Effects of catchment clearing and sedimentation on macroinvertebrate communities of cobble habitat in freshwater streams of south Western Australia

2005·4.5 MB·English

by  K N Armstrong

Journal of the Royal Society of Western Australia, 88:1-11, 2005
Effects of catchment clearing and sedimentation on
macroinvertebrate communities of cobble habitat in
freshwater streams of southwestern Australia
K N Armstrong, A W Storey & P M Davies
School of Animal Biology (M092), The University of Western Australia, Crawley, WA 6009.
(Manuscript received September 2002; accepted Map 2005)
Abstract
The removal of riparian vegetation from along first order streams of the northern jarrah forest
reduced ecological health, as assessed by an examination of cobble communities. Macroinvertebrate
diversity was significantly lower in cleared compared to uncleared reaches. There was also an
associated decrease in the biomass and a change in the composition of epilithon communities, from
those dominated by the angiosperm Potamogeton ?crispus to thin slimes dominated by diatoms.
Extensive growths of filamentous algae were not observed on cobbles in cleared reaches. Species of
Odonata, Ephemeroptera, Trichoptera and Chironomidae were the most notable absences from
cobbles in cleared reaches. Grazer abundance also was reduced. Few species in cobble habitats
appeared to benefit from catchment clearing although there was a significant increase in the
abundance of the undescribed chironomid Orthocladiinae V61, which may be a useful indicator
species. We also tested the effect of short term elevations in suspended sediment through
experimental addition of sediment in an attempt to separate the effects of sediment from others
related to catchment clearing. There was no significant difference in macroinvertebrate diversity
between control and sediment-added cobbles, and both had higher diversity than cobbles in cleared
reaches. We concluded that the changes in the epilithic cover in cleared reaches caused indirectly
the changes in the macroinvertebrate community, either through a change in the composition of
food sources or loss of refugia. Sedimentation in cobble environments might have greater impact in
the longer term than noted in this study, and also might have impact on other stream meso-
habitats.
Keywords: macroinvertebrate communities, freshwater streams, cobble habitats, catchment
clearing, southwestern Australia
Introduction 1972; Karr & Schlosser 1978; Lemly 1982; Campbell &
Doeg 1989; Hogg & Norris 1991; Quinn et al 1992).
The importance of riparian zones is well recognised in Increased sediment loads are known to have detrimental
stream ecology. They provide a critical link between effects on both aquatic macroinvertebrates (e.g.
terrestrial and aquatic ecosystems (Lynch & Catterall McLelland & Briisven 1980; Lenat et al 1981; Lemly 1982)
1999), and largely control the functioning of the latter. and aquatic plants and algae (Lewis 1973a,b; Nutall 1972;
Riparian cover has a strong influence on stream Graham 1990). In many instances, stream
productivity (Bunn et al. 1999a), providing shade that macroinvertebrates respond to increased sediment levels
maintains lower water temperature (Bunn et al 1999b) by drifting (e.g. Gray & Ward 1982; Doeg & Milledge
and reduces light penetration, thereby controlling algal 1991).
growth (Mosisch et al 1999). Riparian zones also provide
Removal of streamside vegetation can have different
an important allochthonous food source for some aquatic
effects among mesohabitat scale communities (Lenat et
animals (Lake 1995) and increase habitat heterogeneity
al 1981; Lemly 1982). The present study focussed on the
(Likens & Bormann 1974; Lake & Marchant 1990;
cobble meso-habitat (sensu Cummins 1962), which has
Gregory et al 1991). Vegetated stream banks are more
been identified as a discrete 'natural unit' due to a 'well
stable than those without vegetation (Abernathy &
defined, abrupt boundary' (Davies 1994). Cobbles are
Rutherford 1999). They moderate the flow of sediment
characterised by a comparatively high projection and
and nutrients into the water column (Gregory et al 1991;
occur in regions of relatively high shear velocity in the
Waters 1995), and filter nutrients from surface and
stream channel (Davies 1994). A unique feature of
groundwater flows (Prosser et al 1999). In eastern
cobbles is the provision of a stable substrate which
Australia, even narrow riparian buffers are effective at
enables growth of epilithic algae and aquatic plants.
trapping overland flows of sediment (Prosser et al 1999).
