Download 7 The Water Column Plankton

7 The Water Column
Plankton
Notes for Marine Biology:
Function, Biodiversity, Ecology
by Jeffrey S. Levinton
©Jeffrey S. Levinton 2001
Plankton: Definitions
• Plankton: organisms living in the water
column, too small to be able to swim
counter to typical ocean currents
Plankton: Definitions 2
• Phytoplankton are photosynthetic plankton
(some ambiguity here, because some groups
like dinoflagellates have non-photosynthesizing
members)
• Zooplankton are animals living as plankton
• Mixoplankton are organisms that may function
as animals but ingest phytoplankton and
maintain the chloroplasts of their food
organisms (phytoplankton), and the
chloroplasts photosynthesize in their new hosts
Plankton: Definitions 3
• Holoplankton - planktonic organisms that
complete their entire life cycle in the plankton
• Meroplankton - spend only a part of their life
cycle in plankton. Most common example:
planktonic larvae of benthic animals
• Neuston - plankton associated with water
surface, such as bacteria in surface film
• Pleuston - plankton that live at surface but
protrude in air, such as Portuguese Man-ofwar, which has a surface float
Plankton: Definitions 4
• Size classes
Ultraplankton
< 2 m
Nannoplankton
2-20 m
Microplankton
20-200 m
Macroplankton
200-2000 m
Megaplankton
> 2000 m
Vertical Position of Plankton - Factors
• Bulk density - regulated by ionic
subsitution, gas secretion and release
(cuttlefish, Nautilus)
• Swimming behavior
• Turbulence stirs plankton through the water
column
Vertical Position of Plankton - Factors
• Size of plankton - smaller plankton live in
world of low Reynolds number
• Low Re means there is a boundary layer
around plankter’s body
• Smaller organisms denser than seawater
sink with a constant velocity, proportional
to organismal volume, although increases of
spines etc. can slow sinking
Phytoplankton
Diatoms
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Occur singly or form chains
Size range of nanno to microplankton
Encased in silica shell consisting of two valves
Usually radially symmetrical
Reproduce asexually by binary fission
Also sexual reproduction
Doubling once or twice per day usually
Diatoms
Coccolithophore
Dinoflagellates
Flagellate Isochrysis
Thalassiosira
Chaetoceros
Asterionella japonica
Phytoplankton
Dinoflagellates
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Secrete organic test and have two flagellae
Size range of nanno and microplankton
Asexual and sexual reproduction
Often many life history stages
Many species are heterotrophic
Often abundant in tropics, mid-latitudes in
summer
• A few species are the cause of red tides
Diatoms
Coccolithophore
Dinoflagellates
Flagellate Isochrysis
Peridinium
Phytoplankton
Other Groups
• Cyanobacteria - abundant in ocean, responsible
for nitrogen fixation
• Coccolithophores - unicellular, nannoplankton,
spherical and covered with calcium carbonate
plates called coccoliths
• Silicoflagellates - unicellular, biflagellate, internal
skeleton of silica scales, often in Antarctic, open
ocean
Diatoms
Coccolithophore
Dinoflagellates
Flagellate Isochrysis
Phytoplankton
Other Groups
• Cyanobacteria - abundant in ocean, responsible for
nitrogen fixation
• Coccolithophores - unicellular, nannoplankton,
spherical and covered with calcium carbonate
plates called coccoliths
• Silicoflagellates - unicellular, biflagellate, internal
