Feeding specialists on fatty acid-rich prey have higher gonad weights: Pay-off in Baltic perch?

Individual specialization is a common phenomenon throughout the animal kingdom. Many studies have identified intraspecific competition as one of the main drivers for individual feeding specialization. These studies have mainly considered the quantity of resources, commonly overlooking qualitative aspects of the diet. For example, highly unsaturated fatty acids of the omega-3 class (omega-3 HUFAs) are related to optimal health and growth in consumers. However, little is known on direct fitness consequences for consumers of natural populations that specialize on high-quality resources, such as those rich in omega-3 HUFAs. Despite being such an important qualitative aspect of the diet, it is still unknown whether natural populations show among-individual variation in their choice on prey items that are either rich or poor in HUFAs, and how it affects individual performances. In this study, we investigated whether there is individual feeding specialization and whether it is related to fitness benefits, in a population of perch (Perca fluviatilis) in the Baltic Sea. The contribution of pelagic planktivorous fish to the diet varied from 17% to 61% among perch individuals, as depicted by stable isotope mixing models. This variation in diet was also qualitative, as the omega-3 HUFA content differed among prey types. Specialization on the high-quality resource pelagic planktivorous fish was associated with the proportions of omega-3 HUFA in the individuals' muscles and individuals among those with the highest proportions of omega-3 HUFAs had the greatest relative gonad weight (gonadosomatic index, GSI), a proxy for reproductive investment. Thus, our results highlight the function of food quality for individual specialization and its potential to have direct fitness benefits, playing a major role in shaping ecological interactions

Fatty acid accumulation in feeding types of a natural freshwater fish population

Fatty acids are widely used to study trophic interactions in food web assemblages. Generally, it is assumed that there is a very small modification of fatty acids from one trophic step to another, making them suitable as trophic biomarkers. However, recent literature provides evidence that many fishes possess genes encoding enzymes with a role in bioconversion, thus the capability for bioconversion might be more widespread than previously assumed. Nonetheless, empirical evidence for biosynthesis occurring in natural populations remains scarce. In this study, we investigated different feeding types of perch (Perca fluviatilis) that are specialized on specific resources with different levels of highly unsaturated fatty acids (HUFAs), and analyzed the change between HUFA proportions in perch muscle tissue compared to their resources. Perch showed matching levels to their resources for EPA, but ARA and especially DHA were accumulated. Compound-specific stable isotope analyses helped us to identify the origin of HUFA carbon. Our results suggest that perch obtain a substantial amount of DHA via bioconversion when feeding on DHA-poor benthic resources. Thus, our data indicate the capability of bioconversion of HUFAs in a natural freshwater fish population

The interaction between metabolic rate, habitat choice, and resource use in a polymorphic freshwater species

Resource polymorphism is common across taxa and can result in alternate ecotypes with specific morphologies, feeding modes, and behaviors that increase performance in a specific habitat. This can result in high intraspecific variation in the expression of specific traits and the extent to which these traits are correlated within a single population. Although metabolic rate influences resource acquisition and the overall pace of life of individuals it is not clear how metabolic rate interacts with the larger suite of traits to ultimately determine individual fitness. We examined the relationship between metabolic rates and the major differences (habitat use, morphology, and resource use) between littoral and pelagic ecotypes of European perch (Perca fluviatilis) from a single lake in Central Sweden. Standard metabolic rate (SMR) was significantly higher in pelagic perch but did not correlate with resource use or morphology. Maximum metabolic rate (MMR) was not correlated with any of our explanatory variables or with SMR. Aerobic scope (AS) showed the same pattern as SMR, differing across habitats, but contrary to expectations, was lower in pelagic perch. This study helps to establish a framework for future experiments further exploring the drivers of intraspecific differences in metabolism. In addition, since metabolic rates scale with temperature and determine predator energy requirements, our observed differences in SMR across habitats will help determine ecotype-specific vulnerabilities to climate change and differences in top-down predation pressure across habitats

Effects of mining activities on fish communities and food web dynamics in a lowland river

