Stable isotopes

Question 1

What do we need to understand about food webs - why stable isotopes are important.

Answer 1

What are the rules and properties of these networks?
What are the regulating/control mechanisms within them?
Do these feeding links define important rules of thumb that may be helpful to management?

Question 2

Strengths of stomach contents data

Answer 2

Strengths

• High taxonomic resolution - identify individual species and sizes of things
• Technologically simple - dissecting kit and microscope
• Tried and Tested - a lot of data has come out of it
• A snapshot in time - detailed snapshot at a point in time and at a location

Question 3

Weaknesses of food web gut contents data

Answer 3

Weaknesses

• Incomplete diet (e.g. mucus) - a lot of things feed on gelatinous things, some things break down and cannot be identified
• Assumed diet trophic level - still. Underlying assumptions we have to make.
• Long term means allowing no variation in space/time (because it is a snapshot). Often have to test these patterns have to be tested spatially.
• Data can be v. limited - have to catch and kill a lot of animals (a lot of predators feed intermittently - 50%
• High sampling effort
• ‘Net contamination’ - opportunistically feed in net

Question 4

Alternatives to stomach content analysis

Answer 4

Other approaches: (1) Scat analysis – useful in large animals e.g. mammals, utilises hard structures e.g. otoliths, beaks

Cephalopods from beaks

(2) Fatty acid profiles - some prey have characteristic FA profiles … used as tracers

Question 5

How do you calculate the trophic level from stomach content analysis?

Answer 5

By looking at the trophic level and the of the diet of known prey in the stomach.

assume: diet fully characterised and quantified (totally made up from the components that you did the calculations with), that the prey TLs correct, and that it reflects the long term mean

Require: long-term diet quantitative data, known prey TPs at the temporal scale of isotopic turnover

Question 6

What is an isotope?

Answer 6

Isotope = atoms of the same element …differing in atomic weight…identical in chemical properties, and in all physical properties except those determined by the mass of the atom.

Most of the carbon in our bodies is carbon 12, with some C13. This has implications on how it forms and breaks bonds.

Question 7

How is the data from stable isotope analysis used?

Answer 7

δ𝐻𝑋=[((“Rsample − Rstandard)” )/”Rstandard”] × 1000

• Delta value – for expressing the ratios of heavy and light isotopes
• Delta 13 carbon – a relative abundance of the heavier isotope
• Multiply by 1000 because the natural abundances of the heavier carbon are very low

International standards

• Standards vary with the element:
• Carbon standard = Pee Dee Belemnite (PDB),
• Nitrogen standard = Atmospheric N2

Question 8

What is isotope fractionation?

Answer 8

• Isotope fractionation = differential partitioning of isotopes between two compounds
• The heavy isotope has higher bond strength and slower reactions.
• Light elements e.g. H, C, N, O, and S. are more likely to exhibit isotopic fractionation than heavy elements.
• The difference in bonding strength and reaction rates proportional to the mass difference

Question 9

Why does fractionation occur as the matter is passed up trophic levels?

Answer 9

Excretion/Respiration: production of metabolites with light isotopes in deamination/transamination (heavy isotopes become concentrated).

Assimilation fractionation: preferential use of heavy isotopes during protein biosynthesis.

Fractionation between diet and consumer assumed constant, level assumed (per ml not % due to the times 1000):
Δdelta 5N = +3.4 ‰ - useful proxy for trophic position
Δdelta 13C = +1.0‰ - good tracer of different sources in the food web.

Question 10

How would you measure the trophic position of an organism using stable isotope data?

Answer 10

The trophic level of the base plus 15 N of the sample - 15 n of base / 3.4

• Assumptions: per-TL fractionation (known and constant, ie. Δδ15N value
• Need to pick a suitable baseline organism and get the trophic level of the baseline organism right.

Question 11

What are the implications and constraints of working out trophic levels using 15N?

