Marine Ecology Applications for
Stable Isotope Analysis
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Susy Honig
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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 Sizedbased analyses
• Body size accounts for a large proportion of
the variance in trophic level compared with
species identity
• Is this a valid assumption?
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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
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• 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
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• Heaviest predator
rarely fed at
highest trophic
level
• Longest food
chains supported
predators with
intermediate body
size
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
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Food Chain Length
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• 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
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•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
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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)
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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, 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
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• 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
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• Differences in 15N and 13C values within and
between individual otters can indicate the extent
of prey specialization and conspecific niche
partitioning
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• High degree
of between
individual
variation
(BIC) ~50%
• Less within
individual
variation
(WIC) ~30%
Seasonal Variability in diet
composition within individuals
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Big Picture
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• Looks like otters
are prey
specialists, but
diet may be
affected by
resource
availability and
season
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