Pathways for algal recolonization in seasonally-flowing streams

Pathways for algal recolonization in seasonally-flowingstreams

BELINDA J. ROBSON, TY G. MATTHEWS, PETER R. LIND AND NIGEL A. THOMAS

School of Life and Environmental Sciences, Deakin University, Warrnambool, Vic., Australia

SUMMARY

1. In semi-arid climates, seasonally-flowing streams provide most of the water required

for human use, but knowledge of how water extraction affects ecological processes is

limited. Predicted alterations in stream flows associated with the impacts of climate

change further emphasize the need to understand these processes. Benthic algae are an

important base for stream food webs, but we have little knowledge of how algae survive

dry periods or respond to altered flow regimes.

2. We sampled 19 streams within the Grampians National Park, south-eastern Australia

and included four components: a survey of different drought refuges (e.g. permanent

pools, dry biofilm on stones and dry leaf packs) and associated algal taxa; a survey of

algal regrowth on stones after flows recommenced to determine which refuges contributed

to regrowth; reciprocal transplant experiments to determine the relative importance of

algal drift and regrowth from dry biofilm in recolonization; direct measurement of algal

drift to determine taxonomic composition in relation to benthic assemblage composition.

3. Algae showed little specificity for drought refuges but did depend on them; no

species were found that were not present in at least one of the perennial pool, dry biofilm

or leaf pack refuges. Perennial pools were most closely correlated with the composition of

algal assemblages once flows resumed, but the loss or gain of perennial pools that might

arise from stream regulation is unlikely to affect the composition of algal regrowth.

However, regulated streams were associated with strong increases in algal density in dry

biofilm, including increased densities of Cyanobacteria.

4. A model for algal recolonization in seasonally-flowing streams identified three

pathways for algal recolonization (drift-dependent, dry biofilm-dependent and

contributions from both), depending on whether streams are diatom-dominated or

dominated by filamentous algae. The model predicted the effects of changes to stream

flow regimes on benthic algal recolonization and provides a basis for hypotheses

testable in streams elsewhere.

Keywords: climate change, drought refuges, freshwater algae, intermittent streams, temporarystreams

Introduction

In Mediterranean and semi-arid climate regions,

seasonally-flowing streams are the most abundant

stream type and provide most of the water required

for human use (Gasith & Resh, 1999). Despite their

economic importance, ecological studies of intermit-

tent or seasonally flowing streams are scarce in

comparison to studies of perennial streams, so we

have little understanding of the effects of water

extraction. Water extraction may prolong dry periods

in streams or increase their frequency and this may

affect the survival of stream biota. Some recent studies

have identified different types of drought refuge for

Correspondence: Belinda J. Robson, School of Life and Envi-

ronmental Sciences, Deakin University, PO Box 423 Warrnam-

bool, Vic. 3280, Australia. E-mail: [email protected]

Freshwater Biology (2008) 53, 2385–2401 doi:10.1111/j.1365-2427.2008.02061.x

� 2008 The Authors, Journal compilation � 2008 Blackwell Publishing Ltd 2385

both algae (e.g. Robson & Matthews, 2004) and

animals (e.g. Magalhaes et al., 2002). However, the

recolonization dynamics of biota in these streams is

still largely unknown.

The stream canopy in Mediterranean and semi-arid

regions is typically more open than in temperate

areas, allowing greater light penetration to the stream

bed and creating ideal conditions for algal growth.

Benthic algae are an important food source in many

streams, including those in arid regions (e.g. Benenati,

Shannon & Blinn, 1998; Bunn, Davies & Winning,

2003), so it is important to understand their response

to impacts such as altered flow regimes. However,

there is little knowledge of algal recolonization pro-

cesses following periods without surface flow. Studies

of algal responses to flow regulation have found that

rapid wetting and drying leads to increased growth of

Cyanobacteria and reduced growth of green algae and

diatoms that are more palatable to grazers (e.g. Blinn

et al., 1995; Benenati et al., 1998; Robson, 2000; Ryder,

2004). Under conditions of rapid drying, algal taxa

respond differently depending on their ability to

retain moisture (Benenati et al., 1998) and the presence

or absence of surface water during dry periods may

affect the density of subsequent algal growth (Robson

& Matthews, 2004).

The rate of water loss when streams dry out is also

likely to influence the source of recolonization (in situ

biofilm regrowth versus recolonization from external

sources, Stanley, Fisher & Jones, 2004). Field experi-

ments examining the impacts of desiccation on algal

recolonization show that when drying occurs rapidly

(i.e. stones removed from water and placed higher on

the bank) there is a devastating effect on algal

recovery and regrowth from dry biofilm (Dodds et al.,

1996; Benenati et al., 1998; Mosisch, 2001). In contrast,

when drying occurred naturally and thereby more

slowly, regrowth from dry biofilm is apparent (Rob-

son, 2000; Robson & Matthews, 2004); similar results

have also been observed with cultured algae (Peter-

son, 1996a and references therein). Peterson (1987)

showed that there may be an interaction between the

current speed to which a developing assemblage is

exposed and its subsequent desiccation resistance;

assemblages sheltered from current showed lower

desiccation resistance. This suggests that, in regulated

streams, prolonged periods of slow flow may encour-

age development of an algal assemblage that is

less resistant to subsequent desiccation than an

assemblage that develops in unregulated streams

with fast currents. Benenati et al. (1998) concluded

that desiccation-induced changes in algal composition

would have strong effects on the quality of the algal

food base of the Colorado River and, hence, on animal

communities. Therefore, alterations to flow regimes of

seasonally-flowing streams may affect the contribu-

tion of primary productivity to stream food webs.

It is difficult to predict the effects of flow modifi-

cation on algal growth without an understanding of

the recolonization process. Stanley et al. (2004) sug-

gested that primary productivity should be examined

at the landscape scale, but their model assumed that

the connectivity of streamflow was uninterrupted

across the landscape. In many catchments where

intermittency predominates, flow patterns are discon-

nected by weirs, pipelines and inter-basin transfers

and the use of streams as irrigation channels (Hughes,

2005). Therefore, to determine effects of these artificial

disconnections and re-connections on algal growth, it

is necessary to understand recolonization processes at

a smaller spatial scale than whole catchments. The

present study aimed to determine the pathways for

recolonization in seasonally-flowing streams and to

evaluate effects of flow regulation on recolonization

and productivity (growth).

