The concentration of dissolved oxygen in aquatic systems helps to regulate biodiversity1'2, nutrient biogeochemistry3, greenhouse gas emissions4, and the quality of drinking water5. The long-term ...declines in dissolved oxygen concentrations in coastal and ocean waters have been linked to climate warming and human activity6,7, but little is known about the changes in dissolved oxygen concentrations in lakes. Although the solubility of dissolved oxygen decreases with increasing water temperatures, long-term lake trajectories are difficult to predict. Oxygen losses in warming lakes may be amplified by enhanced decomposition and stronger thermal stratification8,9 or oxygen may increase as a result of enhanced primary production10. Here we analyse a combined total of45,148 dissolved oxygen and temperature profiles and calculate trends for 393 temperate lakes that span 1941 to 2017. We find that a decline in dissolved oxygen is widespread in surface and deep-water habitats. The decline in surface waters is primarily associated with reduced solubility under warmer water temperatures, although dissolved oxygen in surface waters increased in a subset of highly productive warming lakes, probably owing to increasing production of phytoplankton. By contrast, the decline in deep waters is associated with stronger thermal stratification and loss of water clarity, but not with changes in gas solubility. Our results suggest that climate change and declining water clarity have altered the physical and chemical environment of lakes. Declines in dissolved oxygen in freshwater are 2.75 to 9.3 times greater than observed in the world's oceans6,7 and could threaten essential lake ecosystem services2,3,5'11.
Emerging evidence suggests that zooplankton production is affected by physiological and nutritional constraints due to climate change and eutrophication, which in turn could have broad implications ...for food-web dynamics and fisheries production. In this study, we developed a resource-based zooplankton production dynamics model that causally links freshwater cladoceran and copepod daily production-to-biomass (P/B) ratios with water temperature, phytoplankton biomass and community composition, and zooplankton feeding selectivity. This model was used to evaluate constraints on zooplankton growth under four hypothetical scenarios: involving natural plankton community seasonal succession; lake fertilization to enhance fisheries production; eutrophication; and climatic warming. Our novel modeling approach predicts zooplankton production is strongly dependent on seasonal variation in resource availability and quality, which results in more complex zooplankton dynamics than predicted by simpler temperature-dependent models. For mesotrophic and hypereutrophic lakes, our study suggests that the ultimate control over zooplankton P/B ratios shifts from physiological control during colder periods to strong resource control during warmer periods. Our resource-based model provides important insights into the nature of biophysical control of zooplankton under a changing climate that has crucial implications for food web energy transfer and fisheries production.
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Essential Biomolecules in Food Webs Ruess, Liliane; Müller-Navarra, Dörthe C.
Frontiers in ecology and evolution,
07/2019, Volume:
7
Journal Article
Peer reviewed
Open access
We here review the ecological role of essential nutritional biomolecules fatty acids (FA), amino acids (AA), sterols, vitamins in aquatic and terrestrial food webs, encompassing the forces behind ...their environmental distribution. Across ecosystems, mutualistic relationships frequently ensure exchanges of vitamins between producer and demander, especially between B12 and other B vitamins as well as the AA methionine. In contrast, FA, sterols and most AA are transferred up the food chain via classical predator-prey interactions, and therefore have good biomarker potential for trophic interactions. As biomass-flow depends on the absolute amounts of potential limiting resources, considering solely the relative share in the respective biochemical group may under- or overestimate the availability to consumers. Moreover, if not accounted for, “hidden” trophic channels, such as gut symbionts as well as metabolic conversion of precursor molecules, can hamper food web analyses. Fundamental differences exist between aquatic and terrestrial ecosystems: Vitamin B12 produced by ammonium oxidizing Archaea is essential to many aquatic algae, whereas terrestrial plants escaped this dependency by using B12 independent enzymes. Long-chain ω3 polyunsaturated FA (LC-ω3PUFA) in aquatic systems mainly originate from planktonic algae, while in terrestrial systems, belowground invertebrates can well be a source, also supporting aboveground biota. Interlinks from terrestrial to aquatic ecosystems are of a biochemically totally different nature than vice versa. While biomass rich in proteins and LC-ω3PUFA is transferred to land, e.g., by trophic relationships, the link from terrestrial to aquatic ecosystems provides recalcitrant plant carbon, mainly devoid of essential nutrients, fuelling detrital food chains. Recent global changes influence food webs via altered input and transfer of essential biomolecules, but separating the effects of nutrients, CO2, and warming is not trivial. Current evolutionary concepts (e.g., Black Queen, relaxed selection) considering the costs of metabolic production partly explain food web dynamics, especially for vitamins, whereas adaptations to potential oxidative stress seem to be more important for LC-PUFA. Overall, the provision with essential biomolecules is precious for both heterotrophs and auxotrophs. These nutritional valuable molecules often are kept unaltered in consumer metabolism, including their stable isotope composition, offering a great advantage for their use as trophic markers.
