To assess the significance of nutrient interactions on animal performance, life-history experiments were conducted with the freshwater herbivore Daphnia magna in a system with two limiting nutrients (i.e., sterols and polyunsaturated fatty acids [PUFAs]). By experimentally increasing the availability of dietary cholesterol either in the absence or in the presence of different dietary PUFAs we investigated whether dietary PUFAs differ in their potential to affect the sterol-limited growth kinetics of the herbivore. Sterol-limited growth responses of D. magna were characterized by a higher initial slope of the saturation curves and lower threshold concentrations in the presence of dietary PUFAs. This suggests significant interactions between limiting nutrients and lower dietary sterol requirements in the presence of dietary PUFAs. In contrast to somatic growth rates, which were significantly increased only by additional supplementation with eicosapentaenoic acid (EPA), population growth rates were significantly affected by additional supplementation with linoleic acid, a-linolenic acid, docosahexaenoic acid (DHA), and in particular EPA and arachidonic acid. The fatty acid composition of D. magna reflected that of their diet. However, animals fed the DHA supplemented diet contained considerable amounts of EPA, indicating a retro-conversion of DHA into EPA within their metabolism. The fitness of resource-limited consumers is strongly affected by complex interactions between limiting nutrients and this topic has to be considered in experimental and theoretical studies investigating nutrient-limited growth responses of animals to accurately assess consequences for population dynamics.

Proliferative kidney disease (PKD) caused by the myxozoan parasite Tetracapsuloides bryosalmonae is an emerging salmonid disease implicated in recent declines in salmonid populations. Laboratory experiments have shown that the clinical symptoms of PKD exacerbate with increasing temperature. However, empirical evidence for a relationship between climate change and PKD driven declines in wild salmonid stocks is scarce. The current study uses both empirical data and ecological niche modelling to extrapolate future changes in temperature and precipitation on the spread of PKD in relation to changes in suitability of rivers for its primary bryozoan and secondary salmonid hosts. A 20‐year dataset on brown trout density using standardised method shows a decline of more than 50% in the population of the river Wutach, south‐western Germany at 536–755 m elevation. The decline coincides with higher prevalence of PKD. This decline was temperature related and driven by reduced local survival of parr to yearlings. Kidney hyperplasia was highest at low elevations and correlated negatively with trout density. Niche models based on state‐wide data show strong overlap in the areas suitable for T. bryosalmonae and its primary bryozoan hosts, and a strong link with temperature. Projections based on moderate to high emission‐case climate change scenarios predict a 50%–82% decrease in the area suitable for salmonids within this century, linked to PKD. The empirical data identify temperature‐mediated PKD as the underlying cause of the population decline. This relationship was corroborated by the niche modelling results. The highest losses are expected in projected salmonid suitable areas where salmonid habitats currently overlap with areas suitable for T. bryosalmonae. With ongoing climate change, both current and future populations are at severe risk to T. bryosalmonae and its associated disease PKD. Conservation efforts of salmonid stocks are dependent on climate mitigation and measures to help salmonids adapt to the disease.

The accumulation of cyanobacterial biomass may severely affect the performance of aquatic consumers. Here, we investigated the role of sterols in determining the food quality of cyanobacteria for the invasive clam Corbicula fluminea, which has become a common benthic invertebrate in many freshwater ecosystems throughout the world. In standardized growth experiments, juvenile clams were fed mixtures of different cyanobacteria (Anabaena variabilis, Aphanothece clathrata, Synechococcus elongatus) or sterol-containing eukaryotic algae (Cryptomonas sp., Nannochloropsis limnetica, Scenedesmus obliquus). In addition, the cyanobacterial food was supplemented with different sterols. We provide evidence that somatic growth of C. fluminea on cyanobacterial diets is constrained by the absence of sterols, as indicated by a growth-enhancing effect of sterol supplementation. Thus, our findings contribute to our understanding of the consequences of cyanobacterial mass developments for benthic consumers and highlight the importance of considering sterols as potentially limiting nutrients in aquatic food webs.

