Transcript EVPP 550 Waterscape Ecology and Management – Lecture 10

EVPP 550
Waterscape Ecology and
Management – Lecture 10
Professor
R. Christian
Jones
Fall 2007
Phytoplankton
Patterns of
Abundance
• Seasonal - Winter
– In temperate lakes,
phytoplankton are generally
greatly reduced during
winter due to low
temperature and ice cover
which impedes light
transmission
– However, over the winter
nutrient concentrations
increased due to
decomposition and
sediment release
Phytoplankton
Patterns of
Abundance
• Seasonal - Spring
– With abundant nutrients in
place, rapid growth occurs
in spring when light and
temperature again become
favorable
– In shallow lakes, increase in
ambient light alone is
sufficient to start the bloom
– In deeper lakes, may need
to get stratification before
light and temperature reach
their optima
– In most lakes, spring bloom
is dominated by diatoms
Phytoplankton
Patterns of
Abundance
• Seasonal - Spring
– Spring bloom may
continue for several
weeks, but is
eventually ends when
nutrients become
exhausted which for
diatoms may be either
P or Si
– Grazing may also play
a role in cropping back
the large phytoplankton
populations
Phytoplankton
Patterns of
Abundance
• Seasonal - Summer
– In many oligotrophic
and mesotrophic lakes
a decline occurs in
summer as nutrients
become limiting
– Smaller algae such as
small flagellates and
cyanobacteria
dominate as they are
better able to utilize low
nutrient levels
Phytoplankton
Patterns of
Abundance
• Seasonal - Fall
– In these lakes a second
bloom often occurs in the
fall as nutrients start to be
remixed into the epilimnion
– Diatoms are again often
dominant, but other species
can also occur
– In late fall, light and
temperature decline,
stratification breaks down
and phytoplankton
populations collapse
Phytoplankton
Patterns of
Abundance
• Empirical Data
– A study compiled data
from many lakes and
found that the bimodal
pattern we just described
held very well for
“eutrophic” lakes (here I
would use the term
“mesotrophic/eutrophic”
– However, oligotrophic
lakes did not show as
clear a seasonal pattern
Phytoplankton
Patterns of
Abundance
• SeasonalHypereutrophic Lakes
– In highly productive
systems
(hypereutrophic)
growth may continued
unabated through the
summer forming a
single large peak in
late summer
– Often dominated by
cyanobacteria
Phytoplankton
Patterns of
Abundance
• Interannual
– Cycles are fairly
predictable in a given
lake
– Some variability due to
climatic variation
including flushing
– In this graph the
different lines represent
different diatom
species in Lake
Windermere, UK
Zooplankton - Characteristics
• Taxonomy
– Protozoa
• Single-celled,
heterotrophic,
eukaryotic
• Feed on
bacteria and
small algae
• Ciliates
• Amoebae
• Zooflagellates
Zooplankton - Characteristics
• Rotifers
• Small invertebrates
• Multicellular,
heterotrophic,
eukaryotic
• Suspension feeders
• Rythmically beating
rotating cilia near mouth
creating a feeding
current, also moves
organism through water
• Relatively small (0.2-0.6
mm)
• Generation time: ~ 1 wk
Zooplankton Characteristics
• Rotifers
– Life History
• Have both sexual and asexual (parthenogenetic) reproduction
• Asexual during favorable periods
• Stressful conditions induce sexual reproduction which produces
“resting eggs”
• Resting eggs are resistant to drying, cold, heat, etc. and can
hatch when favorable conditions return
Zooplankton - Characteristics
• Cladocera
– Small invertebrate
arthropods
– Multicellular,
heterotrophic,
eukaryotic
– Use jointed
appendages for
swimming and feeding
– “water fleas”
– Very characteristic of
freshwater
Zooplankton - Characteristics
• Cladocera
– Most are herbivorous filter
feeders
– Filter algae from the water
as they swim in a rather
passive fashion
– Some are raptorial
predators, mainly on other
cladocera
– Adults range from 0.3 mm
up to 3 mm except
Leptodora up to 10 mm
– Generation time as low as
2 weeks when asexual
Zooplankton Characteristics
• Cladocera
– Like rotifers, have both asexual and sexual reproduction
– During favorable conditions, there can be many generations
of asexual reproduction (eggs that don’t need fertilizing)
– When stress occurs, males are produced and sexual
females, meiosis occurs to produce gametes
– Male gametes fertilize eggs in brood chamber producing
sexual (epphipial) eggs
Zooplankton - Characteristics
• Copepods
– Small invertebrate
arthropods
– Multicellular,
heterotrophic,
eukaryotic
– Use jointed
appendages for
swimming and feeding
– Found in freshwater,
estuaries and the
ocean
– Very characteristic of
marine zooplankton
Zooplankton - Characteristics
• Copepods
– Some are passive filter
feeders, but most go
after individual
particles
– Take algae and small
invertebrates
– Adults range from 0.5
mm to 5 mm
– Calanoid & cyclopoid
common in plankton
Calanoid
Cyclopoid
Zooplankton - Characteristics
• Copepods
– No asexual reproduction
– Fertilized egg hatches into a larva called a nauplius
– Nauplius undergoes a series of molts (6) before changing into
a form that looks like an adult (copepodid)
– Copepodid undergoes 6 further molts before becoming an
adult (sexually mature)
– Males and females look similar, but males have clasper
