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Benthic Fauna
Benthic Fauna

Extremely diverse
group of organisms

Many different
groups, difficult to
generalize about
patterns
Distribution and Abundance

Limited by few
general
characteristics
 Food availability
 Type of substrate
 Few phys/chem
factors, especially
dissolved oxygen
Littoral vs. Profundal

Benthic animals living
in littoral region more
varied than those in
profundal region

Reflection of:
 1) abundance of
microhabitats
 2) less stressful
living conditions
Littoral Benthos
 Protozoans,
sponges,
coelenterates,
rotifers,
nematodes,
bryozoans,
decapods,
ostracods,
cladocerans,
copepods,
bivalves, snails,
insects, leeches
Littoral Benthos
 Microbenthos
-
very tiny
 Vastly
outnumber
macrobenthos, and
may contribute up
to 50% of benthic
production
Littoral Benthos
 Seldom
food
limited
 Proximity
to
phytoplankton,
macrophytes
Sublittoral Benthos
 Boundary
between
littoral & profundal
 Species diversity
drops off sharply
 Mussels,
ostracods,
copepods,
cladocerans from
littoral - few typical
dwellers
Profundal Benthos

Very poor diversity
 Oxygen limited

Other stressors:




Colder
Lower pH
Higher CO2, CH4,
organics, P, NH3
Few can survive under
these conditions
Profundal Benthos

Profundal benthos in
eutrophic lakes
resemble those of
grossly polluted
systems
 Low diversity,
monotony - great
number of individuals,
but only 1 or 2
species represented
Typical Profundal
Assemblage

Chironomus midge
larvae

Hemoglobin picks up
limited oxygen

May also be able to
use anaerobic
respiration & excrete
products of this
process
Typical Profundal
Assemblage

Oligochaete worms:
Tubifex, Limnodrilus
 Bury heads in organic
sediments, wave tails
with gills
 Can develop huge
populations
(10,000+/m2)
 Preyed on heavily by
predaceous
Chironomus
Typical Profundal
Assemblage
 Fingernail
clams in
genus Pisidium
 Become dormant
during anaerobic
periods
 May also inhabit
temporary ponds
Typical Profundal
Assemblage

Phantom midge
larvae - Chaoborus
 Temporary occupants
- spend day on
sediments, migrate
into water column at
night to prey on
zooplankton
 Not very tolerant of
anoxia
Typical Profundal
Assemblage

Also several
microscopic forms
that tolerate low
oxygen

Some protozoan
ciliates and
flagellates, some
nematodes
General Standing Crop
 Most
lakes have
profundal benthos
that averages ~5 g
wet weight/m2
 ~1/2
g dry
weight/m2
Less Productive Lakes
 Profundal
benthos
more diverse in
less productive
(oligotrophic) lakes
 Major
reason:
oxygenated
sediment-water
interface
Less Productive Lakes
 More
species of
midge larvae,
oligochaetes,
immature insects
like mayflies
 Hexagenia
burrowing mayfly
Less Productive Lakes
 Also
more
crustaceans like
the amphipod
Pontoporeia
 Makes
vertical
migrations up to
metalimnion at
night (doesn’t eat
zooplankton)
General Benthos Pattern
 Diverse
group in heterogenous,
oxygenated littoral zone
 Less diversity in more homogeneous
profundal zone (less in more productive
lakes)
Maxima of Abundance
Two maxima
- one in littoral
- one in profundal
depth
biomass
Maxima of Abundance
As systems become
more productive, zone of
max. production shifts
from littoral to profundal,
then declines in
depth
profundal
Midges replaced
by oligochaetes
biomass
Seasonal Abundance Patterns

Lowest in summer
(especially in insectdominated
communities)
 Emergence of adults,
high predation
 Maximum densities
and growth typically in
autumn and winter in
temperate zone
Predation by Fish

Predation can
drastically reduce
invertebrate standing
crop
 May be >50% of
populations in some
littoral areas
 Predation losses in
profundal areas
generally much lower
Predation by Fish

Despite intense
predation pressure,
benthos dynamics
and production mostly
controlled by food
supply