Marine Ecology Applications for Stable Isotope Analysis Susy Honig

Many marine food webs are size-based (as opposed to species-based)

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

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?

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

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

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

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

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

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

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

Food Chain Length Heaviest predator rarely fed at highest trophic level Longest food chains supported predators with intermediate body size NEED HELP WITH THIS SLIDE!!!!

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

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

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

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

What affects 15N? Environmental Conditions Physiology

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)

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

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

Isosource Model Results

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

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

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

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

Seasonal Variability in diet composition within individuals

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