The factors that regulate energy transfer between primary producers and consumers in aquatic ecosystems have been investigated for more than 50 years (refs 1,2,3). Among all levels of the food web ...(plants, herbivores, carnivores), the plant-animal interface is the most variable and least predictable link. In hypereutrophic lakes, for example, biomass and energy transfer is often inhibited at the phytoplankton-zooplankton link, resulting in an accumulation of phytoplankton biomass instead of sustaining production at higher trophic levels, such as fish. Accumulation of phytoplankton (especially cyanobacteria) results in severe deterioration of water quality, with detrimental effects on the health of humans and domestic animals, and diminished recreational value of water bodies. We show here that low transfer efficiencies between primary producers and consumers during cyanobacteria bloom conditions are related to low relative eicosapentaenoic acid (20:5ω3) content of the primary producer community. Zooplankton growth and egg production were strongly related to the primary producer 20:5ω3 to carbon ratio. This indicates that limitation of zooplankton production by this essential fatty acid is of central importance at the pelagic producer-consumer interface.
Determining the factors that control food web interactions is a key issue in ecology. The empirical relationship between nutrient loading (total phosphorus) and phytoplankton standing stock ...(chlorophyll a) in lakes was described about 30 years ago and is central for managing surface water quality. The efficiency with which biomass and energy are transferred through the food web and sustain the production of higher trophic levels (such as fish) declines with nutrient loading and system productivity, but the underlying mechanisms are poorly understood. Here we show that in seston (fine particles in water) during summer, specific ω3-polyunsaturated fatty acids (ω3-PUFAs), which are important for zooplankton, are significantly correlated to the trophic status of the lake. The ω3-PUFAs octadecatetraenoic acid, eicosapentaenoic acid (EPA) and docosahexaenoic acid, but not α-linolenic acid, decrease on a double-logarithmic scale with increasing total phosphorus. By combining the empirical relationship between EPA-to-carbon content and total phosphorus with functional models relating EPA-to-carbon content to the growth and egg production of daphnids, we predict secondary production for this key consumer. Thus, the decreasing efficiency in energy transfer with increasing lake productivity can be explained by differences in ω3-PUFA-associated food quality at the plant-animal interface.
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We conducted a series of experiments feeding Daphnia pulex nine different phytoplankton monocultures with widely varying fatty acid composition and nutritional values to test the extent to which ...Daphnia fatty acid composition was affected by diet. In general, Daphnia fatty acid composition matched that of their diet much more closely than it did the fatty acid composition of Daphnia consuming other diets. However, Daphnia had consistently less saturated fatty acids and more arachidonic acid than did their diet, and Daphnia consuming cyanobacteria had substantially less saturated fatty acids and more monounsaturated fatty acids than their diets. Daphnia that consumed cryptophytes, which are rich in ω 3 polyunsaturated fatty acids (PUFAs), had on average$47% \pm 8%$($\pm 1 SD$) ω 3 PUFAs within their fatty acid pool, whereas Daphnia that consumed ω 3 PUFA-poor cyanophytes only had$6% \pm 3% \omega 3$PUFAs. The ratio of ω 3 to ω 6 fatty acids in Daphnia was also strongly dependent on diet, and averaged$\approx 10:1$, 2:1, and 1:1 for Daphnia that consumed cryptophytes, chlorophytes, and cyanophytes, respectively. Furthermore, the sum of C20and C22ω 3 and ω 6 fatty acids in Daphnia was highly correlated with that of their diet (r2= 0.94). These results suggest analyses of Daphnia fatty acid composition may be a powerful means of inferring diet in the field. These results also suggest the nutritional benefits of consuming$\omega 3-rich$phytoplankton will transfer up the food web, making Zooplankton both more efficient at converting phytoplankton biomass to their own biomass as well as much more nutritious for the zooplanktivorous fish that consume them.
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Herbivorous zooplankton production is often constrained by algal food quality, and variation in algal food quality has been attributed to its phosphorus (P) content. To test the hypothesis that ...mineral P limitation is an important constraint on freshwater herbivorous zooplankton production, we conducted a quantitative analysis of published data on lake seston carbon to phosphorus (C:P) ratios and daphnid growth responses to P-limited algal diets. Our summary of 276 observations of mean lake seston C:P ratios showed 38% exceeded a critical threshold of 300, above which daphnid production is often predicted to be limited by algal P content. Our analysis of 91 observations of daphnid growth responses to nutrient-limited algal diets indicates the true daphnid critical threshold lies within the range of 225-375. Algal food quality is consistently reduced at algal C:P ratios above 300; however, the predicted reduction in algal food quality with P limitation is greater than the observed reduction at critical thresholds$<$390. Mineral P limitation of Daphnia production may be quite strong in some lakes; however, the production of most zooplankton taxa would not be expected to be limited by mineral P in a large proportion of lakes. After correcting for the algal C:P ratio, large differences in food quality were found between the major algal taxa, with diatoms and cryptophytes high food quality, chlorophytes intermediate food quality, and small ingestible cyanophytes very low food quality. A Monte Carlo simulation indicated that shifts in algal community composition similar to those seen during seasonal succession or across lake trophic state gradients explained four times more variability in daphnid growth rates across a range of seston C:P ratios representative of natural lakes. Overall these results tend to support the mineral P limitation hypothesis for daphnids, but they also suggest other factors will be the primary determinants of algal food quality for most zooplankters in most lakes.
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