Stable isotopes Flashcards

Question

Overview

Answer 1

## Footnote 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

Answer 2

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

Answer 3

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

Answer 4

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.

Answer 5

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 )

Answer 6

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.

Answer 7

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.

Answer 8

**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?

Answer 9

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.

Answer 10

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.

Answer 11

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

Answer 12

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

Answer 13

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.

Answer 14

**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

Answer 15

* 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

Answer 16

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.

Answer 17

**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.

Answer 18

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

Answer 19

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).

Answer 20

**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

Answer 21

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 -

Answer 22

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)

Answer 23

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.

Answer 24

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 …

Answer 25

**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.

Answer 26

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

Answer 27

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

Answer 28

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

Answer 29

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

Answer 30

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

Answer 31

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

Answer 32

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).

Answer 33

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.

Stable isotopes Flashcards

(58 cards)