Pelagic Food Web

pelagic

water column environment

benthic

the seafloor environment, including coral reefs and rocky intertidals

plankton

unable to swim horizontally against ocean currents but some may be able to move vertically in the water column (e.g. phytoplankton and zooplankton)

nekton

able to swim against ocean currents (e.g. fish, squid and sea turtles)

holoplankton

organisms that live their entire life as plankton

what is an example of a holoplankton

copepod

meroplankton

organisms that spend only part of their life as plankton (e.g. crabs, barnacles and oysters)

why do we focus more on phytoplankton and zooplankton, rather than fish, sharks and whales?

there is a larger proportion of zooplankton than the fish, sharks and whales combined

autotrophs

group of organisms whose energy/carbon for growth comes from non-organic sources

heterotrophs

group of organisms whose energy/carbon for growth comes from previously formed organic carbon material (e.g. carnivores and herbivores)

what is an example of autotrophs

phytoplankton because they use sunlight and co2 for their energy and carbon needs

how do we classify organisms into broad categories in a pelagic food chain?

typically via feeding mode (e.g. splitting into autotrophs and heterotrophs)

how do we assign an organism to a particular trophic level

• first ask if the organism autotrophic or heterotrophic? 

• next ask: is the heterotrophic organism a first consumer or a secondary consumer or a tertiary consumer? use this logic:

  • because animals in fluid suspension do not have time to nibble on their prey, they must consume the prey whole. this puts strong constraints on the size of prey that can be consumed by an organism

  • to determine what trophic level an heterotroph is, look at the size

how to identify if an organism is autotroph or heterotrophic?

if there is chlorophyll, it is autotroph. no chlorophyll = heterotroph

what is the optimal prey size

1/10 of consumer size

what is the feature of pelagic food webs

strongly size-structured

why do we need to understand trophic transfer efficiency

it sets an upper limit on the amount of harvestable fish a given level of primary production can support

what does trophic transfer efficiency depend on

• exploitation efficiency

• production efficiency

what is exploitation efficiency

the efficiency with which a consumer population is able to find, capture and ingest all of the potential prey present in the environment

what is production efficiency

the physiological/biochemical efficiency of converting ingested prey into consumer biomass

how do we calculate trophic transfer efficiency

trophic transfer efficiency = exploitation efficiency x gross production efficiency

what is a strategy to avoid encounters or detection

separate by time and / or space (diel vertical migration)

• much of the zooplankton community migrates up to the surface layer of the ocean at night to feed in the dark while also avoiding visual predators like small fish 

• during the day, zooplankton migrate down to the safety of the darkness found at depth 

why do many zooplankton species exhibit biolumniscence

• due to predator avoidance

• creating a large flash of light as a predator approaches momentarily stuns and confuses the predator long enough for the prey species to make its escape 

what is an example of grazer exploitation efficiencies in spring blooms in temperate north atlantic region

• during long winter periods, large grazers (copepods mainly) sink into the deep ocean and enter a diapause (i.e. hibernation) stage and become decoupled from any variations in primary production above

• in spring, phytoplankton standing stock can initially grow to very high density because it is not held in check by strong grazing pressure until the large grazers have a chance to come out of diapause and reproduce to keep the high numbers of phytoplankton in control → hence there is phytoplankton blooms in early spring

• exploitation efficiency is very low

example of grazer exploitation efficiencies in tropical environments

• small grazers remain active throughout the year and consume phytoplankton as fast as it is made

• any increase in production is quickly met by an increase in grazer abundance and subsequent increase in the consumption of phytoplankton 

• this leaves standing stock of phytoplankton nearly constant throughout the year

• exploitation efficiency is very high since almost all phytoplankton is found and consumed by grazers

what is the rate of trophic transfer efficiency

10%

food chain length of open ocean

• open ocean has low nutrients

• phytoplankton are small and the number of trophic levels (7) to harvestable fish are many - big overall loss due to many trophic transfer steps

food chain length in coastal upwelling regions

• high nutrient environment

• phytoplankton are large and the number of trophic levels to harvestable fish are few - small overall loss due to few trophic transfer steps