Answer 11

Implications:

• rely on fewer samples than stomach content
• give a time-integrated signal of what the animal is feeding on
• length of food-chains
• productivity - if we have two statistically different sources you have the possibility of looking at the relative importance of those two components of the diet inside the food web
• fishing effects or MPAs etc

Constraints:

• 3.4 is a good average, but values can range a lot
• identification of a good ‘baseline’ - knowing what the trophic level of the sample species is

Question 12

Stable isotope proxy of production sources.

How can you work out what a consumer species is eating using C12?

Answer 12

If you know the carbon 12 of the sources, you can work out the ratio of which the carbon in the consumer.

Question 13

What are the assumptions, constraints and implications of working out the stable isotope proxy of production sources?

Answer 13

Assumptions:

• per-TL fractionation (Δδ13C) small (Not always 1 part per ml)
• δ13C of sources accurately measured

Constraints:

• source data may variable
• sources indistinguishable
• more sources more uncertainty
• sources not included

Implications:

• sources of productivity, conservation prioritization, ecosystem modelling etc

Question 14

What is isotopic routing?

Answer 14

• The idea of isotopic ‘routing’: need at least to compare like with like. (muscle with muscle, liver with liver etc)
• Differences in 13C, especially because fats, fractionate differently

Question 15

Explain the issue of variability in stable isotope research issues.

Answer 15

Sweeting CJ et al. (2005) Functional Ecology

Looking at Variability among tissues: European seabass (Dicentrarchus labrax) in the lab.

The standard deviation shows samples from the liver are much more variable than that of the muscle.

Question 16

What variability in different tissues variability when looking at stable isotope data provides an opportunity

Answer 16

Tissues vary in both growth (dilution) and repair (replacement)

Slow tissues integrate any changes over longer periods than fast tissues.

The two tissues may offer proxies at two-time scales in the same consumer

Slow time over tissues will let you know what is going on over longer timescales

Blood or liver is fast turn over, white muscle or sometimes bone is slow turnover

Question 17

Pinnegar JK et al. 2001 Journal of Fish Biology

relationship in 15N of parasite and its host

Answer 17

Some consumers may do it differently e.g. parasites on fish

For these consumers, apparently 15N <<< 15N diet (but do we know what’s going on?)

• biology of parasites - able to be quite selective about what proteins etc they take up form the host

Question 18

Stable isotope research issues

Answer 18

• metabolism of the isotopes is complicated
  
  • digestion, absorption, assimilation into tissues and then excretion and then respiration
  • If the animal is not feeding, they will be excreting more lighter isotopes building in their body - accumulating from their own tissues
  • Starving or intermittent feeders .g. some large carnivores opportunistic feeders.
• The growth rate is an important driver
  
  • Lower 15N in fish growing more rapidly

Question 19

What insights can the timescales stable isotope analysis occurs over give?

Answer 19

Barnes C et al. (2008) Oecologia

Variability within European seabass tissues suggests long term differences in feeding behaviour from individuals.

Significant variability within tissues: not just size, suggests e.g. different feeding strategies.

Dilution – fish keep on growing through there lives,

• Animal isotopic signature derived from the dietary signature
• Atoms generally only go through absorption and assimilation once, therefore HX does not increase indefinitely
• Once constructed a tissue’s component signature is set but the tissue can change

Stomach content data – last 24 hours
Liver – whats happened over a timescale of say a few weeks
Muscles – give the picture over – say a month

Question 20

weaknesses of stable-isotope approaches

Answer 20

Weaknesses

• Robustness/variability of trophic step fractionation? - need to get the value right although there is lots of variabilities. Scientists are trying to find patterns - size ect
• Tissue-specific fractionation?
• Different modes of SI fractionation and tissue turnover effects?
• Simplification of food web complexity?
• looking at different types of prey and production source - low taxonomic detail

Question 21

Strengths of stable-isotope approaches

Answer 21

strengths

• Avoid biases in traditional diet studies, complement other approaches
• ‘Fingerprinting’ of food webs
• Highlight some important pathways
• Important structural details such as trophic level, aspects of omnivory
• Stomach content - may not utilise all the matter in the stomach
  *

