Gergely Boros

The importance of sedimenting organic matter, relative to oxygen and temperature, in structuring lake profundal macroinvertebrate assemblages

We quantified the role of a main food resource, sedimenting organic matter (SOM), relative to oxygen (DO) and temperature (TEMP) in structuring profundal macroinvertebrate assemblages in boreal lakes. SOM from 26 basins of 11 Finnish lakes was analysed for quantity (sedimentation rates), quality (C:N:P stoichiometry) and origin (carbon stable isotopes, δ13C). Hypolimnetic oxygen and temperature were measured from each site during summer stratification. Partial canonical correspondence analysis (CCA) and partial regression analyses were used to quantify contributions of SOM, DO and TEMP to community composition and three macroinvertebrate metrics. The results suggested a major contribution of SOM in regulating the community composition and total biomass. Oxygen best explained the Shannon diversity, whereas TEMP had largest contribution to the variation of Benthic Quality Index. Community composition was most strongly related to δ13C of SOM. Based on additional δ13C and stoichiometric analyses of chironomid taxa, marked differences were apparent in their utilization of SOM and body stoichiometry; taxa characteristic of oligotrophic conditions exhibited higher C:N ratios and lower C:P and N:P ratios compared to the species typical of eutrophic lakes. The results highlight the role of SOM in regulating benthic communities and the distributions of individual species, particularly in oligotrophic systems.

Ontogenetic variation in the body stoichiometry of two fish species

One of the central questions of ecological stoichiometry theory is to what extent animal species maintain constant elemental composition in their bodies. Although several recent studies demonstrate intraspecific variation in animal elemental composition, relatively little is known about ontogenetic changes in vertebrates, especially during early life stages. We studied the intraspecific and interspecific ontogenetic variation in the body stoichiometry of two fish species in two different orders; fathead minnow (Pimephales promelas) and sheepshead minnow (Cyprinodon variegatus), reared under controlled laboratory conditions. During ontogeny, we measured the chemical composition of fish bodies, including carbon (C), nitrogen (N), phosphorus (P), calcium (Ca), and ribonucleic acid (RNA) contents. We found that N and RNA contents were relatively high in early life stages and declined substantially during development. In contrast, body C and C:N ratios were relatively low in embryos, post-embryos and larvae, and increased remarkably thereafter. Concentrations and ratios of some elements (e.g., Ca, P, Ca:P) did not exhibit consistent ontogenetic trends, but fluctuated dynamically between consecutive developmental stages in both species. Specific growth rates correlated significantly with RNA contents in both species. Analyses of the relative importance of different P pools at each developmental stage revealed that RNA was a considerable P pool in post-embryos, while bone-associated P was the dominant body P pool in later stages. Our results suggest that the elemental composition of fish bodies changes considerably during ontogeny. Each ontogenetic stage has its own stoichiometric signature, but the timing, magnitude and direction of ontogenetic changes can vary substantially between taxa.

The role of filter-feeding Asian carps in algal dispersion

The gut contents of filter-feeding fish often contain considerable amounts of viable phytoplankton cells; thus, these animals can act as vectors in the horizontal and vertical transport of algae. In this study, the potential role of the introduced filter-feeding Asian carps (hybrids of silver carp Hypophthalmichthys molitrix and bighead carp H. nobilis) in algal dispersion was studied in the oligo-mesotrophic Lake Balaton (Hungary). We examined the algal composition in the lake water, gut contents (foregut and hindgut), and occasionally in the filtered suspensions collected directly from the gill rakers (filtering apparatus) of fish. Microscopic analyses revealed that the phytoplankton composition of the ingested food differed considerably from what we found in the lake water. Cryptophytes, dinoflagellates, and euglenophytes were observed in both the lake water and foregut samples but were absent in the hindgut samples. However, in the cultured hindgut samples, we found viable cells of several phytoplankton taxa (e.g., diatoms, blue-greens, desmids, volvocalean and chlorococcalean green algae), which managed to survive the physical and chemical digestion. These results imply that the presence of these filter-feeding fish can alter the phytoplankton species composition and promote the dominance of taxa that are able to resist digestion.

Gut content microbiota of introduced bigheaded carps (Hypophthalmichthys spp.) inhabiting the largest shallow lake in Central Europe

Studying the microbiota in the alimentary tract of bigheaded carps (Hypophthalmichthys spp.) gained special interest recently, as these types of investigations on non-native fish species may lead to a better understanding of their ecological role and feeding habits in an invaded habitat. For microbiological examinations, bigheaded carp gut contents and water column samples from Lake Balaton (Hungary) were collected from spring to autumn in 2013. Denaturing Gradient Gel Electrophoresis (DGGE) and pyrosequencing of the 16S rRNA gene were performed to reveal the composition. According to the DGGE patterns, bacterial communities of water samples separated clearly from that of the intestines. Moreover, the bacterial communities in the foreguts and hindguts were also strikingly dissimilar. Based on pyrosequencing, both foregut and hindgut samples were predominated by the fermentative genus Cetobacterium (Fusobacteria). The presence of some phytoplankton taxa and the high relative abundance of cellulose-degrading bacteria in the guts suggest that intestinal microbes may have an important role in digesting algae and making them utilizable for bigheaded carps that lack cellulase enzyme. In turn, the complete absence of typical heterotrophic freshwater bacteria in all studied sections of the intestines indicated that bacterioplankton probably has a negligible role in the nutrition of bigheaded carps.

