Pelagic Food Webs

Question 1

Classic and microbial food webs linked

Answer 1

Classic and microbial pathways are a useful dichotomy for distinguishing the several fates of C PP in marine ecosystems
Recent evidence shows that the microbial food web is a fundamental and almost permanent feature of oligotrophic and eutrophic coastal upwelling areas.

Question 2

Biological Carbon Pump zoop contributions

Answer 2

Sloppy feeding
- excretion
- egestion
- mucous production
- exudates, reproduction
- migration
- death

Question 3

How to quantify C or energy flowing through webs

Answer 3

Bioenergetics, how much energy to metabolism, growth, reproduction
Fatty acid stable isotope analysis, life history info
Measuring primary and secondary production
Ecosystem based modelling

Question 4

Bioenergetic approach

Answer 4

New growth/production (G) = ingestion (R) - Faeces (E) - Excretion (U) - Respiration (T)
or
Net growth/production = assimilated food (AR) - respiration (T)

Question 5

Growth Yield

Answer 5

Ygr = growth/growth + respiration or
Ygr = growth/food intakea
= 10-30%

Question 6

Growth yield depends on

Answer 6

Type of organism
Level of complexity
Swimming ability
Stage of life

Question 7

Trophic yield

Answer 7

Ty = production at trophic level (t+1) / production at trophic level (t)
Total efficiency (Ty)n
Trophic yield = ecological transfer efficiency (10-30%)

Question 8

Trophic Energy Pyramid

Answer 8

Amount of biomass decreases as you move up the trophic levels because only ~10% energy within a trophic level is available for the next trophic level
the amount of PP at the base drives trophic structure

Question 9

Match-mismatch, temporal

Answer 9

In a mismatch there aren’t enough zooplankton to feed all recruits so can’t develop

Question 10

Match-mismatch, spatially

Answer 10

Warm climate conditions reduced spatial overlap, results in lower overwinter survival and recruitment success of juveniles to age 1
Colder climate conditions increased spatial overlap results in higher growth, overwinter survival and recruitment success of juvenile fish to age 1.

Question 11

Up welling ecosystems

Answer 11

Eastern boundary upwelling systems (EBUS) cover <1% of the ocean surface but provide up to 20% of the world’s capture fisheries due to their high levels of production fuelled by nutrient-rich, cold deep water.

Question 12

EBUS, oceanography

Answer 12

Wind blowing pushes warm surface water away from the coast
Displaced water replaced by deep, cold, nutrient rich water rising to the surface.
Nutrient rich water fertilizes surface waters, high PP

Question 13

EBUS ecology

Answer 13

short efficient food chains, lots of fish

Question 14

EBUS, normal climate

Answer 14

Normal:
- Low pressure W Pacific and high pressure E pacific cause trade winds to move surface water to west, upwelling and shallow thermocline

Question 15

EBUS, El Nino

Answer 15

High pressure system weakens, trade winds reduce
Warm surface water flows easy, deepens thermocline and prevents upwelling.

Question 16

EBUS, la nina

Answer 16

Unusually strong trade winds bringing deep cold water to the surface resulting in colder water than usual

Question 17

EBUS, effects of upwelling intensity

Answer 17

Normal, zoop pop grow and control phyto, conventional marine food web
Intense upwelling exports zoop quicker than pop growth.
Strong swimming sardines able to remain and consume phyto, local food web collapse and free nutrients flow off shore
Intense upwelling exports zoop too rapidly, phyto not grazed so die and decay, noxious gas, unused zoop offshore fed on by jelly blooms

Question 18

Cali Food Web

Answer 18

Euphasiids are the most important for energy transfer pathway
Large production of the lower trophic production eaten (green) is transferred to the higher trophic levels (red)

Question 19

Californa current system food web model

Answer 19

Jellyfish act as a production loss pathway
Little of the production consumed was passed upward, jellies arent consumed
If jelly consumption reduced by 20%, productivity increases to upper trophic levels

Question 20

Cali El Nino effect

Answer 20

Deep thermocline
more complex food web
Reduced nutrients and populations

Question 21

Benguela upwelling system

Answer 21

Sardines, anchovies, mackerel and hake fluctuated since the 50s.
Alternation between planktivory and predatory fish
10-fold increase in copepod biomass since 50s
Collapse of sardine fishery
Sardines replaced by mackerel and goby

Question 22

Benguela jellification

Answer 22

Overfishing of small pelagic fish triggered chain of events resulting in rise of jellyfish and the bearded goby
energy flow is diverted away from the higher trophic level (HTL) production towards benthos and detritus

Question 23

Oligotrophic subtropical gyres

Answer 23

Once thought of as deserts, low nutrient inputs and productivity
Subtropical gyres are much more dynamic systems

Question 24

Nutrients and Chl a

Answer 24

Surface nitrate (low f ratio) with clearly defined vertical structure and the DCM
0.05-0.1mg Chl a/m3

Question 25

Production of gyre

Answer 25

Episodic mixing events increase nitrate input and annual production Deep waters: blooms winter due to mixing and nitrate inputs Upper waters: blooms late summer based on N fixing and eddies

Question 26

Gyre picoplankton

Answer 26

Autotrophs dominated by picoplankton, 60-90% of chl a biomass and 60-80% of C fixation due to cells <2um

Question 27

Gyre phtyo >2um

Answer 27

Occasionally larger cells make a significant contribution to production Above line since they get grazed as soon as they reproduce, biomass stays low

Question 28

Gyre Zooplankton

Answer 28

Microheterotrophs, particularly HNF dominate biomass and grazing in oligotrophic systems Cyclopoids and carnivorous copepods Gelatinous zoop

Question 29

Gyre food chains

Answer 29

Both microbial and classic food web pathways co exist Nutrient limitation, microbe loop dominates Nutrient inputs, classic food web briefly dominates

Question 30

Gyre export production

Answer 30

Export production, the amount of OM produced by PP that is not remineralized before it sinks into aphotic zone Composed of faecal pellets, exudates, cells, mucous POC may aggregate to marine snow

Question 31

N Sea change in trophic structure

Answer 31

Evaluated relationships between climate, plankton and cod to determine drivers of change Use CPR to find quality and quantity of plankton for larval cod Survival depends on: - mean size of prey - seasonal timing - abundance Variability in temp affects cod larval survival via its affect on fish's plankton prey

Question 32

Black sea change in trophic structure

Answer 32

Invasive species N american ctenophore via ballast water Ecological events: - 1988, extremely abundant - negative impact on native gelatinous sp - increased impact on commercial fin fish - intro of beroe sp brought numbers under control

Question 33

Mnemiopsis

Answer 33

Good invader in black sea because: - flexible physiology and lobate, easily pumped and transportable - self fertilising simultaneous hermaphrodite - high fecundity, 23000 eggs in 10 days - resists starvation by degrowth - cosmopolitan diet - black sea is a sensitive ecosystem susceptible to invasion - has since spread through europe

Question 34

Limfjorden

Answer 34

Become heavily eutrophicated since 60s Oxygen depletion of near bottom waters has increased in frequency over 100 years since 70s up to 40% becomes anoxic

Question 35

Eutrophication on demersal fish

Answer 35

Landings of demersal fish have decreased a lot In spite of reduction in nutrient load, there has been minimal population recovery.

Question 36

Effect of fish to jellyfish

Answer 36

Biomass of zoop reduced a lot when population of jellyfish increased