Limnology: Lecture 7
Aquatic Ecosystems
Limnology: Lecture 7
I. Ecosystem Ecology
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General discussion of specialties of ecology, origin of
the ecosystem perspective, and its emphasis on
biology, geology, and chemistry, close history with
limnology
A. Structure and Function:
1. Structure: pools, storage, box (mass/area)
2. Function: arrow, process, rate (m/time)
B. Energy (E) Flow & Nutrient Cycling:
1. E-flow as a measure of the functioning =
metabolism = OM dynamics
2. Nutrient cycling = transformation and relocation
of elements
C. Ecosystem perspectives and limnology:
1. Early studies of lakes viewed them as
"microcosms", e.g. Forbes (1887) viewed the
lake as an interactive system with boundaries
2. Functional approach take by Lindeman (1942),
quantified producers and consumers, used Eflow to measure
Limnology: Lecture 7
Lindeman 1942
II. Lake Structure
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A.
B.
C.
D.
Structural divisions of lakes, not always functional units, more related to the distribution and characteristics of biota
photic and aphotic zones: depth of lake including >1% of incident light = photic zone, the zone with <1% of incident light
is the aphotic zone
littoral zone: areas where the shore is in the photic zone: Wetzel's littoral zonation: The Zone of Macrophyte Growth on
the Shore of a Lake (See Figure 7-1)
1. Epilittoral - above the high water level and uninfluenced by spray
2. Supralittoral - above the high water level and subject to spray
3. Eulittoral - the shoreline region between the highest and lowest seasonal water levels
4. Eulittoral plus infralittoral = littoral zone
5. Infralittoral
a. Upper infralittoral - zone where emergent rooted macrophytes exist
b. Middle infralittoral - zone where floating leaved macrophytes exist
c. Lower infralittoral - zone where rooted or adnate macrophytes exist
d. Littoriprofundal - zone where only photosynthetic algae and bacteria exist (often associated with the
metalimnion)
profundal zone - lake bottom w < 1% light, where sediments are free of vegetation
pelagic zone, open-water not impacted by shoreline and/or littoral biota or processes
Limnology: Lecture 7
III. Lake Habitats
A.
B.
C.
D.
E.
Planktonic - free-floating
1. Phytoplankton - free-floating algae
2. Zooplankton - free-floating animals with limited powers of locomotion
3. Tychoplankton - Suspended benthic organisms (not true plankton)
4. Metaphyton - Phytoplankton existing within the macrophyte communities of littoral zones
Nekton - free-swimming
1. Some organisms (such as larvae of fish) can shift between planktonic to nektonic existence within their life cycle
Pleuston - organisms adapted to the habitat of the air-water interface
Neuston - the microscopic component of the pleuston
1. Epineuston or epipleuston are those forms that exist on top of the water's surface
2. Hyponeuston or hypopleuston are those forms that exist under the water's surface but still associated with the
air-water interface
Benthos - organisms that live on the bottom of a lake, stream, or river (See Figure 7.2)
1. Benthic is the adjective that is used to describe organisms that live on the bottom
2. The film of algae, bacteria, fungi, and invertebrates that exist on substrates is known as periphyton
a. Periphyton literally refers to organisms living "on plants" but is most often used in the more general sense
b. The term Aufwuchs is sometimes used for the film of algae, bacteria, fungi, and invertebrates that exist on
substrates, Aufwuchs is a term derived from the German for "growth upon“
c. There is a recent trend to describe these periphyton communities more exactly
i. Epilithic - living on rock substrates
ii. Epipelic - living on fine organic sediments
iii. Epiphytic - living on plants
iv. Epizooic - living on animals
v. Epipsammic - living on sand grains
Limnology: Lecture 7
Limnology: Lecture 7
IV. Standing Crops
A.
B.
C.
Standing crop: - weight of organic matter that can be
sampled or harvested at one time from a given area (not
necessarily the whole plant), called standing stock if
calculated for animals
1. Dry weight - dried in oven at 60°C for at least 24 hours
2. Ash-free dry weight - weight of ash (determined by
weighing the residue after burning sample in furnace at
550°C for at least 4 hours) is subtracted from the dry
weight to determine ash-free dry weight =
Biomass - the weight of all living matter in a unit area at
a given time (includes the whole plant)
3. Estimation of abundance of organisms
a. Enumeration,
b. Volume
c. Weight
4. Nutrient pools - Cellular constituents - calculated as
percentage of standing stock (e.g. carbon = 40-60% for
most plants, nitrogen = phosphorus)
Allochthonous organic matter - organic matter that is
derived from outside the system (in lakes and streams, this is
usually leaves, needles, wood, and twigs from terrestrial
vegetation)
Autochthonous organic matter - organic matter-derived
within the system (in lakes and streams, this is usually algae,
mosses, and aquatic vascular macrophytes)
Limnology: Lecture 7
V. ECOSYSTEM METABOLISM
A.
B.
C.
D.
Primary Production (PP): rate of fixation of inorganic carbon
(i.e. CO2) as newly formed organic carbon - defined as
autotrophy, in lakes = plants of littoral zone, periphyton &
phytoplankton, formula shows PP = CO2 sink and O2 source,
arrow or process
Respiration (R): the rate of destruction of pre-formed organic
matter (i.e. biological oxidation) - all heterotrophs do this, any
biological oxidation contributes to decomposition, but often
specific types of organisms are associated with detrital
consumption = detritivores or decomposers, R = sink for O2
source of CO2
Net Primary Productivity: NPP or PN = amount of new
carbon formed after total amount of fixation is corrected for
system respiration; contrasts with GPP or Gross Primary
Productivity (PG) defined as total amount of energy fixed as
carbon during primary productivity, it follows that PN = PG – R
Fates of PN: what happens to produced organic matter of
plants?
1. Can contribute to growth (change in biomass over time),
may include reproductive tissue = yield
2. Can add to detrital pool via non-predatory mortality
(NPM) = litter fall or other
3. May be consumed by predation or grazing
Limnology: Lecture 7
VI. MEASURES OF PN, PG AND R IN LAKES AND
STREAMS
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In lakes, the form of the lake will determine the distribution of productivity (i.e. among littoral, benthic,
pelagic environments). In rivers, a vast majority of productivity is often benthic. With these assays, one
can measure the change in gases (O2/CO2) over time to derive rates of photosynthesis and respiration
A. light-dark bottles: captured portion of the water column is representative of all the lake
1. light bottle processes: PG of primary producers & R of all biota
2. dark bottle processes: R of all biota
3. combined measure (light-dark): PN of encapsulated system
B. benthic chambers: enclosed portions of the benthos with all biota, circulating water to mimic stream
conditions, dark cloth to mimic night; light - dark to derive estimate of PN
Limnology: Lecture 7
VII. ENERGETIC PROPERTIES OF ECOSYSTEMS
A. Autotrophic - gross primary production within the system exceeds respiration. This is often
expressed as the ratio of production to respiration (PG /R). The P/R ratio of autotrophic
systems is equal to or greater than one
B. Heterotrophic - respiration within the system exceeds gross primary production. The P/R
ratio is less than one in heterotrophic systems
C. Residence Time: (B/P): biomass/production = time, amount of time the average particle of
OM spends in the subsystem with biomass (B) and productivity (P)
D. Turnover Ratio: Production to biomass ratio (P/B) = t-1, the fraction of the pool that "turns
over" during a period of time, essentially a "half-life", also a measure of the metabolic
intensity or activity.
1. High P/B ratio with smaller organisms
2. P/B generally decreases with increases in size and increasing trophic level.
3. Lower P/B in oligotrophic than eutrophic.
Limnology: Lecture 7