Difficulties associated with living in freshwater
habitats
(with particular consideration for those facing the
invertebrates)
In order to explain in your final papers the distribution and abundance of
freshwater organism, you will need to link the following:
Habitats/Localities  Problems/Difficulties  Adaptations/Responses
Problems/Difficulties:
1) Desiccation – Many aquatic habitats are not
permanent
Response to this problem might include:
 Active dispersal
 Passive dispersal
 Temporal ‘dispersal’
What specific adaptations might accomplish these solutions?
(Keep in mind that dispersal ability can be considered a
“difficulty” to explain absence, as well as an adaptation to
explain persistence.)
2) Moving in fluids and preventing fluid from moving you –
Water is a thick fluid
Reynolds number; the physics of water is
different for small organisms
Reynolds number = length X velocity
viscosity
Reynolds numbers describe how a fluid behaves around a solid.
A small organism will experience the surrounding water in the
same way as a large organism would if the large organism was
immersed in a more viscous fluid.
At higher Reynolds numbers, turbulence is greater increasing
drag.
How might small and large aquatic organisms differ in their
response to life at different Reynolds numbers? Hint: how is
drag reduced?
Boundary layers; solid surfaces influence water flow
The boundary layer is the slowing of currents near a surface
due to the fact that a fluid resists slipping over a solid, hence
fluids farther away slip over closer fluids. This low velocity
zone is thinner as water currents overhead increase.
What problem does strong water current pose for aquatic
organisms?
What adaptation would take advantage of the
boundary layer in response to this problem?
What other adaptations might reduce downstream displacement
of individuals in environments with water currents?
What method of dispersal might be effective in compensating
for downstream displacement of individuals?
In what type of habitat would a planktonic lifestyle be most
difficult?
3) Potential food items are often diffuse and on the
move
What is the major input of energy in lentic systems?
What is the major input of energy in lotic systems?
Aquatic organisms can be grouped into “functional feeding
groups” depending on how they obtain their energy (note that
this is an ecological grouping, not a taxonomic one)
Benthic invertebrates:
Shredders depend upon coarse particulate organic matter
(CPOM) for their primary food resource. CPOM is any
material greater than about 1 mm in diameter; examples
include twigs, leaves, fruits and flowers of terrestrial or
aquatic vegetation.
Filtering Collectors depend upon fine particulate organic
matter (FPOM) including plant, animal, and fungal
detritus that is suspended in the water column.
Gathering Collectors (deposit-feeders) depend upon fine
particulate organic matter (FPOM) including plant, animal,
and fungal detritus that has settled out onto the
surfaces of substrates.
Scrapers (grazers) depend upon attached periphyton (i.e.,
attached algae and associated flora and fauna) that
develops on submerged substrates.
Predators (Engulfers) attack living prey organisms; some
ingest whole animals, while others tear off and swallow
large pieces.
Predators (Piercers) require live prey; they have modified
mouthparts that allow them pierce the tissues and suck
up the body fluids.
Herbivores (Piercers) with piercing mouthparts rely on the
fluids from plant tissues, including higher plants, mosses,
and filamentous algae, for their main source of nutrition.
Scavengers and Omnivores are generalists able to utilize a
variety of food sources, including both living and dead
organic materials. In fact, most aquatic organisms are
likely omnivorous at some point in their life cycle. Very
young larvae likely ingest all sorts of material, as long as
it is an appropriate size.
Are these food types likely to be found in the same
proportions within and among habitat types?
Note that aquatic macrophytes might be abundant and
concentrated in some aquatic habitat types, but few
organisms are able to take advantage of this resource
directly.
Planktonic invertebrates
Filter Feeders consume on suspended particles. Larger
zooplankton feed on phytoplankton where as smaller
zooplankton tend to feed on bacteria .
Planktonic Predators are zooplankton that attack other living
zooplankton.
4) Predation is common
Why are freshwater invertebrates particularly vunerable to
predation (i.e. why does Dr. Ensign call them “fish food”)?
How can benthic invertebrates reduce predation?
How can planktonic invertebrates reduce predation?
5) Solubility of gases in water is much different than
in air
 Saturation point is much lower
 Water is much more viscous
 Diffusion is 300,000X slower
What problem does this pose for an aquatic
organism relative to a terrestrial one?
In what type of environments might this problem be
greater?
What adaptation might evolve in response to this problem?
Habitats/Localities  Problems/Difficulties  Adaptations/Responses
How might the considerations discussed above (diseccation, downstream
displacement, food availability, predation, oxygen availability) differ among
following habitat types?
Freshwater Habitat Types:
Lentic
More
predictable
Less
variable
Lotic
Lakes
Reservoirs
Littoral zones
Ponds
Streams
Freshwater
Wetlands
Less
predictable
More variable
Rivers
Temporary
Pools