize the lake, although small-bodied taxa increased in abundance. In contrast, Czarny Staw Polski—where fish remain abundant—showed no signs of recovery: water transparency declined further and zooplankton community structure remained unchanged. Unlike the other stocked lakes, Wielki Staw Polski—the largest and deepest—retains high transparency and large-bodied zooplankton, likely due to low fish density or availability of predator-free refugia. While partial fish control in Przedni Staw Polski yielded rapid benefits, restoring natural ecosystem functioning in these fragile alpine ecosystems will require complete fish eradication.

Cyanobacteria negatively affect zooplankton through several mechanisms including mechanical interference, toxicity, and poor food quality due to a shortage of essential lipids. To understand the nature of each of these mechanisms, they should be examined independently. The goal of our study was to assess the influence of cyanobacteria food quality on the competitive outcomes between the small-bodied Daphnia longispina and the large-bodied Daphnia magna. We conducted life-table experiments to assess R* (population threshold food concentration), competition experiments to determine the outcome of competition, and computer simulation experiments at high levels of food supply, which are difficult to realize in laboratory conditions. We used two types of food: the high-quality green algae Chlamydomonas klinobasis (GREEN) and the cyanobacterium Synechococcus elongatus (CYANO), which contains low levels of essential lipids, but is non-toxic and unicellular. We found that the small-bodied D. longispina was a superior competitor in GREEN, while the large-bodied D. magna was more abundant in CYANO. We established that the species ratio in GREEN was dependent on competitive interaction, while abundances of daphnids in CYANO were controlled by poor food quality. Since cyanobacteria act as a powerful force for structuring cladoceran communities, the role of competition for food between these two Daphnia species greatly declined under their effects.

Competition is one of the key drivers of cladoceran community dynamics. Competitive abilities can be measured as the Threshold Food Concentration (TFC), i.e., concentration of food at which population growth rate equals zero. Species with lower TFC should be superior competitors. However, population TFC can vary with environmental conditions. We hypothesized that algal phosphorus (P) content influences TFC and can alter the outcome of competition between large and small-bodied cladocerans. We conducted competition and life table experiments with single clones of large Daphnia magna and small Daphnia longispina to assess how algal P content affected their TFC and competitive interactions. We also conducted computer simulations to further explore competition between the small and large species. Our experiments showed that TFC varied with the algal P content. P limitation increased the TFC of both species, but this increase was more pronounced for the smaller D. longispina. For this reason, D. magna was a superior competitor at low P content, while D. longispina was superior at high P content. We also found that enhanced food abundance gave an additional advantage to the large-bodied D. magna due to higher reproduction potential at high food concentrations.

The aim of our work was to experimentally test the hypothesis that changes in the thermal stratification in lakes of the temperate zone, expected as a result of progressive global warming, will cause an increase in fitness costs in a planktonic cladoceran Daphnia, associated with diel vertical migrations. In a programmable thermostatic chamber, Daphnia magna have been cultured from the first juvenile instar till the first reproduction, in thermal regimes corresponding to the conditions experienced by non-migratory (remaining in epilimnion day and night) and migrating (spending nights in epilimnion and days in hypolimnion) animals. We exposed Daphnia to thermal regimes: corresponding to the temperatures typically reigning stratified lakes nowadays (20 °C in epilimnion and 8 °C in hypolimnion), and expected at the end of this century (28 °C and 12 °C, respectively). The analysis of life-history parameters confirmed the assumption of an increase in metabolic costs resulting from vertical migration of Daphnia along with the expected increase in temperature differences between epilimnion and hypolimnion, which is a daily anti-predator refuge for migrating animals. This may consequently lead to significant changes in the composition of lake zooplankton communities, i.e., to an increase in the share of non-migrating species with small body sizes.

Exotic cladoceran Daphnia lumholtzi is a highly invasive species in the north and south American continents and can potentially also invade European freshwaters and outcompete native Daphnia populations. However, European waterbodies are frequently dominated by less edible filamentous cyanobacteria including also invaders such as Raphidiopsis raciborskii, which might affect the fitness of D. lumholtzi. Furthermore, temperature may influence the sensitivity of D. lumholtzi to R. raciborskii filaments. In this study, we determined whether the presence of R. raciborskii could obstruct the invasion of Europe by D. lumholtzi, through reducing its fitness, and whether this depends on temperature. We compared the population growth rate (r) and the somatic growth rate of D. lumholtzi maintained at two temperatures (20 or 26°C) and fed with two diets: green microalgae alone or green microalgae mixed with filaments of R. raciborskii. Three clones of D. lumholtzi were used to evaluate potential variation in response to the treatments among different clones. At 20°C, the population growth rate of D. lumholtzi fed with cyanobacterial filaments declined sharply. This reduction was caused by increased egg abortion, egg degeneration, and mortality of newborn daphnids. At 26°C, R. raciborskii lost its harmful effect on the population growth of D. lumholtzi. The presence of cyanobacteria did reduce the somatic growth rate of D. lumholtzi at both temperatures and in all three clones except for one that had a similar somatic growth rate on both diets at 26°C. The presence of filamentous cyanobacteria does affect growth in D. lumholtzi and may thereby substantially reduce its invasive potential, but only at lower temperatures. Therefore, the presence of filamentous cyanobacteria may not present an obstacle to the invasion of Europe by D. lumholtzi in a warmer future climate.

Understanding the factors that regulate phytoplankton and zooplankton is an important goal of aquatic ecologists; however, much remains unknown because of complex interactions between phytoplankton, zooplankton, and fish. Zooplankton, in particular cladocerans, can be regulated by bottom–up factors either via food quantity or food quality in terms of polyunsaturated fatty acids (PUFA) or phosphorus (P) contents in phytoplankton. Fish can recycle nutrients and in turn change the PUFA and P contents of algal resources, thus modifying bottom–up regulation. Furthermore, fish can change phytoplankton structure through consumption of cladocerans which selectively graze phytoplankton. We conducted a mesocosm (300 L) experiment to determine how trophic state and fish affected crustacean dynamics. The mesocosms were filled with water containing natural plankton from the eutrophic Lake Jorzec and mesotrophic Lake Majcz (Northeastern Poland), and we manipulated fish presence/absence. We also conducted a complementary life-table experiment to determine how trophic state and fish nonconsumptively affected demographic parameters of the dominant cladocerans in the mesocosms. Small and large cladoceran species responded differently to food quantity and quality. Small-bodied Ceriodaphnia were regulated mainly by resource concentrations (i.e., food quantity), while large species were limited by PUFAs (i.e., food quality). Fish likely increased food quality in terms of PUFA, primarily eicosapentaenoic acids (EPA), thus providing conditions for more successful development of Daphnia than in the fish-free treatments. Phosphorus in the seston was likely limiting for zooplankton. However, food quality in terms of phosphorus was likely less important than PUFA because zooplankton can accumulate nutrients in their body.