Simultaneous triple stable isotope analysis of carbon, nitrogen and sulphur was employed to study the temporal variation in the food web of a subtidal eelgrass Zostera marina bed in the western Baltic Sea. Samples of 3 potential food sources (eelgrass, epiphytes and seston) and consumer species were collected biweekly from March through September 2011. Temporal variation of stable isotope compositions was observed in primary producers and consumer species. However, variation between replicates, particularly omnivores, often exceeded variation over time. The high degree of omnivory among the generalist feeders in this eelgrass community allows for generalist feeders to flexibly switch food sources, thus enhancing food-web stability. As coastal systems are subject to seasonal changes, as well as alterations related to human disturbance and climate, these food webs may retain a certain resilience due to their plentiful omnivores.
A temporal change in the stable isotope (SI) composition of jellyfish in the Kiel Fjord, Western Baltic Sea, was documented by analyzing δ13C, δ15N and δ34S of bell tissue of Aurelia aurita and Cyanea capillata in the period between June and October 2011. A strong and significant temporal change in all SI values of A. aurita was found, including an increase of ~3 ‰ in δ13C, a decrease of ~4 ‰ in δ15N and sharp decline of ~7 ‰ in δ34S. While knowledge gaps in jellyfish isotope ecology, in particular the lack of reliable trophic enrichment factors, call for a conservative interpretation of our data, observed changes in particular in δ34S, as indicated by means of a MixSIR mixing model, would be consistent with a temporal dietary shift in A. aurita from mesozooplankton (>150 µm) to microplankton and small re-suspended particles (0.8–20 µm) from the benthos. Presence of a hitherto unidentified food source not included in the model could also contribute to the shift. During the 2-month occurrence of C. capillata, its isotope composition remained stable and was consistent with a mainly mesozooplanktonic diet. Mixing model output, mainly driven by δ34S values, indicated a lower proportion of A. aurita in the diet of C. capillata than previously reported, and thus to a potentially lesser importance of intraguild predation among jellyfish in the Kiel Fjord. Overall, our results clearly highlighted the potential for substantial intraspecific isotopic seasonal variation in jellyfish, which should be taken into account in future feeding ecology studies on this group.Electronic supplementary materialThe online version of this article (doi:10.1007/s00227-016-2892-0) contains supplementary material, which is available to authorized users.
Simultaneous analysis of carbon, nitrogen and sulphur stable isotope ratios was applied in this pilot study to examine the food web of a Zostera marina L. system in the western Baltic Sea. Samples of 3 potential food sources: eelgrass, epiphytic algae and seston, as well as 69 consumer species were collected during the growing season of Z. marina from March to September 2011. The measured δ 13 C values of epiphytes were close to δ 13 C values of eelgrass, impeding a clear distinction of those 2 carbon sources, whereas seston δ 13 C values were clearly different. This frequently encountered problem was solved by the additional use of δ 34 S, which resulted in easily distinguishable values for sediment and seawater derived sulphur. The combination of δ 34 S and δ 13 C values made a separation of carbon sources possible and enabled the allocation of potential food sources to consumers and a description of their trophic relationships. The results of stable isotope ratio analysis of this eelgrass community strongly indicate a food web based on epiphyte and seston production. δ 15 N values show a food web consisting of large numbers of generalists and a high degree of omnivory amongst the consumer species analysed. This implies an occupation of every trophic position possible, which is supported by an even distribution of δ 15 N values. Previously described eelgrass food webs may have to be re-evaluated by considering sulphur stable isotope ratios in order to provide a clear picture on primary carbon sources.
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