Tracing Eelgrass Foodwebs: Simultaneous analysis of δ15N, δ13C and δ34S of low biomass samples Agnes Mittermayr, Thomas Hansen

Simultaneous δ15N, δ13C and δ34S measurements of low biomass samples (LBS - SIA)

Analytical assembly for high sensitivity CNS SIA down to 2 µg N, 10 µg C and 0.2- 0.8 µg S (Hansen et al. 2009 and 2007).

The system includes downsized reactors, a temperature program-controlled gas chromatography column and a cryogenic trap to collect small amounts of sulphur dioxide.

Temporal variability in food source preference of functional groups

References: Hansen et al. 2007 Rapid Comm Mass Spec 21:314 – 318 Hansen et al. 2009 Rapid Comm Mass Spec 23:3387 – 3393 Mittermayr et al. 2014a MEPS 497: 93 – 103 Mittermayr et al. 2014b MEPS 505: 95 – 105

Temporal variation of SI composition in Zooplankton species

Conventional CNS stable isotope abundance measurements of solid samples usually require high sample amounts (~ 1 mg C) to achieve exact analytical results. We introduce a high sensitivity elemental analyzer coupled to an isotope ratio mass spectrometer which enables us to simultaneously determine CNS-stable isotope ratios in samples with low carbon content (< 40 µg C).

Application of LBS – SIA to Zostera marina communities

Conclusions: This study highlights the importance of epiphytic microalgae for the carbon flux in temperate Z. marina habitats. The simultaneous triple stable isotope analysis is a unique and comprehensive approach to reveal carbon fluxes and food web structures of marine systems characterized by bentho- pelagic coupling.

Stable isotope values of δ15N vs. δ13C (a) and δ15N vs. δ34S (b) (Mittermayr et al 2014a); units in ‰

a) b)

The triple stable isotope analysis is a successful tool for analysing food webs of systems characterized by bentho- pelagic coupling. The inclusion of δ34S offers a solution to the frequently encountered problem of similarity of δ13C between Z. marina leaves and epiphytes.

The small amount of biomass necessary greatly reduces the sampling effort and enables high temporal resolution of sampling regimes.

SI signatures of zoo- plankton species were significantly similar. Succession patterns in species composition could be observed.

A tight coupling between SI signatures of seston and copepods could be observed over the sampling period. Based on δ34S values, copepods selectively feed on the pelagic portion of seston.

Temporal variation was observed in consumer functional groups; however, variation in SI values within a species, particularly for omnivores, often exceeded variation in consumer group isotopes over time.

Omnivores Predators

Zooplankton Suspension Feeders

Temporal variation in diets (according to MixSIR) and the resulting trophic levels of consumer functional groups differed depending on the position in the food web.

High flexibility and opportunistic switching of food sources in omni- vores are of extra- ordinary impor-tance to food web stability.