The bed of seasonally-flowing streams may provide

a range of microhabitats suitable as refuges for algae

during periods without surface flow (Fig. 1). Many of

these streams retain perennial pools, where algae

continue to grow and proximity to these pools may

influence recolonization (Dodds et al., 1996). Algae

regrow from dry biofilm that remains on stream

stones, once flows resume (Robson, 2000). There are

also places that may be cooler and retain more

moisture than the exposed stone surfaces, such as

patches of moss, accumulations of leaves, seeps and

woody debris. Dry sediments and leaf litter may also

be suitable drought refuges (Davis, 1972). These

refuges differ in their vulnerability to flow regulation:

perennial pools are more likely to disappear with

increased water extraction (Hughes, 2005) whereas

dry biofilm is invulnerable. Conversely, stream regu-

lation may create perennial (weir) pools in streams

that previously dried out completely. An important

aspect of the concept of a refuge is that it must make a

significant contribution to recolonization of the main

habitat areas post-disturbance, otherwise it is merely a

remnant. We therefore identified refuges that are

2386 B. J. Robson et al.

� 2008 The Authors, Journal compilation � 2008 Blackwell Publishing Ltd, Freshwater Biology, 53, 2385–2401

important for algal recolonization and whether dif-

ferent taxa showed specific requirements for particu-

lar refuge types.

For recolonization to occur from these refuges once

flows resume, it is assumed that algae will drift in the

current and attach to stones downstream of their

source (Peterson, 1996a). Algal drift is rarely mea-

sured directly in intermittent streams, but is recog-

nized as an important recolonization mechanism in

perennial streams (McCormick & Stevenson, 1991;

Biggs, 1996). Therefore, we measured algal drift

directly to substantiate it as a viable mechanism for

algal regrowth in seasonally-flowing streams.

In an experiment where the dry biofilm was

removed prior to the resumption of flow, Robson &

Matthews (2004) showed that dry biofilm made a

strong contribution to regrowth, but that this contri-

bution was also an order of magnitude more important

in streams with perennial surface water (pools) than in

streams that dried out completely. This suggested an

interaction between two potential drought refuges;

dry biofilm and perennial pools (Fig. 1). This interac-

tion may be due to increased entrapment of algae

drifting from pools, or increased density of cells in the

dry biofilm in streams with perennial surface water, or

both (Robson & Matthews, 2004). Peterson (1987)

suggested that, by stimulating mucilage production,

desiccation could enhance subsequent entrapment of

drifting cells. Similarly, coating substrata with agar led

to a twofold increase in diatom immigration from the

drift (Stevenson, 1983). This is one mechanism by

which the density of cells in re-wetted dry biofilm

might affect the contribution of drift to algal recol-

onization (Fig. 1) creating an interaction between

refuges. To evaluate the relative contribution of algal

drift and regrowth from dry biofilm, we did three

reciprocal transplant experiments using stream stones

containing established dry biofilm.

In summary, we determined pathways for algal

recolonization in seasonally-flowing streams by

addressing the following questions: first, do algae

depend on refuges (Stanley et al., 2004) and, if so, do

different taxa use particular refuges and do particular

refuges contribute more to subsequent regrowth?

Secondly, do algae disperse from drought refuges

into the drift once flows resume? Thirdly, is the arrival

of drifting algae onto stones a substantial contributor

to algal regrowth and is this contribution affected by

the taxa drifting or by the assemblage already on

stream stones?

Methods

Study area

We sampled 19 intermittent streams within the

Grampians ⁄Gariwerd National Park, western Victoria,

Australia [Table 1; Fig. 2: see descriptions in Robson

(2000), Robson & Matthews (2004) and Cowell,

Matthews & Lind (2006)]. The streams are stony, with

boulder-cobble substrata of Grampians sandstone ⁄siltstone. Vegetation is typical Australian dry sclero-

phyll forest and woodland, dominated by Eucalyptus

species. The climate is semi-arid (400–600 mm average

rainfall per annum) and most streams are seasonal,

flowing from June to December each year.

Study design

The study comprised four components: a survey of

algal taxa present in different drought refuge types; a

survey of algal regrowth on stones after flows recom-

menced to determine which refuge types contributed

to regrowth; reciprocal transplant experiments to

drift

In situ regrowth and/or entrapment of propagules in re-establishing biofilm

stone tops

Dry

leaves

pools sand

wood

Flowing

Fig. 1 Diagram summarizing possible recolonization mecha-

nisms. Left hand side shows potential drought refuges for algae

when stream beds are dry. It is not known which of these ref-

uges contribute most to algal regrowth, nor which taxa use each

refuge type. Right hand side describes three recolonization

methods: (i) drift; (ii) in situ regrowth from dry biofilm and (iii)

enhanced entrapment of drifting algae by regrowing biofilm. It

is not known whether algae enter the drift immediately after

flows resume, or only after in situ regrowth on stones from dry

biofilm followed by active drift. Lastly, neither relative contri-

butions to recolonization by each of these mechanisms are

known, nor is the role of taxonomic composition understood.

Pathways for algal recolonization 2387


determine the relative importance of algal drift and

regrowth from dry algal biofilm in the recolonization

processes; and, lastly, direct measurement of algal drift

to determine taxonomic composition in relation to

benthic assemblage composition. For each component,

data from different streams were used (Table 1; Fig. 2).

This was partly due to the desire randomly to select

streams for each component, thereby ensuring inde-

pendence among components and maximize the

generality of our results. In some cases, it was also

performed to balance sampling designs for our analy-

ses. In all cases algae were quantified by counts of live

cell units per unit area or volume (see Robson, 2000).

Drought refuge survey

Our previous studies have shown that dry biofilm on

stones is the predominant drought refuge in several

Grampians streams (Robson, 2000; Robson & Mat-

thews, 2004; Cowell et al., 2006). However, we recog-

nized that other potential drought refuges may have

also existed. So we surveyed 19 streams during March

& April, 2004 for the frequency of six other potential

drought refuges: leaf packs, woody debris, seeps,

pools, dry sediment and patches of moss. Two people

intensively searched 100 m of streambed counting the

frequency of each refuge type. A further 400 m of

streambed was surveyed upstream for the presence of

pools or perennially flowing sections. We found no

seeps, and moss and woody debris were also rare or

absent from many stream sections, and therefore these

refuges are not considered further. Living algae were

randomly sampled from submerged rocks in six

streams that contained permanent surface water

(Table 1) using methods described in Robson &

Matthews (2004). In addition to pool samples, six

Table 1 Streams used in this study, with their latitude and longitude

Regulated R F M T D Unregulated R F M T D

+ Pools

Reservoir Creek mainstem * * Boggy Creek *

37.347�S, 142.616�E 37.107�S, 142.274�E

2nd Wannon Creek * * *B Bovine Creek * * * *F

37.317�S, 142.541�E 37.239�S, 142.538�E

Dairy Creek * * * Barneys Creek * * * *F

37.209�S, 142.533�E 37.222�S, 142.582�E

Fyans Creek * Mt William Creek * * * *B

37.251�S, 142.536�E 37.322�S, 142.619�E

Brown Creek * Reservoir Creek – South Fork† * *B

37.458�S, 142.19�E 37.347�S, 142.611�E

No. 1 Creek

37.357�S, 142.217�E

) Pools

1st Wannon Creek * * * Muline Creek

37.307�S, 142.540�E 37.207�S, 142.262�E

Cultivation Creek

37.25�S, 142.28�E

Middleton Creek *

37.225�S, 142.532�E

Unnamed Creek * *F

37.259�S, 142.619�E

Bomjinna Creek * * * *F

37.277�S, 142.619�E

Stockyard Creek * * *

37.329�S, 142.519�E

Grevillea Creek * * * *B

37.228�S, 142.547�E

All 19 streams were sampled to determine frequency of drought refuge types. A subset of streams (*) were used for: R, drought refuge

survey; F, stream survey after 1 week of flow; M, Mantel tests of relationship between drought refuges and streams post-flow; T,

reciprocal transplant experiments; D, drift sampling (*B = bacillariophyte-dominated streams; *F = filamentous-dominated streams).