Cobbles also provide a stable platform from which sessile
Loss of or disturbance to riparian vegetation therefore animals, such as filter feeders, may extract suspended
has many effects, one of the more obvious being the food particles from the water column (Wallace & Merritt
periodic increase in turbidity and bed-load sediment 1980).
transport, particularly during rainfall events (Brown
In the northern jarrah forest, macroinvertebrate and
epilithic communities of cobbles are distinct from those
© Royal Society of Western Australia 2005 of other stream habitats, being dominated by animals in
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Journal of the Royal Society of Western Australia, 88(1), March 2005
scraper and filterer feeding guilds in contrast to the general geology and flora of the area has beeq
dominance of collectors in riffles, and collectors and summarised by Bunn (1986) and Storey et al. (1990).
shredders in organic depositional habitats (Davies 1994).
The North Dandalup system has been regulated fron)
Cobbles are also characterised by a greater proportion of
1971 by a small pipehead dam (capacity 20 000 m3).
autochthonous (instream-derived) vegetative material
However, to serve the growing demand from the Pertl)
compared with riffle and organic depositional habitats.
metropolitan area, the pipehead dam was replaced ii)
The production to respiration ratio of cobble habitat is
October 1994 by the North Dandalup Dam, a 62 m high
typically greater than one, indicative of autotrophy (f.e.,
rock-fill dam wall, forming a major water supply
having a carbon source predominantly derived from
reservoir (catchment area 153 km2, 510 ha surface area af
instream photosynthetic processes; Davies 1994). In
full storage and a capacity of 75 GL), supplying 10% of
upland streams of the northern jarrah forest, the low
the annual water consumption of Perth.
nutrient status of soils and shading by the riparian
canopy minimise the standing crop of epilithon
Sampling
compared with streams in eastern Australia (Bunn and
The experimental design comprised three treatments;
Davies 1990). These characteristics make cobbles a useful
undisturbed upstream controls in uncleared reaches
unit for assessing the effects of removal of streamside
(CON), sediment addition in uncleared reaches (SED),
vegetation on both macroinvertebrates and epilithic
and cleared or 'impact' reaches (IMP). Each treatment
vegetation.
was replicated at three sites on each of the three streams,
The construction of the North Dandalup Dam (south Each replicate consisted of three cobbles sampled af
of Perth, Western Australia) provided an opportunity to random from the stream bed, giving a total of nine*
investigate the effects of riparian vegetation removal on cobbles per treatment per stream and a total of 81 cobbler
aquatic macroinvertebrate communities in northern sampled. Storey et al. (1990) have shown that
jarrah forest streams. Dam construction involved the total macroinvertebrate communities of northern jarrah forest
clearing of 430 ha of riparian and upslope vegetation streams are generally homogeneous within the sam^
within the reservoir basin. It was predicted that elevated stream order, and therefore it was anticipated that th^
sedimentation associated with clearing would be three streams would be comparable replicates. All impact
detrimental to both the fauna and epilithon of cobbles. sites necessarily were located downstream of uncleared
We compared the diversity of cobble macroinvertebrates areas but within the same stream order. Sediment and
and the composition of epilithon between cleared and control treatments were interspersed within th^
uncleared reaches. It was also predicted that elevated uncleared reaches. The sediment addition experiment
sediment levels in the water column would have the was performed once only for each cobble between th<*
greatest effect on the cobble habitat, since they are 29th July-3rd of August and the lOth-llth of August,
exposed above the streambed. Thus, we examined the 1995. Other cobbles were sampled whilst moving
short term effect of suspended sediment addition on the upstream at this time, so that no unsampled cobbles were*
aquatic macroinvertebrate communities of cobble left downstream.
substrates in an attempt to distinguish this from other
The short term elevation in turbidity was mimicked
effects resulting from the removal of riparian vegetation.
