marine ecology applications for stable isotope analysis susy honig

Marine Ecology Applications for Stable Isotope Analysis

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 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?




levels




• 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




levels




• 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


levels




• 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) = nb


levels




• Human Impacts

Food Chain Length• 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



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




levels




• 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 fishingimpacts, especially in the absenceof 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



QuickTime™ and a decompressorare needed to see this picture.

Big Picture• Looks like otters

are prey specialists, but diet may be affected by resource availability and season



marine ecology applications for stable isotope analysis susy honig

Documents

prey production

trophic level increases

n value trophic level

interspecific variation

body mass classb

maximum possible trophic

mean body mass of predators

n levels