Dairy Creek was not sampled 1 week after flow (i.e. n = 18 for flow survey after 1 week of flow). Stream names marked ‘†’ were

sampled upstream of weirs marked in Fig. 2. Stream names without ‘†’ were sampled downstream of those weirs.



randomly chosen leaf packs and six randomly chosen

dry rocks with dry biofilm were also collected and

taken back to the laboratory and rewetted to culture

any dormant algae from within the dry biofilm.

Samples were taken only from the thalweg to ensure

that the selected stones were inundated during the

previous winter. Leaf packs and rocks containing dry

biofilm were cultured in distilled water for 48 h before

being sampled and preserved for later counting

(Robson, 2000). This short culture period enabled

algal taxa to be identified and ensured that relative

abundances reflected those of the dry biofilm. A

longer culture period might have allowed interactions

between taxa to alter relative abundances and

increased the risk of contamination from other

sources. There was no evidence of such contamination

(such as novel taxa on laboratory stones) during the

study.

We aimed to sample each refuge type from four

different stream categories (Table 1). The presence or

absence of pools in each stream was determined by

searching 500 m upstream from each study site during

the dry season. Where water was found, sites were

revisited in late autumn to determine whether it had

persisted throughout the dry period. Note that it is

possible with our design that some streams designated

as lacking permanent surface water did have perma-

nent pools at the top of the catchment beyond our

500 m search range. However, we had decided

in an earlier study (Robson & Matthews, 2004) that

pools more than 500 m upstream from sampling

sites were unlikely to influence algal recolonization.

N

To GlenelgRiver

Muline

Cultivation

No.1

LakeBellfield

Bovine

Barneys

Grevillea

Dairy

Middleton

Brown

To WannonRiver

Wannon River

Stockyard

2nd Wannon

1st Wannon

Fyans

Reservoir

Reservoir

south fork

mainstem

Interbasin transfer

Pools present

Pools absent

Perennial

Bomjinna

Unnamed

Mt William

Fig. 2 Schematic diagram showing presence and absence of pools or permanent surface water in 18 of the 19 streams sampled.

Boggy Creek not shown as it was in an isolated catchment. Second-order streams are shown as forked lines, first-order streams are

simple lines. Weirs are indicated with curved solid lines.



Unfortunately, only one regulated stream without

pools was found among the 19 streams surveyed (1st

Wannon Creek; Table 1). Therefore, only the three

remaining stream types were used for univariate ana-

lyses of potential drought refuges (see Data analyses).

Stream survey after 1 week of flow

We used a broad-scale survey of benthic algae in 18

streams to provide an overview of the variation that is

possible among seasonally-flowing streams (Table 1).

Two hypotheses were tested; first, that algal density

and ⁄or algal assemblage structure differ between

regulated and unregulated streams (both with per-

manent surface water) and, secondly, that mean algal

density or algal assemblage structure differ between

unregulated streams with and without permanent

surface water.

To test these hypotheses, three streams were

randomly selected from each of the three stream

categories (nine streams in total) for univariate anal-

yses and two sites were sampled on each of these

streams (Table 1). Six replicate samples were taken

from the tops of individual stones at each site, as

described by Robson & Matthews (2004). In addition,

data from nine streams were used for comparison

with the drought refuge data. That is, data from the

same nine streams (Table 1) were used to determine

the degree of association between the pres-

ence ⁄absence of algal taxa and the assemblage com-

position in drought refuges and on stream stones once

flows resumed.

Reciprocal transplant experiments

Three pairs of streams were used. While streams were

dry, six marked stones were transplanted between Mt

William Creek (unregulated, with pools) and Bomj-

inna Creek (unregulated, no pools), six were trans-

planted between Grevillea Creek (unregulated, no

pools) and Barney Creek (unregulated, with pools)

and six were transplanted between Grevillea Creek

and Dairy Creek (regulated, with pools). In addition,

six stones from each stream were transported to the

laboratory (same algal culture method as drought

refuge samples). Once streams had resumed flow for

1 week, laboratory stones and marked stones were

sampled along with six unmarked, randomly chosen,

non-transplanted stones from each stream (methods

as above). Therefore, for each stream, it was possible

to compare assemblage composition from dry biofilm

in the absence of any external colonization (=labora-

tory stones), with stones exposed to both recoloniza-

tion by drift and from local dry biofilm (=in situ

stones) and, finally, with stones exposed to recolon-

ization by a drift flora that differed from the dry

biofilm on stones (=transplanted stones).

Algal drift

Eight streams were determined a priori to be domi-

nated by either diatoms (four streams) or filamentous

algae (four streams) (Table 1). Drift samples were

collected from each of these streams with a 35 lm

plankton net (0.038 m2 net opening) supported by a

steel frame. While some small stream algae would

pass through a 35 lm mesh net, a finer mesh creates

too much resistance in the stream flow. A wooden

0.045 m block and U-bolt connected the lower tie

down strap to the net frame, providing 5 cm clearance

above the bed (Culp, Scrimgeour & Beers, 1994). The

net was fitted with a threaded flange allowing

attachment of a 120 mL sample container and posi-

tioned in the thalweg for sampling. Placement of the

driftnet ensured avoidance of chutes, boulder clusters,

woody debris, leaf packs and depths <0.08 m. Net

opening height out of the water was measured (to

enable net surface area calculation) and the depth of

the net underwater was recorded when the net was

full. The net was lifted at the end of each sampling

time and the inside was washed into the collection pot

with 500 mL of stream water. After each sample

collection, the pot was removed and the inside of the

net was again flushed with water.

A MiniAir2 flow meter (Schiltknecht Messtechnik

AG, Switzerland), was used to measure water velocity

at the net mouth. Velocity was measured in the net

opening at half net water depth for the first and last

30 s of sample time (10 min – determined by a pilot

study) providing a mean measurement for each 30-s

interval. The net entrance velocity consisted of the

mean of these two 30-s readings. The total volume of

water sampled was calculated as [(net entrance veloc-

ity, m s)1) · (cross-sectional area of net entrance in

m2) · (sample interval in s)] (Faulkner & Copp, 2001).

Streams were randomly allocated for sampling

either in the morning or afternoon (09:00–12:00 hours

and 13:00–16:00 hours) because a pilot study revealed



no clear diel rhythm of drift. Six random drift and six

random epilithic samples (methods as above) were

collected from a randomly selected 50 m stream

length, working upstream. All epilithic and drift

sampling equipment was flushed with distilled water

between use at different streams, to prevent cross-

contamination of samples. Algae from both benthic

and drift samples were preserved, counted and

identified using the methods described above.

