Ecological effects of extreme climatic events on riverineecosystems: insights from Australia

CATHERINE LEIGH*, ALEX BUSH† , EVAN T. HARRISON‡ , SUSIE S. HO§, LAURISSE LUKE¶,

ROBERT J. ROLLS¶ AND MARK E. LEDGER**

*Irstea, UR MALY, Villeurbanne, France†Department of Biological Sciences, Macquarie University, North Ryde, NSW, Australia‡Institute for Applied Ecology, University of Canberra, Bruce, ACT, Australia§School of Biological Sciences, Monash University, Clayton, Vic., Australia¶Australian Rivers Institute, Griffith University, Nathan, Qld, Australia

**School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham, U.K.

SUMMARY

1. Climate extremes and their physical impacts – including droughts, fires, floods, heat waves, storm

surges and tropical cyclones – are important structuring forces in riverine ecosystems. Climate

change is expected to increase the future occurrence of extremes, with potentially devastating effects

on rivers and streams. We synthesise knowledge of extremes and their impacts on riverine ecosys-

tems in Australia, a country for which projected changes in event characteristics reflect global trends.

2. Hydrologic extremes play a major structuring role in river ecology across Australia. Droughts alter

water quality and reduce habitat availability, driving organisms to refugia. Extreme floods increase

hydrological connectivity and trigger booms in productivity, but can also alter channel morphology

and cause disturbances such as hypoxic blackwater events.

3. Tropical cyclones and post-cyclonic floods damage riparian vegetation, erode stream banks and

alter water quality. Cyclone-induced delivery of large woody debris provides important instream

habitat, although the wider ecological consequences of tropical cyclones are uncertain.

4. Wildfires destroy catchment vegetation and expose soils, increasing inputs of fine sediment and

nutrients to streams, particularly when followed by heavy rains.

5. Research on the impacts of heat waves and storm surges is scarce, but data on temperature and

salinity tolerances, respectively, may provide some insight into ecological responses.

6. We identify research gaps and hypotheses to guide future research on the ecology of extreme

climate events in Australia and beyond. A range of phenomenological, experimental and modelling

approaches is needed to develop a mechanistic understanding of the ecological impact of extreme

events and inform prediction of responses to future change.

Keywords: biota, climate change, climate extremes, ecosystem functioning, river

Introduction

The frequency and intensity of extreme climatic events is

increasing (IPCC, 2012). Extreme events are statistically

rare or unusual weather or climatic occurrences, such as

extremes of precipitation or temperature, which can have

severe natural impacts on the environment, including

major floods, hydrologic droughts and fire (IPCC, 2012).

Shifts in the regimes and coincidence of extreme events

could alter the biodiversity and ecosystem functioning of

rivers (Paine, Tegner & Johnson, 1998; Jones, 2013).

Understanding the ecological effects of these events is

essential to predict ecological responses to future change.

In this study, we synthesise the knowledge of extremes

and their impact on riverine ecosystems in Australia, a

continent that has experienced multiple record-breaking

events in the 21st century (Steffen, Hughes & Karoly,

2013).

Correspondence: Catherine Leigh, Irstea, UR MALY, 5 rue de la Doua, CS70077, 69626 Villeurbanne Cedex, France.

E-mail: catherine.leigh@irstea.fr

© 2014 John Wiley & Sons Ltd 1

Freshwater Biology (2014) doi:10.1111/fwb.12515

Australia spans multiple climate zones (Fig. 1a–c) and

its rivers have a correspondingly wide range of flow

regimes, from predictably perennial to unpredictably

intermittent (Kennard et al., 2010). Although many spe-

cies in these systems are adapted to natural variability

in climate and river flow, the persistence of taxa, partic-

ularly those with restricted geographic and climatic

ranges, may be threatened by climate extremes (Hughes,

2011; Morrongiello et al., 2011). As the observed and

projected occurrence of extreme events in Australia

reflects that in many other regions of the world (IPCC,

2012), ecological responses in Australia can help under-

stand events elsewhere.

We focus on six types of extreme events identified by

the Australian Climate Commission as a threat to Aus-

tralian ecosystems (Steffen et al., 2013): heat waves and

hot days, fires, droughts, heavy rainfall and floods, trop-

ical cyclones, and storm surges and coastal flooding. We

examine their effects as reported in the literature, iden-

tify key knowledge gaps and propose hypotheses to

stimulate research.

Heat waves and hot days

Definitions, observations and projections

Heat waves, sometimes referred to as warm spells, are

periods of abnormally hot weather, but more precise

definitions vary regionally (IPCC, 2012; Perkins &

Alexander, 2013). In Australia, heat waves are defined

as at least three consecutive days of high maximum and

minimum temperatures that are unusual for a location

(BOM, 2014a). High temperature extremes also include

hot and very hot days (>35 °C and >40 °C, respectively)

and warm days and nights (temperatures > 90th percen-

tile for any 5-day window across the annual cycle)

(BOM, 2014b). The duration and frequency of heat

waves have increased in Australia since the 1970s, and

the hottest days of heat waves have become hotter (Per-

kins & Alexander, 2013). The warmest year on record

(2013) included a national summer heat wave c. 3 weeks

long (BOM, 2013). Climate projections indicate that the

duration of heat waves and the number of hot days will

continue to increase across Australia, consistent with

projected global trends (Alexander & Arblaster, 2009;

IPCC, 2012).

Ecological effects

The ecological effects of heat waves and hot days are

less well understood than those of hydrologic extremes.

However, long-term research on the European summer

heat wave of 2003 indicated these events can have

severe, lasting impacts on benthic assemblages (Mou-

thon & Daufresne, 2006). In Australia, the prospect of

unprecedented thermal extremes has renewed interest in

upper thermal limits (UTLs) of aquatic biota (e.g. Stew-

art et al., 2013a). UTLs are typically determined experi-

mentally using dynamic, non-lethal, behaviour-based

methods (e.g. critical thermal method) or static, lethal

methods (e.g. lethal temperature), with the former gen-

erally considered the better approximation to field con-

ditions. UTLs and the vulnerability of biota to heat

waves and hot days are likely to depend on a variety of

factors, including antecedent ambient temperature and

the rate and duration of warming in the environment,

but data are scarce (Beitinger, Bennett & McCauley,

2000; Dallas & Rivers-Moore, 2012).

Air temperatures in Australia regularly exceed the

UTLs of many aquatic taxa during the warmest months

of the year, although the periods of exceedance may

only span a few hours in a daily cycle (Davies et al.,

2004), and this may also be the case during heat waves.

Fully aquatic species and those with long aquatic life

stages may be less vulnerable to extreme air tempera-

tures than semi-aquatic species or those with extended

aerial life stages, particularly where water temperatures

remain cool, or at least below UTLs, due to evaporative

cooling (Bogan et al., 2006), riparian shading or ground-

water inputs. However, water temperatures that

approach or exceed UTLs during heat waves, particu-

larly those of extended duration or extreme intensity,

and in hydrologically disconnected, shallow or still

waters, could eliminate sensitive taxa (e.g. Matthews,

Surat & Hill, 1982). The projected intensification of heat

waves threatens the survival of many riverine species

(Fig. 2), and persistence may depend on adaptive man-

agement such as riparian revegetation and provision of

cool-water refuges (Robson et al., 2013).