Fish communities of streams and rivers might be substantially subsidized by terrestrial insects that fall into the water. Although such animal‐mediated fluxes are increasingly recognized, little is known about how anthropogenic perturbations may influence the strength of such exchanges. Intense land use, such as lignite mining, may impact a river ecosystem due to the flocculation of iron (III) oxides, thus altering food web dynamics. We compared sections of the Spree River in North‐East Germany that were greatly influenced by iron oxides with sections located downstream of a dam where passive remediation technologies are applied. Compared to locations downstream of the dam, the abundance of benthic macroinvertebrates at locations of high iron concentrations upstream of the dam was significantly reduced. Similarly, catch per unit effort of all fish was significantly higher in locations downstream of the dam compared to locations upstream of the dam, and the condition of juvenile and adult piscivorous pike Esox lucius was significantly lower in sections of high iron concentrations. Using an estimate of short‐term (i.e., metabarcoding of the gut content) as well as longer‐term (i.e., hydrogen stable isotopes) resource use, we could demonstrate that the three most abundant fish species, perch Perca fluviatilis, roach Rutilus rutilus, and bleak Alburnus alburnus, received higher contributions of terrestrial insects to their diet at locations of high iron concentration. In summary, lotic food webs upstream and downstream of the dam greatly differed in the overall structure with respect to the energy available for the highest tropic levels and the contribution of terrestrial insects to the diet of omnivorous fish. Therefore, human‐induced environmental perturbations, such as river damming and mining activities, represent strong pressures that can alter the flow of energy between aquatic and terrestrial systems, indicating a broad impact on the landscape level.Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659Fischereiabgabe Brandenburg, Ministerium für Landwirtschaft, Umwelt und Klimaschutz des Landes BrandenburgPeer Reviewe

Empirical Correspondence Between Trophic Transfer Efficiency in Freshwater Food Webs and the Slope of Their Size Spectra

The density of organisms declines with size, because larger organisms need more energy than smaller ones and energetic losses occur when larger organisms feed on smaller ones. A potential expression of density-size distributions are Normalized Biomass Size Spectra (NBSS), which plot the logarithm of biomass independent of taxonomy within bins of logarithmic organismal size, divided by the bin width. Theoretically, the NBSS slope of multi-trophic communities is exactly 1.0 if the trophic transfer efficiency (TTE, ratio of production rates between adjacent trophic levels) is 10% and the predator-prey mass ratio (PPMR) is fixed at 104 . Here we provide evidence from four multi-trophic lake food webs that empirically estimated TTEs correspond to empirically estimated slopes of the respective community NBSS. Each of the NBSS considered pelagic and benthic organisms spanning size ranges from bacteria to fish, all sampled over three seasons in 1 yr. The four NBSS slopes were significantly steeper than 1.0 (range 1.14 to 1.19, with 95% CIs excluding 1). The corresponding average TTEs were substantially lower than 10% in each of the four food webs (range 1.0% to 3.6%, mean 1.85%). The overall slope merging all biomass-size data pairs from the four systems (1.17) was almost identical to the slope predicted from the arithmetic mean TTE of the four food webs (1.18) assuming a constant PPMR of 104 . Accordingly, our empirical data confirm the theoretically predicted quantitative relationship between TTE and the slope of the biomass-size distribution. Furthermore, we show that benthic and pelagic organisms can be merged into a community NBSS, but future studies have yet to explore potential differences in habitat-specific TTEs and PPMRs. We suggest that community NBSS may provide valuable information on the structure of food webs and their energetic pathways, and can result in improved accuracy of TTE-estimates

A regime shift from macrophyte to phytoplankton dominance enhances carbon burial in a shallow, eutrophic lake

