1. Factors Determining the Distribution of Freshwater Biota: 1.
ES3053 Freshwater Hydrobiology
Factors Determining the Distribution
of Freshwater Biota: 1.
Professor Nick Gray
Trinity Centre for the Environment
Trinity College Dublin
© Tigroney Press

2. Leaning objectives
To understand the physical and chemical factors affecting the distribution of animals and plants in freshwaters
To appreciate how these factors interact to produce unique habitats and ecosystems

3. Factors affecting distribution of freshwater animals
1. Natural dispersion 2. Biotic factors 3. Abiotic factors

4. 1. Natural Dispersion Do they naturally occur here?
Ireland’s diversity low compared to UK and rest of Europe
Only migratory fish colonized naturally (e.g. eels, salmon, sea trout)
Rest introduced by man (e.g. Pike 1500s; Roach 1889)

5. Exotic species Exotics can have serious effect on ecology and economy
New diseases and parasites
Increased predation and competition
Increased oxygen demand caused by some species
Elimination of indigenous species
Possible damage to fisheries, amenity, navigation

6. 2. Biotic Factors
Can be significant in determining whether a species occurs or not
Species precluded by:
Predation
Competition
Loss of suitable food reserves
Loss of habitat

7. 3. Abiotic Factors
Chemically water is never found in pure state in nature because it is an excellent solvent.
Water quality is a function of precipitation, weathering of rocks, soils, plants, in fact anything water comes into contact with.
Water quality unique to sub-catchment

10. Main determinants of minerals and nutrients found in freshwaters are:
Occurrence of highly soluble weathered minerals (e.g. halite, gypsum)
Distance from coast line
Precipitation: run-off ratio for rivers
Occurrence of peat bogs, wetlands and marshes which release DOM
Other factors such as ambient temperature, thickness of weathered rock, organic composition of soils etc.
Anthropogenic inputs can have major influence on water quality
Water quality depends on characterization of the hydrology, physico-chemical parameters and the biology.

11. Key abiotic factors in rivers
River flow and substrate
Dissolved oxygen and temperature
Suspended solids
Dissolved solids

12. Characteristics (Physical) of Rivers Downstream
Decreasing river slope
Increasing depth
Increasing volume of water
Variable mean flow velocity (ms-1)
Increasing discharge rate (m3s-1)
Decreasing turbulence

14. Current (velocity) is not uniform in a river, so a single measurement gives erroneous estimates of velocity

16. Discharge rate (Q)
The discharge rate equals the cross-sectional area of the river multiplied by the velocity of water through it
Q = AV m3s-1
Q: volume of water past a given point per unit time
A: cross-sectional area of the river, pipe, ditch
V: mean current (velocity) in ms-1

17. So V varies along sections with an overall tendency to increase
Mean current velocity (V ) ( ms-1) is influenced by river slope s mean flow depth d and resistance to flow by banks and substrate f.
Where g is the gravity constant 9.81
So mean current V increases where slope s is steeper, the flow depth d is greater and the resistance to flow f is less
So V varies along sections with an overall tendency to increase

18. Shear Stress (u)
V is not important to benthic animals or in the determination of substrate, these are controlled by local forces known as shear stress or shear velocity.
For an idealized channel where resistance to flow can be ignored µ can be calculated as:
ms-1
Where g is the gravitational constant (9.81), d flow depth (m) and s slope (m m-1).

19. Froude Number (Fr)
Essentially the ratio of inertial to gravitational forces which is a good indicator of hydraulic stress on benthic organisms.
Different to Reynolds Number, which is used to determine whether a flow will be laminar or turbulent.
Where V is mean flow velocity (ms-1), d depth (m) and g the gravitational constant (9.81 ms-1)

20. As the ratio increases the river becomes increasingly erosional.
Example: Compare river site (a) mean velocity 0.1 m s-1 and depth 0.9 m with site (b) mean velocity 0.6 m s-1 and depth 0.2 m.
River (a) Fr = 0.1/2.97 = (depositional)
River (b) Fr = 0.6/1.40 = (erosional)
Excellent for habitat description:
Pools <0.18 Riffles >0.4 Intermediate values classed as glides/runs

21. Fr is not a constant as both V and d vary.
Rainfall causes both V and d to increase.
Generally Fr falls due to d increasing more relative to V, protecting the biota from being scoured away.
Fr is the easiest assessment of shear stress in rivers.

22. Velocity: Depth Ratio (V:d)
Depth measured in metres and Velocity in m sec-1
Excellent for habitat description, more widely used than Froude Number.
Riffle V:D >3.20
Pools V:D <1.24
Intermediate values classed as glides/runs
Like Froud Number avoid measuring values V and d in marginal areas.
More information:
Jowett, I.G. (1993) A method for objectively identifying pool, run and riffle habitats from physical measurements. New Zealand Journal of Marine and Freshwater Research, 27, (2),

23. Rivers can be classified as erosional or depositional in terms of their substrate.
Erosion and deposition at river banks occur whenever there are bends in the channel creating source of solids.

24. So at any one point in the river there will be multiple habitats

25. Fluvial Geomorphology Course:
Yorkshire dales
Fluvial Geomorphology Course:

26. LIFE (Family)/ LIFE (Species)
Why is flow important?
Flow is a critical factor in species distribution, species success, and both community structure and stability.
Some species and families are strongly associated with flow regime
Changing weather patterns due to global warming is changing flow regimes
Increased flood events
Increased drought
Increased urbanization and water abstraction also leading to same effects
All effect riverine flow regimes and so ecology
LIFE (Family)/ LIFE (Species) 
Lotic-invertebrate Index for Flow Evaluation (LIFE) is a method for linking benthic macroinvertebrate data to prevailing flow regimes

27. As current velocity increases, so do LIFE scores.
A flow score (fs) is derived for each species/family (not both) using the species/family abundance and ecological association (flow group) with different flows
LIFE = Sum(fs)/n
where sum(fs) is the sum of the individual taxon flow scores for the whole sample, and n is the number of taxa used to calculate the sum(fs).
As current velocity increases, so do LIFE scores.
LIFE scores <6.00 generally indicate sluggish or still water conditions.  
LIFE scores >7.5 indicate very fast flows.
Ref: Extence, C.A., Balbi, D.M. and Chadd, R.P. (1999) River flow indexing using British benthic macroinvertebrates: A framework for setting hydro-ecological objectives. Regul. Rivers: Res. Mgmt., 15:
Abundance categories (number of individuals)
Flow group
A (1-9)
B (10-99)
C ( )
D/E (1000+)
I Rapid 9 10 11 12
II Moderate/fast 8
III Slow/sluggish 7
IV Flowing/standing 6 5 4 3
V Standing 2
VI Drought resistant 1

28. More information:
See Chapter 2 in Course Text