Download Structure and Productivity of Aquatic Systems

Structure and Productivity of
Aquatic Systems
Functional Lake Zones
Pelagial
Living Things in Lakes

Distribution &
abundance of living
things in lake
controlled by physical
and chemical
conditions in different
zones
Organic Matter in Lakes

Living things make up
only small portion of
organic matter in
lakes
 Most is in form of
non-living detritus
 Both particulate and
dissolved
Organic Matter in Lakes

In most lakes,
dissolved organic
matter is 10 X more
abundant than
particulate
 Living things make up
small portion of
particulate
 Detritus is habitat &
energy resource for
living things
Organic Matter in Lakes

Much of the organic
production of
photosynthesis within
a system is not
consumed, but
becomes part of
detritus reserve
Primary Producers in Lakes

3 major categories of
primary producers:
 Phytoplankton
 Photic zone
throughout lake
 Generally small,
unicellular or colonial
organisms
Primary Producers in Lakes

Emergent
macrophytes
 Shallow portions of
littoral zone
 Roots and lower
portions in water, tops
above water surface
Primary Producers in Lakes

Submersed
macrophytes
 Deeper portions of
littoral zone
 Completely
underwater
Productivity Hierarchy

Emergents most
productive (Carbon
fixed/area/year)
 More productive than
terrestrial grassland,
forest
 Submersed much
less productive
 Phytoplankton least
productive
Phytoplankton
 Cyanobacteria
or
blue-green algae
 Important nitrogen
fixers
 High densities in
late-summer
 Odor (and taste)
problems
Phytoplankton
Desmids
 Green
algae
 Tremendous
diversity
 Planktonic, but can
be attached,
benthic (often
filamentous)
Phytoplankton
 Golden-brown
algae
 Low diversity, but
can be important
segment of
phytoplankton
 Dinobryon
important under
low P conditions
Phytoplankton
 Diatoms
 Very
important
group
 Planktonic and
attached forms
 Cell walls with
silica -- maximum
abundance in
spring when silica
is most abundant
Phytoplankton
 Cryptomonads
 Extremely
small
 May reach high
densities during
cold periods with
low light intensities
(winter under ice)
Phytoplankton
 Dinoflagellates
 Unicellular,
flagellated, with
spines
 Strict requirements
for Ca, pH,
temperature,
dissolved organics
Phytoplankton

Some exhibit
cyclomorphosis seasonal change in
size & form
 Ceratium - more
spines, longer spines,
more divergent spines
as water temperature
increases
 Reduce sinking rate
out of photic zone in
less viscous water
Phytoplankton

Euglenoids
 Unicellular
 Most abundant in
areas with high
ammonia, dissolved
organics
 Shallow farm ponds in
cow pastures
Paradox of the Plankton

Lakes usually have a few dominant species and
many rarer species
 Theoretically should have only single dominant
species (niche overlap leads to competitive
exclusion)
Paradox of the Plankton

Multispecies equilibrium in open waters

4 possible explanations:
Paradox of the Plankton

Environmental change too rapid for competitive
exclusion to occur
 Symbiotic relations among species
(commensalism)
 Selective grazing on competitive dominants by
zooplankton (size-based)
 Some species alternating between plankton and
benthos

Not truly competing with pure planktonic forms
Phytoplankton and
Water Quality

Assemblage indicates
level of nutrient
enrichment
 Desmids and certain
diatoms in nutrientpoor systems
 Different diatoms,
greens, and bluegreens dominate as
enrichment increases
Phytoplankton and
Environmental Factors

Temperature and light
control type,
abundance of
plankton
 Diatoms have lower
temperature optimum,
blue-greens higher
optimum
Phytoplankton and
Environmental Factors



Many can adapt to
changing light
intensities
Chlorella changes
pigments per cell to
maintain same rate of
photosynthesis
Blue-greens regulate
gas pressure in
vacuoles to position
themselves at depth
with optimum light
intensities
Phytoplankton and
Environmental Factors



Some phytoplankton
experience
photoinhibition
High light intensities
near lake surface may
temporarily destroy
enzymes and decrease
photosynthesis
Sunny days - less
photosynthesis near
surface than at greater
depths
Phytoplankton - Seasonal
Succession

Changes in light,
nutrients,
temperature drive a
shift in
phytoplankton
during the year
Phytoplankton - Seasonal
Succession