Ecological regime shifts and carbon cycling in aquatic systems have both been subject to increasing attention in recent years, yet the direct connection between these topics has remained poorly understood. A four-fold increase in sedimentation rates was observed within the past 50 years in a shallow eutrophic lake with no surface in- or outflows. This change coincided with an ecological regime shift involving the complete loss of submerged macrophytes, leading to a more turbid, phytoplankton- dominated state. To determine whether the increase in carbon (C) burial resulted from a comprehensive transformation of C cycling pathways in parallel to this regime shift, we compared the annual C balances (mass balance and ecosystem budget) of this turbid lake to a similar nearby lake with submerged macrophytes, a higher transparency, and similar nutrient concentrations. C balances indicated that roughly 80% of the C input was permanently buried in the turbid lake sediments, compared to 40% in the clearer macrophyte-dominated lake. This was due to a higher measured C burial efficiency in the turbid lake, which could be explained by lower benthic C mineralization rates. These lower mineralization rates were associated with a decrease in benthic oxygen availability coinciding with the loss of submerged macrophytes. In contrast to previous assumptions that a regime shift to phytoplankton dominance decreases lake heterotrophy by boosting whole-lake primary production, our results suggest that an equivalent net metabolic shift may also result from lower C mineralization rates in a shallow, turbid lake. The widespread occurrence of such shifts may thus fundamentally alter the role of shallow lakes in the global C cycle, away from channeling terrestrial C to the atmosphere and towards burying an increasing amount of C

Increased pore size of scaffolds improves coating efficiency with sulfated hyaluronan and mineralization capacity of osteoblasts

Background: Delayed bone regeneration of fractures in osteoporosis patients or of critical-size bone defects after tumor resection are a major medical and socio-economic challenge. Therefore, the development of more effective and osteoinductive biomaterials is crucial. Methods: We examined the osteogenic potential of macroporous scaffolds with varying pore sizes after biofunctionalization with a collagen/high-sulfated hyaluronan (sHA3) coating in vitro. The three-dimensional scaffolds were made up from a biodegradable three-armed lactic acid-based macromer (TriLA) by cross-polymerization. Templating with solid lipid particles that melt during fabrication generates a continuous pore network. Human mesenchymal stem cells (hMSC) cultivated on the functionalized scaffolds in vitro were investigated for cell viability, production of alkaline phosphatase (ALP) and bone matrix formation. Statistical analysis was performed using student's t-test or two-way ANOVA. Results: We succeeded in generating scaffolds that feature a significantly higher average pore size and a broader distribution of individual pore sizes (HiPo) by modifying composition and relative amount of lipid particles, macromer concentration and temperature for cross-polymerization during scaffold fabrication. Overall porosity was retained, while the scaffolds showed a 25% decrease in compressive modulus compared to the initial TriLA scaffolds with a lower pore size (LoPo). These HiPo scaffolds were more readily coated as shown by higher amounts of immobilized collagen (+ 44%) and sHA3 (+ 25%) compared to LoPo scaffolds. In vitro, culture of hMSCs on collagen and/or sHA3-coated HiPo scaffolds demonstrated unaltered cell viability. Furthermore, the production of ALP, an early marker of osteogenesis (+ 3-fold), and formation of new bone matrix (+ 2.5-fold) was enhanced by the functionalization with sHA3 of both scaffold types. Nevertheless, effects were more pronounced on HiPo scaffolds about 112%. Conclusion: In summary, we showed that the improvement of scaffold pore sizes enhanced the coating efficiency with collagen and sHA3, which had a significant positive effect on bone formation markers, underlining the promise of using this material approach for in vivo studies. © 2019 The Author(s)

Comprehensive analysis of chemical and biological problems associated with browning agents used in aquatic studies

Inland waters receive and process large amounts of colored organic matter from the terrestrial surroundings. These inputs dramatically affect the chemical, physical, and biological properties of water bodies, as well as their roles as global carbon sinks and sources. However, manipulative studies, especially at ecosystem scale, require large amounts of dissolved organic matter with optical and chemical properties resembling indigenous organic matter. Here, we compared the impacts of two leonardite products (HuminFeed and SuperHume) and a freshly derived reverse osmosis concentrate of organic matter in a set of comprehensive mesocosm- and laboratory-scale experiments and analyses. The chemical properties of the reverse osmosis concentrate and the leonardite products were very different, with leonardite products being low and the reverse osmosis concentrate being high in carboxylic functional groups. Light had a strong impact on the properties of leonardite products, including loss of color and increased particle formation. HuminFeed presented a substantial impact on microbial communities under light conditions, where bacterial production was stimulated and community composition modified, while in dark potential inhibition of bacterial processes was detected. While none of the browning agents inhibited the growth of the tested phytoplankton Gonyostomum semen, HuminFeed had detrimental effects on zooplankton abundance and Daphnia reproduction. We conclude that the effects of browning agents extracted from leonardite, particularly HuminFeed, are in sharp contrast to those originating from terrestrially derived dissolved organic matter. Hence, they should be used with great caution in experimental studies on the consequences of terrestrial carbon for aquatic systems