Data analyses

All univariate analyses were performed using SYSTATSYSTAT

version 10. Assumptions of ANOVAANOVA (normality and

homoscedasticity) were checked by inspecting resid-

ual plots and data were log-transformed if necessary

(Quinn & Keough, 2002). The Peritz procedure was

used for post hoc pairwise comparisons of significant

treatments after ANOVAANOVA (Quinn & Keough, 2002).

Multivariate analyses (with data square-root transfor-

mations to reduce the influence of very abundant

taxa) used Bray–Curtis similarities using PRIMERPRIMER

software (version 5; Clarke & Gorley, 2001).


The dependent variables total density, species rich-

ness and the total density of Cyanobacteria, chloro-

phytes and bacillariophytes were compared across the

three stream categories (fixed, three levels: regulated

streams with pools; unregulated streams with pools;

and unregulated streams without pools) for two of the

three refuge types (i.e. omitting the pool refuge type

which did not occur in all three stream categories).

Secondly, we compared the dependent variables

across all three drought refuge types in regulated

and unregulated streams with pools. We used a

partly-nested mixed-model ANOVAANOVA with the main

effects: stream category (fixed, two levels: regulated

streams with pools versus unregulated streams with

pools), refuge type (fixed, three levels: leaf packs,

pools and dry biofilm) and Stream (stream category)

(random, three streams nested within each stream

category).

Algal community composition was analysed in two

ways: comparison of algal assemblages across refuge

types by pooling data across stream categories (10

streams including 1st Wannon Creek); and compari-

son of algal assemblages in refuge types across

regulated and unregulated streams with pools (i.e.

streams without pools excluded = six streams rather

than 10) using a combined factor (refuge type · stream

category; see Robson & Matthews (2004) for method).

Stream Survey after 1 week of flow

The same dependent variables used for the drought

refuge survey were compared across sites, streams

and stream categories using a three-factor fully nested

ANOVAANOVA. The three factors were stream category (fixed,

three levels: regulated streams with pools; unregu-

lated streams with pools; unregulated streams with-

out pools), Stream (stream category) (random, three

streams nested within each stream type) and Site

[Stream (stream category)] (random, two sites nested

within each of three streams nested within each of

three stream categories, two levels) (Table 1).

Ordination of samples from 18 of the 19 streams

was used to compare assemblage composition across

all four stream categories (Dairy Creek did not flow

for long enough for algae to be sampled). A hypoth-

esis test of differences in assemblage composition

between stream categories was performed using the

same nine streams as for the univariate tests.

Relationship between assemblage composition of drought

refuges and streams after 1 week of flow

The composition of algae in each of the three dom-

inant drought refuge types (dry biofilm, leaf packs

and pools) was related to community composition of

algae after 1 week of flow using Mantel tests

(RELATE function in PRIMERPRIMER) on square-root trans-

formed data for nine streams (Table 1). The weighted

Spearman rank correlation was used (Clarke & War-

wick, 1994), with 999 permutations.

Reciprocal transplant experiment

Single factor ANOSIMANOSIM was used to analyse algal

assemblage composition data from the three

reciprocal transplant experiments. Univariate analy-

ses were not used for these data, because the purpose

of these experiments was to identify the dominant

colonization source based on taxonomic composition.

Total abundances would have differed among treat-

ments as a simple product of the experimental design

(i.e. stones cultured in the laboratory would always



have had fewer individuals). We had no a priori

reason to believe that the number of species would

vary among any of the treatment levels.

For each experiment (pair of streams), the factor

‘treatment’ was analysed with six levels: laboratory

stones stream 1, laboratory stones stream 2, in situ

stones stream 1, in situ stones stream 2, stones

transplanted into stream 1 and stones transplanted

into stream 2. All three experiments showed signifi-

cant differences among treatments, so pairwise com-

parisons were used to compare treatment levels.

Relationship between algal drift and benthic algal

composition

Differences between the composition of algal drift and

epilithic assemblages were examined using a Bray–

Curtis similarity matrix. Presence ⁄absence data were

used to eliminate the effect of systematic differences

in abundances between drift and epilithic samples.

Differences in taxonomic composition between drift

and epilithic samples were examined by two-way

ANOSIMANOSIM. Factors used were stream and sampling

method (drift ⁄epilithic).

Results

Relative frequency of occurrence of refuge types

Only three drought refuges were common and exten-

sively available to algae, including dry biofilm on

stones, dry biofilm on leaf packs and living algal

biofilm on stones in pools. No species were found in

the post-flow stone sampling that were not found in

one of these three refuges. Therefore, only these three

refuge types were considered further.


Patterns of total algal density, total Cyanobacteria

density and total bacillariophyte density were similar

for both the pools and leaf pack refuge types (Figs 3 &

4). These three dependent variables showed signifi-

cant stream-to-stream variation in both pool (density:

F4,30 = 16.7, P < 0.01; Cyanobacteria: F4,30 = 26.2,

P < 0.01; Bacillariophyta: F4,30 = 7.5, P < 0.01) and

leaf pack refuge types (density: F4,30 = 13.5, P < 0.01;

Cyanobacteria: F4,30 = 3.4, P < 0.01; Bacillariophyta:

F4,30 = 12.2, P < 0.01), but did not vary significantly

0

1

2

3

4

5

6

7

Unregulated + poolsUnregulated –pools

Regulated + pools

0

0.5

1

1.5

2

2.5

3

3.5

40

1

2

3

4

5

60

2

4

6

8

10

12

140

1

2

3

4

5

6

7 Dry biofilmLeaf pack

Lo

g10

(T

ota

l alg

al d

ensi

ty)

Lo

g10

(Cya

no

bac

teri

a)L

og

10 (C

hlo

rop

hyt

es)

Lo

g10

(Bac

illar

iop

hyt

es)

Sp

ecie

s ri

chn

ess

Fig. 3 Log-transformed [log10(x)] total algal densities, species

richness, Cyanobacteria, Chlorophyta and Bacillariophyta sam-

pled from the three stream categories for two of the three

drought refuge types (i.e. pool refuge type was omitted leaving

dry biofilm and leaf packs refuge types, each bar represents a

single stream, n = 6, units = no. individuals per 36.3 cm2). All

data represent least square means ± 1 SE.



across the three stream categories. The only significant

variation across the three stream categories existed for

both total algal densities (F1,4 = 8.7, P < 0.05) and the

Cyanobacteria in the dry biofilm (F1,4 = 13.1,

P < 0.01). Total algal densities in the dry biofilm were

higher in regulated streams with pools than in the

unregulated stream types, which did not differ (Peritz

Pairwise comparison) (Figs 3 & 4). Total cyanobacte-

rial density differed across all three stream category

combinations (Peritz Pairwise comparisons) and was

highest in regulated streams with pools and lowest in

unregulated streams with pools (Fig. 3). Total chloro-

phyte density was not significantly influenced by

stream category, but did show stream-to-stream var-

iation in pools (F4,30 = 3.8, P < 0.05) and in dry biofilm

(F4,30 = 4.1, P < 0.01; Fig. 4).

