7 The Water Column Plankton

1. 7The Water ColumnPlankton 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: Definitions2 • 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: Definitions3 • 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: Definitions4 • Size classes

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. 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 Porpita (ca. 10 cm wide) Physophora (50 mm high) By-the-wind-sailor Velella 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 1h 2h 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 Zooplankton2 • 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 Zooplankton3 • 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. 0 Night 100 200 Depth (m) Day 300 Twilight 400 0 1 2 3 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 Zooplankton5 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 Zooplankton6 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