Research to determine UTLs for stream invertebrates

has revealed marked contrasts in vulnerability to

extremes both within and among taxonomic groups. For

instance, UTLs of Ephemeroptera and Plecoptera

nymphs are generally lower than those of Odonata and

Coleoptera larvae (e.g. Stewart et al., 2013a), although

the mean UTL for each order varies substantially among

geographic regions (e.g. southern versus northern taxa;

Davies et al., 2004). Some Ephemeroptera from south-

east Queensland can survive water temperatures >30 °C

for at least 72 h (L. Luke, unpubl. data), which is well

above the global mean UTL for this group (c. 22 °C;

Davies et al., 2004; Stewart et al., 2013a).

© 2014 John Wiley & Sons Ltd, Freshwater Biology, doi: 10.1111/fwb.12515

2 C. Leigh et al.

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© 2014 John Wiley & Sons Ltd, Freshwater Biology, doi: 10.1111/fwb.12515

Extreme events and riverine ecosystems 3

Vulnerability to heat waves can be influenced by

behavioural or physiological responses to warming.

For example, high temperatures may influence the

dispersal of some species (e.g. Hassall & Thompson,

2008), limiting escape of intolerable conditions. Sublethal

effects could also alter sex ratios in surviving populations.

For instance, temperature extremes forecast for the year

2100 simulated in a mesocosm experiment increased

emergence rates of male Ulmerophlebia pipinna mayflies

more so than females, implying a change in the sex ratio

that could have demographic and fitness consequences

and cause local extinctions (Thompson et al., 2013a).

Knowledge gaps

Knowledge of thermal tolerance in aquatic organisms is

limited to coarse levels of taxonomic resolution, and less

is known about thermal tolerance among closely related

species or across populations (Chessman, 2009, 2013;

Stewart et al., 2013a). Organismal responses to extreme

temperature may be context dependent (Caissie, 2006;

Verberk & Bilton, 2013), varying among habitats, and

contingent upon the modifying influence of other

extreme events or stressors. For instance, thermal

impacts on biota may be exacerbated during drought

when instream temperatures are less buffered against

temperature changes in air (Davies et al., 2004; Fig. 2),

but data are scarce. More research is needed to identify

heat wave and hot day refugia for the most sensitive

taxa to direct management efforts, and there is a critical

absence of research specifically examining effects of heat

waves and hot days at the community and ecosystem

levels.

Droughts

Definitions, observations and projections

Hydrological drought is a period of below-typical flow

that is unusual in its duration, extent or intensity

(Humphries & Baldwin, 2003). Drought is seasonal in

many systems, but here, we focus solely on ‘supra-sea-

sonal’ events: unpredictable droughts extending beyond

seasonal periods, sometimes lasting >10 years (‘mega-

droughts’) (Lake, 2003; IPCC, 2012). Drought is a key

element of climatic and hydrological variability in Aus-

tralia. The Millennium Drought in south-eastern Austra-

lia was one of the most severe hydrological and

meteorological mega-droughts on record, lasting

between 1997 and 2009 (Gallant & Gergis, 2011; Timbal

& Fawcett, 2012; Fig. 1g). Mega-drought has also

affected the south-western region of Western Australia,

which has been in drought since the mid-1970s, with

rainfall 10–20% below long-term averages (NCC, 2010).

Climate projections indicate southern Australia will

Fig. 1 Australia has a range of climate zones (a), with regional variation in spatiotemporal patterns of temperature and rainfall (b, c).

Examples of extreme events include Cyclone Larry in March 2006 (d: aerial image of Upper North Johnstone River, post-event, showing

treefall and leaf denudation in the riparian zone; credit S. Turton), flood across much of south-east QLD in January 2011 (e: Gregors Creek,

post-event, during which flood waters reached the mature treeline above river banks; credit L. Luke), the 2003 wildfires in the ACT

(f: aerial image of Cotter River, post-event, showing burned vegetation across the catchment; credit T. Nelson), and the Millennium Drought

(g: a drying waterbody in the Ovens River catchment, 2009; credit S.S. Ho). Climate-zone mapping data and summary statistics sourced

from www.bom.gov.au, with statistics calculated for the reference period 1 January 1961 to 31 December 1990. ACT, Australian Capital

Territory; NSW, New South Wales; NT, Northern Territory; QLD, Queensland; SA, South Australia; TAS, Tasmania; VIC, Victoria; WA,

Western Australia.

Heat waves & hot days

Δ Emergence rates of aqua�c

macroinvertebrates

Disrup�on or ins�ga�on of spawning or

dispersal cues in some biota

Δ Aqua�c community composi�on & ↑ threat to species’ persistence

Death of some biota

Droughts(see Fig. 3)

↑ Air & water temperature

↑ Risk that upper thermal limits of biota (including

aqua�c, riparian & terrestrial taxa and life stages) are

exceeded

↑ Risk of sub -lethal effects of

hot extremes on biota

Δ Reproduc�on & recruitment

Fig. 2 Effects of heat waves and hot days on rivers, as based on

links among drivers and responses identified in the Australian lit-

erature (see text for detail). Responses may be affected by the inten-

sity, duration and/or timing of heat waves and hot days. Solid

arrows show direct links, broken arrows where one event often

coincides with or increases the likelihood or intensity of the linked

event. Ovals show where effects are hypothesised (event-specific

evidence scarce in the Australian literature). D, change in.

© 2014 John Wiley & Sons Ltd, Freshwater Biology, doi: 10.1111/fwb.12515

4 C. Leigh et al.

experience future droughts of increased duration and

intensity (Risbey, 2011; Steffen et al., 2013). Similar

changes may also occur in parts of Europe, the Mediter-

ranean, the Americas and Africa (IPCC, 2012, 2014).

Ecological effects

Supra-seasonal river droughts are ‘ramp’ disturbances,

increasing in strength and spatial extent over time (Lake,

2003). In the initial stages of drought, riparian habitats

disconnect from channels and littoral habitat dries (Boul-

ton, 2003). Lateral and longitudinal hydrological connec-

tivity is reduced or lost as flow declines, creating

isolated pools (Fig. 3). In downstream river sections and

estuaries, reduced flow can limit the delivery of nutri-

ents from upstream, alter productivity and trophic struc-

ture and increase sediment accumulation (Lake, 2011).

Once surface flows cease, many rivers enter a lentic

phase associated with major changes in water physico-

chemistry (e.g. Sheldon & Fellows, 2010). Pools recede,

and surface water can disappear completely (Fig. 1g).

Declining groundwater levels and increasing salinity in

groundwater-dependent systems and floodplains can

lead to loss of riparian and floodplain vegetation (e.g.

Murray et al., 2003; Fig. 3). In floodplain forests of the

Murray–Darling Basin (MDB), stands of drought-tolerant

Eucalyptus camaldulensis suffered increased mortality

during the Millennium Drought (Horner et al., 2009).

