Core Lecture 5: Energy Flow in Marine Food Webs and Global Patterns of Production

trophic pyramid

• used by ecologists to follow the flow of energy from one trophic level to the next

• relative size of each layer represents the relative biomass of organisms in that trophic level

what does the amount of energy available to higher trophic levels depend on

• amount of primary production

• trophic transfer efficiency

• number of trophic levels

how is production at highest trophic levels correlated to primary production

• positively correlated

• differences in the amount of energy at bottom translate to differences in amount of energy available at top

trophic transfer efficeincy

• fraction of energy in one trophic level that gets transferred to the next (higher) level

• (production in TLn)/(Production in TLn-1)X 100

why are trophic transfer efficiencies so low

• feces

• respiration

• dies uneaten

• TTE in marine foodwebs often averages ~10%

how does # of trophic levels affect amount of energy at top of pyramid

• each additional trophic level decreases the energy available at the top by an order of magnitude

• more trophic levels -> less energy available at top

factors regulating primary production

• light: quantity and quality

• nutrient availability: macro and micro nutrients

• grazing pressure: from meso and microzooplankton

PAR

• photosynthetically active radiation

• The band (400-760nm) of solar energy used for photosynthesis

what does light penetration depend on

• wavelength of the light:

  • red light: shallower

  • blue light: deeper

• water clarity:

  • the more particulate/dissolved matter in the water, the most rapidly light is absorbed/scattered

• light penetrates deeper in open ocean than in coastal waters

which colour pentrates the deepest

blue

euphotic zone

• uppermost layer of ocean with enough light for photosynthesis

• base usually defined as depth at which 1% of surface light remains

euphotic zone in coastal waters

often less than 30m

euphotic zone in open ocean

can be at >150m deep

fraser river plume

• lots of silt

• very productive

• low light penetration

• euphotic zone <10m

mariana islands

• far from land

• low productivity

• high light penetration

• euphotic zone 150m

how does light limit primary production

• daily production is positive where and when there is enough light for photosynthesis to exceed daily losses to respiration

compensation depth

• depth at which production = respiration

• corresponds roughly to base of euphotic zone

nutrients

• chemical substances required by phytoplankton to live in grow

• inorganic nutrients generally present as dissolved salts

• used by primary producers as precursors for synthesis of energy-rich organic compounds, cell walls, and skeletons

key macronutrients required by phytoplankton

• nitrogen, phosphorus, silicon

• nitrogen is nutrient that most commonly limits phytoplankton production in ocean

where are inorganic nutrient concentrations lowest

near surface where they get taken up by phytoplankton

nutricline

depth of rapid change in nutrient concentration

why are inorganic nutrient concentrations usually higher at depth

• organic matter gets re-mineralized by bacteria as it sinks

• phyto can’t photosynthesize wthout light

fundamental challenge for phytoplankton

• need light and nutrients at same time

• light high at surface, nutrients low at surface

stratification

• retains phytoplankton in well-lit surface layer

• restricts nutrient resupply from depth

• favourable light, but low nutrients

mixing/upwelling

• transports nutrients toward the surface

• can supply nutrients to well-lit layer (if mixing is not too deep)

• brings nutrients and light together

what are two reasons why some parts of the ocean are more productive than others

• variability in light

• variability in nutrients

microzooplankton

• single celled, eukaryotic organisms

• usually solitary (some form colonies)

• range in size from few um-3mm

• kew component of microbial loop

  • feed on bacteria, detritus and small phytoplankton

  • important prey source for larger zooplankton

• very diverse group taxonomically

three most important marine groups of microzooplankton

• foraminiferans

• radiolarians

• ciliates

role of bacteria in marine food webs

• approx 10^6 bacteria per ml of seawater

• 95% are heterotrophic, accounts for 70% of living carbon in ocean

• heterotrophic bacteria play critical roles as decomposers of organic material, and recyclers of inorganic nutrients

importance of microbial loop

• recycles inorganic nutrients

  • particularly important in nutrient poor regions

  • decomp of DOM returns inorganic nutrients to the water column

  • inorganic nutriens can be taken up by phytoplankton

• increases overall food chain efficiency by re-directing bacterial carbon back into classic marine phyto-zoop-fish food chain

marine viruses

• extremely abundant - millions to tens of millions per ml of seawater

• most infect microorganisms, especially bacteria and phytoplankton

• cannot reproduce independently - use host cell’s machinery

• infection often ends swith lysis of host cell

viral shunt

redirects organic matter away from larger consumers and back into microbial food web

variations in primary production - polar oceans

• very strong seasonality

  • winter: light is key limitation

  • macronutrients generally abundant

• PP is light limited

  • increases during spring/summer

  • antarctic PP > arctic PP

• result is a single short pulse of primary production in spring

variations in primary production - subtropical gyres

• very little seasonality

  • nutrients always low

  • strong thermocline prevents mixing

  • light always abundant

• PP is nutrient limited at low steady rate

• nutrient recycling via microbial loop is very important

• exceptions to this pattern:

  • equatorial and coastal upwelling

  • coral reefs

variations in primary production - midlatitude (temperate) oceans

• winter:

  • PP: light limited

  • abundant nutrients

• spring:

  • adequate light

  • major phytoplankton bloom

• summer:

  • PP: nutrient limited by stratification

• autumn:

  • nutrient replenishment (increased wind)

  • smaller phytoplankton bloom

• PP control alternates between light and nutrients

how does zooplankton production compare to primary production seasonal pattern

follows similar seasonal pattern with much lower biomass and lagged by few weeks

PP in subtropical gyres

• lots of light

• stratification prevents nutrients from being mixed into euphotic zone

• pp is always low

PP in polar regions

• usually lots of nutrients (strong wind mixing)

• PP limited by light except for a short period in spring/summer when there is a burst of PP

PP in temperate regions

• PP limited by light in winter

• when water column stabilizes - string bloom

• PP becomes nutrient limited in summer due to lack of mixing

• smaller autumn bloom occurs when fall winds mix water and supply nutrients but light is becoming limiting again

why are food chains longer in open ocean

• coastal waters dominated by diatoms

  • well mixed conditions

  • lots of nutrients

  • large cells

• open ocean dominated by picoplankton

  • low mixing rates

  • very low nutrients

  • tiny picos are better at nutrient uptake

• diatoms can be large enough to be grazed directly by zooplankton and fish

• picoplankton are too small and must first be consumed by protozoans

  • adds extra steps to food chain

upwelling system

• nutrient rich upwelled water supports large phytoplankton and a short food chain

• allows a relatively large fraction of primary production to reach fish

temperate coastal system

• a mixed phytoplankton community supports an intermediate-length food chain and a moderate transfer of primary production to fish

• moderate nutrients

subtropical gyre

• in nutrient poor, strongly stratified waters, very small phytoplankton support a long food chain, so a relatively small fraction of primary production reaches fish