Toxicity and Starvation Induce Major Trophic Isotope Variation in Daphnia Individuals: A Diet Switch Experiment Using Eight Phytoplankton Species of Differing Nutritional Quality

Abstract

Stable isotope values can express resource usage by organisms, but their precise interpretation is predicated using a controlled experiment-based validation process. Here, we develop a stable isotope tracking approach towards exploring resource shifts in a key primary consumer species Daphnia magna. We used a diet switch experiment and model fitting to quantify the stable carbon (δ13C) and nitrogen (δ15N) isotope turnover rates and discrimination factors for eight dietary sources of the plankton species that differ in their cellular organization (unicellular or filamentous), pigment and nutrient compositions (sterols and polyunsaturated fatty acids), and secondary metabolite production rates. We also conduct a starvation experiment. We evaluate nine tissue turnover models using Akaike’s information criterion and estimate the repetitive trophic discrimination factors. Using the parameter estimates, we calculate the hourly stable isotope turnover rates. We report an exceedingly faster turnover value following dietary switching (72 to 96 h) and a measurable variation in trophic discrimination factors. The results show that toxic stress and the dietary quantity and quality induce trophic isotope variation in Daphnia individuals. This study provides insight into the physiological processes that underpin stable isotope patterns. We explicitly test multiple alternative dietary sources and fasting and discuss the parameters that are fundamental for field- and laboratory-based stable isotope studies.

Dissolved nitrogen uptake versus nitrogen fixation: Mode of nitrogen acquisition affects stable isotope signatures of a diazotrophic cyanobacterium and its grazer

“…In contrast to δ 15 N values, the δ 13 C values of starving Daphnia increased over time, which has been reported previously [ 60 , 64 ]. Enrichment in 13 C during starvation may result from increased metabolization of 13 C-depleted lipids in order to meet energetic demands [ 63 ].…”

“…After the diet switch to cyanobacteria, δ 13 C values of Daphnia continuously decreased in all DIN treatments, with no clear relationship with DIN concentration. Decreasing δ 13 C values after a diet switch to cyanobacteria have been reported previously [ 60 ]. One possible explanation could be that the animals incorporated 13 C-depleted lipids from the cyanobacterial food but were unable to grow and reproduce due to sterol limitation and thus did not further metabolize the accumulating lipids and also could not allocate them to reproduction.…”

“…However, the stable isotope data showed that the animals incorporated and processed the carbon and nitrogen provided by the cyanobacterium, suggesting that the animals were able to at least meet their basic energy requirements by consuming the cyanobacterium. δ 15 N and δ 13 C values of consumers can change due to food quantity- and quality-related isotope fractionation [ 60 ]. During periods of nutritional stress, enrichment in 15 N may result from the segregation of 14 N, i.e., the excretion of isotopically light urea [ 61 , 62 ].…”

Understanding amphipod reproductive health drivers in the Baltic Sea: A Study Based on Structural Equation Modeling

“…Other studies report that both niche expansion and compression can occur in stressed populations, including those facing chemical exposure in polluted environments, with outcomes depending on the nature of the stress and confounding factors, such as food availability and quality (Helmer et al, 2022).…”