Question 22

Opportunities of stable-isotope approaches

Answer 22

Opportunities

• Large-scale changes in fish and fishery trophodynamics and food webs
• Long-term studies (e.g. bones)
• Different time scales e.g. fast and slow tissues
• Fundamental understanding of food web processes
• Compound-specific approaches
• variation in population with respect to feeding strategies of individuals. Big divergences could have big impacts on things like competition. - could tell us how adaptive a population is to future changes in the environment.
  *

Question 23

Compound-specific approaches -

Answer 23

• usually look at bulk tissues - booking at the isotope signatures of a whole bunch of biochemicals. By looking at compounds specific you may be able to cut out a lot of variabilities. Essential amino acids don’t change in their stable isotope composition whereas things synthesised by the host animal do. By comparing these essential and trophic amino acids you might be able to derive an estimate of trophic position. MIght also be a much better estimate of trophic position.

PCA shows great power to discriminate among major production sources in the sea e.g. algae vs terrestrial plants

Question 24

What essential amino acids can be used as a good indicator of source in compound-specific approaches?

Answer 24

Trophic’ (e.g. glutamic acid) vs ‘source’ (e.g. phenylalanine) amino acid-specific data

By looking at the difference between trophic and source you are getting some indication of the trophic position as they are N15 data but for the individual amino acid.

Question 25

Overview

Answer 25

Overview

Food webs are important models of how ecological communities are structured, but data traditionally come from gut contents data that have many shortcomings.

Stable isotope data also have their limitations but their strengths include informing a range of food-web issues including measurement of trophic position, food sources, feeding strategies over extended periods and spatial and temporal variation in these.

Comprehending the full potential of these approaches requires appreciation of the variety of ways in which food is processed by consumer organisms, time scales over which it is assimilated and manner in which it may be ‘routed’ to different tissues

Question 26

What can stable isotope analysis show for the deep ocean? paper by will

Answer 26

Reid WDK et al. (2012) Marine Ecology Progress Serie

Range of species including fish and invertebrates (~22 species in all) from otter trawl and trapping at 2400-2750m (2009)

Stable isotope data help
distinguish consumers which are predator-scavengers (reliant on elevated biomass along ridge) from those based on phytodetritus (downwelling phytoplankton materials).

Two stations ~690 km apart, N and S of Charlie Gibbs Fracture Zone.

Abyssal grenadier - carbon signature increases with size in two locations but not a third. As the animals grow they are doing different things. Different species differ with diets and the productions scources.

Differences between stations differing in sea surface primary productivity

Question 27

NIck study : chemotrophic

Answer 27

Higgs ND et al. (2016) Current Biology 2

analysis of 15 N show the spiny lobsters contained a chemotrophic production source - which turned out to be from chemotrophic clams

Question 28

historical reconstruction

Answer 28

Compare different annual bands in otoliths or scales or bones.

Have particular individuals showed the same patterns over time - consistency.

• When ALL tissues are formed they develop an isotopic signature
• Most tissue signatures change through life by dilution and replacement
• Some tissues once formed are metabolically stable and do not change
• Can collect a time series of tissue samples
• Scales and otoliths widely archived because provide records of age structure, growth rate etc for stock assessment
• SEFAS - huge database od scale for these kind of studies.

Question 29

NIck study harbour popose

Answer 29

Harbour porpoise (Phocoena phocoena) skeletons from strandings.

Omitting pups and buried carcass: 84 individuals show a significant decline in trophic level over time?

Skeletal data – particularly useful for marine mammals
Over the years = on av sig decline in trophic levels
The shift in time could be to do with a greater reliance from fishery discards.

Discontinuity analysis indicates three periods of relative stability: 1948-58, 1971-78, 1978-2002. Between the 1940s and early naughties!

Implies significant shifts over time.

(can also Analysing material along the length of baleen plates
Delta 13 carbon, derived from ageing work
Significant changes over time. Much of the variation is occurring on an annual basis )

Question 30

Comparing ecosystems study metaanalysis

Answer 30

Vander Zanden & Fetzer (2007) Oikos

Useful metaanalysis of stable isotope data, based on the notion of the trophic position and the length of chains can be derived from 15N.

