Download Lecture 28- River Continuum Concept

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•Intensive aquaculture can
produce yields that are
orders of magnitude
beyond natural
How to maximize energy flow to fish
Increased nutrient loading—fertilization + ammonia and anoxia tolerant species
Shortening the food chain—primary consumers (eg carps, tilapia or mullets)
Don’t rely on natural recruitment and managing the life cycle—stocking/hatcheries
Increasing consumption efficiency—small pens intensive feeding
Increased assimilation efficiency—feeding with easy to digest food pellets
Increased production efficiency—low activity species that don’t mind crowding,
, highly turbid water
Lepeophtheirus salmonis
Many aquaculture proponents
argue that aquaculture reduces
harvesting pressure on wild
Salmonid aquaculture not very trophically efficient,
food pellets made from by-catch of wild species
Major water quality issues—nutrient pollution
from cages, anti-fouling paint, antibiotics, habitat
Transmit diseases to wild salmonids—bacteria,
viruses, protozoans, fungi, “fish lice” –parasitic
copepods and other Crustacea
Genetic problems when domestic escapees
compete with or interbreed with wild fish
Summarizing concepts on Secondary production
•The organic matter produced by primary producers (NPP) is used by
a web of consumers
•NPP is used directly by primary consumers (herbivores and detritivores), which are in
turn consumed by carnivores.
•Measurement of 2o Production is done by estimating the rate of growth of individuals
and multiplying by the number of individuals per unit area in the cohort (age or size group).
•The efficiency of secondary production ranges from 5-20% (Avg 10%)
at each trophic level.
•Efficiency depends on several factors--palatability, digestibility, energy requirements
for feeding (activity costs)(eg homeotherms vs poikilotherms , other limiting factors
eg water, and nutrient quality of food.
•Trophic efficiency can be represented as the product of CE*AE*PE, each of which
is dependent on one or more of the above factors.
•The yields of many important fisheries depends on a combination of NPP, the
length ofthe food chain leading to the fish being harvested, and the efficiency
of each step.
•Many of the species that we harvest or very high in the food chain, so a great deal
of NPP is required to support them.
•Lakes have zonation structured by physical forces such as light, wind and waves.
•different zones in the lake had different types of plants and animals
•Zones in a river system are less
•But they are functionally very
The River Continuum Concept
Physical forces change gradually along a river
•Elevation ↓
•Slope ↓
•Temperature and nutrients ↑
•Drainage area and discharge ↑
•Width of channel and floodplain ↑
•Mean velocity ↑
•Mean depth ↑
•Turbidity ↑
•Sediments, erosional, alluvial, to depositional
•Shading ↓
•Periphyton, macrophytes ↑, then ↓
•Phytoplankton and zooplankton ↑
•Coarse detritus input highest upstream
•Fine detritus accumulates downstream
•Benthic invertebrate community changes
shredders, grazers, collectors
•Fish community changes
•Cold water to warm water species
Allochthonous input—Detritus processing
•Dead plant biomass breaks down slowly
and their nutrients can remain tied up in as
organic detritus for long periods of time
•Primary production in many ecosystems
depends more on its recycling rate ie mainly
decomposition of plant detritus, than on
loading rates
•Aquatic plants break down more rapidly
than terrestrial plants, and woody plants are
very slow to decompose because they
contain lignin, which most bacteria and fungi
can’t digest.
Leaf processing
•Wetting and breadown of cuticle
•Leaching of soluble components (DOM)
•Colonization by bacteria and fungi
•Increase in protein content
•Colonization by invertebrates
•Enhances microbial action
•Breakdown into small fragments
Invertebrate detritiivores find
leaves much more to their
liking after they have been
colonized by bacteria and
Detritus processing in a stream
Shredders enhance
microbial action
(bacteria & fungi)
•convert CPOM to FPOM
•Food for microdetritivores
Processing of FPOM by microdetritivores
Filter-feeders, deposit-feeders
Litter bag experiments have been used to study decomposition of detritus
•Nutrient content of the
detritus, especially N
greatly increases
decomposition rate,
•as does increased
•and mesh size
100 %
remaining %
Larger invertebrates get
into the litter bags if the
mesh is coarse
0.5 mm mesh
2 mm mesh
The interplay between the autochothonous and the allochtonous food chain
Allochthonous input
Autochthonous input
The River Discontinuum: Dams and wiers
Stream Fragmentation, A wier blocking fish movement
a hanging culvert can block fish movement
Dams/Reservoirs interrupt the river continuum
•create entirely new habitats