The predominant taxa sampled during the drought

refuge survey were the diatoms Fragilaria spp. and

Gomphonema spp. and some unidentified Cyanobacte-

ria taxa (Table 2). Variation in algal composition was

clearly lower among leaf pack samples than from the

dry biofilm or living biofilm on pool stones (Fig. 5)

and there were significant differences in algal com-

position across the three refuge types (Global

R = 0.29, P = 0.001). The greatest differences were

between leaf packs and pools (R = 0.39, P = 0.001)

and leaf packs and dry biofilm (R = 0.37, P = 0.001).

Differences in algal composition between pools and

dry biofilm were small, but significant (R = 0.09,

P = 0.004).

Algal composition differed across regulated and

unregulated streams with pools for each of the three

refuge types (Global R = 0.28, P = 0.001; pools,

R = 0.225, P = 0.002; leaf packs, R = 0.561, P = 0.001;

dry biofilm, R = 0.15, P = 0.02; Table 2). There was

small, but significant variation in algal composition

(data pooled across the three drought refuges) across

the three replicated stream categories: between unreg-

ulated and regulated streams with pools (R = 0.11,

P = 0.001) and unregulated streams with and without

pools (R = 0.09, P = 0.005), but no difference was

detected between regulated streams with pools and

unregulated streams without pools (R = 0.04,

P = 0.06).

Stream survey after 1 week of flow

None of the dependent variables (total algal density,

species richness or Cyanobacteria, Chlorophyta and

0

1

2

3

4

5

6

7

Regulated + poolsUnregulated + pools

0

0.5

1

1.5

2

2.5

3

3.5

40

1

2

3

4

5

60

2

4

6

8

10

12

14

16

180

1

2

3

4

5

6

7 Dry biofilmLeaf packsPools

Lo

g10

(T

ota

l alg

al d

ensi

ty)

Lo

g10

(Cya

no

bac

teri

a)L

og

10 (C

hlo

rop

hyt

es)

Lo

g10

(Bac

illar

iop

hyt

es)

Sp

ecie

s ri

chn

ess

Fig. 4 Log-transformed [log10(x)] total algal densities, species


pled from the three drought refuge types in regulated and

unregulated streams with pools (each bar represents a single

stream, n = 6, units = no of individuals per 36.3 cm2). All data

represent least square means ± 1 SE. Note: unregulated streams

without pools had to be omitted due to the absence of pool

refuges in these streams.



Bacillariophyta densities) differed across the three

stream categories after flows had resumed (Table 2;

Fig. 6). There was significant stream-to-stream vari-

ability for species richness (F6,9 = 10.1, P < 0.01),

chlorophytes (F6,9 = 8.0, P < 0.01) and the Cyano-

bacteria (F6,9 = 3.9, P < 0.05), and significant site-to-

site variation for total density (F9,90 = 2.1, P < 0.05),

chlorophytes (F9,90 = 2.1, P < 0.05) and the Cyano-

bacteria (F9,90 = 2.5, P < 0.05) in some streams (Fig. 6).

The predominant taxa sampled after flow

resumed were similar to those sampled during the

survey of drought refuges (Table 2). Differences in

algal composition were detected across the four

stream categories using 18 streams (Fig. 7), but

hypothesis tests showed that these differences were

small (e.g. Global R = 0.08, P = 0.001) regardless of

whether 1st Wannon Creek was included or

excluded from analyses.

Relationship between algal composition in drought

refuges and in-stream algal composition after 1 week

of flow

Neither leaf pack nor dry biofilm refuges were

strongly correlated with the assemblage composition

(species · abundance) that developed a week after

flows resumed in any of the stream categories (leaf

pack q = 0.184, P = 0.001; dry biofilm q = 0.178,

P = 0.001). In addition, algal assemblage composition

in these two refuge types was strongly correlated

(q = 0.446, P = 0.001) indicating substantial similarity

in assemblage composition. In contrast, algal assem-

blage composition of pool refuges was more strongly

correlated with assemblages that grew on stones in

streams after flows resumed (q = 0.332, P = 0.001).

Reciprocal transplant experiments

For the experiment in Mt William Creek and Bomjinna

Creek, treatments differed significantly (Global

R = 0.484, P < 0.001). Differences in assemblage

composition occurred between laboratory stones

Leaf packsPoolDry biofilm

Stress: 0.13

Fig. 5 Non-metric dimensional scaling (NMDS) ordination

comparing algal assemblages sampled from leaf packs, dry

biofilm and pools (pooled across stream categories, n = 6 sam-

ples of each refuge type within each stream). These multivariate

data include 1st Wannon Creek, the only regulated stream

without pools available for sampling (i.e. inclusion of all 10

streams, Table 1).

Table 2 Dominant algal taxa collected during drought refuge study (all 10 streams) and 1 week after flow (all 18 streams)

Drought Survey Stream category Dominant algal taxa

Leaf packs Regulated with pools Fragilaria spp., Gomphonema spp., Cyanobacteria sprout., unknown sp. 1,

unknown Cyanobacteria sp. 1.

Unregulated with pools Fragilaria spp., Gomphonema spp., Tabellaria sp., unknown sp.,

Cyanobacteria sprout

Pools Regulated with pools Fragilaria spp., unknown Cyanobacteria sp. 1, Gomphonema spp.

Unregulated with pools Unknown sp. 2, Fragilaria spp., Gomphonema spp. Stigonema sp.

Dry biofilm Regulated with pools Unknown Cyanobacteria sp. 1, Stigonema sp., unknown sp. 2, Gomphonema spp.

unknown sp. 1

Unregulated with pools Fragilaria spp., unknown sp. 2, unknown sp. 1, Gomphonema spp.

Cyanobacteria sprout

1 week after flow Regulated with pools Fragilaria spp., Gomphonema spp., unknown Cyanobacteria sp. 1

Unregulated with pools Fragilaria spp., Gomphonema spp., unknown Cyanobacteria sp. 1

Regulated without pools Unknown Cyanobacteria sp. 1, Gomphonema spp., Fragilaria spp.

Unregulated without pools Fragilaria spp., Gomphonema spp., unknown Cyanobacteria sp. 1, Stigonema sp.,

unknown sp. 1

Taxa are listed in order of decreasing contribution to within sample variation. Only those contributing to >5% are shown.



(R = 0.435, P = 0.006), with each stream having a

distinct assemblage dominated by diatoms. In both

streams, diatom assemblages on laboratory stones

differed from those left in situ in each stream (Mt

William R = 0.624, P = 0.002; Bomjinna R = 0.367,

P = 0.004). This is likely to have resulted from the

different lengths of time each treatment was exposed

to inundation, as the laboratory stones were only

cultured for 48 h and had lower abundances of algae

than the in situ stones, which were exposed to flow for

a week. Interestingly, algal assemblages that grew on

transplanted stones in both streams did not differ from

those on in situ stones for that stream (Mt William

R = 0.046, P = 0.247; Bomjinna R = 0.128, P = 0.123).

