Marine Ecology Applications for Stable Isotope Analysis

1. Marine Ecology Applications for Stable Isotope Analysis Susy Honig

3. Size-based Nitrogen Stable Isotope Analysis can be used for: • Intra- and inter-specific variation in trophic levels • Predator-prey size ratios • Transfer efficiency • Food chain length • Human Impacts

4. Principle Assumption of Sized-based analyses • Body size accounts for a large proportion of the variance in trophic level compared with species identity • Is this a valid assumption?

5. Size-based Nitrogen Stable Isotope Analysis can be used for: • Intra- and inter-specific variation in trophic levels • Predator-prey size ratios • Transfer efficiency • Food chain length • Human Impacts

6. No significant relationship between species identity and 15N value (trophic level) in a North Sea food web • On the other hand, trophic level increases continuously with body mass

7. Size-based Nitrogen Stable Isotope Analysis can be used for: • Intra- and inter-specific variation in trophic levels • Predator-prey size ratios • Transfer efficiency • Food chain length • Human Impacts

8. Predator Prey Mass Ratios • PPMR= ratio of the mean body mass of predators in a food web to the mean body mass of their prey=n(/b) Where  = mean PPMR, n = the base of lognbody mass class,  = the fractionation of 15N, and b = the slope of the relationship between 15N and lognbody mass class. • Important b/c can predict strength of biotic interactions, food chain length, and pathways of energy transfer

9. Size-based Nitrogen Stable Isotope Analysis can be used for: • Intra- and inter-specific variation in trophic levels • Predator-prey size ratios • Transfer efficiency • Food chain length • Human Impacts

10. Transfer Efficiency • TE = how much prey production is converted into predator production =P+1 / P • P= B x (P/M) • P is production in each body mass class • B is biomass • P/M is individual biomass production (can be calculated if you know body mass) • TE calculated from slope of relationship between lognP (y) and 15N (x) = nb

11. Size-based Nitrogen Stable Isotope Analysis can be used for: • Intra- and inter-specific variation in trophic levels • Predator-prey size ratios • Transfer efficiency • Food chain length • Human Impacts

12. Food Chain Length • Heaviest predator rarely fed at highest trophic level • Longest food chains supported predators with intermediate body size

13. Food Chain Length, cont. • Trophic level increases with body mass, but you can’t calculate the maximum possible trophic level in a community (ie the food chain length) just using the largest individual

14. Food Chain Length • PPMR is smaller in longer food chains and less variable environments • Longer food chains with smaller PPMR ratios are often more stable

15. Size-based Nitrogen Stable Isotope Analysis can be used for: • Intra- and inter-specific variation in trophic levels • Predator-prey size ratios • Transfer efficiency • Food chain length • Human Impacts

16. Human Impacts: Fishing • Reduction in biomass of large fishes in North • Sea compared to predicted baseline (using PPMR and TE) • Good tool for assessing fishing • impacts, especially in the absence • of historical baseline data

17. What affects 15N? • Environmental Conditions • Physiology

18. Take-Home Message • Size-based Nitrogen Stable Isotope analysis is a good tool for macroecological research, especially in marine food webs • Assumptions about base 15N levels should be made carefully (account for environmental conditions and food availability)

20. Quick Summary • Loggerheads can be in immature neritic stage for >20 years • During this period, have mostly carnivorous diet, but lots of variation (mollusks, crustaceans, even fish from discarded bycatch) • Used 15N and 13C to describe diet composition of immature loggerheads and see if variation in growth rate was related to inter-individual variation in diet selectivity

21. More on Turtles… • Analyzed 77 blood plasma samples from 49 individual turtles • Also analyzed potential prey (blue crab, whelk, spider crab, horseshoe crab, cannonball jellies, and two locally important fish species) • Measured growth rates of 15 turtles • Used mixing model to generate and explain potential source contribution to diet

22. Isosource Model Results

23. Lots of variation in 15N and 13C values for immature loggerheads, but no significant relationship with body size or growth rates

24. The Big Picture • Isotope signatures show us that immature loggerhead turtle growth rates were not related to the trophic level in which individuals fed • Diet composition was variable, but blue crab and whelk (and not fish) are important components

26. Differences in 15N and 13C values within and between individual otters can indicate the extent of prey specialization and conspecific niche partitioning

27. High degree of between individual variation (BIC) ~50% • Less within individual variation (WIC) ~30%

28. Seasonal Variability in diet composition within individuals

29. Big Picture • Looks like otters are prey specialists, but diet may be affected by resource availability and season