by the slow addition of fine (<2 mm) sediments to
channel reaches containing cobbles. Approximately 60 kg
of lateritic soil was washed into the water column over a
Methods
two-hour period at distance of c. 1 m above the cobbles to
be sampled. Total suspended sediment level was
Study area measured from water samples (dry weight per litre)
The study area, situated c. 100 km south of Perth, collected downstream of the point of sediment addition
consisted of three first order streams (Foster and Wilson to assess the effectiveness of the treatment. Cobbles were
Brooks and the North Dandalup River; Fig 1) in the sampled two hours after the commencement of the
North Dandalup catchment, a system of about 40 km in sediment addition by removal of the cobble, using a D-
length. The headwater streams arise in swamps on the frame pond net (250 mm mesh aperture) placed
Darling Range (300 m above sea level), the south-western immediately downstream.
edge of the Great Plateau of Western Australia (Jutson Within the cleared reaches, there were numerous
1950). The streams flow over lateritic soils and granite cobbles that had only recently entered the stream
bedrock before descending through an incised river through bank erosion/slumping or through the influence
valley to the Swan Coastal Plain. The headwaters are of heavy machinery. These lacked an obvious layer of
located in the northern jarrah forest, a dry sclerophyll
epilithon, were not embedded in the stream and
forest dominated by jarrah Eucalyptus marginala Sm. and therefore were not sampled; only cobbles of lateritic
to a lesser extent marri Corymbia calophylla (Lindl.) K.D.
composition with an established epilithic layer were
Hill & L.A.S. Johnson. The riparian understorey is
sampled. Attached fauna were removed into a basin by
composed of dense sclerophyllous shrubs, including
scrubbing the cobble with a soft brush, and epilithon was
Agonis linearifolia (DC.) Sweet, Lepidosperma tetraquetrum
removed with a harder brush and immediately preserved
Nees and Hypocalymma angustifolium (Endl.) Schauer. The
in 5% formalin. Macroinvertebrate specimens were
upper catchment has been selectively logged since identified to lowest possible taxonomic level (species,
European settlement and was the site of a small gold
where possible), and vouchers lodged with the School of
strike in the late 1800s. The area has a Mediterranean
Animal Biology, The University of Western Australia.
climate with wet, mild winters, hot, dry summers, and
Taxa were then allocated to their appropriate functional
an annual rainfall of approximately 1100 mm. The
feeding groups (ie. filterer and grazer; sensu Cummins
2
Armstrong, Storey & Davies: Macroinvertebrates of stream cobbles
Figure 1. Location of the North Dandalup catchment and first order streams sampled in Western Australia. Bars indicate extent of
catchment clearance in each stream, closed circles indicate sites where impact cobbles were sampled and open circles where control
and sediment treatment cobbles were sampled.
1973, Merritt & Cummins 1978, Bunn 1985, 1986). To assay using the molybdenum blue method; Wetzel &
standardise the abundance of fauna between different¬ Likens 1979) and total Kjeldahl nitrogen (by digestion
sized cobbles, the surface area of each cobble was with ammonia subsequently assayed using an ammonia-
determined in the field by wrapping it in aluminium foil sensitive electrode; Bremner & Mulvaney 1982) were
and relating the weight of the foil to surface area by measured from each stream. Two samples from the water
regression. Only the proportion of the rock surface surface were taken at the downstream end of each
exposed to the water was measured. treatment area.
Physico-chemical variables were measured at each
Analysis
cobble before sampling. Conductivity (EC) dissolved
oxygen (DO) and pH were measured with WTW For statistical analysis, the type of epilithic cover was
conductivity, dissolved oxygen and pH meters recorded as either 'algae', 'diatom matrix','Potamogeton',
respectively, and stream velocity at 5 cm above the 'Nostoc' or 'no epilithon'. Chi-square analysis was used
substrate with a Marsh McBirney portable water flow to detect departures in observed from expected frequency
meter (model 201). Total Phosphorus (by potassium of occurrence for each epilithon type in each treatment.
