7 The Water Column Plankton Notes for Marine Biology: Function, Biodiversity, Ecology


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7 The Water Column Plankton

  • Notes for Marine Biology: Function, Biodiversity, Ecology

  • by Jeffrey S. Levinton


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-of-war, which has a surface float



Plankton: Definitions 4

  • Size classes



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

  • 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







Phytoplankton

  • 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







Phytoplankton





Phytoplankton

  • 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

  • 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

  • Numerous other groups, including many flagellated types



Zooplankton

  • 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

  • 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



Zooplankton



Zooplankton

  • 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



Zooplankton

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



Zooplankton

  • 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



Zooplankton



Zooplankton

  • Porpita (ca. 10 cm wide) Physophora

  • (50 mm high)



Zooplankton

  • Known as comb jellies

  • Microcarnivores - feed on smaller zooplankton, planktonic eggs, invertebrate larvae

  • 8 rows of meridional plates, some have two long tentacles



Zooplankton



Zooplankton

  • 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





Zooplankton

  • Torpedo shaped, a few cm in length

  • Rapid swimmers, carnivorous



Zooplankton

  • Holoplanktonic snails

  • Swim by means of lateral projections from foot

  • Suspension feed or are carnivorous, depending upon species



Zooplankton

  • Have very well developed parapodia



Zooplankton

  • 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

  • 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

  • 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



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





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





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

  • 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

  • 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

  • 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

  • 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

  • 5. Surface mixing hypothesis -



The End




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