Week 9- Pelagic Food webs

What is a simple linear food chain

Sun, primary producers, then prey etc etc.

Classic

What is wrong with a linear food web

Just doesn’t exist

What is more realsistic but harder to contruct and quanitfy and requires good knowledge than a linear food chain

Food web

What are the two types of food web

Classic and microbial

• They are not isolated and are linked (e.g. by dic)

What is the difference between classic and microbial food web

Micorbial is dominated by viruses, bacteria and protosomes, was assumed to only be in nutrient poor areas but now both exist in all ecosystems

What is microbial food web like

Efficient, lots of recyling/regeneration, keeps it energy efficient

What is zooplankton role in biological carbon pump

• Sloppy feeding (Not completely ingested instead broken up relasing organic material)

• Excretion

• Egestion

• Mucous predation

• Exudates, reproduction (exoskeletons)

• Migration

• Death

How can we quantify carbon/energy flowing through food webs

1. Bioenergetics

2. Fatty acid stable isotope analysis

3. Measuring primary/secondary production

4. Ecosystem based modelling

What is the bioenergetic approach

Energy in ingested food, some is egested and some is digested/assimilated of the assimilative food it is lost through excretion, metabolism, growth (somatic/moulting/reproductive)

How to calculate net growth/production (G)

(G) = Ingestion - faeces - Excretion - Respiration

or

(G) = Assimilated food- respiratoion

What is another way of expressing bioenergetic approach

Growth yield- proportion/percentage that growth is of the growth/respiration terms

How to calculate growth yield

Growth/growth+ repsiration

or

Growth/ food intake

What value does growth yield tend to be

10-30%

Depends on: Organism type, complexity level, swimming ability, life stage

What is trophic yield

How energy goes from one level to another

How do you calculate trophic yield

Production at trophic level (t+1) / production at trophic level (t)

Whats another name for trophic yield

Ecological transfer energy

What are the feataures of a trophic energy pyramid

Amount of biomass decreases up trophic level as only 10-30% of energy from one trophic level is available for next

What does a tuna sandwich show

Effect of trophic/energy pyramid from a fisheries perspective

Do all ecosystems have 6 trophic levels

No

Open ocean tends to have 6 (low nutrients)

Continental shelf tend to have 4

Upwelling have tend to have 3 (higher nutrients) (most fish production due to much higher primary production and short food chain)

Does primary production have a major impact on fish production

Yes

What do short efficient food chains mean

Lots of fish

When is cod recruitment greatest

When plankton anomalies are positive

What is Cushings match-mismatch hypothesis temporally

If timing of larval food coincides with larval herring= match (food available can grow)

Delay in food for larval herring= mismatch (starve and unsuccessful recruitment)

How does Cushings match-mismatch hypothesis work spatially

Warm climate= reduced spatial overlap in lower overwinter survival and recruitment success of juvenile fish to 1

The opposite in cold climate

How much of ocean surface do eastern boundary upwelling systems cover

<1%

What impact do eastern boundary upwelling systems have

Provide 20% of world’s capture fisheries

Why does eastern boundary upwelling systems provide so much fish

High productivity

How do upwelling systems work

• Wind blows warm surface water away from coast

• Displaced water replaced by deep, cold, nutrient rich water rising to surface

• Nutrient rich water fertilises surface water

How do el nino/la nina (climate variability) effect upwelling systems

Normal- low pressure w. pacific and high in e.pacific, trade winds move surface water to west= upwelling and shallow thermocline

El nino- high pressure weakens, trade winds reduce, warm water flows east= deepens thermocline and prevents upwelling

La nina- unusually strong trade winds, bring deep cold water to surface= cold water than normal

What are the effects of upwelling intensity and fish stocks

Normal- zooplankton population grow and control phytoplankton= conventional local marine food web

Intense upwelling and lots of fish- zooplankton exported too fast, strong swimming fish stay and consume phytoplankton= local food web collapses

Intense upwelling and no fish (e.g. been fished) - zooplankton exported too fast, phytoplankton not grazed so dies and produces noxious gad, unused zooplankton offshore fed on by jellyfish blooms

What is measured at California current system

• Inter annual variation in food web structure

• Energy transfer from bottom to top of web modelled by data sets

• Different scenarios applied to highlight role and impacts of groups as energy transfer paths

What was found at californian current system from food web model

Euphausiids are most important for energy transfer transfer pathway

Large proportion of low trophic production eaten is transferred to higher trophic levels

Jellyfish are a production loss pathway

How many zones in benguela upwelling system split into

4

What is there lots of in the Benguela Upwelling System

Fisheries

Fisheries monitoring

What are some of the numbers from the Benguela upwelling system

Collapse of sardine fishery in North and replaces by mackrel and foby

In south the fish have fluctuated

V. big difference

What has overfishing of small pelagic fish in N.Benguela lead to

Jellification= rise of jellyfish/bearded goby

• Energy flow diverted away from higher trophic level production towards benthos/detritus

What happens at oligotrophic subtropical gyres in complete contrast to upwelling

Low nutrient and nitrate= low productivity- but they are slightly more dynamic

How many subtropical gyres are there

5 - circular ocean currents formed by global

What is chlorophyll and nitrate like in gyres

Very low with defined vertical structure and DCM

What can lead to changes in subtropical gyres to productivity

Episodic mixing events increase nitrate input

Deep waters: blooms winter due to mixing/nitrate input

Upper waters: blooms late summer due to n fixing organisms

In subtropical gyres what do the ohytoplankton tend to be like

Small e.g. picoplankton (dinoflagellates, cocxolithophores, hapthophytes)

What happens occasionally during mixing events at subtropical gyres

Blooms of larger cells e.g. diatoms and can make a significant contribution to production

WHat zooplankton dominate

Microheterotrphs - can grow and repsond to change quick and domiante grazing in oligotrophic

Carnivorous copepods

Gelatinous zooplankton

Do the microbial and classic food chains co exist

Limited nutrients- microbial dominates

Nutrient inputs- classic will briefly dominate

What is export production

Amount of organic matter produced by primary production that is not recycled e.g. faecal pellets, exudates, cells from messy feeding, mucous, marine snow

Does marine snow vary

Yes

How and why does tropic structure chanfe

1. Climate variability

2. Invasive species

3. Fishing

4. Eutrophication

Describe case study for climate variability in N.atlantic/n.sea

• Two indicator species of copepods

• Cold water= larger more biomass

• Being pushed further north

• Then impacts larval recruitment

Describe case study for invasive species in black sea

Ctenophore (mnemiopsis) was introduced by ballast tank of ship

1988- extremely abundant

Negative impact on native gelatinous spp.

Increased impact on commercial fin fish

Introduction of Beroe brought numbers under control

Why is mnemiopsis a good inasive species in black sea

• Flexible physiology (tolerates lots of conditions)

• Self fertilising simultaneous hermaphrodite (breeds easily)

• High fecundity

• Shrinks during temporary starvation

• Cosmopolitan diet

Describe case study for eutrophication in Limfjodren

• Loads of nitrogen run off from land to fjords since 1960s

• Very anoxic system

• Oxygen depletion near bottom waters

• Negative effect on demersal fish e.g. plaice

• Tried to fix- but not yet recovered

• Now dominant is from fish to jellyfish

• Jellyfish then consume lots of zooplankton= reduced zooplankton abundance