That is, transplanted stones grew a diatom assemblage

characteristic of local conditions, not resembling the

assemblage that originated from their dry biofilm.

In Grevillea Creek and Barney Creek, experimental

treatments also differed significantly (Global R =

0.432, P < 0.001). Algal assemblages on laboratory

stones differed between streams (R = 0.507, P = 0.006),

with Barney Creek stones being dominated by fila-

mentous chlorophytes whereas Grevillea Creek stones

were diatom-dominated. When compared to in situ

stones, Barney Creek laboratory stones did not differ

from in situ in Barney Creek (R = 0.161, P = 0.067),

Unregulated + poolsRegulated + poolsUnregulated – poolsRegulated – pools

Stress: 0.1

Fig. 7 Non-metric dimensional scaling (NMDS) ordination

comparing algal assemblages after 1 week of flow across the

four stream categories. All 18 streams are included in this

example (n = 6 at each of two sites per stream).

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1

2

3

4

5

6

0

1

2

3

4

5

6

0

1

2

3

4

5

6

1 2 3 4 5 6 7 8 9Unregulated + pools Unregulated – pools Regulated + pools

0

1

2

3

4

5

60

Lo

g10

(T

ota

l alg

al d

ensi

ty)

Lo

g10

(Cya

no

bac

teri

a)L

og

10 (C

hlo

rop

hyt

es)

Lo

g10

(Bac

illar

iop

hyt

a)L

og

10 (S

pec

ies

rich

nes

s)Fig. 6 Log-transformed [log10(x)] total algal densities, species


pled after 1 week of flow from regulated streams with pools,

unregulated streams with pools and unregulated streams with-

out pools (each bar represents a site, each paired bar represents a

stream, n = 6 rocks sampled from each of two sites per stream,

units = no. individuals per 36.3 cm2). All data represent least

square means ± 1 SE. Note: there was only one regulated stream

without pools available for sampling (1st Wannon Creek,

Table 1), so it was omitted from the analysis. Note: scale on y-

axis differs for species richness.



whereas Grevillea Creek laboratory stones did differ

from their in situ counterparts (R = 0.35, P = 0.011), as

in the experiment above. However, when in situ stones

from Grevillea Creek were compared to stones trans-

planted from Grevillea Creek to Barney Creek, there

was only a small, though significant, difference in the

algal assemblage (Barney Creek stones R = 0.235, P =

0.019) and the transplanted stones differed strongly

from those they were surrounded by in Barney Creek

(R = 0.563, P = 0.002). Therefore, in Barney Creek

(which was dominated by filamentous chlorophytes),

the in situ stones grew filamentous chlorophytes

similar to laboratory stones. The transplanted stones

placed in Barney Creek from Grevillea Creek grew a

diatom assemblage similar to that in Grevillea Creek,

indicating that regrowth of algae on these stones was

also dependent on the dry biofilm and that there was

little or no recolonization from the drift in Barney

Creek. In contrast, in Grevillea Creek, diatom drift was

the dominant pathway because the stones trans-

planted from Barney Creek to Grevillea Creek did

not differ from those in situ in Grevillea Creek

(R = 0.039, P = 0.316) and differed strongly from those

in Barney Creek (R = 0.68, P = 0.002), showing no

sign of the filamentous chlorophyte assemblage that

originated from Barney Creek dry biofilm.

Lastly, treatments for the Grevillea Creek and Dairy

Creek experiment also differed significantly (global

R = 0.676, P < 0.001). Both streams were diatom-

dominated, with distinct assemblages (laboratory

stones from both streams: R = 0.989, P = 0.002). Lab-

oratory stones and in situ stones from both streams

showed significant differences in their algal assem-

blages (Dairy Creek R = 0.35, P = 0.004; Grevillea

Creek R = 0.35, P = 0.011), probably because of the

different length of immersion, with notably lower

abundances on the laboratory stones. However, algal

assemblages on transplanted stones from the two

streams did not differ from each other (R = 0.276,

P = 0.05), but their in situ assemblages did differ

(Dairy Creek in situ compared with stones trans-

planted to Grevillea Creek R = 0.794, P = 0.002; Gre-

villea Creek in situ stones compared with stones

transplanted to Dairy Creek R = 0.659, P = 0.004),

showing that sufficient recolonization from algal drift

was occurring to create an intermediate assemblage

on the transplant stones that was a mixture of

regrowth from dry biofilm and arrivals from the drift.

Interestingly, while this intermediate assemblage on

transplanted stones differed markedly from the in situ

stones in Dairy Creek (R = 0.887, P = 0.002), it did not

differ from the in situ stones in Grevillea Creek

(R = 0.29, P = 0.058) or from the laboratory stones

from Grevillea Creek (R = 0.29, P = 0.05), because it

was more variable than the other treatments. There-

fore, diatom recolonization was occurring from both

the drift and dry biofilm in both streams, but diatom

taxa prevalent in Grevillea Creek were more success-

ful colonists than those from Dairy Creek, regardless

of the recolonization pathway.

Relationship between algal drift and benthic algal

composition

Individual drift and epilithic sample densities ranged

from 1.53 to 64.71 algal units per litre (n = 48) and

1870–1 788 480 algal units per sample (n = 48), respec-

tively. A total of 25 individual morphogenera from

four algal divisions were identified (Table 3), com-

prising similar taxa to the earlier components of the

study. Bacillariophyta was the dominant division

within both drift and epilithic samples, as well as

being represented in all samples collected from the

eight creeks (Table 1). Diatoms were also observed

‘rafting’ on both chlorophyte and Cyanobacteria fila-

ments, and also on leaf fragments. Chlorophyta was

the second most dominant division, with an uniden-

tified chlorophyte (sp. 11) present in both sampling

methods across all creeks. Green algae also contri-

buted a greater proportion of the algal units present in

the drift than their proportion of the epilithic assem-

blage. Cyanobacteria were a minor component of both

drift and epilithic samples. Stigonema sp. was the most

common cyanobacterial taxon, being absent only from

drift samples in 2nd Wannon Creek (Table 3).

Stream drift assemblages from the eight streams

showed significant differences (Global R = 0.792,

P = 0.001). Pairwise comparisons of drift assemblages

found that 2nd Wannon Creek was significantly

different to all other creeks (all R > 0.864, all

P = 0.002). There was a range of differences between

the bacillariophyte-dominated streams with Grevillea

Creek and Mt William Creek being significantly

different (R = 0.988, P = 0.002) whereas Mt William

Creek and Reservoir South Creek were more similar

(R = 0.327, P = 0.009). Grevillea Creek and an

Unnamed Creek were the only two that were not

clearly different (R = 0.259, P = 0.059).



Algal assemblages from epilithic surfaces differed

from drift assemblages (Global R = 0.144, P = 0.001).