Compare different types of the ecosystem (lakes, streams and open marine)

Stable isotope data - no obvious difference in the average food chain length of estuarine vs coastal vs marine ecosystems - although excluded marine mammals which add 2/3 of a trophic level.

Question 31

What do variations in C13 and 15 N imply about a population?

Answer 31

C13

• low variation - reliant on one particular source in diet
• High variation - individuals reliant on different sources of food

15 N

• Low variation - same trophic level
• High variation - different trophic level

A= Obligate specialist or generalist (all individuals doing the same things on the same time scale). Low variability in both.

B = Trophic level omnivory – individuals consistently feeding at different trophic levels. Depending on the nature of data could be ontogenetic. High N low C.

C = Source omnivory - individuals consistently feeding on prey reliant on different trophic levels. Depending on the nature of data could be ontogenetic. High C low N.

D = Facultative specialist (individuals doing different trophic things on the same time scale). HIgh of both.

Question 32

15 n data – shows us that they are positively correlated – species a at trophic position b in the Celtic sea it is likely to be near that in the north sea. But there is also a significant amount of variation, raises a question – the difference in stable isotope data – does this actually show a difference in trophic position. How has this been tested?

Answer 32

Jennings S, Warr (2003) Marine Biology 142(6)

Size related changes and the trophic position they operate that can be related to the ontogeny of the animals.

Estimation of TP using bulk isotope data requires food web ‘baseline’ species e.g. scallop (Aequipecten opercularis) trophic position 2.5.

Are differences in 15n actually representative of differences in trophic level or whats going on at the base of the food chain?

The only way to test this is to test the base of the food web and see if anything varies there too.

Found that scallops 15N data do vary spatially - which is related most strongly to salinity - probably related to freshwater inputs ect.

A way to normalise data?

Question 33

Jennings S, Warr (2003) Marine Biology

What are the findings.

Answer 33

Scallop δ15N varies spatially: higher values around coasts especially, lowest away from land

Salinity and temperature effects (riverine inputs of nitrate and ammonia, land-derived ammonia from agriculture and industry [higher δ15N])

Need to develop isoscapes for major pathways.

Isocape - stable isotope scale on map - i think

76% and 79% of spatial variance in dab and whiting explained by variation in queen scallop δ15N baseline data

Why scallop rather than plankton
Plankton are so seasonally variable so determining their baseline value is very difficult. They are time integrating variability in phytoplankton.

Question 34

What animal gets used to test variations in 15 N at the base of food webs?

Answer 34

Lion’s mane jellyfish Cyanea capillata mesogloea

scallops can be difficult to find

Driven by global NEMO-MEDUSA physics-biogeochemistry model.
Explain spatial variations in zooplankton data quite well.

Have to take spatial differences into account when studying over large scales.
Use isoscape data – this study can track where cod come from.

Question 35

overview spatial scales

Answer 35

Overview

Bulk SI data help reveal changes in relation to body size that in the bathyal vary a lot spatially in some species
Archival materials allow some historical reconstruction of trophic ecologies, as illustrated by mammals and fish
SI data indicate global patterns in food-chain length among different ecosystems
The data also help elucidate feeding strategies within populations
Isotopic baselines are being developed for some marine areas to allow correct interpretation of spatial differences in trophic ecology

Question 36

Understanding types of production sources

Answer 36

Some large isotopic differences among production source types - some very negative values. (seagrass, recent sediments etc.

Question 37

Polunin NVC et al. (2001) Mar Ecol Prog series

Of Ibiza in western med
Main constituents of the ecosystem – planktonic invertebrates, benthic invertebrates and fish.

Some correlation between 15N
and 13C indicates a single ultimate type of source material with fractionation: ‘marine snow’?

Linearity of 15N
vs 13C plot can indicate common production source

15 N and 13 C – things at the top are reliant on things at the bottom
Why is the relationship so linear? Things high up in the food web are reliant on one type of production source.