persulphate digestion with subsequent orthophosphate Two-way ANOVA was used to test for treatment and
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Journal of the Royal Society of Western Australia, 88(1), March 2005
sub-catchment (stream) differences in epilithon biomass Epilithon
(per square metre), as well as macroinvertebrate
Four dominant types of epilithic growth wer^
community parameters (abundance, richness, Shannon-
recorded. Most cobbles in uncleared reaches supported
Weiner diversity (Shannon & Weiner 1949), chironomid
growths of the aquatic angiosperm Potamogeton ?crispus
richness and abundance, and the abundance of filterers
(Potamogotonaceae). When present, this specie^
and grazers), with 'treatment' and 'stream' as fixed
dominated the cobble epilithon community. On cobble$
factors. Where there were significant main effects,
where Potamogeton ?crispus was absent, chlorophytes
Tukey's HSD multiple range tests were applied a
were the dominant epilithic growth. In this study,
posteriori to detect treatment and sub-catchment
chlorophytes were classified into two main types; ‘NostPd
differences in community parameters. All data were
which had a nodular growth form similar to that of
l°8io(x+l) transformed prior to analysis to achieve
Nostoc and 'algae' which included all remaining
equality of variances (Cochran's C test for
Chlorophyta. The majority of cobbles in cleared reaches
homoscedasticity). All the above analyses were
lacked an epilithon layer although many were covered iq
undertaken in the SAS statistical package (SAS, 1994).
a thin, slimy matrix of diatoms, bacteria and accumulated
Ordination of samples based on macroinvertebrate
sediments. The sediment addition was a short term
community composition was undertaken using hybrid
treatment, and therefore had no effect on epilithon.
non-metric Multi Dimensional Scaling (MDS) in the
PATN analysis program (Belbin 1995), with pairwise There was a significant departure from expected
similarities amongst samples calculated using the Bray- frequency for the types 'Potamogetort, 'diatom matrix'
Curtis similarity index. The separation of treatments in and 'no epilithon' (Table I). In each instance, tlm
ordination space was tested using Analysis of Similarity departure from expected occurred in the impact reach,
(ANOSIM) (Belbin, 1995). Gradients in environmental with more than the expected number of cobbles
variables (DO, EC, pH and stream velocity) and dominated by 'diatoms' or with 'no epilithon', and fewer
community descriptors were fitted through ordination than expected cobbles being dominated by 'Potamogeton'
plots using the Principle Axis Correlation (PCC) option (Table 1). Associated with the change in epilithon type
in PAFN, with Monte Carlo randomisations (n=100) of was a decrease in biomass in the impact treatment (Table
the data performed to test the significance of the 2).
gradients. A separate ordination was performed for each
subcatchment (stream) due to the observed differences Macroinvertebrate fauna
between streams. Ordinations were performed to The total number of taxa and total abundance per m2
minimise dimensions and stress, and plotted to display recorded in controls (98 taxa and 26 556 individuals m2)
those dimensions that illustrated the greatest separation was greater than in both sediment and impact treatments
of treatments.
(87 taxa and 26 748 individuals nr2, and 72 taxa and 12
498 individuals m2 respectively). The proportion of taxa
in common between control and impact treatments was
Results 66 %, and between control and sediment addition
treatments was 86 %. Notable absences from impact
Physico-chemical cobbles compared to controls included the mayfly
The turbidity of the water column two metres Bibu/mena kad/ina, the trichopteran P/ectrotarsus minor, the
downstream from the point of sediment addition dragonfly Argioiestes minimus, the caddisfly Ecnomina
increased significantly during sediment addition sentosa type group and the chironomid Rietiiia Fzepiandica.
compared with control and cleared sites (two-way All community parameters showed a significant
ANOVA: F225 = 4.5, p = 0.03; means ± S.E.: SED 26.8 ± 4.0 treatment effect, and the majority a significant stream
mg L*1; CON 12.0 ± 4.6 mg L1; IMP 11.0 ± 2.4 mg L’1). effect, with no significant interactions (Table 2). In
Bedload transport was also observed to be much higher general, the impact treatment had the lowest taxonomic
(as might be expected) but was not quantified. Total richness and total densities of any parameter (Figure 2).
phosphorus and total Kjeldahl nitrogen were both <10 There were 20 taxa with significant subcatchment and/or
mgL 1 for all treatments (K. Armstrong unpublished treatment differences in abundance (Table 3). Of these,
data).