Die Effekte von struktureller Komplexität auf ökologische und evolutionsbiologische Prozesse in Flachseesystemen

Most of the world’s lakes are small and shallow. Shallow lakes can have two alternative equilibria, either a macrophyte-dominated and clear-water state or a turbid, phytoplankton-dominated state. Littoral zones (defined as the areas colonized by macrophytes) may therefore differ between shallow lakes of alternative stable states in their extent and structural complexity. Littoral zones act as ecological boundaries, connecting lake ecosystems with their terrestrial surrounding. Pulsed annual fluxes of terrestrial particulate organic carbon (tPOC) in the form of leaf litter enter temperate lakes at the littoral zone, and food webs may obtain a significant proportion of carbon via this allochthonous subsidy. In the first part of my thesis, I present the results of whole-lake experiments mimicking leaf litterfall. We added maize leaves (Zea mays) as an isotopically distinct carbon tracer into one half of a turbid and one half of a clear-water shallow lake, each divided by a plastic curtain. During the subsequent year, carbon isotope values for benthic macroinvertebrates and fish were significantly higher with maize additions than in the reference side of each lake, demonstrating experimentally that tPOC was incorporated up to higher trophic levels of the lake food webs. I furthermore established the existence of reciprocal fluxes of tPOC back into the terrestrial surroundings. Larval Chironomidae fed on the added maize leaves and after emergence, served as prey for spiders living in the riparian reed belt. These findings indicate a close functional coupling of aquatic ecosystems with the adjacent terrestrial habitats. In the second part of my thesis, I consider the structural complexity provided by macrophytes as a selective agent, directly and indirectly shaping populations of freshwater fish. The structural complexity of near-shore habitats provides a more diverse set of resources compared to the open-water zones and a resource-driven polymorphism has been reported for numerous fish species along the littoral- pelagic axes of lakes. In my thesis, I studied whether a similar polymorphism occurs among lakes that vary in structural complexity. I further considered predation pressure as a selective trait known to alter the phenotype of an organism. I analyzed the body shape of omnivorous roach (Rutlius rutilus) from four shallow lakes, which differed in littoral structural complexity and predator abundance (pike Esox lucius). Roach from the lake with the highest predation pressure showed the most distinct body shape, characterized by a more streamlined body and caudally inserted dorsal fins; features that facilitate predator escape. Surprisingly, diet composition was not associated with morphology and I concluded that a variable morphotype facilitating the efficient uptake of a variety of spatially and temporarily scattered resources seems to be favored in these small aquatic systems. Altogether, this thesis adds an important body of knowledge to our understanding of the importance of the littoral zone for species interaction and food web dynamics in shallow lake ecosystems.Die meisten Seen der Welt sind klein und flach. Flachseen kommen in zwei alternativen stabilen Zuständen vor: Sie sind entweder klar und von Makrophyten dominiert oder trüb und von Phytoplankton dominiert. Folglich unterscheidet sich das Litoral (definiert als die Zone, welche von Makrophyten besiedelt ist) in seiner Ausdehnung sowie strukturellen Komplexität zwischen Flachseen unterschiedlicher stabiler Zustände. Das Litoral wirkt als ökologische Grenzzone, welche das Ökosystem des Sees mit dem terrestrischen Umland verbindet. Terrestrischer partikulärer organischer Kohlenstoff (tPOC) in Form von Blättern wird einmal jährlich über die Litoralzone in das Seeökosystem eingetragen. Die aquatischen Nahrungsnetze können somit einen signifikanten Anteil ihres Kohlenstoffs über diesen allochthonen Beitrag erhalten. Im ersten Teil meiner Doktorarbeit präsentiere ich die Ergebnisse von Großexperimenten zur Simulation des Laubfalls in Flachseen. Es wurden Maisblätter (Zea mays) in jeweils eine Hälfte eines trüben und eines klaren Flachsees eingebracht, welche durch eine Plastikplane geteilt wurden. Im darauf folgenden Jahr waren die Kohlenstoff-Isotopensignaturen von Makroinvertebraten und Fischen in den Seehälften der Maiszugabe im Vergleich zu den Referenzseiten signifikant erhöht. Damit wurde experimentell bewiesen, dass tPOC bis in die höheren trophischen Ebenen genutzt wird. Chironomidenlarven fraßen von den zugefügten Maisblättern und wurden nach ihrer Emergenz Beute von Spinnen, welche den Schilfgürtel bewohnten. Diese Ergebnisse weisen auf eine enge funktionelle Kopplung von aquatischen Ökosystemen mit den angrenzenden terrestrischen Habitaten hin. Im zweiten Teil meiner Doktorarbeit habe ich überprüft, ob die durch Makrophyten hervorgerufene strukturelle Komplexität eines Sees als Selektionsfaktor wirkt, der direkt und indirekt Fischpopulationen formen kann. Die ufernahen Habitate mit ihrer strukturellen Komplexität bietet im Vergleich zur Freiwasserzone ein diverseres Nahrungsangebot. Für zahlreiche Fischarten ist ein ressourcenbasierter Polymorphismus entlang der litoral-pelagischen Achse innerhalb von Seen bekannt. In meiner Arbeit habe ich getestet, ob ein ähnlicher Polymorphismus auch zwischen Seen mit unterschiedlicher struktureller Vielfalt auftritt. Des Weiteren habe ich den Prädationsdruck als selektiv wirkendes Merkmal für Änderungen im Phänotyp von Fischen betrachtet. Dazu habe ich die Körperform von omnivoren Rotaugen (Rutilus rutilus) aus vier Flachseen untersucht, welche sich in der strukturellen Vielfalt der Makrophyten sowie der Abundanz von räuberischen Hechten (Esox lucius) unterscheiden. Hoher Prädationsdruck führte außerdem zu einer starken Änderung der Körperform von Rotaugen hin zu einem stromlinienförmigem Körper und einer caudal angefügten Dorsalflosse, eine Körperform, die die Flucht erleichtert. Überraschenderweise war die Nahrungszusammensetzung nicht mit der Morphologie verknüpft. Offensichtlich wird in diesen kleinen aquatischen Systemen ein variabler Morphotypus gefördert, der die effektive Aufnahme einer Vielfalt von räumlich und zeitlich verteilten Ressourcen erleichtert. Insgesamt trägt diese Dissertation zum Verständnis der Bedeutung des Litorals für die terrestrisch- aquatische Kopplung, die Interaktion von Arten, sowie die Dynamik von Nahrungsnetzen in Flachseen bei