Question 38

Study of Corsica

Answer 38

More diffuse 15N vs 13C plot indicates many types of source material: littoral benthic vs planktonic.

Maxroalage spread in terms of delta 13 carbon values.
Planktonic invertebrates and benthic invertebrates, a cormorant and fish.
Not a linear relationship due to a range of production sources. Things are likely to be supported by what is below them on the graph.

Question 39

Nick study - NW Mediterranean: River Rhone inputs

Answer 39

Darnaude AM et al. (2004) Oecologia 1

Looking at material entering the med from a river –
Look at the POM from phytoplankton coming down the rhone river
Looking at the fractionation that occurs form those sources, you can see the overlap of species dependant on eoither of the food types. A triangle of possible organisms dependant on this POM.
Red Sole – at least part of biomass built of material coming down the rhone.

4 flatfish species’ production supported by phytoplankton production, but sole supported by riverine detritus

Question 40

How reliant are some of the big predators in reef systems on matter derived from reefs?

Answer 40

• Palmyra Atoll ~ 1700 km SW of Hawaii: 3 fish species constituting 85% of large predatory fish biomass
• 15N vs 13C used to characterise the food-web across lagoon, forereef and outer reef habitat
• Use 13C to identify principal production sources
  
  • Shark C. amblyrhynchos diet ~86% of from pelagic
  • Snapper L. bohar diet ~69% forereef, 29% pelagic, 11 lagoon
  • Shark C. melanopterus diet ~67% forereef, 22% pelagic, 11% lagoon

Important because reefs are thought to be very productive and self-sustaining, whereas large predators are very reliant of the pelagic
Local sources of upwelling around islands, giving sources of nutrients in oligotrophic environments – explaining where this input is coming from

Question 41

Why is reef pelagic coupling interesting?

Answer 41

Interesting because: Boundaries around habitat are quite ‘leaky’ and driving significant

• Coral reefs long noted for their productivity but thought to exist in an oceanic desert: ‘Darwin’s paradox’
• Fish production mostly from reef algal production maintained by nutrient recycling?
• Often high abundances of plankton feeding fishes
• And unexpectedly high planktonic production observed in some areas e.g. ‘island mass effect’
• Reefs static thus advected water, even if with low plankton concentration, could means of transporting pelagic material to the reef
• • Another study shows a change in diet of Caledonia reef fish to phytoplankton as you move away from the reef.

Question 42

How are delta carbon 13 values separate benthic and benthopelagic feeders?

Answer 42

Trueman CN et al. (2014) Proceedings of the Royal Society

NW Atlantic (W of Ireland): muscle 13C data from 30 fish species, 500-1800m depth. Separation increases with depth.

With depth, there is a greater separation between benthic feeders with lower less negative carbon values and benthopelagic feeders more negative values of delta 13 carbon, indicative of them being more reliant on the water column, rather than what is settled.

Strong bentho-pelagic coupling to 750m, but beyond much smaller proportion of production is from benthic sources.

Question 43

Why could biomass - pelagic coupling studies be important?

Answer 43

Important because:

• Global peaks in biomass of fish at mid-slope depths are likely to be important in affecting the number of carbon sources that support deeper oceanic layers. Deep-sea fishing, remove methods of sequestering carbon.
• Over 50% of the demersal fish community at 500-2000m supported by biological rather than detrital materials
• Loss of these species may affect long-term carbon storage in the deep ocean, reduction in nutrients transferred to benthic community
• Reduction in benthic biomass

Question 44

What does looking at benthic-pelagic coupling in north sea fish show and why is this important?

Answer 44

Duffill-Telsnig J et al. Journal of Animal Ecology 8

North Sea: 13 fish species from IBTS

International Bottom Trawl Survey data

Using muscle δ13C data and mackerel and plaice as indicator species of benthic and pelagic food sources

The 13 species range from heavily pelagic to heavily benthic sources

Ranked according to pelagic dependance
Species reliant solely on the pelagic sources (herring) – are likely to be more variable, as the production source is more variable. Those ultimately derived from plankton but processed in a different way on the benthos show a much more stable source (Cod, sole (lemon sole)).
Those in the middle which rely on both are most likely to be stable because they have the capacity to draw on these different sources of production (Dab, haddock).

