Food Webs Flashcards
(35 cards)
What is a pelagic food web and why is it important to marine ecology?
A pelagic food web represents the complex trophic relationships in the open ocean, primarily involving phytoplankton, zooplankton, and higher consumers like fish. It is crucial for understanding carbon flow, nutrient cycling, and fisheries productivity.
How do food chains differ from food webs?
Food chains are linear representations of trophic interactions, while food webs illustrate the more complex and realistic interconnections among multiple species and feeding relationships.
What distinguishes classic food webs from microbial food webs?
The classic pathway involves direct transfer of energy from phytoplankton to zooplankton to fish. The microbial web involves recycling of dissolved organic carbon via bacteria and microzooplankton, often dominating in oligotrophic and eutrophic upwelling zones.
What evidence supports the ubiquity of the microbial food web?
Studies like Calbet & Landry (2004) and Vargas et al. (2007) show that microbial food webs are permanent features even in nutrient-rich coastal upwelling systems.
What methods are used to measure carbon or energy flow in marine food webs?
Methods include bioenergetics, fatty acid and stable isotope analysis, measuring primary and secondary production, and ecosystem-based modeling.
What is the bioenergetic equation for net growth or production in organisms?
Net Growth (G) = Ingestion (R) - Faeces (E) - Excretion (U) - Respiration (T), or alternatively, G = Assimilated food (AR) - Respiration (T).
What biological processes are included in growth or production?
Somatic growth, moulting, and reproduction.
How is growth yield defined, and what factors influence it?
Growth yield (Ygr) = Growth / (Growth + Respiration) or = Growth / Food Intake. It varies from 10–30% depending on organism type, complexity, swimming ability, and life stage.
What is trophic yield, and how is it calculated?
Trophic yield (Yt) = Production at trophic level t+1 / Production at trophic level t. Total yield = product of yields across levels, representing ecological transfer efficiency (~10–30%).
Why does biomass decrease up the trophic pyramid?
Only ~10% of energy is transferred between levels; the rest is lost to metabolism, respiration, and waste. Thus, less energy supports fewer organisms at higher levels.
How does the energy pyramid relate to fisheries?
Top predators like tuna require vast amounts of primary production at the base. Efficient food chains mean more fish biomass per unit of energy input.
What is the match-mismatch hypothesis in marine ecology?
It proposes that the survival of fish larvae depends on the timing and spatial overlap with abundant zooplankton prey. Temporal and spatial mismatches reduce survival and recruitment.
How do climate conditions influence recruitment success?
Warm conditions reduce overlap and survival; cold conditions enhance overlap and promote recruitment of juvenile fish to age-1.
What are eastern-boundary upwelling systems (EBUS) and their significance?
Covering <1% of the ocean, EBUS contribute up to 20% of global fisheries due to nutrient-rich cold waters fueling high primary production.
How does upwelling occur?
Winds push surface waters offshore, and cold, nutrient-rich deep waters rise to replace them, fertilizing surface waters.
Why do upwelling systems support short, efficient food chains?
High phytoplankton biomass allows direct grazing by zooplankton and planktivorous fish, maximizing energy transfer efficiency.
What are the oceanographic impacts of El Niño and La Niña on upwelling?
El Niño reduces trade winds and upwelling, leading to a deep thermocline and low productivity. La Niña enhances upwelling, increasing nutrient input and surface productivity.
What happens when upwelling intensity becomes too strong?
Excessive upwelling can export zooplankton offshore before they can reproduce. Phytoplankton remains ungrazed, leading to decay, gas release, or jellyfish blooms.
What drives productivity in the California Current?
Seasonal wind-driven upwelling causes bursts of productivity and energy flow from euphausiids (krill) to fish.
How do euphausiids and jellyfish differ in energy transfer roles?
Euphausiids effectively transfer energy up the food web. Jellyfish consume production but are not consumed themselves, creating dead ends in the web.
What effect does El Niño have on the California Current food web?
It disrupts normal energy flow, reducing production and altering species composition, ultimately affecting fisheries.
What happened during the collapse of the Benguela sardine fishery?
Sardines were replaced by less efficient species like horse mackerel and bearded goby. Zooplankton biomass rose 10-fold, and jellyfish blooms increased due to energy diversion.
Why were gyres once considered ‘deserts of the sea’?
Low nitrate levels and productivity suggested low biological activity, but they are now known to be dynamic systems with episodic nutrient inputs.
What supports production in these gyres?
Vertical mixing, mesoscale eddies, and nitrogen-fixing organisms like Trichodesmium contribute to episodic primary production.