Similarly, drought-related low flows have increased

salinity levels in the lower lakes of the MDB (Mosley

et al., 2012) and contributed to the loss of vulnerable fish

populations (Nannoperca obscura; Wedderburn, Hammer

& Bice, 2012). Ultimately, there is a shift from aquatic to

terrestrial habitat as organisms from surrounding ripar-

ian areas move into dry channels (Steward et al., 2012).

Biotic responses to drought vary depending on many

factors, but in general, organisms that inhabit non-peren-

nial systems have adaptive traits of resistance or resil-

ience to dry periods and may be more likely to survive

supra-seasonal drought than those from perennial sys-

tems (Lake, 2003; Rolls, Leigh & Sheldon, 2012; Fig. 3).

Fish species that tolerated the Millennium Drought in

the MDB tended to have traits associated with adapta-

tion to warm environments (e.g. high spawning temper-

atures), the ability to switch diets and life-history

strategies including delayed maturation, short spawning

seasons and high fecundity (Chessman, 2013). The Mil-

lennium Drought also altered macroinvertebrate com-

munities in Victorian streams, with conditions favouring

taxa tolerant of low flow and poor water quality (Rose,

Metzeling & Catzikiris, 2008; Thomson et al., 2012). Des-

iccation-resistant life-history stages also enable some

taxa (e.g. zooplankton and aquatic plant species, Brock

et al., 2003) to survive drought, although survival can

depend on drought duration. Cladoceran egg banks

from Australian dry land rivers became depleted when

drought lasted more than 6 years, which likely slowed

recovery of microinvertebrate communities and altered

post-drought food webs (Jenkins & Boulton, 2007).

Drought survival depends on the type, quality and

quantity of refugia and the hydrological history and con-

nectivity of habitats (Robson, Chester & Austin, 2011;

Bogan, Boersma & Lytle, 2014). The temporal availability

of refugia and their position in river networks influence

macroinvertebrate community resistance and resilience

to drought (Marshall et al., 2006; Sheldon et al., 2010;

Chester & Robson, 2011; Fig. 3). The rate of water

decline may also affect the ability of some organisms to

find refugia and influence the availability of basal

resources for riverine food webs (Bunn et al., 2006).

↑ Buildup of organic & non-organic material in

dry channels & disconnected pools Heavy rainfall

& floods(post-drought)

(see Fig. 5)

Droughts

↓ Connec�vity of aqua�c habitats

↓Flow & flow permanence

↓ Size &/or availability of

aqua�c habitats & refugia

Groundwater decline

↑ Risk of hypoxic blackwater

events

↑ Risk of fish kills & threat

to other aqua�c taxa

↓ or Δ water quality

↑ Mortality of riparian & floodplain vegeta�on

Δ Community composi�ons (↓sensi�ve & flow-

dependent, ↑ tolerant biota & terrestrial biota)

↓ Influx of nutrients from

upstream to downstream

reaches

Fig. 3 Effects of droughts on rivers, as based on links among driv-

ers and responses identified in the Australian literature (see text for

detail). Responses may be affected by the duration, extent and/or

intensity of droughts. For example, build-up of material on dry

channels and in pools and the risk of hypoxic blackwater events

increase with duration and extent; disconnection among habitats

increases and the size and availability of habitats decline with

increased duration, intensity and extent; survival of aquatic taxa

declines with increased duration. See Fig. 2, for explanation of

arrows, shapes and symbols.

© 2014 John Wiley & Sons Ltd, Freshwater Biology, doi: 10.1111/fwb.12515

Extreme events and riverine ecosystems 5

Knowledge gaps

Although the long-term (decadal) consequences of

supra-seasonal drought are still poorly understood

(Bond, Lake & Arthington, 2008), recent research sug-

gests prolonged events can elicit novel community tra-

jectories and extirpate sensitive species (Bogan & Lytle,

2011; Bogan et al., 2014). However, research is frequently

opportunistic, sporadic and short term (e.g. <1–2 years),

and there is a pressing need for comprehensive monitor-

ing programmes to assess drought impacts and recovery

processes systematically across regions and taxa (Rose

et al., 2008; Lake, 2011; Thomson et al., 2012). Research

typically focuses on macroinvertebrates, fish and ripar-

ian vegetation, with less emphasis on other groups such

as microbes and microinvertebrates. While drought

impacts on the structure of some key assemblages are

well described, consequences for ecosystem processes

and higher levels of biological organisation are less well

known (e.g. Ledger et al., 2013; Woodward et al., 2012).

Incorporating species resistance and resilience traits

into biomonitoring approaches will enhance mechanistic

and predictive understanding of drought impacts (e.g.

Sheldon, Marsh & Rolls, 2012; Chessman, 2013). In many

cases, however, trait information is incomplete or held

in separate repositories and literatures (Crook et al.,

2010). We also need to know how traits interact and bet-

ter understand relationships between traits and biotic

interactions (Verberk, Van Noordwijk & Hildrew, 2013).

For example, a trait such as air breathing that aids sur-

vival of anoxic conditions during drought may increase

predation risk where movement to the water surface

increases encounter rates (Robson et al., 2011). Water-

quality guidelines specific to drought periods are also

required to manage and protect surface and subsurface

drought refugia, thereby supporting species persistence

(e.g. Sheldon & Fellows, 2010).

Fires

Definitions, observations and projections

Much of Australia is vulnerable to wildfire due to the

dry, hot weather and volatile compounds in many of the

native plant species. In the north, the fires generally

occur in winter and spring (the dry season in northern

Australia), whereas in the south, they generally occur in

summer and autumn (BOM, 2009). Hot, dry and windy

conditions increase fire danger (Clarke, Smith & Pitman,

2011), linking heat waves, drought and strong winds to

fire. Since the 1970s, extreme fire weather has increased,

particularly in the south-east, and projections indicate

this trend will continue, with the annual fire season

starting earlier and lasting longer (Lucas et al., 2007;

CSIRO & BOM, 2014). Projections are similar for New

Zealand and increased frequency of fire risk is projected

for much of the Americas (IPCC, 2012).

Ecological effects

One of the most significant and commonly reported

effects of wildfire is the large-scale destruction of

terrestrial vegetation (Fig. 1f). In severely burned and

devegetated catchments, exposed soils become hydro-

phobic, increasing run-off and associated inputs of fine

sediment and nutrients (e.g. Lane, Sheridan & Noske,

2006; Fig. 4). For instance, severe fires in Victoria sub-

stantially elevated stream phosphorus concentrations

and conductivity (Chessman, 1986). Similarly, severe

burning in experimental forests in New South Wales

increased stream nitrate concentrations by an order of

Fires

Destruc�on of catchment &

riparian vegeta�on

Erosion

Fine sediment input

Nutrient input

Burned vegeta�on input

Habitat smothered

Heavy rainfall & floods

(post-fire)(see Fig. 5)

Water quality

Abundance & mortality of

sensi�ve species

Periphytoncover

Composi�on of func�onal feeding

groups of consumers

Aqua�c community composi�on ( sensi�ve,

tolerant species)

Droughts(see Fig. 3)

Heat waves & hot days

(see Fig. 2)

Habitat for riparian terrestrial

fauna & bio�c community

composi�on

Shading

Organic ma�er quality &/or

input

↑

↑ ↑

↑

↑

↓

↓

↓

↓↓Δ Δ

Δ

Δ

↓

Fig. 4 Effects of fires on rivers, as based on links among drivers

and responses identified in the Australian literature (see text for

detail). Responses may be affected by the intensity and/or extent

of fires, for example by increasing the extent and quantity of vege-

tation destroyed or reducing the survival or abundance of species.