Low growth in winter
Diatoms and
cryptophytes dominate
in spring
Greens take over in
summer, joined or
replaced by bluegreens as N runs low in
productive lakes
Less productive lakes few greens, blue-greens,
only peaks of diatoms
spring and fall (silica)
Phytoplankton - Seasonal
Succession


Seasonal abundance
varies much more in
temperate (1000 X)
than in tropical (5 X)
lakes, but total
populations are much
greater in tropical lakes
Selective grazing by
zooplankton can
influence succession

Eating some, providing
nutrients for others
Phytoplankton - Nutrient
Enrichment





Enrichment can greatly
increase productivity (per
volume) up to a point
Eventually self-shading
develops and thickness of
photic zone reduced
Inhibits further increases
Productivity/m2 of surface
remains virtually
unchanged
Photosynthetic efficiency
low (<1% of incident light)
Phytoplankton - Variation in
Production

More production in
littoral zones than
pelagial areas
 Peak production
during midday (except
at surface - earlier in
day)
 Seasonal production
peaks in summer
Macrophytes
 Restricted
to the
littoral zones
 In small, shallow
lakes with no
profundal zone,
macrophytes may
occur basin-wide
Emergent Macrophytes
 Rooted
in water or
saturated soil with
aerial leaves/stems
 Upper littoral - out
to 1.5 m depth
 Typha - cattail
Emergent Macrophytes
 Special
category
occupying midlittoral region - 0.53.0 m
 Floating-leaved
plants
 Water lily
Submersed Macrophytes
 All
depths within
photic zone down
to ~10 m for
vascular plants
 Macroalgae - may
occur slightly
deeper
 Coontail, curlyleaf
pondweed, Elodea
Free-floating Macrophytes
 Not
rooted
 May have welldeveloped
submersed roots,
or no roots
 Lemna - duckweed
Aquatic vs. Terrestrial
 Aquatics
mostly
similar to terrestrial
macrophytes
 One major
difference - rooting
tissues grow in
anaerobic
substratum
Aquatic vs. Terrestrial

Roots need O2 to
respire
 Only can get it by
transporting it from
tissues in other parts
of plant
 Extensive system of
intercellular gas
lacunae for gas
transport, exchange
Aquatic vs. Terrestrial

Emergent
macrophytes have
leaf structure similar
to terrestrial plants
 Linear, thick leaves no problem obtaining
light, CO2
 High transpiration lose lots of water
Aquatic vs. Terrestrial




Submersed
macrophytes often look
much different than
terrestrials
>70% of volume is
intercellular lacunae
Leaves very thin,
divided and broadened
to increase surface
area to volume ratio
Better absorb sunlight,
CO2
Aquatic vs. Terrestrial

Some submersed
forms also capable
of assimilating
bicarbonate for use
in photosynthesis
 Based on relative
scarcity of free CO2
in most
environments
Nutrient Needs

Most nutrients
required by
macrophytes come
from sediments
 Free floaters get it
from water
Nutrient Needs


Interstitial waters
generally contain much
higher concentration of
nutrients than waters
above sediments
(anoxic conditions)
Most macrophytes can
assimilate nutrients
from water if
concentrations rise (just
like phytoplankton)
Leaky Macrophytes




Submersed
macrophytes are very
leaky
Lose nutrients to
surrounding water
during active growth
Developed on land and
not adapted to water?
Compromise improved light, CO2
uptake at cost of losing
some nutrients?
Light Limitations

Emergent macrophytes are seldom lightlimited - tremendous capacity for production
 Submersed macrophytes are light-limited
 Depth distribution regulated by light, in part
Depth Limitations



Even in systems with light penetrating to great
depths (unproductive systems), macrophytes only
occur down to ~10 m
Results from hydrostatic pressure - doubles
atmospheric pressure by 10 m
Inhibits movement of gas through lacunae
Macrophytes vs.
Phytoplankton




Phytoplankton productivity may be very low in littoral
areas with many macrophytes - 3 reasons:
1) Competition for nutrients
2) Shading
3) Release of inhibitory organic chemicals by
macrophytes
Macrophytes vs. Algae

Productivity of some types of algae may be
very high in close proximity to macrophytes
 Grow attached to macrophytes and live off
materials leaking out