Question 45

Brnthopelagic coupling - a tropical reef

Answer 45

Skinner C et al. Journal of Animal Ecology (in press)

• Another reef study
• A range of predatory species
• Plankton is very important for all these species

Outside the atoll, you will be more likely to find things reliant on the pelagic – but this study shows lots of evidence for organisms in the middle of the atoll relying on the pelagic.

Both groupers and snappers and jacks are nearly always most reliant on pelagic sources of plankton, then reef sources, with sponges showing the lowest dependence.

Interesting because:

• Reefs derive substantial pelagic inputs
• Pelagic inputs will be affected by external factors e.g. ocean currents, upwelling
• But may make reef food-webs more resilient e.g. to local changes such as coral bleaching and reef degradation arising therefrom

Question 46

Why is size an important ecological factor?

Answer 46

If you look right across marine organisms right from the smallest to the largest, they will span around 20 orders of magnitude. This is important ecologically as smaller body sizes have a greater metabolic rate, greater capacity for a population increase, typically higher levels of natural mortality and a smaller lifespan.

The big things - opposite -

Question 47

What relationship does delta 15N have with body size?

Answer 47

Max body mass is fixed for a species. Max body mass does not seem to lead to variation in delta 15 N. Rather than by species if you break it down by size you create a very strong relationship between delta 15 N and body mass.

Weak or non-existent relationship at the species level contrasts strongly with what happens when you break the whole community down purely by their body size.

(Infaunal and epifaunal invertebrates and demersal and pelagic fishes)

Question 48

Implications of size structuring in relation to trophic level.

Answer 48

Small high-TL and large low-TL species rare

TL of fish increases with body size across the whole community

Individuals at A on average tend to feed on individuals at B
Mean predator:prey body mass ratio (PPMR) = 23.4/slope= 109:1

3.4 standard fractionation which is assumed, 1 tropic level between a and b, creates a linear pattern between body size and trophic level.

Because we know quite a lot of how body size affects productivity we can say the productivity of A is going to be x, and b, y. and the relationship between those two things is the trophic transfer efficiency.

Question 49

Trophodynamics of whole food webs

uses of size structuring

Answer 49

Trophic transfer efficiency (TE) is the proportion of prey production (Pp) that is converted to consumer (predator) production (Pc), thus TE = Pc/Pp
Useful for estimation of

• potential fisheries yield
• primary productivity required to sustain fisheries
• maximum food chain length - lots of models that are really begging for accurate information on things like trophic transfer efficiency.
• parameterise models

The conventional calculation requires predator to prey mass ratios derived from costly and labour intensive diet and ecosystem modelling studies, but stable isotope data …

Question 50

An estimate of the level of production - nick study

Answer 50

Jennings S et al. (2002) Marine Ecology Progress Series

An estimate of the level of production in g per m2 per year as a function of delta 15N which they then use as a proxy for trophic level.

As the trophic level increases the production declines. This ideas that the transfer efficiency is low from one level to the next.

This study found the food chain efficiency is roughly 3.7 %.

Infaunal and epifaunal invertebrates and demersal and pelagic fishes.

Sampled by a range of gears: core, anchor dredge, beam trawl, GOV trawl, acoustics.

Transfer efficiency against size classes creates a slope. Smaller animals are comparatively efficient.

The higher the trophic level, the lower the production per unit area.