Intrinsic processes causing periodic changes in stability in a shallow biomanipulated lake

Shallow lakes experience alternative states in their biotic organisation over time. In this study, we analysed a long-term dataset on the recovery from eutrophication of a shallow hypertrophic lake (Lake Major, Hungary) following fish manipulation. Disturbances in the food web triggered a shift, markedly affecting both abiotic and biotic variables. Clear and turbid states of Lake Major over the period 1999–2009 were defined by concentrations of chlorophyll-a. Lake Major in a clear-water state had higher transparency and submerged macrophyte cover, lower turbidity and fish biomass, and total phosphorus was relatively low compared to that shown during the turbid-water state. It follows from our study that a regime shift from a turbid state to a clear state can be expected in response to fish manipulation. Reduction in fish biomass increases light conditions, decreases internal nutrient loading and promotes macrophyte dispersion. Food-web effects appear to be considerable in these shallow lakes containing benthic fishes. The roles of phosphorus and submerged macrophytes are essential in maintenance of alternative states of vegetation in shallow lakes of various climatic zones.

Extension of employing ITS region in the investigation of Hungarian Fridericia species

Abstract Same species of genus Fridericia (Oligochaeta: Enchytraeidae) collected from different localities, can have small differences in their morphology, for example, the number of nephridia or length and width of the spermathecal ectal duct. During the identification of enchytraeid worms, several characteristics were investigated at the same time but presence of small variations in only one or two main characteristics can cause taxonomic difficulties or may raise the possibility of encountering a subspecies. Using molecular biological techniques can help answer these questions. In preceding researches of the authors ITS (Internal Transcribed Spacer) came into focus. ITS is mainly a non-coding region of ribosomal DNA located between the 18S and 28S rDNA genes. Based on these previous studies, in the present analysis, the authors extended their investigation to include examination of the morphology and genes of various Fridericia species from different parts of Hungary. Phylogenetic trees were also created by Maximum Parsimony (MP), Neighbor-Joining (NJ), and Bayesian analyses. In the present study, we established that molecular biological techniques are suitable to confirm that individuals from the same species (but collected from different places) have the same genetic profile, while very similar species can be shown to clearly segregate on the phylogenetic tree based on the divergence of certain external or internal characteristics. Even though partial sequences give us only rough information about the phylogenetic position of the species, our aim is to show how the ITS region is suitable to investigate closely related enchytraeid worms and to differentiate the morphologically similar species.

Source of bigheaded carp (Hypophthalmichthys spp.) in Lake Balaton, Hungary: natural recruitment or continuous escapement from aquaculture?

Abstract The worldwide spread of bigheaded carp (Hypophthalmichthys spp.) is considered an ecological threat and problem in many countries. The aim of this study was to consider the likelihood of bigheaded carp reproduction in Lake Balaton and its catchment area (Hungary). The within-year trends of gonadosomatic index values show that female bigheaded carp increase ovary masses from spring until midsummer, after which a decline is typical. This trend may indicate bulk egg release, but the lack of a sharp drop in fecundity (number of eggs in the ovary) values during and after the potential spawning period suggests that a general and considerable ovary evacuation does not likely explain the declining gonadosomatic index values. Rather, it is more probable that female bigheaded carp resorb their eggs, supported by the presence of atretic oocytes in the ovaries after the potential spawning season. In turn, postovulatory follicles were present in the posterior ovary segments of 2 individuals, indicating that mature eggs were released by some females. The evidence of sporadic egg release does not necessarily imply that successful reproduction has occurred, however. The genetic analyses showed remarkable deviation from Hardy-Weinberg equilibrium within the stock, suggesting that bigheaded carp do not constitute a natural reproductive community in Lake Balaton. Thus, the present bigheaded carp stock in Lake Balaton likely derives primarily from external origin (i.e., from the surrounding fish ponds).

Influence of environmental factors and individual traits on the diet of non-native hybrid bigheaded carp (Hypophthalmichthys molitrix × H. nobilis) in Lake Balaton, Hungary

Abstract Planktivorous silver carp and bighead carp (collectively, the bigheaded carps) have been stocked worldwide and their invasion has caused severe impacts on many freshwater ecosystems. Exploiting the chance provided by the specific hybrid bigheaded carp stock in Lake Balaton (Hungary) covering the entire morphological range between the two species (including gill raker morphology), we implemented a comprehensive study (1) to reveal the feeding habits of hybrid bigheaded carps living in a mesotrophic, lacustrine habitat and (2) to assess how biotic and abiotic environmental factors and gill raker morphology affect diet composition. We found that all bigheaded carps utilized primarily zooplankton and neglected the scarce and inefficiently digestible phytoplankton, irrespective of gill raker morphology. Moreover, we observed strikingly high levels of inorganic debris consumption, but the proportion of inorganic matter in the guts was not associated directly with the concentration of suspended inorganic particles. Variance in the diet composition of bigheaded carps was related mostly to environmental factors, including the wind-induced resuspension of inorganic particles and seasonally variable availability of food resources. In conclusion, the effects of abiotic environmental factors and available food resources could overwhelm the effect of gill raker morphology in shaping the feeding habits of bigheaded carps.

First record of reproduction by fragmentation in the genusMarionina: (Oligochaeta: Enchytraeidae)

Abstract Hermaphrodite enchytraeids usually breed sexually, but a few species can also multiply asexually by fragmentation (architomy). These species can alternate sexual breeding with asexual breeding by fragmentation. In Hungary, a Marionina species was found in the botanical garden of University of Szeged in 209, which is also able to reproduce asexually by architomy. No mature specimens of this species have been found so far. Fragmentation was observed rarely in these individuals and a strategy comparable to other fragmenting species is not known.