The taxonomic composition of epilithic and drift

samples differed within each stream (R = 0.2– 0.6,

all P = 0.001) with the exception of Bomjinna Creek,

where the composition of the drift samples resembled

epilithic samples closely (R = )0.02, P > 0.05). In six

of the eight streams, densities of drifting algae

reflected epilithic algal densities, but drift densities

in the remaining two streams were either lower

(Reservoir Creek) or higher (2nd Wannon Creek) than

expected (Fig. 8).

Discussion

Drought refuges for stream algae

All taxa present on stream stones after 1 week of flow

were also found in at least one of the three drought

refuges (i.e. leaf packs, dry epilithic biofilm and living

biofilm in pools). Therefore, other potential drought

refuges (woody debris, moss and dry sediment) were

probably unimportant in algal recolonization because

they covered a much smaller area of the streambed

than leaf packs or epilithic biofilm. However, these

other drought refuges may be prominent in other

rivers; for example, woody debris has been shown to

be important in the lowland Murrumbidgee River,

Australia, and may provide a refuge under conditions

of moderately frequent fluctuations in river level

(Ryder, 2004).

The composition of assemblages in leaf pack and

dry biofilm refuges correlated poorly with assem-

blages that developed a week after flows resumed.

However, there was a stronger correlation with the

pool flora. This was possibly the product of higher cell

densities on pool stones compared to stones or leaves

cultured for 48 h, because there was a large overlap

between the flora of pools and cultured dry biofilm.

However, assemblages in these refuges were not

Table 3 Taxa identified in epilithic (E) and drift (D) samples

Taxa 2nd Wannon Bovine Grevillea Barneys Bomjinna Unnamed Mt William Reservoir South

Chlorophyta

Chlorophyta sp. 1 ED ED D ED ED ED ED ED

Chlorophyta sp. 2 ED D D ED E D E ED

Chlorophyta sp. 4 ED – – E ED – – D

Chlorophyta sp. 5 D D – ED ED E D –

Bulbochaete sp. E – – D ED – – D

Closterium sp. D D – – ED – – –

Chlorophyta sp. 10 ED D D D E D – –

Chlorophyta sp. 11 ED ED ED ED ED ED ED ED

Chlorophyta sp. 12 D – – – – D – –

Chlorophyta sp. 13 D ED ED ED – ED ED ED

Chroodactylon sp.* – – – – – – E D

Bacillariophyta

Achnanthidium sp. ED ED ED ED ED ED ED ED

Gomphonema sp. 1 ED ED D ED ED – ED ED

Gomphonema sp. 2 D ED – ED ED – D E

Fragilaria sp. 1 E D ED ED ED ED ED ED

Fragilaria sp. 2 ED ED D ED ED ED ED ED

Fragilaria sp. 3 ED D – E – D ED –

Navicula sp. – ED – ED ED – ED ED

Cyanobacteria

Stigonema sp. E ED ED ED ED ED ED ED

Hapalosiphon sp. E ED – E – D – –

Cyanobacteria sp. 1 E D D – – D – –

Loefgrenia anomala Gomont – – – – D E – D

Cyanobacteria sp. 2 – – – – – D D –

Cyanobacteria sp. 3 – – – – – D – –

Nostoc sp. – – – – – D D D

Bold letters indicates species found either in drift or in epilithic samples; –, not collected.

*Rhodophyta.



particularly distinct, nor were they very different to

what grew after flows resumed, so it appears that

these taxa showed relatively little specificity for

different refuges. The predominance of diatoms in

leaf packs was interesting, but probably arose more

from this relatively unstable, short-lived refuge being

a poor substratum for filamentous algae than to any

‘preference’ shown by diatoms.

The role of permanent surface water (pools) in these

streams was less significant than expected, since few

taxa were found only in pools. Furthermore, pools

appeared to have limited influence on the density or

composition of regrowth once flows resumed across

the 18 streams analysed. Therefore, we found limited

evidence to support the conservation of pools (either

natural or artificial weir pools) as refuges. However,

pools are a vital refuge for some stream-dwelling

animals.

The effect of regulation

Robson & Matthews (2004) showed a strong positive

relationship between the presence of permanent

surface water and the density of algal regrowth after

a week of flow. However, the role of regulation was

unclear, as both streams with permanent surface

water were also regulated. Results from the present

drought refuge survey of a larger number of streams

confirm that higher cell densities within dry biofilm

coincide with stream regulation and that permanent

pools have a limited influence on density in epilithic

biofilm. Unregulated streams did not differ in mean

dry algal density, regardless of the presence or

absence of permanent pools, but regulated streams

with permanent pools did have higher densities.

Therefore, it is unlikely that the contribution of

drifting cell units washed from permanent pools is

responsible for the increased algal regrowth observed

by Robson & Matthews (2004). Rather, it appears that

permanent pools in regulated streams alter the role of

epilithic dry biofilm refuges by increasing their

quality (density of cells) and hence, capacity for

regrowth and possibly the rate of entrapment (Peter-

son, 1987).

This was particularly apparent for Cyanobacteria,

which were most dense in regulated streams with

pools and least dense in unregulated streams with

pools; unregulated streams without pools had inter-

mediate dry biofilm densities. Cyanobacteria are

known to be very resilient to frequent and rapid

drying (Blinn et al., 1995) and to be strongly reliant on

their persistence in the dry biofilm for regrowth

(Robson, 2000). They are also less prone to senescence

(and sloughing, Peterson et al., 1994), thereby increas-

ing the persistence of cyanobacterial mats. Therefore,

it is likely that part of the increased cell density in dry

biofilm in regulated streams is often, but not always,

due to Cyanobacteria responding positively to rapid

declines in water depth, as opposed to other taxa that

cannot tolerate rapid fluctuations in water levels.

Other algal taxa can also be abundant in the dry

biofilm of regulated streams, so this is not the only

mechanism responsible for increased cell densities in

regulated streams.

Several other mechanisms might explain increased

biofilm cell densities in regulated streams. Increased

reliance on dry biofilm could be induced by frequent

cessation and resumption of flows resulting from

5

10

15

20

25

30

0 200 000 400 000 600 000 800 000 1 000 000 1 200 000

Epilithic algal densities (units sample–1)

Algal density in drift (units. L–1)

1. 2nd Wannon 2. Bovine 3. Grevillea 4. Barneys 5. Unnamed 6. Bomjinna 7. Mt William 8. Reservoir South

1

7

3

4

2

68

5

Fig. 8 Average drift densities versus

average epilithic algal densities per stream

(±1 SE).



regulation (Dairy Creek and First and Second Wan-

non Creeks; B.J. Robson and T.G. Matthews, pers.

obs.). Regulation may also alter the onset of drying in

these streams: stream drying is likely to occur earlier

toward the end of the flow season for regulated

streams (spring–early summer) rather than later in the

summer for unregulated streams. This is likely to

reduce rates of desiccation and, hence, improve

successful regrowth from the dry biofilm in regulated

streams following the recommencement of flow.