See Fig. 2, for explanation of arrows, shapes and symbols.

© 2014 John Wiley & Sons Ltd, Freshwater Biology, doi: 10.1111/fwb.12515

6 C. Leigh et al.

magnitude (Mackay & Robinson, 1987). Changes to

riparian zones may also affect incident light and alter

temperature regimes.

The ecosystem consequences of fire can be exacer-

bated by heavy rains (Cooper et al., 2014), with the com-

bined, interactive impacts of these two extremes creating

a compound event (sensu IPCC, 2012) characterised by

intense sedimentation and turbidity (Fig. 4). In the Cot-

ter River catchment of the Australian Capital Territory, a

large storm 1 month after severe wildfire raised stream

turbidity to an unprecedented level of 3000 NTU (White

et al., 2006). By contrast, fires not followed by heavy rain

tend to have less impact on rivers, as was the case in

Australia’s wet–dry tropics, where fires occurring early

in the dry season had little effect on water quality

(Townsend & Douglas, 2000).

Declines in abundances and richness of instream

fauna are often observed following wildfire (Fig. 4). For

instance, macroinvertebrate communities in the most

severely burned areas of the Cotter River catchment

were impoverished compared with those in unburned

areas (T. Nelson, unpubl. data). However, the scale of

the effects depends on the intensity of fire and post-fire

rainfall (Papas, 1998). A range of conditions in post-fire

streams, including heavy sedimentation and poor water

quality (e.g. dissolved oxygen sags, chemicals from

burned material), has also been associated with reduced

survival and abundance of fish (Carey et al., 2003; Lyon

& O’Connor, 2008).

Fires also affect the availability and quality of trophic

resources. In south-eastern Australian streams, sodium

concentrations increased in burned Eucalyptus viminalis

leaves during post-fire decomposition, and changes in

microbial activity ultimately reduced the amount of bio-

available carbon (P. Love, unpubl. data). In turn, fire-

induced changes in the quality and quantity of trophic

resources can alter the composition of functional feeding

groups (Verkaik et al., 2014; Fig. 4). For example, sub-

stantial inputs of fine suspended sediment after fire in

the Cotter River catchment scoured and smothered

periphyton, reducing the occurrence of herbivorous

scrapers (Peat, Chester & Norris, 2005; T. Nelson, un-

publ. data).

Knowledge gaps

The effects of fire can be immediate and also extend to

the short term (<1 year), mid term (1–10 years) or long

term (>10 years) (Minshall, Brock & Varley, 1989). Least

is known about the long-term ecological consequences of

fire, although there is also some uncertainty regarding

short-term impacts, which may be underestimated when

post-fire sampling is delayed (Verkaik et al., 2014).

Research on the influence of land use on post-fire recov-

ery is rare, even though this likely determines recovery

rates. In the Lower Cotter River catchment, for example,

stream turbidity recovered more quickly in naturally

regenerating, native forest than in plantation pine forest

(Harrison et al., 2014). Research on interactions between

fire and other extreme events such as drought and flood

is also needed. The trajectory of community recovery

from fire during drought periods may be altered by

burning of dewatered benthic sediments (Cowell, Mat-

thews & Lind, 2006) or ash physicochemistry (Verkaik

et al., 2014), but our understanding of such interactions

is incomplete.

Heavy rainfall and floods

Definitions, observations and projections

Heavy rainfall events are periods of prolonged and/or

intense rainfall within a specific location relative to

recorded averages and are often defined by expected

return intervals over decades or centuries (Steffen et al.,

2013). The events are often associated with inland flood-

ing because surface water accumulations and run-off

can rapidly inundate dry lands and floodplains. Austra-

lian indices used to describe these events include heavy

and very heavy precipitation days (≥10 and 30 mm,

respectively) and very wet and extremely wet days

(annual total precipitation when daily precipitation

>95th and 99th percentiles, respectively) (BOM, 2014b).

Several of these extremes occurred across eastern Aus-

tralia in 2010–2012, breaking the Millennium Drought.

Severe flooding occurred in central and southern

Queensland in 2010–2011, during a period including the

state’s wettest December on record (NCC BOM, 2011;

Fig. 1e). Both central and northern Australia experienced

the wettest dry season on record in 2010 and record

heavy rainfall and flooding occurred in most states and

territories in 2010, 2011 and 2012 (NCC BOM, 2011,

2012; BOM, 2012a). Heavy rainfall intensity is expected

to increase in Australia in the 21st century, whereas the

number of events may increase, decrease or remain

unchanged depending on region (Alexander & Arblast-

er, 2009), matching projections elsewhere (IPCC, 2012;

Donat et al., 2013). Such changes in rainfall patterns will

likely affect flood regimes, altering the intensity, dura-

tion, frequency and timing of events.

© 2014 John Wiley & Sons Ltd, Freshwater Biology, doi: 10.1111/fwb.12515

Extreme events and riverine ecosystems 7

Ecological effects

The ecological importance of floods in riverine ecosys-

tems is well documented, both within Australia and

internationally. Floods influence ecosystem processes,

affecting within-channel productivity, particulate organic

matter retention, decomposition, nutrient transforma-

tions and microbial processes (Baldwin & Mitchell, 2000;

Hladyz et al., 2012). Floods also affect terrestrial flood-

plain biota (Horrocks et al., 2012) and influence surface

water and groundwater quality and recharge (Cend�on

et al., 2010) (Fig. 5). Rising waters increase hydrological

connectivity at various spatial scales, particularly in arid,

semi-arid and wet–dry zones (Douglas, Bunn & Davies,

2005; Leigh et al., 2010). This facilitates transport of

materials, such as propagules, nutrients, organic matter

and woody debris, both downstream and to adjacent

floodplains and riparian zones (Ballinger & Lake, 2006;

Erskine et al., 2012; Fig. 5). Floods may also cause

siltation and ‘sand slugs’, alter channel morphology,

scour substrata, remove in-situ basal resources such as

macrophytes and detritus and translocate, injure or kill

biota (Lake, 2000; Fig. 5).

Much research has focused on flood-driven hydrologi-

cal connectivity and food webs. Floods drive spatial sub-

sidies in food resources and affect the relative

importance of autochthonous and allochthonous carbon

sources (Bunn et al., 2006; Reid et al., 2008). In some

cases, mobile consumers such as fish and birds may

transfer resources between habitats connected by inun-

dation (Jardine et al., 2012). River–floodplain connections

can also facilitate ‘boom-bust’ cycles in basal productiv-

ity and invertebrate emergence, with flow-on effects to

other components of the food web (Boulton & Lloyd,

1992; Bunn et al., 2006; Fig. 5). The floods of 2010–2011

increased taxonomic richness and abundance of zoo-

plankton in several southern-MDB rivers (Ning et al.,

2013). In central Australia, post-flood booms in fish pro-

duction subsequently fuelled high levels of heterotrophic

bacterial production once waters (and fish) retracted to

isolated waterholes (Burford et al., 2008).

