Floods and disturbances in aquatic communities Outcomes H Discuss the practices that exacerbate floods, and flood control measures H Describe the effects.

Floods and disturbances in aquatic communities

OutcomesDiscuss the practices that exacerbate floods, and flood control measuresDescribe the effects of floods, particularly on aquatic communitiesUse ‘disturbance ecology’ as a theoretical framework to assess the extent and severity of a disturbance

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)

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.

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

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

Floods as disturbances in aquatic systemsFloods as disturbances in aquatic systems

Group assignments- 5 minsGroup 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 o’clock 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.

The disturbance regime is influenced by a number of The disturbance regime is influenced by a number of factorsfactors

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

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.

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 transferProvision of nursery areas for fishFertilisation of the floodplainCreation 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

Effects on biotaEffects 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

Recovery and resistance to floodsRecovery 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)

(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.

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):

DrainageAfforestation development (preparation, growth, harvesting)DamsChannelisationDredging and straightening of streamsFlood alleviation schemes.

Summary

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