Tropic Ecology

Question 1

Define a food chain

Answer 1

22:14 Food chain - a linear sequence that reveals which organisms consume which other organisms in an environment

Question 2

Define a food web

Answer 2

Food web – a more complicated diagrammatic representation of overall pattern of feeding interactions

Question 3

Define ecology

Answer 3

Ecology - how organisms interact with one another and their environment

Question 4

All animals are Heterotrophs. All marine consumers feed in one of what five basic ways

Answer 4

1. Grazers – herbivores directly consuming plant material. Copepods, gastropod molluscs 2. Predators –carnivores hunting smaller animals. Several levels, herring, tuna, sharks 3. Scavengers – searching for dead organic matter. Common amongst benthic invertebrates 4. Filter feeders (and suspension feeders) – obtain food by removing suspended particles from the water. Barnacles Deposit feeders –extracting food particles contained in the sediment. Worms, starfish

Question 5

How do bacteria fix carbon?

Answer 5

• Split in bacteria, fixing carbon either aerobically or anaerobically. ○ Aerobic bacteria require oxygen to produce nitrate (NO3-) from ammonia(NH3) Anaerobic bacteria liberate oxygen from sulphate (SO42-) and produce hydrogen sulphide gas (H2S)

Question 6

Calculating Trophic Levels

Answer 6

• Require / Assume: diet fully characterised and quantified, prey TLs correct, and reflect long term mean. Use data from stomach content analysis • To appreciate what’s going on in a food web is to calculate trophic levels. This can be done by quantifying food webs. ○ The most basic way of doing this would see where its food comes from. Stomach content analysis etc.. ○ Calculate the percentage of each prey species that makes up the diet ○ Take an average of the trophic levels of the prey item and sum them up.

Question 7

Weaknesses f stomach content analysis

Answer 7

• Assumed diet trophic level (will take a very long time to establish a trophic level of something high up) • Long term mean allowing no variation in space/time - takes a snapshot, but diet will change • Unknown base of production. Can’t run stomach content analysis on each prey species - so we don’t know where the base of production actually starts (algal / bacterial) • Data extremely limited (e.g. vacuity). When fish are pulled up in nets the stress causes vomit and excretion. This would give an incomplete diet. • High sampling effort (e.g. opportunistic feeders). A lot needs to be done to establish the whole picture for a population Net Contamination - can opportunistically feed in net

Question 8

Strengths of stomach content anaysis

Answer 8

Strengths • High taxonomic resolution - with the stable isotope analysis you never know what the species actually are. • Technologically simple • Tried and Tested - used throughout history Snapshot in time

Question 9

energy transfer between trophic levels

Answer 9

• Chemical energy is passed upwards through the trophic levels • Results in an increase in biomass of a region • Energy transfer is not efficient, 90% lost per level. • Biomass lower at the top of the chain/web • Energy will be lost through excretion, indigestible material such as the CaCO3 tests of a foram, or silicate tests of a diatom. If we want to estimate potential production at the top of a food chain, i.e. fisheries production, it is necessary to tabulate these energy losses

Question 10

how can you estimate potental fish populations in an area?

Answer 10

• Rates of production and energy transfer efficiency can then be used to estimate potential fish populations in a region • Useful guides to explain how much production you can expect form a fishery, however, the numbers are not usually accurate as these estimates are far more complicated.

Question 11

Alternatives to Stomach Contents

Answer 11

Alternatives to Stomach Contents • Scat analysis – useful in large animals e.g. mammals, utilises hard structures e.g. otoliths, cephalopod beaks ○ Can be difficult in the marine environment, disperses quickly ○ Less destructive method • Fatty Acid Profiles - some prey have characteristic FA profiles…used as tracers Look at fractionation of fatty acids within an organism, as these fatty acids are usually taken from other organisms and formed into long chains.

Question 12

What is isotope fractionation?

Answer 12

• Isotope fractionation = differential partitioning of isotopes between two compounds • Heavy isotopes have a higher bond strength and therefore react slower. The differing in bond strength will give you a rate proportion to the mass difference. The greater the mass difference the greater the difference in the rate of transfer between them.

