Floods and disturbances in aquatic communities

1

• Floods and disturbances in aquatic communities
• Outcomes
• Discuss the practices that exacerbate floods, and
  flood control measures
• Describe the effects of floods, particularly on
  aquatic communities
• Use disturbance ecology as a theoretical
  framework to assess the extent and severity of a
  disturbance

2
Floods as catastrophes

• Variations in stream flow lead to floods and
  droughts.
• Floods are natural earth processes, amongst,
  earthquakes, volcanoes, fires, drought etc.
• Why do we see an increase in flood incidence?
• Less mortality, but more damage due to
• changes in land use patterns (overconfidence in
  flood control)
• urbanisation
• overpopulation
• changes in magnitude and frequency (climate
  Bangladesh)

3

• Definitions and terms
• River discharge is simply a measure of the amount
  of water moving down a channel past a given point
  per unit time (m3 s-1). It is related to stream
  width, depth, current velocity and roughness of
  the substrate.
• Flood- when stream flow exceeds bankfull
  discharge and water spreads out onto adjacent
  land or floodplain
• During rainfall events, flood hydrographs are
  used to measure base flow, the time taken to
  reach peak discharge, and the time taken to
  achieve base flow when rainfall ceases.
• Floodplain- should not be considered as dry land
  that is damaged, but a natural extension of the
  riverbed that is less frequently used.

4

• Causes
• INCREASE IN STREAM DISCHARGE (proximal cause)
• snow melt
• arterial drainage, waterway improvement both act
  to reduce time of rise to peak discharge.
• Increase in rainfall
• Major features of floods
• the flood hydrograph
• relationship between time and flood severity
• overhead
• Response time related to
• catchment size
• catchment shape
• gradient
• vegetation
• soil and permeability

5
Floods as disturbances in aquatic systems

• What is a disturbance?
• A relatively discrete event that in time that is
  characterised by a frequency, severity and
  intensity that lies outside a predictable range
  for the system (Resh et al 1988)
• focuses on physical phenomenon
• Any relatively discrete event in time that
  through its frequency, severity or intensity lies
  outside a predictable range for the system, and
  that removes organisms and opens up space or
  other resources that can be utilised by
  individuals of the same or different species
  (Townsend 1989)
• focuses on physical and biological
• Intermediate Disturbance Hypothesis

6
(No Transcript)
7
Group assignments- 5 mins

• ASSUMPTIONS
• Imagine that you are an organism whose most
  benign environment is to drink 24 hours/day,
  7days /week.
• However, you experience a disturbance that
  prevents you from doing this- a lecture (which
  you have to attend)
• 3-5 oclock on Wednesdays and Fridays- happy
  hour
• TASK
• Design a high, medium and low disturbance regime
  for such an organism (use your imagination!)
• Specify the underlying processes that make a
  particular regime more or less harsh.

8
The disturbance regime is influenced by a number
of factors

• Magnitude. The intensity or strength of the
  disturbing force, it consists of two main
  components
• Intensity- a measure of the strength of the
  disturbing force
• Severity- a measure of the damage caused
• The physiological and morphological
  characteristics of organisms
• Frequency. Number of disturbances per unit
  time. Separate terms are used for the average
  frequency of disturbance at the local and the
  regional spatial scales.
• Random point frequency
• Regional frequency
• Different types of rivers have different
  frequencies of flood

9

• The disturbance regime- contd
• Predictability (mentioned in definition) results
  from three main processes
• events that occur with a constant probability
  e.g. snow melt, winter rain
• predictable cycles in climate or weather (storms,
  temperature, rain, El Nino, glacial periods)
• biological processes with predictable cycles e.g.
  production/ accumulation of biomass
• predicatability in space (upper vs lower river)
• If a flood is very predictable, it may not be a
  disturbance!
• Areal extent.
• The absolute and relative size of the disturbed
  area, and the shape of the disturbed area, have
  an important effect on recolonisation.
• E.g. hurricane vs thunderstorm, climate and
  catchment
• Larger scale disturbances are rarer in
  occurrence.

10
Effects of floods on physical system

• There are a number of beneficial aspects for the
  ecology of both the river and lowland system
  (Giller 1996)
• Nutrient and energy transfer
• Provision of nursery areas for fish
• Fertilisation of the floodplain
• Creation and maintenance of specialised habitats
• Creation of patchiness (e.g. the Amazon lowland
  forest).
• Substrate and o.m. in riffle areas (shallow
  fast-flowing and steep) dislodged
• severe flooding scours the stream bed removing
  vegetation/animals
• removal of sediments to depths between 20cm and
  2m
• In pools and glides (slow and low slope) large
  amounts of sediment redeposited

11
Effects on biota

• Case and netbuilding, locomotion, territoriality,
  respiration,
• Dislodgement and downstream transport
• Mortality
• Impact may be related to life stage
  (..predictability..)
• e.g. greatest impact on fish is on eggs (buried
  in substrate) or fry, and loss of habitat or food
  resources
• Usually, recovery can take in the region of
  months
• Catastrophic floods
• Yoshino, Japan (typhoon) 1959 32g/ 0.25m2
  1964 0.5g/ 0.25m2
• see overheads

12
Recovery and resistance to floods

• Adaptations to strong flow
• life-history strategies (and timing of flood)
• recolonisation fastest when areal extent small
  at the usual time of year
• upstream areas, flight from downstream areas
• REFUGIA
• Flood plain (especially for fish, in lowland
  floodplains)
• Hyporheic zone
• Flow refugia (see handout)

13

• (modern) Flood Alleviation
• Realises that physical simplification of river
  system makes catchment more vulnerable to flood
  (WHY?), and less likely to recover (WHY?)
• use the floodplain to dissipate energy
• physical complexity of the river system spreads
  the risk and avoids simultaneous catastrophes.

14
Summary

• Floods are natural phenomena that form part of
  the normal part of many aquatic systems. The
  impacts of floods will depend on several factors,
  the more important of these being the size of the
  disturbed area, the nature of the river system,
  and the timing, frequency and duration of the
  flood.
• Human activities are continuing to alter the rate
  at which river water enters stream channels, and
  these alterations become most apparent during
  heavy rainfall and normal flood events. These
  activities include (after Giller 1996)
• Drainage
• Afforestation development (preparation, growth,
  harvesting)
• Dams
• Channelisation
• Dredging and straightening of streams
• Flood alleviation schemes.

15

• Floods and disturbances in aquatic communities
• Outcomes
• Discuss the practices that exacerbate floods, and
  flood control measures
• Describe the effects of floods, particularly on
  aquatic communities
• Use disturbance ecology as a theoretical
  framework to assess the extent and severity of a
  disturbance