Developing triple (13C, 15N, 34S) stable isotope analysis as a tool in aquatic food web studies: seagrass meadows as model system (TRIAS)
Final Report Abstract
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 provide only snap-shots in time and have specific limitations. Therefore, techniques to track feeding relationships via biomass composition of the different organisms have gained importance. The use of stable isotopes (usually 13C and 15N) has become particularly successful. By additionally analysing δ34S signatures we could clearly distinguish the isotopic fingerprints of epiphytes and Z. marina, which previously has been impeded by the similarity in their δ13C and δ15N signatures. Analysing three instead of two stable isotopes also enhances mixing model outputs. A pilot study was conducted in an eelgrass system in Falckenstein (western Baltic Sea) to test the applicability of the simultaneous analysis of δ13C, δ15N and δ34S isotopes. The successful use of this newly developed method enabled me to confidently allocate potential food sources to consumers and describe their trophic relationships. The data suggest that the studied eelgrass community strongly depends on epiphyte and seston production. A strong temporal variation of stable isotope signatures could be observed in primary producers and consumers in the Falckenstein eelgrass system. This led to the description of how 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. Spatial and temporal changes of two eelgrass systems in Pleasant Bay, Cape Cod, USA could be described in primary producers. However, these variations did not affect diet choice of consumers but did translate into variations in trophic levels of all functional groups. This indicates that a change in top- consumer trophic level does not necessarily mean a change in diet. The significant variabilities we 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 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.
Publications
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(2014) Simultaneous analysis of δ13C, δ15N and δ34S ratios uncovers food web relationships and the trophic importance of epiphytes in an eelgrass Zostera marina community. Marine Ecology Progress Series 497:93 – 103
Agnes Mittermayr, Thomas Hansen and Ulrich Sommer
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(2014) Temporal variation in stable isotope signatures (δ13C, δ15N and δ34S) of a temperate Zostera marina community. Marine Ecology-Progress Series 505:95 – 105
Agnes Mittermayr, Sophia E Fox and Ulrich Sommer
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Temporal dietary shift in jellyfish revealed by stable isotope analysis. Marine Biology, May 2016, 163:112
Jamileh Javidpour, Ashlie N Cipriano, A Mittermayr, Jan Dierking