Question 13

What is Stable isotope analysis

Answer 13

• Because of processes and mechanisms within an organism fractionation occurs between heavier and lighter isotopes. Lighter or heavier ions can be use preferentially. • Lighter ions are used because ○ They are more common ○ More likely to see the fractionation when there is a comparatively bigger ratio of weight between the ions. • Nitrogen 14 and 15 are used a lot. • This uses =Delta Notation d (‰) ○ `dHX = [(Rsample - Rstandard) / Rstandard] × 1000 ○ Testing the parts per 1000 difference between the standard from the environment and the sample. ○ Carbon Standard = Pee Dee Belemnite (PDB) (expected to know) ○ Nitrogen Standard =Atmospheric N2

Question 14

Trophic Step Fractionation

Answer 14

• 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 • CONVENTIONALLY ASSUMED ○ Δδ15N = +3.4 ‰ (3.4 part per thousand difference in nitrogen will give you a different trophic levels) Δδ13C = +1.0‰ (1 part per thousand difference in carbon will give you a different food source)

Question 15

What are the advantages of Trophic Step Fractionation

Answer 15

• independent of gut-contents data (easy to get samples, less destructive and using different body parts means you may be able to calculate trophic levels at different points in its life) • materials largely invisible in gut data can be evaluated, no species id needed • integrate short-term changes into one variable • materials involved are those actually assimilated, doesn’t include accidental swallowing • simple first-order tests of spatial, temporal and ontogenic trends (one easy comparable number)

Question 16

WIDER READING: Diet composition and trophic levels of marine mammals

Answer 16

Diet composition and trophic levels of marine mammals, D. Pauly et al 1998

• The study:
• Looked at the stomach composition and scat of marine mammals from previously published papers and sorted this into eight categories: benthic invertebrates, large zooplankton, small squids, large squids, mesopelagic fishes, miscellaneous fishes and higher invertebrates.
• They used composition of prey to determine the trophic level of the species.
• They estimated the trophic levels and compared these to already published figures- which were derived from stable isotope ratios.
• The results:
• The trophic levels in Ostrom et al’s 1993 study are lower than this one in three species (sperm whale, pygmy sperm whale and Sowerby’s beaked whale)
• The values however are higher for five other species (beluga whale, minke whale, humpback whale, common dolphin and white beaked dolphin) so they conclude that the stable isotope is not a completely accurate method of determining trophic levels.
• The results regarding stomach contents can be used in the future in various ways: calorific contents of marine mammal diets; global food consumption by marine mammals and energy requirements of marine mammals.

Question 17

WIDER READING: Stable Isotope

Answer 17

Stable isotopes in ecosystem studies, B. Peterson & B. Fry, 1987 (review paper)

The review:

• Shows how isotopes of C, N, S, H and o can be used to solve biogeochemical problems as they can be used as tracers, and compositions can be accurately determined using mass spectroscopy.
• In isotopic compositions, if the value of δ increases, then the compound or substance contains more of the heavier isotope, and vice versa.

Question 18

WIDER READING: Feeding relationships - stable nitrogen and carbon isotope data

Answer 18

Feeding relationships in Mediterranean bathyal assemblages elucidated by stable nitrogen and carbon isotope data, N. Polunin et al, 2001.

The study:

Explored two main hypotheses:

Food-web structure differs between depth zones

Deep-sea food webs respond to seasonal changes in the photic zone

There is only one main food source in the Mediterranean deep sea, i.e. marine snow.

Fish and invertebrates were collected from three distinct depth zones on the continental slope south west of the Balearic Islands. These three depth zones were 200-800m, 800-1425m and 1425-1800m, and were done in October 1996 and again in may 1998

The animals caught were processed in various ways, eg ground up and frozen, and then tested for isotope concentrations. These concentrations were tested for using continuous-flow isotope-ratio mass spectrometry.

Mainly, concentrations of nitrogen and carbon were tested.

The results:

Temporal changes:

Only 2 significant changes between the two dates, which were both increases in δ15N in 1998.

δ15N of surface plankton were higher in the spring due to upwelling events which bring 15N to the surface which is then absorbed by phytoplankton.

When upwelling stops, the water becomes oligotrophic and primary production relies on nitrogen in the form of ammonia, which lacks in 15N.

Depth changes:

Calanoid copepods and other zooplankton had higher δ15N values at a greater depth, which may indicate that food web basal materials become δ15N-enriched at depth.

Also, changes in feeding type of the copepod and length of food chain may explain the pattern, as there are more carnivorous organisms at greater depths.

Food web structure:

Strong correlations between δ15N and δ13C suggest that marine benthic organisms are highly supported by a primary source material.

This is in contrast to surface waters, where there is a weaker correlation, due to the multitude of production sources which are available there.

Trophic level:

Calanoid copepods were used as a reference level and it was assumed that there trophic level was 2.

δ15N increased by 3.4% with every level.

The maximum trophic level exhibited was 4.4, which was the shark Centroscymnus coelolepsis in 1996 and the macrourid Nezumia aequalis in 1998.

Bathyal fishes forage over 2 o 3 full trophic levels.

Decapods, and zooplankton fed over 2 trophic levels.

Gelatinous plankton showed very low levels of δ15N which suggests that they have a greater reliance on microzooplankton for food than other organisms.

Overall: zooplankton at shallower depths showed a lower δ15N value (3.5%) than those at greater depths (6%). This was the case for decapod crustaceans, fishes and mysids. Therefore, variation in the concentrations of δ15N in benthic organisms is responsible for changes in trophic levels.