skeleton of silica scales, often in Antarctic, open
ocean
Diatoms
Coccolithophore
Dinoflagellates
Flagellate Isochrysis
Phytoplankton
Other Groups
• Cyanobacteria - abundant in ocean, responsible for
nitrogen fixation
• Coccolithophores - unicellular, nannoplankton,
spherical and covered with calcium carbonate
plates called coccoliths
• Silicoflagellates - unicellular, biflagellate, internal
skeleton of silica scales, often in Antarctic, open
ocean
Phytoplankton
Other Groups
• Numerous other groups, including many
flagellated types
Zooplankton
Crustacean zooplankton (Arthropods)
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Crustaceans have an external chitin skeleton
Some degree of segmentation
Paired jointed appendages (e.g., legs, antennae)
They possess antennae, mandivles and maxillae
as head appendages
• Usually have compound eyes
• Include copepods, krill, amphipods (crabs,
lobsters, sowbugs - not in plankton)
Zooplankton
Crustaceans - Copepods
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Largest group of crustaceans in zooplankton
Range from <1 - a few mm long
Planktonic forms belong to the Calanoida
Long pair of antennae
Swim mainly with aid of 5 pairs of thoracid appendages
Lack compound eyes, medial naupliar eye
Feed on phytoplankton or smaller zooplankton, depending
on the species
Zooplankton
Copepod Feeding
Low Reynolds number - viscosity dominates
Feeding current (green) generated by thoracic appendages
Maxilliped reaches out and grabs particles entrained in current
Zooplankton
Copepods
Females of different species with eggs
Zooplankton
Crustaceans - Euphausids (Krill)
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Shrimplike, up to 5 cm long
Abundant in Antarctic and in upwelling regions
Main food of baleen whales in Antarctic
Feed on phytoplankton and smaller zooplankton
Feeding by means of group of appendages that
form a basket - appendages have setae and smaller
setules, hairs that capture particles
Zooplankton
Crustaceans - Euphausids (Krill)
Zooplankton
Gelatinous Zooplankton
• Jellies include a wide variety of distantly related
groups, all have gelatinous material used for
support (skeleton)
Zooplankton
Gelatinous Zooplankton - Cnidaria
• Planktonic Cnidaria are mainly Scyphozoan
jellyfish, but also include Hydrozoan jellyfish
(some meroplanktonic jellyfish stages) and
siphonophores, specialized colonial and
polymorphic cnidarians such as Portuguese manof-war
• Jellyfish are mainly carnivores, feeding with aid of
nematocysts - stinging cells - on tentacles
Zooplankton
Gelatinous Zooplankton - Cnidaria
Note muscular bell and tentacles
Zooplankton
Gelatinous Zooplankton - Cnidaria
By-the-wind-sailor
Velella
Porpita (ca. 10 cm wide) Physophora
(50 mm high)
Siphonophores
Zooplankton
Gelatinous Zooplankton - Ctenophores
• Known as comb jellies
• Microcarnivores - feed on smaller zooplankton,
planktonic eggs, invertebrate larvae
• 8 rows of meridional plates, some have two long
tentacles
Zooplankton
Gelatinous Zooplankton - Ctenophores
Zooplankton
Gelatinous Zooplankton - Salps
• Related to benthic sea squirts, but have incurrent
and exit siphons on opposite ends of body
• Solitary or colonial (up to 2 m in length)
Gelatinous Zooplankton - Larvacea
• Have a tail, typical of tunicate swimming larvae
• Small, only a few mm long
• Tail generates current through house, current is
strained by fine fibers that trap food
Colonial Salp Pegea sp.
