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Marine Ecology Applications forStable Isotope Analysis

Susy Honig

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Size-based Nitrogen StableIsotope 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 ofthe variance in trophic level compared withspecies identity

• Is this a valid assumption?

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Size-based Nitrogen StableIsotope 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 significantrelationship betweenspecies identity andδ15N value (trophiclevel) in a North Seafood web

• On the other hand,trophic level increasescontinuously with bodymass

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Size-based Nitrogen StableIsotope 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 preyµ=n(Δ/b)

Where µ = mean PPMR, n = the base of lognbodymass class, Δ = the fractionation of δ15N, and b =the slope of the relationship between δ15N andlognbody mass class.

• Important b/c can predict strength of bioticinteractions, food chain length, and pathways ofenergy transfer

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Size-based Nitrogen StableIsotope 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 intopredator production ε=Pλ+1 / Pλ

• 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 δ15N (x) ε= nΔb

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Size-based Nitrogen StableIsotope 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 athighest trophiclevel

• Longest foodchains supportedpredators withintermediate bodysize

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Food Chain Length, cont.

• Trophic levelincreases with bodymass, but you can’tcalculate themaximum possibletrophic level in acommunity (ie thefood chain length) justusing the largestindividual

Food Chain Length

• PPMR is smaller inlonger food chainsand less variableenvironments

• Longer food chainswith smaller PPMRratios are often morestable

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Size-based Nitrogen StableIsotope 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 inbiomass of largefishes in NorthSea compared topredicted baseline(using PPMR andTE)•Good tool forassessing fishingimpacts, especiallyin the absenceof historicalbaseline data

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What affects Δδ15N?• Environmental Conditions• Physiology

Take-Home Message

• Size-based Nitrogen Stable Isotope analysisis a good tool for macroecological research,especially in marine food webs

• Assumptions about base δ15N levels shouldbe made carefully (account forenvironmental conditions and foodavailability)

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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, evenfish from discarded bycatch)

• Used δ15N and δ13C to describe diet compositionof immature loggerheads and see if variation ingrowth rate was related to inter-individualvariation in diet selectivity

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More on Turtles…

• Analyzed 77 blood plasma samples from 49individual turtles

• Also analyzed potential prey (blue crab, whelk,spider crab, horseshoe crab, cannonball jellies, andtwo 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

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• Lots of variation inδ15N and δ13Cvalues forimmatureloggerheads, butno significantrelationship withbody size orgrowth rates

The Big Picture

• Isotope signatures show us that immatureloggerhead turtle growth rates were notrelated to the trophic level in whichindividuals fed

• Diet composition was variable, but bluecrab and whelk (and not fish) are importantcomponents

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• Differences in δ15N and δ13C values within andbetween individual otters can indicate the extentof prey specialization and conspecific nichepartitioning

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• High degreeof betweenindividualvariation(BIC) ~50%

• Less withinindividualvariation(WIC) ~30%

Seasonal Variability in dietcomposition within individuals

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Big Picture• Looks like otters

are preyspecialists, butdiet may beaffected byresourceavailability andseason