Question 51

Overview (3)

(Coral reef: relationship between 15N and body size weak for all fishes, although stronger within separate carnivore and herbivore groups)

The positive relationship is different for carnovores and herbivores

Answer 51

Overview

SI data cost-effective means of exploring ontogenetic and large-scale spatial, temporal changes in fish populations

Body size important determinant of who eats who in the North Sea and elsewhere (e.g. Gulf of Oman, Bahamas data), and possible to estimate the average TP of predators of a given size

Since body size also drives other things like growth/production per unit biomass, SI data help derive patterns of fish production and derive crucial estimates of food-chain transfer efficiency

These linear 15N vs body-size relationships not universal (e.g. Maldives data) but valuable where they occur

Question 52

Kelp as a trophic resource for marine suspension feeders:

whole study

Answer 52

Kelp as a trophic resource for marine suspension feeders: A review of isotope-based evidence – Miller and Page, 2012 – review – TRACKING THE SOURCE OF FOOD USING CARBON 13
Summary:
• Kelp forests house large numbers of suspension feeders – large biomass and productivity
• This enables suspension feeders to provide the trophic link between benthic and pelagic communities
• Often thought that kelp forest detritus is the primary food source for suspension feeders in kelp forests – however studies have showed that inshore phytoplankton provides most of the food to suspension feeders
• Used carbon 13 isotope to track the source – However, traditionally inshore phytoplankton uncontaminated with detritus is difficult to get hold of – therefore used offshore phytoplankton to compare to
• Separating detritus from phytoplankton has long be problematic - Detrital particles are similar in density and often in size to phytoplankton, and filtration or centrifugation methods have been ineffective, which has made microscopy the only method for quantifying detritus
• Inshore phytoplankton have been shown to be enriched in carbon 13 compared to offshore phytoplankton – Unaccounted for variation may have skewed the results and cause a bias of the contribution of kelp detritus to suspension feeders’ diet

Question 53

Lecture 7: Isotope ecology

summary

Using ocean models to predict spatial and temporal variation in marine carbon isotopes
(Magozzi et al 2017)

Answer 53

Lecture 7: Isotope ecology

Summary:

• Marine pelagic accurate interpretation of stable isotope data is hampered by a lack of reliable, spatiotemporally distributed measurements of baseline isotopic composition
• This study presents carbon 13 isotope composition of phytoplankton across the global ocean at one degree and monthly resolutions
• NEMO-MEDUSA model used

Question 54

Isotope ecology - lecture 7

results

Using ocean models to predict spatial and temporal variation in marine carbon isotopes
(Magozzi et al 2017)

Answer 54

Results:
• Predicted annual average 13CPLK values range between 31‰ and 16.5‰ across the global ocean
• the mean SD predicted annual average 13CPLK value is 23.4‰ ± 3.7‰ for the global ocean
• Variation across latitudes – more variation at higher latitudes and temperate latitudes than at tropical latitudes
• >60 degrees latitude showed the highest carbon 13 data values
• transfer of temporal baseline variations to animal tissues generally decreases with decreasing tissue isotopic incorporation rate and increasing trophic level - potentially leading to the temporal averaging of substantial variability in baseline isotope values in high-trophic-level organisms
• Mismatches between modelled and measured values may indicate the presence of additional carbon sources within food webs
• Capture of major spatial carbon isotopic baselines – implies the model can be used to capture broadscale temporal variation in 13C
• Could be used to predict 13C spatial and temporal variation in baseline carbon isotopes over a foraging range and seasonal variation – this variation can then be included in diet portioning

Question 55

Implications of scaled δ15N fractionation

study

Answer 55

Implications of scaled δ15N fractionation for community predator-prey body mass ratio estimates in size-structured food webs – Reum et al., 2015 – Evidence against the constant delta 15N value between each trophic level

Summary:

• When estimating TL, researchers had assumed that fractionation of δ(15) N (Δδ(15) N) did not change with TL.
  