– Generation time: months to one year
Zooplankton
Factors Affecting Growth
• Two methods have been used to
measure zooplankton performance
– Population growth rate (r)
• N(t) = N(0) ert where r is the growth rate of the
population in units of 1/time
– Filtration rate
• Filtration rate = volume of water cleared of
particles per unit time, mL or % per unit time
Zooplankton
Factors Affecting Growth
• Food concentrations and Temperature
– Zooplankton growth often seems to be limited by food
and temperature
– In the study cited below, r increased with temperature at
each food concentration and with food concentration at
each temperature
– Growth rate at the highest T and food was over 7x that
at the lowest combination
Zooplankton
Factors Affecting Growth
• Food quantity and quality
– Both the quantity and quality of food are important
– r = b – d (birth rate – death rate)
– At the lowest food concentration, birth rate was very low and death
rate quite high
– As food concentration increased, birth rates increased and death
rates declined strongly
– The green alga Chlamydomonas supported highest birth rates and
lowest death rates
Zooplankton
Factors Affecting Growth
• Filtering rates are a function of temperature and
body size
• In the data shown below, larger individuals filter
much more water than smaller ones
• For this species, filtration rates increase to 20oC
and then decline
Zooplankton
Factors Affecting Growth
• Food concentrations and Temperature
– Zooplankton growth often seems to be limited by food
and temperature
– In the study cited below, r increased with temperature at
each food concentration and with food concentration at
each temperature
– Growth rate at the highest T and food was over 7x that
at the lowest combination
Zooplankton
Patterns of Abundance and Activity
• Some zooplankton
populations grow in a
synchronized pattern
• This is particularly true in
the temperate and polar
areas with strong
seasonality
• In these areas there may
be only one or two
generations per year
• Graph on the right shows
a copepod population in
a Norwegian lake which
has one well
synchronized cohort per
year
Zooplankton
Patterns of Abundance and Activity
• Here is a second one
with two synchronized
populations and a resting
stage
• This is most common in
copepods which require
sexual reproduction
• In the cladocerans and
rotifers, there is less
synchrony generally
partially due to
continuous asexual
reproduction under
favorable conditions
• It’s also harder to discern
the different stages in
cladocerans
Zooplankton
Patterns of Abundance and
Activity
• Other factors affect
zooplankton abundance
and acitivity in the field
such as predation
• Here is a data set which
found that predation by
Leptodora was a major
controlling factor on
Daphnia populations
• Note the very high birth rate
(b) in July meaning they
were producing lots of eggs
• But r was near 0, implying a
high death rate
• The period of high death
rate corresponded with the
maximum for the
predaceous cladoceran
Leptodora
Zooplankton
Patterns of Abundance and Activity
• Predation by fish is also an
important regulatory factor
• It has strong effects on
behavior
• In a lake with fish present,
a strong diel migration of
zooplankton was observed
with zooplankton exiting the
top layers during the day,
presumably to avoid fish
predation
• In a similar nearby lake
without fish, zooplankton
remained in the upper
layers all day which
presumably allows them to
feed longer
Zooplankton
Patterns of Abundance and Activity
• In addition to these
depth patterns of
avoidance, there seem
to be other behaviors
for avoidance of fish
predation
• Zooplankton cluster
within macrophyte
beds during the day,
but venture into open
water at night
Zooplankton
Patterns of Abundance and Activity
• Presence or absence of
fish in a lake has a
strong effect on the
species and sizes of
zooplankton
• An important early study
looked at the size
structure of lakes in
Connecticut with and
without anchovy
• This study led to the
concept of “top-down”
control of food webs by
which predators as
opposed to food sources
control biological
communities
Zooplankton
Patterns of Abundance and Activity
• While top-down control
seems to regulate the
types and sizes of
zooplankton, the total
biomass of zooplankton
is strongly related to food
supply
• Here, we see a graph
showing a positive
correlation between TP
vs. zooplankton
• The inference is that P
fuels phytoplankton
growth which fuels
zooplankton growth, a
bottom up pattern
Zooplankton
Patterns of Abundance and Activity
• A typical seasonal
pattern of zooplankton
activity involves a late
spring-early summer
maximum (see
phytoplankton seasonal
pattern earlier in lecture)
• Note that all 4 groups of
zooplankton can play a
role during the year
• The numbers attained
tend to be inversely
proportional to the size
of individuals
Zooplankton
Patterns of Abundance and Activity
• Zooplankton can exert
heavy grazing pressure
on phytoplankton and
create their own “topdown” effect
• Their effect varies
strongly with seasonal
and depth patterns in
abundance
Grazing/filtering rates above 50%/day
would exert a major control over
phytoplankton. That would imply that 50%
were removed on a daily basis.