Furthermore, regulated streams generally have a

lower discharge and the effects of small-medium

spates are damped compared to unregulated streams

(Hughes, 2005). This may reduce losses to scour

during the flow season, leading to higher cell densities

at cessation of flow. Matthaei, Guggelberger & Huber

(2003) showed that local disturbance history had a

greater effect on epilithic algal biomass than did

substratum particle size, water depth or near-bed

velocities. Peterson (1987) found that diatom-domi-

nated algal assemblages grew more densely in areas

sheltered from current in a regulated river, suggesting

that low, constant regulated flows might increase dry

biofilm biomass. However, Peterson (1987) also

showed that assemblages that developed in slower

flows were less resistant to both short-term desicca-

tion and scour.

In the past, it has been assumed that prolonged

periods of drying in seasonally-flowing streams that

result from water abstraction will not be a problem,

because biota will be adapted to this drying. This

appears to be true for algae since they are able to use a

range of refuge types, are adapted to desiccation and

produce a dry biofilm that will be viable in the

following year. However, as mentioned previously,

there are many instances where reduced flows in

regulated streams appear to favour the development

of a dry biofilm with a higher biomass and proportion

of Cyanobacteria. This is of concern since Cyano-

bacteria are less palatable to grazers than most algae

(e.g. Benenati et al., 1998; Peterson & Boulton, 1999).

Sources of recolonization

The reciprocal transplant experiment showed differ-

ent sources of colonists in different streams. In part,

this could be related to the structural composition of

the biofilm assemblage, as also shown by Peterson

(1996b). For example, recolonization in Barney Creek

occurred entirely from the dry biofilm on both in situ

and transplanted stones. This may be explained by

Barney Creek being dominated by filamentous green

algae and very few diatoms, so that drift, at least in

the first week of flow, was minimal. In contrast,

other streams were diatom-dominated and showed

either dependence on drifting propagules or a

mixture of taxa arriving from the drift or regrowing

from biofilm.

Sampling drifting cells later in the flow season

showed that propagule numbers were proportional

to benthic density in most streams, but both epilithic

and drift densities were lower than those reported in

previous studies of perennial streams (e.g. Peterson

et al., 1994; Peterson, 1996b). Cyanobacteria were

equally rare on the bed and in the drift, whereas

they were more common in drought refuges and

also during the first week of flow. This is consistent

with the ability to colonize by gliding (Peterson,

1996a) and to resist both scour and desiccation

(Robson, 2000; Stanley et al., 2004). It also suggests a

pattern of passive, rather than active drift (Peterson,

1996b) and, hence, Cyanobacteria appear to be more

dependent on recolonization from dry biofilm than

drift.

Both chlorophytes and diatoms were common in

winter drift, and chlorophytes also drifted more

frequently than their benthic densities suggested.

Filaments rather than spores dominated, and were

probably broken from parent plants by the current.

However, these broken filaments cannot reattach to

surfaces and so cannot colonize stones. Therefore, it

seems likely that most initial regrowth of filamentous

chlorophytes is from dry biofilm. In contrast, diatoms

were shown both to regrow from biofilm and to

colonize from the drift. The process of recolonization

used by diatoms may depend on the specific taxa

present or on characteristics of past or present flow

regimes.

Drift sampling showed that the composition of

algal drift differed from benthic composition in most

streams. This indicates that only a subset of the algal

assemblage is drifting under normal (non-spate)

conditions and, as there was no consistent difference

in drift rates between diatom-dominated and fila-

mentous-algae dominated streams, this subset of taxa

is a mixture of algal divisions. Therefore, this

suggests that, although there is little specificity

among these algae for particular drought refuges,



there are differences in colonization pathways among

taxa. Different diatom species have been shown to

differ in their patterns of active drift (Peterson,

1996b).

A model for pathways of recolonization in intermittent

streams

Together, results from the four components of this

study provide a model for the recolonization path-

ways for epilithic algae in seasonally-flowing streams

including modifying effects of stream regulation

(Fig. 9). Our results clearly show that stream algae

depend on refuges, but there appears to be consider-

able flexibility with respect to their use during

droughts. Of the three refuges studied, leaf packs

appear to contribute least to post-flow algal growth.

The relative importance of in situ regrowth and

recolonization via drift varies between streams. Recol-

onization does occur immediately from refuges (dry

biofilm or pools) after flows resume, due to ‘wash

out’, but this is mainly diatoms, not other algal

divisions. Arrival of these drifting algae onto stones

makes a substantial contribution to algal regrowth in

some streams. For example, drifting algae, rather than

regrowth from the biofilm, was the dominant recol-

onization source in Bomjinna, Mt William and Gre-

villea Creeks (Fig. 9). Why some diatom-dominated

streams show a stronger dependence on drift than

others (e.g. Dairy Creek) is less clear. However, Dairy

Creek is a regulated stream with a particularly erratic

flow regime (B.J. Robson & T.G. Matthews, pers. obs.),

so the increased dependence on dry biofilm that

appears to result from stream regulation (discussed

above) may lead to the apparent mixture of diatom

regrowth observed from both drift colonists and dry

biofilm.

By identifying the pathways of recolonization, we

have constructed a model that can be tested

elsewhere (Fig. 9). Although we sampled a large

number of streams, one weakness of our sampling

was that we did not sample over more than one

dry-to-flow transition. Knowledge of recolonization

processes is fundamental to understanding how

algae will adjust to changes in flow regime, includ-

ing those arising from climate change and increased

abstraction. For example, in regions where the

climate is predicted to become drier and where

rainfall is predicted to occur in shorter, more intense

bursts, an increased reliance on regrowth from dry

biofilm may be anticipated because such changes

will be similar to those that have already occurred in

regulated streams. Further challenges remain,

including testing of the general applicability of this

model and to determine whether these changes in

colonization processes have subsequent impacts on

assemblages of stream animals.

Acknowledgments

This research was funded by the Glenelg-Hopkins

Catchment Management Authority and conducted

with appropriate permits from Parks Victoria. We

would like to thank Amanda Cowell and Brett

Downey for assistance in the field and Gerry Quinn

for helpful comments on the manuscript.

Dry UnregulatedFlowing

RegulatedFlowing

leaves

dry biofilmon stonetops

pools

driftingdiatoms

driftingdiatoms

Driftingdiatoms

regrowth

regrowth regrowth

in situregrowth insitu

Fig. 9 Model for the pathways of recolonization for epilithic algae in seasonally-flowing streams. When streams are dry, three

refuge types are sufficient to ensure the survival of all algal taxa subsequently growing on stones. Once streams resume flow,

both drifting algae and regrowth from dry biofilm assist recolonization, the relative magnitude of each depends on the

composition of the algal assemblage in each stream. Pools and dry biofilm are the main source of drifting colonists. Flow

regulation increases the dependence on regrowth from dry biofilm. Thicker arrows and bold type indicate stronger pathways.



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(Manuscript accepted 13 June 2008)



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Pathways for algal recolonization in seasonally-flowing streams

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