PP 8: Zooplankton community dynamics

seasonal succession graph

• Generalization  

• Copepods, have a higher percentage of predatory species 

• Predators: 15% of the population  

• This is why copepods have that steady presence, and also because they live longer than the other species  

• Rotifers and cladocerans have those peaks, have low winter density 

• Rotifers peak in the spring, cladocerans peak in the fall 

• Respond to the seasonality of phytoplankton  

Community ecology: seasonal succession

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Community ecology: resource partitioning

-Habitat partitioning in Polyarthra species, Lake Skarshult  

• Closely related species may populate different areas of a lake  

• They need to separate themselves so they can get ahold of the nutrients they need 

Feeding and assimilation rates: filter feeding rate

-1. terms of expression 

• Feeding rate  

  • Count the number of algae cells per time, etc. How many cells of algae is it eating per hour 

• Filtering rate  

  • Instead of counting cells, you’re taking a volume of water and then letting them filter the water  

  • number of liters of water filtered per animal per day  

-2. Methods of measurement  

• Direct counting method 

  • Count algal cells at beginning (t₀) and end (t_n) of feeding period; calculate # cells removed per L per h  

• Radioisotope method  

  • Label algae (or other food, e.g. yeast) with ³²P or ¹⁴C  

  • Incubate known algal quantity with zooplankton in chamber 

  • Collect and radioassay zooplankton (can separate by species) 

Feeding and assimilation rates: assimilation rate (non-assimilated food in defecation)

-Respiration 

• Measured with O2 probe 

-Defecation  

• Collect defecation and combust it and get a calorie count 

-Radioisotope method 

• Label food, feed it to them, once the gut clears whats left inside the body is the radioisotope signature  

population dynamics for a given species

-Population growth rate: r 

• b= birth rate 

• d= death rate 

• e= emigration rate  

• i= immigration rate 

-r=b-d-e+i 

population dynamics: controlling factors

• Birth rate 

  • Zooplankton are temp dependent. With a high temp, molting will speed up, this speeds up birth rates.  

  • Increased food supply = increased number of offspring, increased brood size 

• death rate 

  • Predation can be major, the big driving factor of population of zooplankton  

  • Longevity is greater at lower temps 

  • Growth rate slows down, but more likely to live longer 

•  Emigration  

  • Depends on your system  

  • Common with outflow systems  

  • Depends on stream discharge lake relative to lake volume  

    • Ex. Dam being open or closed 

• immigration  

  • Stream coming into a lake  

  • Invasive species, things hitch rides on boats  

  • Birds/mammals can transport things 

population dynamics: practical quantification

-Instantaneous growth rate: 

• In most lakes, immigration and emigration are negligible  

• If birth rate is relatively rapid, then over short periods death rate is negligible  

• Therefore, birth rate alone is often used as an estimate of “instantaneous growth rate”: r=b 

birth rate is a function of:

-# of adults in the total population (population is one species, multiple species would be a community) 

-Average brood size (# of eggs per female) 

 

-Egg development time  

• b=log_n [1 + (#adults/pop size x average brood size/egg dev time)] 

• The egg ratio method was pioneered by W.T. Edmonson  

• Measured in a lab, extrapolated into the field 

-graph note: if there’s no predators theres usually an exponential growth curve

Don Hall’s surprise

• Because these are such predatorial systems, death is in fact an important factor 

• So things that had ignored death were not true  

• Top one is a generalized pattern of population dynamics  

• Bottom graph, something besides food is controlling the daphnia population because it deviates from the generalized pattern.  

