Core Lecture 4: Ocean Productivity and Marine Food Webs

primary production

• process by which primary producers convert inorganic carbon into organic material

• in surface ocean, occurs via photosynthesis

• biomass produced by primary producers available to be consumed by primary consumers

photosynthesis equation

sunlight + 6CO2 + 6H2O + 6O2

plankton

• plants and animals that live suspended in the water, floating or drifting with the currents

• eg. algae and bacteria

nekton

• animals that swim freely and purposefully

• eg. fish, whales

benthos

• organisms that live on, in, or attached to the seafloor

• eg. sea stars, clams, anemones, etc

phytoplankton

• single celled plants

• primary producers

zooplankton

• planktonic animals

• primary consumers

what makes something planktonic

• cannot swim against a current (drifters)

• most are very small, usually <1cm (some can be very large, 1m)

• two functional groups: phytoplankton and zooplankton

• plankton are the main producers and consumers in marine ecosystems, analogous to the plants and animals in terrestrial ecosystems

phytoplankton autotrophic t/f

true

main characteristics of phytoplankton

• unicellular microscopic photosynthetic organisms

• most exist as individual cells

• some form chains

• most are eukaryotic

• well-defined nucleus and organelles

• chlorophyll in chloroplasts

• most <100um, range from <2um to 2mm

diatoms

• largest phytoplankton (10s to 100s of um)

• outer covering (frustule) made of silica

• dominate in well mixed nutrient rich waters

• often form chains

two main types of diatoms

• centrics: radially symmetric (pill-box shaped)

• pennates: bilaterally symmetric and elongated

dinoflagellates

• usually smaller than diatoms (10s of um) and often have an outer covering of cellulose

• have 2 flagella which aid in locomotion

• cane swim vertically in response to sunlight

• many are mixotrophic - have chlorophyll and can photosynthesize but can also feed on other plankton

• abundant in well-stratified, often nutrient poor waters

red tides

• dinoflagellates become so abundant that they form dense blooms that colour the sea surface red

Harmful algal blooms (HABs)

• some (not all) blooms known as “harmful algal blooms”

• contain toxins that can poison fish and even humans

• in coastal bc, shellfish harvesting is often suspended due to PSP caused by blooms of dinoflagellate which produces a potent neurotoxin

bioluminescence

• many dinos flash in response to mechanical stimulation

• chemical reaction involves protein lucifran

• thought to be primarily a defence mechanism against predators

coccolithophorids

• small group of phytoplankton (size and diversity)

• exterior covered with CaCO3 plates (coccoliths)

• CaCO3 = chalk, sensitive to dissolution via ocean acidification

• although small, can often form blooms large enough to be visible from space

how to oceanographers quantify phytoplankton abundance

• collect them and count them

• measure the amount of chlorophyll with a fluorometer

• measure ocean colour by satellite

collect and count them method

• can be sampled with very fine mesh nets

• niskin bottles can be used to collect water at discrete depths

• can also be sampled using submersible pumps

measure the amount of chlorophyll with a fluorometer method

• plants appear green because chlorophyll a absorbs in blue light and reflects green light

• when the Chl in phytoplankton are exposed to blue light about 1-2% of it is fluoresced back as red light

• by measuring the amount of red light that is produced, one can estimate the amount of chlorophyll present

standing stock/biomass

• How much is there at a particular time (eg. chlorophyll concentration

productivity

• how fast new biomass is produced (e.g. primary production)

• more biomass does not necessarily mean higher productivity

• productivity matters more for food web

picophytoplankton

• extremely small phytoplankton: <2um

• not discovered until 1980s

2 particularly important marine cyanobacteria

• prochlorococcus

• synechococcus

prochlorococcus

• photosynthetic cyanobacterium

• among smallest phytoplankton

• part of the picophytoplankton

• probably most abundant photosynthetc organism o nearth

• despite tiny size, collectively responsible for a substantial fraction of ocean primary production

  • tiny individual cells x enormous abundance = major contribution to ocean productivity

where does prochlorococcus thrive

• especially abundant in warm tropical and subtropical oceans

• dominates in nutrient poor waters

• extremely small size gives high surface-area:volume ratio, helping cells acquire scarce nutrients effeciently

• different ecotypes are adapted to different light levels, allowing prochlorococcus to occupy most of euphotic zone

zooplankton

• planktonic animals that cannot swim against a current

• range in size from <1mm to>1m

• heterotrophic: consumers rather than producers

• as primary consumers, main link between primary producers and higher trophic in marine food webs

how can zooplankton be classified functionally

• herbivores: consume phytoplankton

• carnivores: consume other zooplankton

• omnivores: consume both

trophic level

• a group of organisms occupying the same position in a food web

• energy flows from lower to higher trophic levels

how do oceanographers collect zooplankton

• generally use nets

• different mouth and mesh sizes depending on what trying to catch

microzooplankton

• tiniest grazers

• highly diverse group taxonomically

• most are heterotrophic, some can be mixotrophic

• consume small phytoplankton, bacteria, other microzooplankton, and are fed upon by larger zooplankton

copepods

• most abundant zooplankton (>80% in terms of abundance)

• along with microzooplankton, main consumers of phytoplankton

• mainly herbivorous or omnivorous, some entirely carnivorous

• many species migrate 100s of metres vertically each night

• size: 1-10 mm in length, most <5mm

• key prey items of young fish - represent a main link from the plankton to hgiher trophic levels in most marine food webs

copepods diversity

• about 20,000 species

• morphologically very diverse

• about 33% are parasitic

euphausiids (aka krill)

• among largest zooplankon (1-10 cm)

• generally omnivorous: filter phytoplankton and smaller zooplankton

• unlike most zoop, krill have multi-year life cycles (3-5 yrs)

• undertake extensive vertical migrations each night, forming dense swarms near surface

• key prey for fish, seabirds, baleen whales, others

• can create a very short food chain

diel veritcal migration

• although unable to swim against a horizontal current, many zooplankton undertake extensive vertical migrations each day

• feed at surface during night

• dawn descent

• dusk ascent

• reside near bottom by day

gelatinous zooplankton

• diverse collection of species:

  • cnidarians (true jellyfish)

  • ctenophores (comb jellies) both fish for smaller zooplankton with tentacles

  • salps (primitive chordates) are filter feeders that form dense patches

• although most are quite small, some jellies can be >1m in diameter

holoplankton

• zooplankton that spend their whole life cycle in the plankton

meroplanton

• many benthic and nektonic species have planktonic larval stages that spend days to months in the plankton

why would bethnic species need plantonic larvae

• plantonic larvae provide bethnic species a way to disperse

• also main reason that marine populations are more open or connected than many terrestrial populations

food webs

• more complex, including numerous food chains of different lengths

• a species may be part of several different food chains within a single food web

what do high levels of redundancy and omnivory due to the stability of marine food webs

increase it