Floods also alter water quality. Hypoxic blackwater

events can result from the microbial breakdown of

deposited or leached organic matter, although their

intensity and extent are often exacerbated by pre-flood

drought and human activities (Whitworth, Baldwin &

Kerr, 2012; Figs 4 & 5). As the Millenium Drought was

breaking in late 2010, one such event killed many native

crustaceans and fish and led to the emergence of native

crayfish from affected waters (King, Tonkin & Lieshcke,

2012; McCarthy et al., 2014). However, non-native fish

such as Cyprinus carpio were apparently unaffected.

Floods can facilitate the spread of non-native and

invasive species. Widespread flooding has probably

facilitated the spread of Phyla canescens, an invasive

riparian plant, across eastern Australia (Murray, Stokes

& Van Klinken, 2011; Fig. 5). By contrast, remnant popu-

lations of native freshwater mussels are threatened by

extreme floods, particularly where events exacerbate

existing impacts of catchment degradation (Jones & By-

rne, 2010). However, accumulation of large woody deb-

ris during flooding can facilitate post-flood recovery of

native riparian vegetation (e.g. in Australia’s semi-arid

savannah; Pettit & Naiman, 2005) and, in some cases,

floods favour native over non-native species (Costelloe

et al., 2010; Ho, Bond & Thompson, 2013). In south-east-

ern Australia, flooding increased species richness of

native plants in E. camaldulensis stands, with twice as

Heavy rainfall & floods

Catchment & channel erosion

Movement of large amounts of

organic & inorganic ma�er (including biota)

Booms in produc�on & trophic interac�ons

Spread of (e.g. invasive)

species

Groundwater recharge

Water quality

Watering of riparian & floodplain vegeta�on

Connec�vity of aqua�c habitats

Triggers dispersal,

spawning & recruitment

opportuni�esDroughts

(pre-flood)(see Fig. 3)

Tropical cyclones

(see Fig. 6)

Risk of hypoxic blackwater

events

& threat to other aqua�c taxa

Community composi�on ( sensi�ve, tolerant biota)

↑ ↑

↑↑

↑ Risk of fish kills

↑↓Δ

Δ

Δ

Fig. 5 Effects of heavy rainfall and floods on rivers, as based on

links among drivers and responses identified in the Australian lit-

erature (see text for detail). Responses may be affected by the inten-

sity, timing and/or extent of these events. For example, the extent

of habitat connectivity and amount of channel erosion and scouring

increase as these events increase in intensity and extent; timing of

these events relative to drought will affect the risk of hypoxic

blackwater events; timing and intensity of events relative to life-his-

tory stages and age of biota may affect the chance of translocation,

injury or death. See Fig. 2, for explanation of arrows, shapes and

symbols.

© 2014 John Wiley & Sons Ltd, Freshwater Biology, doi: 10.1111/fwb.12515

8 C. Leigh et al.

many natives found in flooded as unflooded areas

(Horner et al., 2012).

Knowledge gaps

Floods that are aseasonal, recurrent or interact with

other extreme events (e.g. fire and drought) or land-use

degradation, can markedly affect river biodiversity.

However, explicit studies of such phenomena are scarce.

The timing of heavy rainfall and extreme floods relative

to key stages of aquatic life cycles may influence recruit-

ment, dispersal and foraging opportunities (e.g. in fish,

Humphries, King & Koehn, 1999), and shifts in flood fre-

quency and intensity may affect the persistence or

spread of invasive species, for example by providing

opportunities for passive or active dispersal within and

between river networks. However, more research is

needed to determine the vulnerability and dispersibility

of invasive species during floods. Impacts of extreme

floods on genetic diversity of threatened populations are

also uncertain, although it has been proposed that more

large-scale floods could increase gene flow, while more

intense floods could create population bottlenecks (Wor-

thington Wilmer et al., 2011). Research on ecological out-

comes of environmental flows featuring managed

flooding may help us understand impacts of future flood

regimes, but existing data are biased towards particular

taxa (e.g. fish and floodplain vegetation) and systems

(e.g. MDB rivers; Aldous et al., 2011). We require more

rigorous, high-resolution hydrological models (Pittock

et al., 2006), and these must be better integrated with

ecological models to better predict relationships between

changing rainfall, future flood regimes and ecological

responses.

Tropical cyclones

Definitions, observations and projections

Tropical cyclones are low-pressure systems that develop

over warm, tropical marine-waters and are associated

with gale-force winds (sustained at ≥63 km h�1 or in

gusts >90 km h�1) (BOM, 2014c). Winds sustained at

≥118 km h�1 (or gusts >165 km h�1) qualify events as

severe tropical cyclones (STCs), also known as hurri-

canes or typhoons. Well-documented STCs of the last

50 years in Australia include Tracy, Larry (Fig. 1d),

Monica and Yasi (BOM, 2014d). STC Monica was associ-

ated with maximum recorded winds of 130 km h�1 and

rainfall of up to 340 mm in 24 h. Although no clear

trends in tropical cyclone characteristics have been

found in the Australian region, projections for tropical

zones in Australia and beyond suggest they may become

less frequent but more intense (CSIRO & BOM, 2012;

IPCC, 2012). The proportion of severe events is therefore

likely to increase.

Ecological effects

As with fire, one of the most commonly reported effects

of tropical cyclones is vegetation damage, with riparian

zone vegetation being particularly susceptible (Fig. 6).

In the forests of Magela and Ngarradj Creek catch-

ments, Northern Territory, wind gusts during STC

Monica created significant treefall and riparian zones

sustained greater damage than did other areas (Saynor

& Erskine, 2008; Staben & Evans, 2008). In some parts

of the Ngarradj Creek catchment, the entire riparian

rainforest was destroyed. STC Monica occurred at the

end of the tropical wet season when riparian zone soils

were saturated and streams inundated, so these factors

probably exacerbated the damage (Saynor & Erskine,

2008; Fig. 6).

Riparian vegetation was also damaged during STC

Larry (Fig. 1d). Six months after that event, pioneer spe-

cies in north-east Queensland rainforests were more

severely damaged than other species. They suffered dis-

proportionately more uprooting, particularly along

stream edges (Pohlman, Goosem & Turton, 2008). This

was associated with sandy soils near streams, producing

loose root systems, and bank erosion during post-cyclo-

nic floods. However, while damage to riparian vegeta-

tion can be substantial, the associated delivery of large

woody debris to rivers contributes important habitat

and increases channel complexity (Erskine et al., 2012;

Fig. 6). After STC Monica in the Ngarradj Creek catch-

ment, the number of pieces of large wood per metre of

channel more than doubled (Erskine et al., 2012).

