BIO321
Zooplankton II: Seasonal changes in zooplankton communities
Overview
1.
2.
3.
I.
Cyclomorphosis
Migration - vertical and horizontal
Succession - seasonal cycles
Cyclomorphosis
A. Defined: a seasonal (cyclic) change in the morphology of planktonic populations of some species.
B. Review : We have already seen similar in sexual processes - parthenogenetic vs sexual as a characteristic of all but the
copepods
C. Occurs in several groups
1. Cladocerans - an example, but also a major feature of this group
a. changes in helmet size (esp. Daphnia)
b. Increased tail spine length
c. Both of these increase in spring and decrease in fall
d. Does not occur in tropical lakes
2.
Why ? What types of explanations have been broached?
a. 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
b. The balance hypotheses
i.
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
ii.
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
3.
Rotifers also show cyclormorphosis
a. increased # spines
b. 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.
4.
Ceratium (algae) also shows cyclomorphosis
a. increased # spines, increased length, & increased spine divergence
b. increased size to decrease grazing
c. increased size to increase stabilization in water column (how change sinking rate?)
5.
Cyclomorphosis is absent in copepods. Why?
a. are highly motile predators/grazers - skitter away as a pre-contact defense.
b. B. cladocerans and rotifers are less motile and rely on "post-contact" defenses?
i.
increased body size,
ii.
iii.
iv.
II.
increased spines and length to frustrate predators,
protection of young in brood pouches
short generation time
Vertical Migration
A. 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?
B. Patterns and factors in vertical migrations:
1. The stage of development is important
• immature forms migrate more
2. Time of year is important
• migration maximum varies for different groups
3. Region and amplitude of migration is group specific
• Mysis migrates up to the thermocline, but not beyond
4. Gender is important
• Females migrate more than males
5. Size is important
•Bigger migrate further
rotifers up to 1 m
Daphnia 10 to 20 m
C. Why does migration occur - Hypotheses
1. Early hypotheses were mostly proximate explanations:
a. Temperature - Zoop escape warm O2 depleted layers in the day (not true)
b. Dissolved gases CO2 &  O2 in the hypolimnion and they want to "escape" these conditions
c. Turbulence theory
d. Pressure hypothesis
e. To avoid toxic metabolites
f. To avoid predators or to find prey[differential apparent size growth - helmet vs carapace]
2.
Current Theories on Why vertical migration occurs
a. 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
3.
Ultimate reason theories
a. Visibility hypothesis: predation is geared to visible prey and zooplankton migrate  day to visibility (time of
day and turbidity are involved).
lux
Depth
Follow the 1 lux light level during the
day
time
b.
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.
c.
Energy bonus hypothesis: (growth efficiency hypothesis - energetic reasons): Migrations allows the best use of
energetic resources at the lowest possible cost.
i.
feed in warm waters during night
ii.
and digest in cooler water during the day at lower metabolic cost
PROBABLY is a combination of all three - the differential growth - feeding-protection hypothesis
d.
III.
IV.
Horizontal migration
A. 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
2. Cause 2 - Interactions with antagonistic responses to macrophytes, wastes, toxins, and salinity changes etc.
Season cycles in Zooplankton
A. Overwintering populations: N is low,  food, mostly adult & immature copepods with cladocerans in diapause or
epphipium
B. Spring  T
1. Cladocerans & rotifers emerge from overwintering eggs with rapid amictic growth leading to a fast population peak.
Fig 13-9
Copepod growth is slower leading to a later peak
Horne
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