Feeding specialists on fatty acid-rich prey have higher gonad weights : Pay-off in Baltic perch?

Individual specialization is a common phenomenon throughout the animal kingdom. Many studies have identified intraspecific competition as one of the main drivers for individual feeding specialization. These studies have mainly considered the quantity of resources, commonly overlooking qualitative aspects of the diet. For example, highly unsaturated fatty acids of the omega-3 class (omega-3 HUFAs) are related to optimal health and growth in consumers. However, little is known on direct fitness consequences for consumers of natural populations that specialize on high-quality resources, such as those rich in omega-3 HUFAs. Despite being such an important qualitative aspect of the diet, it is still unknown whether natural populations show among-individual variation in their choice on prey items that are either rich or poor in HUFAs, and how it affects individual performances. In this study, we investigated whether there is individual feeding specialization and whether it is related to fitness benefits, in a population of perch (Perca fluviatilis) in the Baltic Sea. The contribution of pelagic planktivorous fish to the diet varied from 17% to 61% among perch individuals, as depicted by stable isotope mixing models. This variation in diet was also qualitative, as the omega-3 HUFA content differed among prey types. Specialization on the high-quality resource pelagic planktivorous fish was associated with the proportions of omega-3 HUFA in the individuals' muscles and individuals among those with the highest proportions of omega-3 HUFAs had the greatest relative gonad weight (gonadosomatic index, GSI), a proxy for reproductive investment. Thus, our results highlight the function of food quality for individual specialization and its potential to have direct fitness benefits, playing a major role in shaping ecological interactions