Food density drives diet shift of the invasive mysid shrimp, Limnomysis benedeni.

Understanding the diet preferences and food selection of invasive species is crucial to better predict their impact on community structure and ecosystem functioning. Limnomysis benedeni, a Ponto-Caspian invasive mysid shrimp, is one of the most successful invaders in numerous European river and lake ecosystems. While existing studies suggest potentially strong trophic impact due to high predation pressure on native plankton communities, little is known of its food selectivity between phyto- and zooplankton, under different food concentrations. Here, we therefore investigated the feeding selectivity of L. benedeni on two commonly occurring prey organisms in freshwaters, the small rotifer zooplankton Brachionus calyciflorus together with the microphytoplankton Cryptomonas sp. present in increasing densities. Our results demonstrated a clear shift in food selection, with L. benedeni switching from B. calyciflorus to Cryptomonas sp. already when the two prey species were provided in equal biomasses. Different functional responses were observed for the two food types, indicating somewhat different foraging mechanisms for each food type. These findings provide experimental evidence on the feeding flexibility of invasive mysid shrimps and potential implications for trophic interactions in invaded ecosystems.

Environmental changes associated with drying climate are expected to affect functional groups of pro- and microeukaryotes differently in temporary saline waters.

Temporary ponds are among the most sensitive aquatic habitats to climate change. Their microbial communities have crucial roles in food webs and biogeochemical cycling, yet how their communities are assembled along environmental gradients is still understudied. This study aimed to reveal the environmental drivers of diversity (OTU-based richness, evenness, and phylogenetic diversity) and community composition from a network of saline temporary ponds, soda pans, in two consecutive spring seasons characterized by contrasting weather conditions. We used DNA-based molecular methods to investigate microbial community composition. We tested the effect of environmental variables on the diversity of prokaryotic (Bacteria, Cyanobacteria) and microeukaryotic functional groups (ciliates, heterotrophic flagellates and nanoflagellates, fungi, phytoplankton) within and across the years. Conductivity and the concentration of total suspended solids and phosphorus were the most important environmental variables affecting diversity patterns in all functional groups. Environmental conditions were harsher and they also had a stronger impact on community composition in the dry spring. Our results imply that these conditions, which are becoming more frequent with climate change, have a negative effect on microbial diversity in temporary saline ponds. This eventually might translate into community-level shifts across trophic groups with changing local conditions with implications for ecosystem functioning.

The effects of interspecific interactions between bloom forming cyanobacteria and Scenedesmus quadricauda (chlorophyta) on their photophysiology.

Eutrophication and enhanced external nutrient loading of lakes and seas are most clearly reflected by increased cyanobacterial blooms, which are often toxic. Freshwater cyanobacteria produce a number of bioactive secondary metabolites, some of which have allelopathic properties, significantly influencing the biological processes of other algae, thereby affecting species composition and succession of the phytoplankton. The goal of this work was to investigate the influence of bloom-forming cyanobacterial exudates on the photophysiology of the green alga Scenedesmus quadricauda by chlorophyll fluorescence analysis. We were able to prove the effect of algal cell-free filtrates on the performance of S. quadricauda and demonstrate for the first time that the freshwater picocyanobacterium Cyanobium gracile has strong negative impact on the coexisting green alga. Neither the cyanotoxin (MYC, CYN and ATX) producing, nor the non-toxic strains showed any systematic effect on the production of S. quadricauda. Various strains of the cyanobacterium Cylindrospermopsis raciborskii inhibited the performance of the green alga independently of their origin. Our results urge further studies for a better understanding of the factors affecting the release of allelopathic compounds and the mechanisms of their effects on target organisms.

Unusual behaviour of phototrophic picoplankton in turbid waters.

Autotrophic picoplankton (APP) abundance and contribution to phytoplankton biomass was studied in Hungarian shallow lakes to test the effect of inorganic turbidity determining the size distribution of the phytoplankton. The studied lakes displayed wide turbidity (TSS: 4-2250 mg l-1) and phytoplankton biomass (chlorophyll a: 1-460 µg l-1) range, as well as APP abundance (0 and 100 million cells ml-1) and contribution (0-100%) to total phytoplankton biomass. Inorganic turbidity had a significant effect on the abundance and contribution of APP, resulting in higher values compared to other freshwater lakes with the same phytoplankton biomass. Our analysis has provided empirical evidence for a switching point (50 mg l-1 inorganic turbidity), above which turbidity is the key factor causing APP predominance regardless of phytoplankton biomass in shallow turbid lakes. Our results have shown that turbid shallow lakes are unique waters, where the formerly and widely accepted model (decreasing APP contribution with increasing phytoplankton biomass) is not applicable. We hypothesize that this unusual behaviour of APP in turbid waters is a result of either diminished underwater light intensity or a reduced grazing pressure due to high inorganic turbidity.

Picophytoplankton predominance in hypersaline lakes (Transylvanian Basin, Romania).

The occurrence and importance of photoautotrophic picoplankton (PPP, cells with a diameter <2 µm) was studied along a trophic and salinity gradient in hypersaline lakes of the Transylvanian Basin (Romania). The studied lakes were found to be rich in PPP, with abundances (maximum 7.6 × 10(6) cells mL(-1)) higher than in freshwater and marine environments of similar trophic conditions. The contribution of PPP to the total phytoplankton biovolume did not decrease with increasing trophic state as it was generally found in other aquatic environments. Regardless of the trophic conditions, the contribution of PPP could reach 90-100 % in these hypersaline lakes. We hypothesized that the PPP predominance might be the result of the low grazing pressure, since heterotrophic nanoflagellates (the main grazers of PPP) were absent in the studied samples. There were significant differences in community composition among the lakes along the salinity gradient. CyPPP predominated in less saline waters (mainly below 5 %), while EuPPP were present along the entire salinity range (up to 18.7 %), dominating the phytoplankton between 3 and 13 % salinity. Above 13 % salinity, the phytoplankton was composed mainly of Dunaliella species.

Unique picoeukaryotic algal community under multiple environmental stress conditions in a shallow, alkaline pan.

Winter phytoplankton communities in the shallow alkaline pans of Hungary are frequently dominated by picoeukaryotes, sometimes in particularly high abundance. In winter 2012, the ice-covered alkaline Zab-szék pan was found to be extraordinarily rich in picoeukaryotic green algae (42-82 × 10(6) cells ml(-1)) despite the simultaneous presence of multiple stressors (low temperature and light intensity with high pH and salinity). The maximum photosynthetic rate of the picoeukaryote community was 1.4 µg C µg chlorophyll a (-1) h(-1) at 125 µmol m(-2) s(-1). The assimilation rates compared with the available light intensity measured on the field show that the community was considerably light-limited. Estimated areal primary production was 180 mg C m(-2) d(-1). On the basis of the 18S rRNA gene analysis (cloning and DGGE), the community was phylogenetically heterogeneous with several previously undescribed chlorophyte lineages, which indicates the ability of picoeukaryotic communities to maintain high genetic diversity under extreme conditions.