Tropical cyclones producing heavy rainfall change

water quality in rivers (Fig. 6). Rainfall following Tropi-

cal Cyclone Celeste in north-east Queensland (January

1996) rapidly increased water level and discharge in the

Burdekin River (Alexander et al., 2001). River water

became highly turbid, with suspended solids concentra-

tions >4000 mg L�1. Conductivity decreased rapidly but

recovered to pre-event conditions within 1–2 weeks. In

contrast, increased suspended solids concentrations were

observed up to a year following STC Monica (Evans &

Moliere, 2010). Differences in the effects of Celeste and

Monica on water quality were likely influenced by dif-

ferences in their timing relative to the first, flushing

flows of the wet season (Alexander et al., 2001), their

© 2014 John Wiley & Sons Ltd, Freshwater Biology, doi: 10.1111/fwb.12515

Extreme events and riverine ecosystems 9

intensity (e.g. erosive forces on soils and sediments) and

the amount of damage caused to riparian vegetation

(Fig. 6).

Recharge of water stores in northern Australia and

aquifers in central Australia relies heavily on tropical

cyclones, and thus recharge can be sporadic depending

on event timing, frequency and intensity (Barron et al.,

2011; McGrath et al., 2012; Fig. 6). Past long-term

declines in tropical cyclone frequency are associated

with ‘water storage droughts’ and desertification (Nott,

2011; McGrath et al., 2012). Tropical cyclones in northern

Australia also deliver rains to more southerly regions as

they degenerate to extropical lows during their overland

trajectories. These rains supply water that produces

flows and sustains biota in temporary wetlands of dry-

land and semi-arid river systems (Roshier et al., 2001)

and in catchments that discharge into the Great Barrier

Reef lagoon (Boer, 2010).

Knowledge gaps

Much of our knowledge of tropical cyclone effects is

drawn from vegetation studies. We know little about

how they affect instream biota (e.g. fish, invertebrates;

although see Mallin et al., 1999) and ecosystem pro-

cesses, either directly or via indirect effects on riparian

zones and water quality (Fig. 6). Although we have

some evidence suggesting cyclone (and flood) timing

affects ecological responses (e.g. Alexander et al., 2001;

Saynor & Erskine, 2008), our understanding of this

aspect and cyclone–flood interactions is quite limited.

Moreover, the lack of studies combining pre- and

post-event data limits understanding of impacts and

recovery.

Storm surges and coastal flooding

Definitions, observations and projections

Coastal flooding results from high water levels pro-

duced by sea-level fluctuations during short-term,

localised storm-surge and wave events, and longer term

sea-level rise (Eliot, 2012). The primary cause of surge-

related coastal flooding is the landfall of low-pressure

systems with strong winds (CSIRO & BOM, 2007).

More intense events occur when storm surge coincides

with high tides, and events become more frequent as

sea level rises (S�anchez-Arcilla et al., 2008; Fig. 7).

Surge-related coastal flooding can occur anywhere

topography allows (CSIRO & BOM, 2007; BOM, 2012b).

However, projected changes in storm-surge and inun-

dation impacts vary regionally and can depend on the

method by which storm-tide intervals are estimated

(CSIRO & BOM, 2007). It is very likely that mean sea-

level rise will continue to contribute to global upward

trends in extreme coastal high water levels (IPCC,

2012).

Heavy rainfall & floods(see Fig. 5)

Damage to catchment &

especially riparian vegeta�on

Groundwater recharge

Intensity of winds &/orwind gusts

Erosion

Fine sediment, organic ma�er &

nutrient input

Water quality

Tropical cyclones

Delivery of large woody

debris to streams

Storm surges & coastal flooding

(see Fig. 7)

Sandy riparian

soils

Satura�on of riparian

soils

Intensity & extent of stream

& floodplain inunda�on

Aqua�c community composi�on

Instream habitat for aqua�c biota &

channel complexity

Diversity of fish & macroinvertebrates

Shading

Wetland replenishment

↑

↑

↑

↑

↑

↑

↑

↑

↓

Δ

Δ

↑

↑

Fig. 6 Effects of tropical cyclones on rivers, as based on links

among drivers and responses identified in the Australian literature

(see text for detail). Responses may be affected by the intensity,

timing and/or frequency of these events. For example, saturated

soils are more likely during wet-season events; events before the

wet season’s first flushing flows increase inputs of sediment,

organic matter and nutrients to streams and the time taken to

return to pre-event conditions; the timing and frequency of events

affect the sporadicity of groundwater recharge. See Fig. 2, for

explanation of arrows, shapes and symbols.

© 2014 John Wiley & Sons Ltd, Freshwater Biology, doi: 10.1111/fwb.12515

10 C. Leigh et al.

Ecological effects

Storm surges alter habitats by pushing sea water into

estuaries, brackish lagoons and wetlands, which in turn

can redistribute sediment, contribute to erosion (Cahoon,

2006; Howes et al., 2010) and force saline water into fresh-

water river sections (CSIRO & Bureau of Meteorology

(BOM), 2007; Kang & King, 2012; Fig. 7). Surge-related

effects of increased salinity and geomorphological

changes on riverine biota (e.g. aquatic macroinverte-

brates) are, however, understudied (Kang & King, 2012),

and most research has been performed outside Australia,

primarily on aquatic macrophytes (e.g. Frazer et al., 2006).

Knowledge gaps

Effects of storm surge and coastal flooding on riverine

ecosystems are poorly known in Australia and globally.

Studies from North America suggest aquatic vegetation

in coastal fresh waters is susceptible to salinity changes

and erosion from storm surge (e.g. Frazer et al., 2006).

However, these studies have focussed on wetlands, and

their applicability to rivers is unclear (Fig. 7). Responses

to coastal flooding may also differ among and within

taxa, but again, knowledge is incomplete. While short-

term shifts in fish community composition were

observed following saltwater surge into freshwater

reaches of rivers near Hurricane Katrina’s landfall zone

(Schaefer et al., 2006), amphibian species in freshwater

coastal wetlands showed resistance to water chemistry

changes (Gunzburger et al., 2010). Recovery of freshwa-

ter fauna following saltwater flooding is likely influ-

enced by dispersal characteristics, with active dispersal

(e.g. swimming) contributing to rapid recovery (Watana-

be et al., 2014).

Understanding of storm-surge effects in rivers could

be informed by knowledge of species salinity tolerances,

and by studies of tsunamis (e.g. Watanabe et al., 2014) or

salinisation from progressive saltwater intrusion and

sea-level rise. However, ramp disturbances such as pro-

gressive salinisation may have different impacts from

pulsed events like storm surges, where recovery to pre-

disturbance (freshwater) conditions can occur rapidly.

On the other hand, storm surges can rework sediments

and channels and may even cause mass mortalities

requiring long recovery periods (Cahoon, 2006). Intru-

sion of saline waters into ground water during surges

may also prolong recovery of aquatic biota in groundwa-

ter-fed rivers (Fig. 7). Furthermore, surge-induced flood-

ing may be followed by highly turbid floods (of inland

origin) associated with the same cyclone or storm (e.g.

Gong & Shen, 2009), causing abrupt changes from fresh-

water to saline water to turbid freshwater conditions.

