Presentation on theme: "Floods and disturbances in aquatic communities"— Presentation transcript:
1 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
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)urbanisationoverpopulationchanges in magnitude and frequency (climate: Bangladesh)
3 Definitions and termsRiver 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 floodplainDuring 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 CausesINCREASE IN STREAM DISCHARGE (proximal cause)snow meltarterial drainage, waterway improvement both act to reduce time of rise to peak discharge.Increase in rainfallMajor features of floods:the flood hydrographrelationship between time and flood severityoverheadResponse time related tocatchment sizecatchment shapegradientvegetationsoil 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 phenomenonAny 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 biologicalIntermediate Disturbance Hypothesis
7 Group assignments- 5 mins ASSUMPTIONSImagine 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’TASKDesign 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 forceSeverity- a measure of the damage causedThe physiological and morphological characteristics of organismsFrequency 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 frequencyRegional frequencyDifferent types of rivers have different frequencies of flood
9 If a flood is very predictable, it may not be a disturbance! The disturbance regime- contdPredictability (mentioned in definition) results from three main processes:events that occur with a constant probability e.g. snow melt, winter rainpredictable cycles in climate or weather (storms, temperature, rain, El Nino, glacial periods)biological processes with predictable cycles e.g. production/ accumulation of biomasspredicatability 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 catchmentLarger 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 transferProvision of nursery areas for fishFertilisation of the floodplainCreation and maintenance of specialised habitatsCreation of patchiness (e.g. the Amazon lowland forest).Substrate and o.m. in riffle areas (shallow fast-flowing and steep) dislodgedsevere flooding scours the stream bed removing vegetation/animalsremoval of sediments to depths between 20cm and 2mIn pools and glides (slow and low slope) large amounts of sediment redeposited
11 Usually, recovery can take in the region of months Effects on biotaCase and netbuilding, locomotion, territoriality, respiration,Dislodgement and downstream transportMortalityImpact 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 resourcesUsually, recovery can take in the region of monthsCatastrophic floods:Yoshino, Japan (typhoon) 1959: 32g/ 0.25m2 ; g/ 0.25m2see overheads
12 Recovery and resistance to floods Adaptations to strong flowlife-history strategies (and timing of flood)recolonisation: fastest when areal extent small at the usual time of yearupstream areas, flight from downstream areasREFUGIAFlood 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 energyphysical complexity of the river system spreads the risk and avoids simultaneous catastrophes.
14 SummaryFloods 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.
15 Floods and disturbances in aquatic communities Outcomes Discuss 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