Aquatic Ecosystem Overview:
• We need to understand the physical (e.g. hydrodynamics) and
chemical environment that ultimately control the productivity,
interactions, and evolution of life in aquatic ecosystems.
• Organisms reside in habitats characterized by their physicochemical
setting, perform a particular niche or function, and represent a
particular trophic level.
• Individual organisms are members of a population, which interacts
with other populations within a community.
• Ecosystems may possess multiple community types based on
groupings of common habitat and/or niche characteristics.
– Littoral (fringe with light)
– Plankton (drifting)
– Detritus (dead organics)
- Benthic (bottom)
- Nekton (swimming)
- Groundwater (subsurface; hyporheic)
The Lake
Example:
Know your lake regions
for benthic and open
water habitats.
(Cole, 1994; Fig 2.1)
Temperate lake formed by
geological faulting, depicted
during the summer season.
Density differences due to
temperature result in stable
layers, or strata. This process
is called stratification.
Why the difference in
O2 and CO2?
Biological Productivity
(Rawson Diagram):
Volumetric units: mg C/m3/d
Areal units: mg C/m2/d
(Cole, 1994; Fig 1.1)
Productivity and Trophic Status
• Ecosystem productivity is generally classified according to its trophic
state for convenience:
– Oligotrophic (low)
– Mesotrophic (medium)
– Eutrophic (high)
• These states represent ranges within a continuum of potential
productivity within a system.
• The impetus for identifying trophic state came from observations that
certain biota were often associated with specific nutrient levels.
• Being able to classify systems is convenient for comparison of
different areas as well as for defining management goals and
reference points.
• Trophic state is generally defined as a function of phytoplankton
biomass / chlorophyll concentrations, nutrient concentrations, and/or
water clarity.
Probability Distribution
(Dodds, 2002; Fig. 17.2)
Fixed Boundary Classifications
OECD = Organization for Economic Cooperation and Development
Total P = phosphorous in dissolved and solid forms; inorganic and organic.
Mean Chl = annual average of chlorophyll a.
Mean Secchi = average depth to which a black and white disc can no longer be seen.
(Dodds, 2002)
Continuous Value Scale
(Dodds, 2002; Fig. 17.2)
Stream Ecosystems
• Unless flow is slow enough, stream systems
usually do not have a truly planktonic biomass of
primary producers. Trophic state is measured by:
– Attached benthic algae (periphyton) biomass
– Nutrient (N & P) concentrations
• Due to dependence of streams on terrestrial
(allocthonous) organic matter, primary producers
are not always a good indicator of system
productivity.
Properties of Water
•
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•
•
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Polarity & H-bonds
Water Density
Water as a Solvent
Heat
(Sensible vs Latent)
Surface Tension
Reynolds Number
(Inertia to Viscosity)
Molecular Diffusion
Flow Type
(Laminar vs Turbulent)
Light Attenuation
Water Density
Stratification by Temperature
or Dissolved Ions (salts):
Water as a Solvent
Geologic Weathering
Solute solubility increases
with increasing temperature
due to increases frequency of
molecular interaction and less
H-bonding between water
molecules and more hydration
of solutes by water molecules.
Gas solubility decreases
with increasing temperature
due to increased vibration and
expansion of molecules plus
the reduced partial pressure
favors a gas equilibrium shift
from water to air, something
solutes don’t experience
because water binds solutes
electrostatically.
Sensible vs Latent Heat
High Heat Capacity
Water is our natural thermal buffer!
Surface Tension
•At water-air or water-solid interfaces
molecules of water are nearly
completely H-bonded together, like a
molecule thick layer of ice.
• This is added tension force is
utilized by many aquatic & semiaquatic arthropods
Water strider (right)
Fishing Spider (left)
• It is also what explains waters ability
to be drawn against gravity when in
narrow spaces; capillary action.
Reynolds Number (Re)
• Viscosity: resistance of a liquid to change of form.
• Inertia: resistance of an object (body) to a change in its state
of motion.
• These two forces together influences an organism’s ability to
move in water and how water flows.
• The ratio of inertial force to viscous force is Re.
Fi
SU
U l


Fv

 SU 


 l 
2
• U = velocity of object relative to the fluid; l = object length;
S = surface area; µ = dynamic viscosity; p = fluid density
• High Re values are associated with dense, fast and long
(streamline) objects.
• Low Re values are associate with small and slow objects,
particularly those with a high surface to volume (S:V) ratio.
• The Y-axis below could be switched from velocity to body size
(length) a similar trend would be seen.
Plankton
Viscosity
Nekton
Inertia
Molecular Diffusion:
• Water molecules move constantly
in a vibrating fashion, called
Brownian Motion.
• Browning Motion is increased by
increased temperature.
• Solutes in water also experience
Brownian Motion.
• Molecular diffusion it that solely
due to solute and solvent Brownian
Motion (expressed as the diffusion
coefficient at a given temperature;
D) and the solute concentration
gradient between two locations.
• The rate of molecular diffusion (J)
is expressed by Fick’s Law.
• Small organisms at low Re are
dependent on molecular diffusion.

C1  C2 
J D
x1  x2 
Laminar versus Turbulent Flow
• Laminar flow is unidirectional, turbulent flow is more chaotic.
Laminar Flow
Turbulent Flow
• At larger spatial scales flow is often turbulent. Eddies may form.
Solute diffusion becomes dominated by eddy transport for water
parcels, not solely molecular diffusion.
• At smaller spatial scales flow is more often laminar, due to the
viscous force dampening turbulence.
• Flow across a surface experience a viscous (frictional) force and
becomes more laminar.
• The transition between turbulent and laminar flow is called the flow
boundary layer.
Flow Boundary Layer Thickness:
• It ↓ with ↑ velocity.
• It ↓ with ↓ surface
roughness.
• It ↓ with ↓ object
size.
• It ↓ with ↓ distance
from the upstream
edge.
Relationships
between Re
and flow type.
• For any given object; lower
Re will translate to more
laminar flow.
• Re is lower when velocity
decreases.
• Flow is more laminar at
higher Re when the object is
streamline (long).
Colonial Diatom
Light Penetration & Attenuation
(Io)
Euphotic zone
Note refraction
1% Io
Amount reflected depends on wave
conditions and ice cover.
Light that penetrates the surface may then
be absorbed, reflected of transmitted.
Light intensity decreases logarithmically with
depth; a process called attenuation
Attenuation (or extinction)
coefficient (k is defined as:
k = (ln Io – lnIz) / z
Light intensity at a given
depth can be calculated as:
Iz = Ioe-kz
• Different wavelengths of light
absorb differently in pure water.
Blues and greens penetrate
deepest and reds the least.
Why the “deep blue sea”?
• Particles and colored dissolved
organic matter will further
influence how quickly different
colors of light are absorbed in
natural waters.
Secchi Depth and Trophic Status
Secchi depth (ZSD) can be used to estimate the attenuation coefficient (k).
k = 1.7 / ZSD ; This translates into the 1% light level being 2.7 times ZSD, or
that 16% of surface irradiance remains at the ZSD.
(k)