  • However, a recent meta-analysis indicated that this assumption was not as well supported by data as the assumption that Δδ(15) N scales negatively with the δ(15) N of prey.
• We collated existing fish community δ(15) N-body size data for the Northeast Atlantic and tropical Western Arabian Sea with new data from the Northeast Pacific.
• These data were used to estimate TL-body mass relationships and PPMR under constant and scaled Δδ(15) N assumptions, and to assess how the scaled Δδ(15) N assumption affects our understanding of the structure of these food webs.
• Adoption of the scaled Δδ(15) N approach markedly reduces the previously reported differences in TL at body mass among fish communities from different regions.
• Delta 15N enrichment in predator species declined with increasing Delta 15N in prey species – e.g. predators feeding on prey with δ15N values of 8 and 15‰ would, on average, be enriched by 3·8 and 1·9‰, respectively – Hussey et al., 2014
• obtained community δ15N–body mass relationships from the published literature and also estimated a new relationship for a coastal food web in the Northeast Pacific
• For all food webs, we calculated TL based on the assumptions of (1) constant Δδ15N and (2) scaled Δδ15N
• calculated PPMR for each community with TL estimates based on these assumptions.
• predator to prey body mass ratio = PPMR = used to describe ratios between the mean mass of predators and the mean mass of their prey

Question 56

Implications of scaled δ15N fractionation

results

Answer 56

Implications of scaled δ15N fractionation for community predator-prey body mass ratio estimates in size-structured food webs – Reum et al., 2015 – Evidence against the constant delta 15N value between each trophic level

• With scaled Δδ(15) N, TL-body mass relationships became more positive and PPMR fell.
• Results implied that realized prey size in these size-structured fish communities are less variable than previously assumed and food chains potentially longer.
• The adoption of generic PPMR estimates for calibration and validation of size-based fish community models is better supported than hitherto assumed, but predicted slopes of community size spectra are more sensitive to a given change or error in realized PPMR when PPMR is small.

Results:
• scaled Delta 15N as proposed by Hussey et al., 2014 led to lower estimates of PPMR than those that resulted from using the constant Delta 15N method

• estimates of PPMR and TL at mass were less variable among regions when scaled, rather than constant, Delta 15N was assumed
• TL at body mass was underestimated when we assumed constant rather than scaledDd15N, and the degree of underestimation increased with body size
• Results implied that realized prey size in these size-structured fish communities are less variable than previously assumed and food chains potentially longer

Question 57

Rescaling the trophic structure of marine food webs

Answer 57

Rescaling the trophic structure of marine food webs – Hussey et al., 2014
Summary:
• Measures of trophic position (TP) are critical for understanding food web interactions and human‐mediated ecosystem disturbance
• a range of consumers spanning zooplankton to apex predators were sampled in two distinct marine food webs and analysed for δ15N
• All zooplankton, teleost and elasmobranch species were sampled from the KwaZulu‐Natal continental shelf food web in South Africa and from Cumberland Sound, Baffin Island in the Canadian Arctic
• All teleosts and elasmobranchs were measured and a white muscle tissue sample was taken anterior to the first dorsal fin (or from the wing margin for rays/skates) and frozen (−20 °C).

Question 58

Rescaling the trophic structure of marine food webs

Answer 58

Rescaling the trophic structure of marine food webs – Hussey et al., 2014

Results:
• Apex predator TP estimates were markedly higher than currently assumed by whole‐ecosystem models, indicating perceived food webs have been truncated and species‐interactions over simplified. - validated by known predator–prey relationships
• Previous studies using conventional constant demonstrates elasmobranchs off south Africa and Short-fin Mako shark in NE Atlantic at trophic levels - pex sharks (5.1 ± 0.5) and common shark prey species (4.4 ± 0.4) given known levels of omnivory. These TP estimates conflict with conventional stomach content TP estimates of 4.4 ± 0.1 and 4.2 ± 0.1 for predators and prey, respectively, which suggest feeding over an unrealistic range of 0.2TL. – Also places them one TL above Zoopanktivores fish – which suggests this is the majority of the diet- not the case
• Similarly, in the northeast Atlantic, TP of shortfin mako, was estimated at 4.0 using a Δ15N of 3.4‰, placing them one TL above zoopanktivores, which is inconsistent with their known diet of piscivorous bluefish (Pomatomus saltatrix)
• Scaled delta 15N values result in markedly higher TP estimates for large fish and extending food web length compared to the conventional constant discrimination approach
• By accounting for directional Δ15N narrowing within food webs, our ability to accurately measure absolute TP variation is substantially improved enabling ecologists and resource managers to better understand and conserve aquatic ecosystems.