Floods and disturbances in aquatic communities Outcomes

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

7. Factors affecting species richness/diversity over large spatio-temporal scales • Time • Environment • favourability (mean conditions) • stability (variance in conditions, little fluctuation) e.g. stable climate, Abysal ocean depths • Disturbance (Grime and Connell): rainforests, various vegetation types, coral reefs • Spatial structural heterogeneity • Area • Productivity • Competition/Predation • (Further info: Chapter 8 of Giller: Community Structure and the Niche, available in library)

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.