Lecture 16 - (Freshwater Ecology) - Ecological impacts of altered river flows

How does natural river flow regimes vary?

• flow regimes vary year round

• low flow periods occur naturally - communities are adapted to this

  • river systems can recover quickly - but climate change / regulation can lead to long term change

What causes droughts to have varying impacts on rivers?

• duration and magnitude

• timing (season) - affects the organisms and their life cycles

• river type

• frequency

  • management practices

what are droughts classed as?

A ‘ramp disturbance’ = where effects get worse over time

What is ‘Summer low flow’

a period in summer when a river or stream reaches its lowest water level and discharge

• water isn’t moving fast downstream = lower spiral of nutrients

  • more point source pollutants will be less diluted

What are the phases of drought?

1.recession from riparian veg

2. isolation from riparian veg

3. loss of riffle habitat

   1. drying of pools

Define ponded reach as a type of drought

a section of river where water slows down and pools

define extreme low flows

when rivers discharge drop to unsual or critically low levels

Define isolated pools

patches of water left behind in the riverbed after flow has stopped between them

Define dry reach

a section of a river, where there is no surface water

What are the effects of reduced depth / wetted areas?

• increased competition over resources / territory

  • changes in habitat composition (ie less riffles, flow velocity)

How does reduced flow velocity affect organisms

reduces repiration, locomotion, drift, food availability

How does siltation of substrates affect organisms?

reduced living spaces, clogging of spawning gravels - but increased food supply for detrivores

how does concentration of nutrients / pollutants affect organisms?

toxic effects for fish, eutrophication, anoxia (low O2)

How does increased light penetration affect organims?

increases algal production

How are invertebrates adapted to drought periods?

• utilising dissolved oxygen - done by chironmidae (has haemoglobin type pigments)

• adapted gills / respiratory behaviour - done by mayflies

• Aestivation - hibernation but when under pressures from low flows - done by chronmidae (some go into the hypoeric zone)

  • increase in diatoms - positive response to light, nutrients

What are some impacts of droughts on fish?

• death - from lack of O2 / decay of plants

• eggs are sensitive to siltation

  • fish can swim away - looking for shaded pools

How do humans impact river regulation?

dams are built for:

• Public water supply

• irrigation

  • hydro elective power

What are the impacts of river regulation upstream?

• change from a riverine to a lacustrine environment (aka more lake like)

• high nutrient concentrations (leading to anoxia)

  • greatly impacts fish migration (often cannot travel past dams)

what are the impacts of river regulation on flow regime?

Example: Colorado river = has multiple dams on it = reducing the flow regime lower down.

• may wash out fish eggs / invertebrates

  • hydro peaking dams - release water based on demand - can be catastrophic on downstream biota

How does river regulation impact temperature regime?

• reduction in variation - typically lower in summer and higher in winter

  • however will depend on stratification pattern / outlet points on the dam

How does river regulation impact sediments?

sediment is held back by resevoir

• effects floodplain vegetation - silt is rich in nutrients essential for regeneration e.g eucaluptus forests

How does flow regulation impact water chemistry?

• low dissolved oxygen in river if released from hypolimnion (bottom layer of lake)

  • turbulent flow = oxygen levels rapidly restored

How does flow regime impact algae?

leads to increase in filamentous green algae

How does flow regulation umpact macrophytes?

• increased clarity / be stability

  • ‘rolling stones may gather no moss’ = reduced habitat availability

How does flow regulation impact floodplain vegetation

sediment is rich in nutrients = reduced silt means less floodplain vegetationH

How does flow regulation impact macroinvertebrates?

• reduction in diversity (from lower oxygen levels)

• periphyton growth + increased FPOM as food supply

What impacts does flow regulation have on fish?

• reduces spawning habitats available

• delays / prevention of fish migration to spawning grounds

• upper layers of resevoirs too warm if stratified

  • fish ladders can help - but many don’t make it

give an example showing how human flow regulation can impact rivers

Lake Kariba - Zimbabwe

• huge store of nutrients = led to invasive weed the water hyacinth

• biological controls / harvesting for use as animal feed to keep water hyacinth under control