Description: 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 three potential food sources: eelgrass, epiphytes and seston, as well as consumer species were collected biweekly from March through September 2011. Temporal variation of stable isotope signatures was observed in primary producers and consumer species. However, variation within a species, 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.

Description: A temporal change in stable isotope (SI) composition of jellyfish in the Kiel Fjord, Western Baltic Sea, was documented by analyzing delta13C, delta15N and delta34S 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 ~3permille in delta13C, a decrease of ~4permille in delta15N and sharp decline of ~7permille in delta34S. Sampling from 18 m to surface.

Description: © The Author(s), 2016. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Marine Biology 163 (2016): 112, doi:10.1007/s00227-016-2892-0.

Description: 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.

Description: This project was funded by the German Research Foundation (DFG, JA2008/1-1). JD received financial support from the Cluster of Excellence “The Future Ocean” and the BONUS project BIO-C3, funded jointly by the EU and the BMBF (Grant No. 03F0682A).

Description: 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 three 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 δ13C values of epiphytes (-14.1‰ ± 1.8 SD) were close to δ13C values of eelgrass (-11.6‰ ± 1.8 SD), impeding a clear distinction of those two carbon sources, whereas seston δ13C values (-20.9‰ ± 3.5 SD) were clearly different. This frequently encountered problem was solved by the additional use of δ34S, which resulted in easily distinguishable values for sediment and seawater derived sulphur. Values of primary producer δ34S ranged from 5.6‰ (± 2.3 SD) for Z. marina leaves to 14.2‰ (± 1.6 SD) for epiphytes and 11.9‰ (± 3.3 SD) for seston. The combination of δ34S and δ13C 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 data of stable isotope ratio analysis of this eelgrass community strongly indicate a food web based on epiphyte and seston production. δ15N 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 a continuous distribution of δ15N values. Previously described eelgrass food webs may have to be re-evaluated to include sulfur in order to provide a clear picture on primary carbon sources.

Description: 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.

Description: 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.

Description: Resolving the structure of complex food webs is a classic challenge for ecology which has gained renewed priority in view of climate change, species invasions, biodiversity loss and other anthropogenic impacts influencing ecological communities. Traditional methods (direct observation, gut contents analysis, feeding experiments) provide only snap-shots in time and have their specific limitations: direct observation is restricted to large organisms, gut contents analysis is biased by different digestibility, and feeding experiments are too time- consuming to cover all links in a food web. Therefore, techniques to track feeding relationships via biomass composition of the different organisms have gained importance. The use of stable isotopes (usually 13 C and 15 N) has become particularly successful. By additionally analysing δ34 S signatures I could clearly distinguish the isotopic fingerprints of epiphytes and Z. marina, which previously has been impeded by the similarity in their δ13C and δ15 N signatures. Analysing three instead of two stable isotopes also enhances mixing model outputs. Chapter 1 describes the pilot study in which I tested the applicability of the simultaneous analysis of δ13C, δ15N and δ34 S isotopes to examine the food web of a Zostera marina system in Falckenstein (western Baltic Sea). I sampled three potential food sources and collected 69 consumer species over the course of 6 months. The combination of δ34 S and δ13 C values made a separation of epiphytes and Z. marina as a food source possible. This enabled me to confidently allocate potential food sources to consumers and describe their trophic relationships. The data suggest that this eelgrass community strongly depends on epiphyte and seston production. δ15 N values show a food web consisting of large numbers of generalists and a high degree of omnivory. Chapter 2 describes the temporal variation of stable isotope signatures that I could observe in primary producers and consumers in the Falckenstein eelgrass system. I also describe how this variability affects food web structure and trophic levels. The dominance of omnivory in this eelgrass community allows generalist feeders to flexibly switch food sources, thus dampening the temporal shifts in higher trophic levels and enhancing food web stability. However, variation within a species, particularly omnivores, often exceeded variation over time. Chapter 3 describes spatial and temporal changes of two eelgrass systems in Pleasant Bay, Cape Cod, USA. I sampled 75 species over the course of 5 months and by applying simultaneous triple stable isotope analysis of δ13C, δ15N and δ34 S I could determine striking similarities across habitat boundaries. Temporal variation was more pronounced than spatial variation and the signatures of 7 primary producers followed similar trends at both sites. However these variations did not affect diet choice of consumers but did translate into variations in trophic levels of all functional groups. Thus, a change in top- consumer trophic level does not necessarily mean a change in diet. The significant variabilities I found in stable isotope values across time in both Kiel and Cape Cod have implications on past and future studies. Only samples from the same seasons should be compared and used for mixing models. Spatial differences could only be found on a global scale. The high degree of omnivory among the generalist feeders in the studied eelgrass communities allows for these feeders to switch food sources as availability changes, thus stabilizing trophic dynamics. 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.

Description: Seagrass meadows are widely distributed in coastal zones worldwide. They represent highly productive autotrophic communities and provide many ecosystem services to humans such as shoreline protection and nurseries for commercially important fish. Here we aim at understanding how the structure of a Zostera marina food web changes from March to September. The study area is an eelgrass meadow in the Kiel Fjord (western Baltic Sea). We applied simultaneous triple stable isotope analysis of carbon, nitrogen and sulfur to identify feeding preferences of species. Data were collected biweekly and allowed to construct 15 food webs. We characterized the structure of these food webs through network analysis and studied the trends of different properties with respect to time and average water temperature preceding each sampling. In the second part of the temporal series, the food webs display lower link densities (trophic interactions per species), shorter trophic chains, and lower degrees of omnivory. In summer, trophic specialization prevails and species feed at lower trophic levels. Studying the temporal relationship linking temperature to food web structure may be relevant under climate change conditions.