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Benthic–pelagic coupling mediated by bivalves has been shown to increase the flow of energy towards the benthos. To assess the capability of clams to process and therewith modify the quality of pelagic food sources for subsequent use by benthic invertebrates, we conducted a growth experiment in which juvenile Gammarus roeselii were raised either directly on sedimented pelagic autotrophs (algae, cyanobacteria) or on the same autotrophs biodeposited by the invasive freshwater clam Corbicula fluminea either as feces or pseudofeces. We show that growth and survival of G. roeselii are significantly improved when autotrophs are offered as biodeposition material and suggest that this clam-mediated upgrading of food quality is due to both an increased bioavailability of pelagic food particles, which are packed in mucus during clam processing, and an increased dietary provisioning with essential lipids (sterols and (or) polyunsaturated fatty acids) originating from the clams. Hence, filter-feeding bivalves provide a crucial link between the pelagic and benthic food web not only by deflecting energy fluxes, but also by processing and upgrading pelagic food (increased bioavailability, improved biochemical composition) for benthic invertebrates.

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1. The Danube streber, Zingel streber, is a threatened and data-deficient percid fish endemic to the Danube catchment. The study provides the first data on distribution, life history, and genetic structure of the species at the upstream limit of its historic distribution (south-western Germany). A 3-year surveyeffort with 143 fishing events identified several small, fragmentary populations covering only 7% of the historical range of the species. Census population sizes (N c ) of these subpopulations were estimated from mark-recapture data at <200 individuals. Effective population sizes (N e ), calculated from genetic data (microsatellite genotyping), were much smaller still, at <15 individuals, resulting in an N c /N e ratio of <0.25, strongly indicating that populations are seriously affected by genetic drift and inbreeding, and are thus facing a severe extinction risk. 3. Life-history parameters recorded during the study indicate a rapid life cycle, with both sexes probably attaining sexual maturity at the age of 1 year or older. Spawning commenced at the beginning of April and fecundity was low (~300-400 eggs per female). 4. Genetic analysis and mark-recapture data indicate that subpopulations of the streber live in effective isolation, separated by impassable weirs that significantly reduce genetic connectivity between subpopulations. 5. The species is rheophilic, and limited to sites with flow velocities of~0.7 m s -1 . Hydropower infrastructure may thus also have diminished the availability of suitable habitat by reducing flow rates. 6. Only 32% of the historical range of the Danube streber is now estimated to be morphologically suitable for the species. Furthermore, relevant parts of this range are located upstream of dams and are therefore not accessible for natural recolonization. 7. The availability and accessibility of suitable habitats seem to be factors limiting the size of the remaining subpopulations. 8. Conservation actions should address the restoration of degraded river habitats and increase the connectivity between isolated subpopulations of the Danube streber.

RATIONALE: Dietary sterol deficiencies can have severe life history consequences for consumers. Compound-specific stable isotope analysis (CSIA) was applied to the exploration of the sterol metabolic constraints and bioconversion capacities of the amphipod Gammarus roeselii. Evaluating structural sterol requirements has great potential to improve our understanding of the ecological relevance of sterols as limiting nutrients. METHODS: Juvenile G. roeselii were reared on food mixtures consisting of different ratios of the two algae Scenedesmus obliifuus (cultivated with 13 C-labeled NaHC0 3 ) and Nannochloropsis limnetica (unlabeled), which have been shown previously to differ in food quality. We measured the sterol content and composition using a gas chromatograph equipped with a flame ionization detector and the o 13 C values of sterols using compound-specific isotope ratio mass spectrometry to examine potential sterol-mediated nutritional constraints of G. roeselii. CONCLUSIONS: We provide evidence that the the ~7-phytosterols present in S. obliifUUS cannot be metabolized to cholesterol in G. roeselii, resulting in the accumulation of lathosterol in the animals and potentially in sterol-limited growth. These findings emphasize the advantage of CSIA in revealing the physiological mechanisms associated with n utritional constraints.