BIO321

Zooplankton II: Seasonal changes in zooplankton communities

Overview

  1. Cyclomorphosis
  2. Migration - vertical and horizontal
  3. Succession - seasonal cycles
  1. Cyclomorphosis
  1. Defined: a seasonal (cyclic) change in the morphology of planktonic populations of some species.
  2. Review : We have already seen similar in sexual processes - parthenogenetic vs sexual as a characteristic of all but the copepods
  3. Occurs in several groups
  1. Cladocerans - an example, but also a major feature of this group
  1. changes in helmet size (esp. Daphnia)
  2. Increased tail spine length
  3. Both of these increase in spring and decrease in fall
  4. Does not occur in tropical lakes
  1. Why ? What types of explanations have been broached?
  1. The form resistance hypothesis (increased Tdecreased densityincreased size to decrease sinking rate. [Sars 1890, Wesenberg-Lund Hypothesis 1900]
    •evidence is that below 10 - 15°C there is no helmet extension
  2. The balance hypotheses
  1. increased helmet size is a response to increased turbulence and the large helmet helps to "keep its head up" [Woltereck 1913]

•helmet size is larger in spp found in turbulent waters

  1. increased helmet size improves orientation for feeding (esp in rotifers) where spine length and spread increases to allow for "more stable" feeding during the summer low density period [Manuilovo & Enman 1972]

c. Light hypotheses - an increase in light causes an increase in helmet size [Jacobs 1962]

•No cyclomorphosis in dark turbulent waters

d. Predator-Prey interactions causes: [Brooks 1966]

small prey are less visible and thus get eaten less by fish larvae (but of course eaten more by copepods and insects

But all need to grow leading to increased size and increased predation

i. predator-prey hypothesis #1: Visibility selection hypothesis: zooplankton put energy into increasing the size of "invisible" parts of the body like the helmet and spines, and do not increase "visible" body size, which would lead to increased predation by larger organisms. [differential apparent size growth - helmet vs carapace]

ii. predator-prey hypothesis #2: The visual predator hypothesis: Predators in aquatic systems select prey not by size, but by visibility (which is linked)

• helmeted individuals  decreased eye pigmentation

• non-helmeted individuals increased eye pigmentation and increased predation

  1. Rotifers also show cyclormorphosis
  1. increased # spines
  2. increased divergence of spines (increased body size, increased stabilization, & decreased predation

•Gilbert (1950's) examined popns of two spp of rotifers

Brachionus - found an increased # of and divergence of spines when the second species, Asplanchna was present.

  1. Ceratium (algae) also shows cyclomorphosis
  1. increased # spines, increased length, & increased spine divergence
  2. increased size to decrease grazing
  3. increased size to increase stabilization in water column (how change sinking rate?)
  1. Cyclomorphosis is absent in copepods. Why?
  1. are highly motile predators/grazers - skitter away as a pre-contact defense.
  2. B. cladocerans and rotifers are less motile and rely on "post-contact" defenses?
  1. increased body size,
  2. increased spines and length to frustrate predators,
  3. protection of young in brood pouches
  4. short generation time
  1. Vertical Migration
  1. Defined: A vertical pattern of movement through the water column on a diel basis, typically in zooplankton up during the night and down during the day.
  1. Groups involved are mainly the meso- & macro- zooplankters and especially Cladocerans
  2. Crepuscular migrations also occur with a migration up at dawn and dusk.

• a few phytoplankton like Volvox also migrate, but the pattern is opposite - up day and down night to maximize pigment system photosynthesis?

  1. Patterns and factors in vertical migrations:
  1. The stage of development is important

• immature forms migrate more

  1. Time of year is important

• migration maximum varies for different groups

  1. Region and amplitude of migration is group specific

• Mysis migrates up to the thermocline, but not beyond

  1. Gender is important

• Females migrate more than males

  1. Size is important

•Bigger migrate further

rotifers up to 1 m

Daphnia 10 to 20 m

  1. Why does migration occur - Hypotheses
  1. Early hypotheses were mostly proximate explanations:
  1. Temperature - Zoop escape warm O2 depleted layers in the day (not true)
  2. Dissolved gases -

CO2 &  O2 in the hypolimnion and they want to "escape" these conditions

  1. Turbulence theory
  2. Pressure hypothesis
  3. To avoid toxic metabolites
  4. To avoid predators or to find prey[differential apparent size growth - helmet vs carapace]
  1. Current Theories on Why vertical migration occurs
  1. Light is the regulating factor (proximate explanation)

• but there is no evidence that they are trying to stay in an "optimal" light environment for visual or other reasons

  1. Ultimate reason theories
  1. Visibility hypothesis: predation is geared to visible prey and zooplankton migrate  day to visibility (time of day and turbidity are involved).
  1. Migration hypothesis: Zooplankton migrate day to avoid "overharvesting" phytoplankton and to avoid toxic metabolites of photosynthesis (esp. BGA's) and Zoop. Sneak up to eat them when they are sleeping.
  1. Energy bonus hypothesis: (growth efficiency hypothesis - energetic reasons): Migrations allows the best use of energetic resources at the lowest possible cost.
  1. feed in warm waters during night
  2. and digest in cooler water during the day at lower metabolic cost
  1. PROBABLY is a combination of all three - the differential growth - feeding-protection hypothesis
  1. Horizontal migration
  1. Pelagic species migrate away from the littoral zones to the open water zones, especially Cladocerans and Copepods.
  1. Cause 1 - light - shore interferes with light and fouls up their orientation finders

Higher horizons cause movement towards the center of the lake

  1. Cause 2 - Interactions with antagonistic responses to macrophytes, wastes, toxins, and salinity changes etc.
  1. Season cycles in Zooplankton
  1. Overwintering populations: N is low,  food, mostly adult & immature copepods with cladocerans in diapause or epphipium
  2. Spring  T
  1. Cladocerans & rotifers emerge from overwintering eggs with rapid amictic growth leading to a fast population peak. Copepod growth is slower leading to a later peak
  2. body size in late spring

C. Stratification leads to a proliferation of indigestible greens and BGA leading to an increase in predators like the cladoceran Diaptomus or Leptodora