Mittermayr, Agnes (2014) Eelgrass Foodwebs – A comprehensive study employing ∂13C, ∂15N and ∂34S stable isotopes. (PhD/ Doctoral thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 114 pp.

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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 (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.

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