Future directions and concluding remarks

Extreme events are locally and globally important, with

impacts often disproportionate to their duration (Smith,

2011). The ecological effects of extremes can be difficult

to determine from opportunistic studies, and more

research is needed to develop a sound mechanistic

understanding of responses to inform prediction of

future events, where they occur singly or in combina-

tion. Among event types, research on floods, droughts

and fires is more extensive than for heat waves, tropical

cyclones and storm surges. While research on all events

is needed, these latter events remain a particular priority

for future research.

As the frequency of extreme events increases, the his-

tory of past events will become increasingly important in

shaping responses to subsequent extremes (IPCC, 2012).

Compound events will be particularly deleterious, as has

been observed where heavy rains and flooding follow fire

or drought. The intensification and coincidence of

extreme events may elicit regime shifts, novel interactions

and/or abrupt transitions in species distributions and

Heavy rainfall & floods

(post-surge)(see Fig. 5)

Storm surges & coastal flooding

Death or of saline-intolerant species, &/or

composi�on

Sediment redistribu�on &

Abrupt salinity in lowland river

sec�ons (fresh to saline to fresh)

Intrusion of saline water into groundwater &

groundwater-dependent

riversWater

quality

Tropical cyclones (see Fig. 6)

Sea-level rise

High �des

↑ salinity in ↑ erosion

Δ

Δ aqua�c community

Δ

↓ abundance

Fig. 7 Effects of storm surges and coastal flooding on rivers, as

based on links among drivers and responses identified in the Aus-

tralian literature (see text for detail). See Fig. 2, for explanation of

arrows, shapes and symbols.

© 2014 John Wiley & Sons Ltd, Freshwater Biology, doi: 10.1111/fwb.12515

Extreme events and riverine ecosystems 11

Table

1Hypotheses

andpotential

approaches

toexplore

theim

pactofextrem

eclim

atic

even

tsonfreshwater

hab

itat

andacross

levelsofbiological

organ

isation

Even

tsan

dfuture

projections

Hypotheses*

Approaches*

Heatwav

esan

dhot

day

swillincrease

innumber

andduration

Habitat

Increasedim

portan

ceofcool-water

refuges

forheat-sensitivetaxa

Species

andpopu

lations

More

freq

uen

texceed

ance

ofupper

thermal

tolerance

limitsofbiota

Red

ucedresilien

ce(recovery)ofheat-sensitivepopulations

Localextinctionorcontractionin

distributionsofheat-sensitivetaxa

Com

munities

Loss

offavourable

hab

itat

andsp

eciesan

dim

migrationofnew

specieswillaltercommunity

structure

Functioning

Organ

icmatterprocessingmay

alterdueto

localextinctionsan

dbeh

aviouraleffectsofheatstress

on

primaryconsu

mers

Measu

remen

tofhab

itat

characteristicsto

iden

tify

thermal

refugia

Experim

ents

applyinghightemperature

extrem

esofdifferentdurationsan

dfreq

uen

cies,

downscalingclim

atemodelsto

gen

eraterealistic

temperature

extrem

esan

dregim

esModellingto

predictch

anges

indistributionsof

heat-sensitivetaxa,

particu

larlyforknown

vulnerab

letaxa

Droughts

willincrease

inintensity,extent

andduration

Habitat

andevents

Increasedextentofhydrological

disconnection(terrestrialisation);hab

itat

loss,includingdrought

refugia;increasedim

portan

ceofremainingrefugia

Increasedinciden

ceofdroughts

interactingwithother

extrem

eev

ents

(compoundev

ents)

Species

andpopu

lations

Loss

orcontractionin

distributionsofvulnerab

letaxa(includingflow-dep

endan

ttaxa)

Com

munities

Chan

ges

inrich

nessan

dcommunitycompositionresu

ltingfrom

chan

ges

inhab

itat

and/orlocal

speciesextinctions,

includingloss

offlow-dep

enden

ttaxafrom

stream

networksan

dch

anges

inveg

etationtype,

andincreasedlikelihoodofnovel

communitytrajectories

Functioning

Accumulationoforgan

icmatterondry

stream

bed

s;increasedoccurren

ceofhypoxic

blackwater

even

ts;alteredorgan

icmatterprocessingratesin

stream

s;increasedfuel

load

foran

drisk

offire;

potentially

drivingsh

ifts

inbeh

aviour(e.g.foraging)an

dcompositionofterrestrialbiota

Extended

periodsofreducednutrientinputinto

downstream

(e.g.lowland)reaches

Chan

ges

infood-w

ebstructure

anddynam

icsvia

loss

ofrare

taxaan

dpredators

Experim

ents

applyingdifferenthydrological

(dis)

connectivitytreatm

ents

anddryingdisturban

ces

ofdifferentintensities

over

multiple

spatiotemporalscales

Experim

ents

applying‘droughtdisturban

ces’

incombinationwithother

extrem

eev

enttreatm

ents

Experim

ents

ondry-stream

organ

icmatter

break

down,pre-conditioningan

dconsu

mptionor

use

bybiota

Modellingto

predictch

anges

inav

ailabilityan

dconfigurationofdroughtrefugia

andtheir

accessibilityto

vulnerab

letaxa

Fires

willincrease

infreq

uen

cy,therisk

of

fire

willincrease,an

dan

nual

fire

seasons

willstartearlieran

dlast

longer

Habitat

andevents

Accumulationoffinesedim

entonstream

bed

san

dreductionin

stream

hab

itat

size

andquality

Increasedexposu

reofstream

chan

nelsto

sunfrom

decreasein

canopycover,increasingstream

temperaturesan

dexacerbatingeffectsofheatwav

esan

dhotday

sIncreasedinciden

ceoffire–fl

oodinteractions(compoundev

ents)

Species

andpopu

lations

Loss

orcontractionin

distributionsofheat-sensitivetaxaan

dthose

sensitiveto

finesedim

ent

accu

mulation

Com

munities

Chan

ges

inrich

nessan

dcommunitycompositionresu

ltingfrom

chan

ges

inhab

itat,localsp

ecies

extinctionsan

d/orresp

onsesto

chan

ges

inorgan

icmatterqualityan

dquality(see

Functioning)

Chan

ges

inveg

etationtypean

dstructure

inriparianzo

nes

Functioning

Increasedquan

tity

ofrefractory

carbon(charcoal)an

dreducedqualityoforgan

icmatterin

stream

san

dresu

ltan

tch

anges

tofoodweb

s

Experim

ents

man

ipulatingthequan

tity

andquality

oforgan

icmatterin

stream

sExperim

ents

man

ipulatingsedim

entan

dash

inputs

under

both

dry-an

dwet-stream

conditions

Modellingto

predictextentofsedim

ent

accu

mulationan

dtran

sport

instream

s,including

regionswithvulnerab

letaxa,

communitiesan

d/

orecosystem

s

© 2014 John Wiley & Sons Ltd, Freshwater Biology, doi: 10.1111/fwb.12515

12 C. Leigh et al.

Tab

le1

(Continued

)

Even

tsan

dfuture

projections

Hypotheses*

Approaches*

Heavyrainfall(and

floods)

willincrease

innumber

and

intensity

Habitat

andevents

Increasedfreq

uen

cy,intensity

andinundationextentan

dnovel

timingoffloods(hydrological

connectionev

ents)

