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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