| 1. |
- Galloway, Aaron W. E., et al.
(author)
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A Fatty Acid Based Bayesian Approach for Inferring Diet in Aquatic Consumers
- 2015
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In: PLOS ONE. - : Public Library of Science (PLoS). - 1932-6203. ; 10:6
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Journal article (peer-reviewed)abstract
- We modified the stable isotope mixing model MixSIR to infer primary producer contributions to consumer diets based on their fatty acid composition. To parameterize the algorithm, we generated a 'consumer-resource library' of FA signatures of Daphnia fed different algal diets, using 34 feeding trials representing diverse phytoplankton lineages. This library corresponds to the resource or producer file in classic Bayesian mixing models such as MixSIR or SIAR. Because this library is based on the FA profiles of zooplankton consuming known diets, and not the FA profiles of algae directly, trophic modification of consumer lipids is directly accounted for. To test the model, we simulated hypothetical Daphnia comprised of 80% diatoms, 10% green algae, and 10% cryptophytes and compared the FA signatures of these known pseudo-mixtures to outputs generated by the mixing model. The algorithm inferred these simulated consumers were comprised of 82% (63-92%) [median (2.5th to 97.5th percentile credible interval)] diatoms, 11% (4-22%) green algae, and 6% (0-25%) cryptophytes. We used the same model with published phytoplankton stable isotope (SI) data for delta C-13 and delta N-15 to examine how a SI based approach resolved a similar scenario. With SI, the algorithm inferred that the simulated consumer assimilated 52% (4-91%) diatoms, 23% (1-78%) green algae, and 18% (1-73%) cyanobacteria. The accuracy and precision of SI based estimates was extremely sensitive to both resource and consumer uncertainty, as well as the trophic fractionation assumption. These results indicate that when using only two tracers with substantial uncertainty for the putative resources, as is often the case in this class of analyses, the underdetermined constraint in consumer-resource SI analyses may be intractable. The FA based approach alleviated the underdetermined constraint because many more FA biomarkers were utilized (n < 20), different primary producers (e.g., diatoms, green algae, and cryptophytes) have very characteristic FA compositions, and the FA profiles of many aquatic primary consumers are strongly influenced by their diets.
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| 2. |
- Persson, Jonas, et al.
(author)
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Food quantity and quality regulation of trophic transfer between primary producers and a keystone grazer (Daphnia) in pelagic freshwater food webs
- 2007
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In: Oikos. - : Wiley. - 0030-1299 .- 1600-0706. ; 116:7, s. 1152-1163
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Journal article (peer-reviewed)abstract
- The transfer of energy and nutrients from plants to animals is a key process in all ecosystems. In lakes, inefficient transfer of primary producer derived energy can result in low animal growth rates, accumulation of nuisance phytoplankton blooms and dissipation of energy from the ecosystem. Most research on carbon transfer efficiency in pelagic food webs has focused on either food quantity or food quality, with the latter considered separately as either elemental stoichiometry or biochemical composition. The natural occurrence and magnitude of these types of growth limitations and their combined effects on Daphnia, a keystone grazer in pelagic freshwater ecosystems, are largely unknown. Our empirical models predict that the strength and nature of food quantity and quality limitation varies greatly with lake trophic state (total phosphorus, TP) and that Daphnia growth rates and thus energy and nutrient transfer efficiency are highest in lakes with intermediate trophic status (TP 10-25 μg l−1). We predict that food availability place the greatest constraint on Daphnia growth in nutrient poor lakes (TP≤4 μg l−1). Phosphorus limitation of Daphnia growth increased with decreasing TP, but the overall effect was never predicted to be the dominant constraining factor. Eicosapentaenoic acid (EPA, 20:5ω3) limitation was predicted to occur in both nutrient poor and nutrient rich lakes and placed the primary constraint on food quality in the most productive lakes. Two contrasting EPA-models gave different results on the magnitude of EPA-limitation, implying that additional food quality factors decrease Daphnia growth at high TP. In conclusion, the model predicts that Daphnia growth should peak in mesotrophic lakes, food quantity will place the greatest constraint on growth in oligotrophic lakes and EPA will primarily limit growth in eutrophic lakes.
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| 3. |
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| 4. |
- Ravet, Joseph L., et al.
(author)
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Threshold dietary polyunsaturated fatty acid concentrations for Daphnia pulex growth and reproduction
- 2012
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In: Inland Waters. - 2044-2041 .- 2044-205X. ; 2:4, s. 199-209
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Journal article (peer-reviewed)abstract
- Knowing when, where, and why food quality limits herbivore growth and reproduction is the core question in nutritional ecology. We investigated dietary threshold concentrations of polyunsaturated fatty acids (PUFAs) for the herbivorous zooplankter Daphnia pulex when fed the cyanobacterium Microcystis aeruginosa or the green algae Scenedesmus obliquus. Phosphatidylcholine liposomes were used to supplement diets with arachidonic acid (ARA, 20:4 omega 6), stearidonic acid (SDA, 18:4 omega 3), or eicosapentaenoic acid (EPA, 20:5 omega 3) at concentrations (per mg carbon) ranging from 0.04 to 10 mu g (mg C)(-1). ARA supplementation had no effect on Daphnia somatic growth or reproduction. SDA supplementation did not affect Daphnia growth, however egg production dropped below a 50% saturation threshold when dietary SDA concentrations were <0.34-0.39 mu g SDA (mg C)(-1). Daphnia egg production dropped below a 50% saturation threshold when EPA concentrations for the Microcystis and Scenedesmus treatments were <0.25 and 0.17 mu g EPA (mg C)(-1), respectively. Growth declined below the 50% threshold when EPA concentrations for the 2 diets were <0.14 and 0.04 mu g EPA (mg C)(-1), respectively. These results show the demands for EPA are more intense for reproduction than for somatic growth. We used the EPA results to develop an overall model between Daphnia production and dietary EPA: normalized production = (EPA/C)/(0.15 + EPA/C), R-2 = 0.72, n = 40. The boot-strapped 50% saturation threshold for this model was 0.15 +/- 0.03 (+/- SE) mu g EPA (mg C)(-1). Our results suggest Daphnia pulex dietary EPA limitation may be common in eutrophic lakes.
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