Zooplankton
Arrow worms
• Torpedo shaped, a few cm in length
• Rapid swimmers, carnivorous
Zooplankton
Pteropods
• Holoplanktonic snails
• Swim by means of lateral projections from foot
• Suspension feed or are carnivorous, depending
upon species
Zooplankton
Planktonic polychaetes
• Have very well developed parapodia
Zooplankton
Protistan zooplankton - Foraminifera
• Secrete skeleton of calcium carbonate, sometimes
with great ornamentation
• Common in plankton
• Range from ca. 1 mm to a few mm in size
• Contractile pseudopodia stream from body wall,
trap food particles
• Form sediment in deep sea from skeletons
Zooplankton
Protistan zooplankton - Radiolaria
• Secrete skeleton of silica, sometimes with great
ornamentation, occurs singly and as colonies, depending
on species
• Common in plankton
• Range from ca. 50 m to a few mm
• A membrane separates interior cell from exterior
cytoplasm, which streams out something like foraminifera
• Form sediment in deep sea from skeletons
Zooplankton
Protistan zooplankton - Ciliates
• Common in plankton, feed on bacteria, smaller
phytoplankton, some mixotrophic
• Elongate, ranging from size from about 50 m to over 1
mm in length, covered with rows of cilia
Strombidium, 80m long
Strombidium sp. under
ultraviolet light, showing
ingested chloroplasts in red
Patchiness of the Plankton
• Plankton rarely distributed homogeneously
in the water column
• Plankton occur in spatially discontinuous
patches, sometimes distinct aggregations,
sometimes concentrated at interfaces
between water bodies
Patchiness of the Plankton 2
• Spatial changes in physical conditions
(light, temperature, salinity) - behavioral
responses and population growth/mortality
responses
• Water turbulence and current transport
• Spatially discontinuous levels of grazing
• Localized reproduction
• Social behavior
Patchiness of the Plankton 3
• Phytoplankton - patchiness a function of
population growth versus rate at which
turbulence spreads out the population
• If turbulence is limited,and if growth
conditions are optimal, plankton grow fast,
producing a bloom
10m
100m
4h
2h
1h
No current
1h 2h
4h
Current
Example of spread of plankton with general turbulence (left)
Or in a current (right)
Diurnal Vertical Migration of
Zooplankton
• Zooplankton rise to shallow water at night, sink to
deeper water during the day
• Found in many different groups of zooplankton
• Zooplankters usually start to sink before dawn,
and start to rise before dusk
• Cycle is probably an internal biological clock that
must be reinforced by day-night light changes
Diurnal Vertical Migration of
Zooplankton 2
• Zooplankton rise to shallow water at night, sink to
deeper water during the day
• Found in many different groups of zooplankton
• Zooplankters usually start to sink before dawn,
and start to rise before dusk
• Cycle is probably an internal biological clock that
must be reinforced by day-night light changes
Diurnal Vertical Migration of
Zooplankton 3
• Zooplankton rise to shallow water at night, sink to
deeper water during the day
• Found in many different groups of zooplankton
• Zooplankters usually start to sink before dawn,
and start to rise before dusk
• Cycle is probably an internal biological clock that
must be reinforced by day-night light changes
0
Night
100
200
300
400
0
Day
Twilight
1
2
Distance (km)
Vertical migration of planktonic shrimp Sergia lucens
3
Diurnal Vertical Migration of
Zooplankton 4
• Zooplankton rise to shallow water at night, sink to
deeper water during the day
• Found in many different groups of zooplankton
• Zooplankters usually start to sink before dawn,
and start to rise before dusk
• Cycle is probably an internal biological clock that
must be reinforced by day-night light changes
Diurnal Vertical Migration of
Zooplankton 5
Cause of Vertical Migration?
• 1. Strong light hypothesis - plankton migrate away
from strong light
• Problem: plankton migrate to great depths, below
those where light damage is likely
Diurnal Vertical Migration of
Zooplankton 6
Cause of Vertical Migration?
• 2. Phytoplankton recovery hypothesis zooplankton migrate downward for a time to allow
phytoplankton to recover
• Problem: Why not cheaters who would stay up to
feed on phytoplankton. Hypothesis requires
cooperation, even among many species
Diurnal Vertical Migration of
Zooplankton 7
Cause of Vertical Migration?
• 3. Predation hypothesis - zooplankton migrate
downward to avoid visual predation during day
• This idea is consistent with avoidance of
predation. Many visual predators feeding on
zooplankton.
• Site in Washington where copepods have a reverse
migration, moving up during the day, apparently
in response to arrow worm predators, which
themselves carry a more typical diurnal vertical
migration
Diurnal Vertical Migration of
Zooplankton 8
Cause of Vertical Migration?
• 4. Energy conservation hypothesis - zooplankton
migrate downward to avoid higher surface
temperatures during the day, which saves energy
• Consistent with calculations of energy budget of a
planktonic copepod, which would benefit from the
vertical migration, relative to the cost of
swimming and loss of feeding in the surface for
part of the day
• Consistent with fact that copepods tend to sink
beneath the surface after spring phytoplankton
increase, suggesting an adaptation to save energy
in the lower temperature deeper waters
Diurnal Vertical Migration of
Zooplankton 9
Cause of Vertical Migration?
• 5. Surface mixing hypothesis -
The End