1. 7 The Water Column Plankton
Notes for Marine Biology: Function, Biodiversity, Ecology
by Jeffrey S. Levinton
©Jeffrey S. Levinton 2001

2. Plankton: Definitions
Plankton: organisms living in the water column, too small to be able to swim counter to typical ocean currents

3. 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

4. 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-of-war, which has a surface float

5. Plankton: Definitions 4
Size classes
Ultraplankton
< 2 m
Nannoplankton
2-20 m
Microplankton m
Macroplankton m
Megaplankton
> 2000 m

6. 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

7. 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

8. Phytoplankton Diatoms 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

9. Diatoms
Dinoflagellates
Coccolithophore
Flagellate Isochrysis

10. Asterionella japonica
Thalassiosira
Chaetoceros
Asterionella japonica

11. Phytoplankton Dinoflagellates
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

12. Diatoms
Dinoflagellates
Coccolithophore
Flagellate Isochrysis

13. Peridinium

14. 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

15. Diatoms
Dinoflagellates
Coccolithophore
Flagellate Isochrysis

16. 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

17. Diatoms
Dinoflagellates
Coccolithophore
Flagellate Isochrysis

18. 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

19. Phytoplankton Other Groups
Numerous other groups, including many flagellated types

20. Zooplankton Crustacean zooplankton (Arthropods)
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)

21. Zooplankton Crustaceans - Copepods
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

22. Zooplankton Copepod Feeding Low Reynolds number - viscosity dominates
Feeding current (green) generated by thoracic appendages
Maxilliped reaches out and grabs particles entrained in current

23. Zooplankton
Copepods
Females of different species with eggs

24. Zooplankton Crustaceans - Euphausids (Krill)
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

25. Zooplankton
Crustaceans - Euphausids (Krill)

26. Zooplankton Gelatinous Zooplankton
Jellies include a wide variety of distantly related groups, all have gelatinous material used for support (skeleton)

27. 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 man-of-war
Jellyfish are mainly carnivores, feeding with aid of nematocysts - stinging cells - on tentacles

28. Zooplankton Gelatinous Zooplankton - Cnidaria
Note muscular bell and tentacles

29. Zooplankton Gelatinous Zooplankton - Cnidaria Siphonophores
By-the-wind-sailor
Velella
Porpita (ca. 10 cm wide) Physophora
(50 mm high)
Siphonophores

30. 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

31. Zooplankton
Gelatinous Zooplankton - Ctenophores

32. 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)
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
Gelatinous Zooplankton - Larvacea

33. Colonial Salp Pegea sp.

34. Zooplankton Arrow worms Torpedo shaped, a few cm in length
Rapid swimmers, carnivorous

35. Zooplankton Pteropods Holoplanktonic snails
Swim by means of lateral projections from foot
Suspension feed or are carnivorous, depending upon species

36. Zooplankton
Planktonic polychaetes
Have very well developed parapodia

37. 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

38. 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

39. 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 sp. under
ultraviolet light, showing
ingested chloroplasts in red
Strombidium, 80m long

40. 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

41. 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

42. 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

43. 10m
100m 4h 4h 2h 2h 1h 1h
Current
No current
Example of spread of plankton with general turbulence (left)
Or in a current (right)

44. 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

45. 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

46. 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

47. Night
100
Depth (m)
200
Day
300
Twilight
400
Distance (km)
Vertical migration of planktonic shrimp Sergia lucens

48. 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

49. 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

50. 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

51. 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

52. 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

53. Diurnal Vertical Migration of Zooplankton 9
Cause of Vertical Migration?
5. Surface mixing hypothesis -

54. The End