Size selective predation on zooplankton

-Planktivorous fish are size selective 

• Visual predators 

• This impacts zooplankton species structure  

• Fish can only eat things that fit in their gape size (mouth size) 

• They're going to eat the biggest thing they possibly can 

• With secondary effects on phytoplankton grazing results  

-Classic size selection experiments of Brooks and Dodson:  

• When fish were introduced to previously fishless lakes, they strongly reduced all zooplankton species greater than 1mm in size 

• Smaller zooplankton species become more dominant 

• Community composition changed  

size in the water column graph

• Zooplankton move up and down in the water column. In the day they move down, at night they move up  

• The biggest ones come out at night. They hide in the day from the fish 

• Small ones come out during the day 

Fish predation on zooplankton  

pic

Alewife (herring): a size-selective predator  

What can pass past the gill rakers is what they’re eating  

Trophic cascase

• Idea of something at top driving whats below it, top down control 

• Piscivorous fish= fish eating fish 

• Planktivorous fish= fish eating plankton  

Top down control

• Some argue system is controlled by the nutrients, not by the predators 

• It can be both 

top down trophic cascade

pic

evolutionary question

-Why should large species dominate a zooplankton community in absence of fish predation?  

-Advantages of relatively large body size in zooplankton:  

• Greater range of food availability and feeding efficiency- with larger filtering apparatus, both small and large algae etc. are consumable  

• Lower respiration rate  

  • Lower surface area to mass ratio 

• Larger brood size  

  • If she can carry more babies, she might still have an edge even if predation is high for her because of her body size 

  • Probably the most important factor  

relative competition fitness vs predation avoidance fitness

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vertical migration of zooplankton

• Most zooplankton species exhibit cyclic vertical migration in the water column, generally associated with day/night cycles. The largest species migrate the most  

• Largest species can also move faster and better in the water column 

• Primarily the cladocerans and copepods in freshwater 

• Distances: several meters per day  

• Velocities: up to nearly 2 cm per sec (= approx. 1 meter per min). This is a very energy intense behavior, and therefore must have resulted from major evolutionary selection pressures  

vertical migration of zooplankton: migration patterns

-Nocturnal  

• Up a night, down during the day 

• Most common 

-Twilight  

• Up a dawn and dusk, down during day at night 

-Diurnal  

• Down at night, up during the day 

• Most rare  

• Smaller species, less predation prone  

vertical migration of zooplankton: controllers for nocturnal type

-Primarily a combination of  

• Negative geotaxis  

  • Movement away from gravity 

• Negative phototaxis 

  • Movement away from light 

-Phototaxis controllers  

• Principally blue light (they are red blind) 

• Rate of change in light -> initiation and velocity 

  • How quick the light changes will drive the speed 

• Maximum intensity -> depth of migration (but will not descend into anaerobic hypolimnion)  

Hypotheses for evolutionary (and other) significance of vertical migration

-1. sampler avoidance: (in light) 

• When you throw a net, they can swim away  

-2. predator avoidance  

• Natural tendency for light avoidance in predator vulnerable species  

• Probably the best theory  

-3. feeding efficiency- McLaren hypothesis  

• Feeding is more efficient at high temp  

• Growing is more efficient at low temp (low resp.) 

-4. food quality- Haney hypothesis  

• Phytoplankton make more protein and divide faster at night -

Vertical migration of zooplankton Cont.  

-General practical implication of nocturnal migration: daytime sampling of zooplankton in the epilimnion (especially near surface) only will greatly underestimate population sizes. Also, there can be major horizontal patchiness in distribution:  

• Zooplankton are concentrated by (and attracted to) Langmuir convergence zones  

  • Lots of zooplankton in the slick 

  • If you sample in the downwelling outside the slick, there will be less zooplankton 

• There is avoidance of the littoral zone in some (predator avoidance?) 

Vertical migration of zooplankton graph

See different patterns bc theres different species  

vertical migration graph

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seasonal changes in vertical migration

-Algae is somewhat predictable, zooplankton are much more complex 

endogenous rhythm of migration behavior

• Still moving up and down without “seeing” light a day 

• Are genetically programmed to still move 

response to predator exudates

Theres a big change in behavior once you add fish juice. Shows there's a predatory response  

impact of an invader on the zooplankton community

Predator introduced, zooplankton drops down