12.2 How are marine organisms classified?

marine organisms can be classified according to where they live (habitat) and how they move (mobility). organisms that inhabit the water column can be classified either as plankton (drifters) or nekton (swimmers). all other organisms = benthos (bottom dwellers)

Plankton (Drifters)

plankton: includes all organisms - algae, animals, bacteria - that drift w/ ocean currents

• individual organism = plankter

**just bc plankters drift doesn’t mean they can’t swim - many plankters can swim but either move only weakly or only move vertically —> cannot determine horizontal position in ocean

• plankton are hugely abundant and incredibly important w/in marine environment

**MOST of earth’s biomass (mass of living organisms) consists of plankton in oceans even though 98% of marine species are bottom dwelling

Types of Plankton

plankton classified based on feeding styles:

• if an organism can photosynthesize and produce its own food = autotrophic = phytoplankton

  • can range in size from microscopic algae to larger species of drifting kelp

• if an organism cannot produce its own food and relies on food produced by other organisms = heterotrophic = zooplankton

  • includes drifting marine animals

plankton also include bacteria

• scientists have discovered that free-living bacterioplankton are much more abundant and far more widely distributed than previously thought

Phytoplankton = dinoflagellates, diatoms, coccolithophores

Zooplankton = radiolarians, foraminifera, copepods

Plankton also include viruses = virioplankton

• virioplankton are an order of magnitude smalelr than bacterioplankton and are similarly little-known - only recently through advanced sampling methods has the role of viruses in marine planktonic communities been better understood

• viruses surprisingly abundant in marine ecosystems; in some locations = most abundant biologic entities in ocean —> can strongly influence structure of marine microbial assemblages by limiting abundance of other types of plankton through infeciton

• viruses probably infect and kill approx 20% total marine biomass per day

• marine viruses provide a governing force that influences patterns of oceanic nutrient and energy cycling

• marine viruses affect gas exchanged between atmosphere and ocean surface —> viruses may play a key role in ocean’s response to human-caused climate change

although plankton can be classified as phyto, zoo, bacterio, or virioplankton, they can also be classified according to the portion of their life cycle spent as plankton

• holoplankton = organisms that spend their entire lives as plankton

• meroplankton = many organisms that spend their lives as nekton or benthos spend juvenile and/or larval stages as plankton

plankton can also be classified based on size

• macroplankton = 2-20cm

• picoplankton = 0.2-2 microns

Nekton (Swimmers)

nekton include all animals capable of moving independently of the ocean currents by swimming or other means of propulsion. they’re capable not only of determining their own positions w/in ocean but also often undertake long migrations

• Ex. most adult fish, marine mammals, marine reptiles, some marine invertebrates (squid)

• gradual changes in temp, salinity, viscosity, and availability of nutrients limit lateral range

  • death of large numbers of fish can be caused by temporary horiz. shifts of water masses in the ocean

fish may appear to exist everywhere in the oceans but they’re most abundant near continents and islands and in colder waters. some fish i.e. salmon ascend freshwater rivers to spawn; many eels do the reverse, growing to maturity in freshwater and then descending to streams to breed in great depths of the ocean

Benthos (Bottom Dwellers)

benthos: describes organisms living on or in ocean bottom

epifauna = live on surface of sea floor, either attached to rocks or moving along the bottom

infauna = live buried in sand, in discarded shells, or w/in mud that exists on sea bottom

nektobenthos = live on bottom but also have ability to swim or crawl through water above ocean floor i.e. flatfish, octopuses, crabs, sea urchins

• shallow coastal ocean floor contains wide variety of physical and nutritive conditions, allowing a great # of animal species to develop

• moving across bottom from the shore into deeper wate4, # of benthos species per square meter may remain relatively constant, but biomass of benthos organisms decreases bc shallow sea floor areas receive sufficient sunlight, supporting many species of large marine algae that are attached to the bottom

throughout most of deeper parts of sea floor, animals live in perpetual darkness, where photosynthesis doesn’t happen - they must feed on each other or on whatever outside nutrients fall from highly productive upper sunlit surface zone of the ocean

deep-sea bottom = environment of coldness, stillness, and darkness. under these conditions, life progresses slowly, and organisms that live in the deep sea usually are widely distributed bc physical conditions vary little on deep-ocean floor, even over great distances

Hydrothermal Vent Biocommunities

• prior to discovery of deep-sea hydrothermal vents in 1977, scientists believed that only sparse and small life existed on deepocean floor

• discovery of hydrothermal vent demonstrated that high concentrations of abundant and large deep-ocean benthos are possible

• primary factor that limits life on deep-ocean floor = sparse food supply —> scientists wondered how hydrothermal vent organisms obtained enough food to exist

• bacteria-like archaea (don’t use photosynthesis, thrive on sea floor chemicals) = bhase of this food web —> size of individuals and total biomass in hydrothermal communities far exceed those previously known for deep-ocean benthos

RECAP

marine organisms can be classified according to their habitat and mobility as plankton (drifters), nekton (swimmers), or benthos (bottom dwellers)

CONCEPT CHECK 12.2

(1) describe lifestyles of plankton, nekton, and benthos. why does plankton account for a much larger percentage of ocean’s biomass than benthos and nekton combined?

• plankton = drifters, can’t control horizontal motion

  • high turnover rate, rapid reproduction, base of marine food web/chain

• nekton = free-moving, can control horizontal motion except when nutrient availability, temp, or pressures say otherwise

• benthos = bottom dwellers

(2) list subdivisions of plankton and benthos and criteria used for assigning individual species to each.

Plankton:

• phyto: autotrophs (photosynthesize)

• zoo: heterotrophs

• bacterio: bacteria

• virio: viruses

benthos

• epifauna: live on sea floor

• infauna: live in sea floor

• nektobenthos = live on sea floor but can swim in water column right above it to move

(3) for the following marine organisms, determine if they’re plankton, nekton, or benthos:

• shark: nekton

• octopi: nektobenthos so benthos

• clams = benthos

• diatoms = plankton

• corals = benthos

• crabs = benthos

• giant kelps = benthos

• jellies = nekton

• dolphins = nekton