Increasedoccurren

ceofsu

ccessivefloodsan

dfloodsoccurringin

combinationwithother

extrem

eev

ents

(compoundev

ents)

Increasedoccurren

cesofch

annel

erosion

Species

andpopu

lations

Increasedfreq

uen

cyof‘hab

itat-connecting’ev

ents

willfacilitate

spread

ofsp

ecies,

including

invasives,an

dthetran

sport

ofother

materials

(organ

ican

dinorgan

ic)

Novel

timingofev

ents

may

trigger

spaw

ningcu

esin

somesp

eciespossibly

lead

ingto

unsu

ccessful

recruitmen

t,forexam

ple

ifsp

awningoccurs

when

seasonal

conditions(e.g.temperatures)

arenot

suitab

lefordev

elopmen

tCom

munities

Chan

ges

inrich

nessan

dcommunitycompositionresu

ltingfrom

chan

ges

inhab

itat,localsp

ecies

extinctionsan

d/orinvasions

Functioning

Increasedtran

sport

oforgan

ican

dinorgan

icmaterials

within

andbetweencatchmen

ts

Experim

ents

applying‘flooddisturban

ces’

successivelyan

d/orin

combinationwithother

extrem

eev

enttreatm

ents

Experim

ents

applyingdifferenthydrological

connectivitytreatm

ents

over

multiple

spatial

scales

Modellingto

predictch

anges

inrecruitmen

tsu

ccessan

ddistributionsofvulnerab

letaxaan

dinvasives

Tropical

cyclones

will

increase

inintensity

Habitat

Increaseddestructionofriparianveg

etation(treefallan

dother

winddam

age)

andincreaseddelivery

oflargewoodydeb

risto

stream

sIncreasedsh

elteringhab

itat

forfish

andmacroinvertebratesan

dincreasedsu

bstrate

forbiofilm

growth

(foodforconsu

mers)

Increasedintensity

ofwater-qualitych

anges

followingev

ents

(e.g.extrem

eincreasesin

turbidity),

withprolonged

recovery

Species

andpopu

lations

Creationof(tem

porarily)unsu

itab

lehab

itat

forturbidity-sen

sitiveaq

uatic

biota,potentially

causing

localextinctions

Substan

tial

dam

ageto

orloss

ofwind-sen

sitivetaxain

riparianzo

nes

andincreasedrisk

ofdam

age

towind-toleranttaxa

Com

munities

Chan

ges

inrich

nessan

dcommunitycompositionresu

ltingfrom

chan

ges

inhab

itat

and/orlocal

extinctions

Functioning

Altered

food-w

ebdynam

icsresu

ltingfrom

chan

ges

inriparianveg

etationan

dorgan

icmatterinputs

Experim

ents

applying‘w

indtreatm

ents’of

differentstrength

anddurationunder

high-flow

andcontrol-flow

conditions

Experim

ents

andsu

rvey

sto

establish

how

loss

and

recoveryofriparianveg

etation,an

dch

anges

inturbidity,alterfoodweb

san

daq

uatic

community

composition

Modellingto

predictextentan

ddurationof

turbidityan

dother

water-qualitych

anges

during

andafterev

ents

ofdifferentintensity

Storm

surges

will

increase

infreq

uen

cyan

dcoastalflooding

willincrease

inextent

Habitat

Increasedfreq

uen

cyofab

ruptch

anges

insalinityin

freshwater

sectionsofriversan

dincreased

likelihoodofev

ents

affectingsectionsfurther

upstream

than

previouslyknown

Increasedtemporalan

dsp

atialvariabilityin

chan

nel

morphology

Species

andpopu

lations

Loss

orcontractionin

distributionsofsalinity-sen

sitivetaxa

Com

munities

Terrestrial

andaq

uatic

communitiesofaffected

riversbecomemore

similar

tothose

typically

found

inestuarinehab

itats

Experim

ents

applyingab

ruptwater-quality

chan

ges

Modellingto

predictextentan

dpersisten

ceof

saltwater

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© 2014 John Wiley & Sons Ltd, Freshwater Biology, doi: 10.1111/fwb.12515

Extreme events and riverine ecosystems 13

ecosystem functioning (IPCC, 2014). Successive events of

the same type may alter the organisation of food webs

and disrupt the successional development of biological

communities (e.g. Ledger et al., 2012, 2013) or initiate

novel development trajectories (Turner et al., 1998).

Understanding of compound events and long-term

effects of extreme events could be enhanced by exploit-

ing long-term data collected by monitoring programs for

other purposes such as bioassessment but which capture

pre- and post-event information (e.g. Rose et al., 2008).

Standardisation and maintenance of monitoring pro-

grams across regions, along with question-driven pro-

grams and adaptive designs would also improve the

utility of long-term datasets to elucidate mechanisms

underlying ecological responses to and recovery from

extreme events (Lindenmayer & Likens, 2010). While

such survey data are important, it is clearly impractical

to rely solely on phenomenological research of rare

unpredictable events. A range of experimental

approaches is also required to test hypotheses and estab-

lish a solid, mechanistic understanding of the effects of

extreme events in fresh waters (cf. Wernberg, Smale &

Thomsen, 2012; Stewart et al., 2013b; Table 1).

Extreme events are key elements of the natural vari-

ability that shapes the ecology of rivers and streams.

Freshwater organisms can recover from disturbances,

including extreme events, yet an intensification of distur-

bance regimes in a warming world could exceed the

capacity of communities to rebound. Because future

events are likely to be more extreme than those today

(unprecedented events) and more frequent or novel

combinations are projected (IPCC, 2012), there is an

increased likelihood that extreme events will push some

species or ecological processes beyond critical thresh-

olds, threatening species persistence, altering ecosystem

functioning or tipping communities and ecosystems into

novel states (Paine et al., 1998; Table 1). As the resilience

of aquatic organisms is lessened by a range of anthropo-

genic stressors, the effects of extreme events on riverine

ecosystems must be considered alongside those of

human activities (e.g. Balcombe et al., 2011; Leigh et al.,

2013; Reich & Lake, 2014). As extreme events also occur

against a backdrop of gradual climate change, outcomes

of future events must be considered in the light of these

longer term trends in climate, especially progressive

warming. Studies that apply experimental extreme

events and monitor long-term recovery patterns, apply

experimental extreme events repeatedly over realistic

timeframes (e.g. Ledger et al., 2011), apply experimental

treatments that incorporate extreme events into future

climate scenarios (e.g. Thompson et al., 2013b) or assess

the combined effects of multiple stressors (e.g. Lunt, Jan-

sen & Binns, 2012) will help fill knowledge gaps and

better enable us to predict responses to the events and

novel regimes of the future.

Acknowledgments

We thank Pettina Love, Tom Nelson and Anthea Flo-

rance for information on fires from their Honours

research at the University of Canberra; Steve Turton,

Campbell Clarke and Tom Nelson for photographs; and

Andrew Boulton, Sarah Boulter, Dave Strayer and two

anonymous reviewers for their comments and edits that

improved the manuscript.

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