10 species had a higher abundance in control than impact
Table 1
Analysis of the frequency of occurrence of dominant epilithon types on cobbles from Control, Impacted and Sedimented reaches,
giving % statistic, significance level and Observed/Expected number of occurrences in each treatment. *: indicates the cell that has the
greatest contribution to the total x2-statistic.
Epilithon Type X2 P Control Impact Sediment
Algae 0.75 ns 3/3 4/3 2/3
Diatom matrix 15.32 0.0005 0/2.3 7 / 2.3* 0/2.3
Nostoc 1.28 ns 6/4.3 3/4.3 4/4.3
Potamogeton 32.34 < 0.0001 18/13 1/13* 20/13
No epilithon 24.37 < 0.0001 0/4.3 12 / 4.3* 1 / 4.3
4
Armstrong, Storey & Davies: Macroinvertebrates of stream cobbles
Table 2
Analysis of various community parameters by sub-catchment (F: Foster, W: Wilson and N: Nth Dandalup) and treatment (C: Control, I:
Impact, S: Sedimented). Tukey's multiple comparison test was used to locate between-level differences for significant main effects. The
order of levels within each parameter has been standardised according to that indicated for the first parameter, and a common line joins
levels not significantly different. Significance of main effects and interaction terms are indicated by: ns: not significant, *: P < 0.05, **: P <
0.01 and ***: P < 0.001.
Parameter Sub-catchment Treatment Interaction Sub-catchment Treatment
(df=2) (df=2)
Epilithon biomass (m2) 7.0** 9.4*** 0.7ns W N F C S I
Abundance (m2) 7.4** 10.7*** 1.7ns
Number of taxa 4.3* 26.2*** 2.5ns
Shannon-Weiner Diversity 0.9ns 16.7*** 1.7ns
Chironomid richness 14.5*** qx-x-x- 1.3ns
Chironomid abundance (m2) 2.5ns 3.4* 1.1ns
Grazer abundance (m2) 18.9*** 18.2*** 2.5ns
Filterer abundance (m2) 3.8* 5.1** 0.03ns
Table 3
Differences in the abundance of selected invertebrate species (F: Foster, W: Wilson and N: Nth Dandalup) and treatment (C: Control, I:
Impact, S: Sedimented) on log]0(x+l) abundance of each taxon (ns: p>0.05, *: p<0.05, **: p<0.01, ***: p<0.0001). The order of levels within
each parameter has been standardised according to that indicated for the first parameter, and levels of main effects not significantly
different, as determined by Tukeys multiple range (a<0.05) are joined by a common line. Voucher specimens are lodged in the Aquatic
Research Laboratory, School of Animal Biology, The University of Western Australia.
Taxon Sub-catchment Treatment Interaction Sub-catchment Treatment
A090102 ns X- ns W N F C S I
Austrosimulium sp. X x- x-x ns
Ceratopogonidae sp. A x- x-x- ns
Ecnomus turgidus ns x-x- ns
ND 307 Hydracarina ND ? x- X- ns
Neboisiphlebia sp. Ngunurra sp. combined *** x-x-x- ns
Ostracoda sp. x- ns ns
Smicrophylax australica x-x- ns ns
Tasmanocoenis tillyardi x-x-x- ns ns
Botryocladus bibulmun ns x-x- ns
Thienemanniella sp.V19 x-x-x- ns ns
Nilothauma sp.V21 x-x- ns ns
?Paralimnophyes sp.V31 x-x-x- x-x-x- ns
Limnophyes pullulus V42 x-x- ns ns
Dicrotendipes sp.V47 x-x- ns ns
Riethia sp. nov. V5 ns x-x-x- ns
Tanytarsus spp.V6 x-x-x- x-x- ns
Orthocladiinae V61 ns X- ns
Stempellina ?australiensis ns X- ns
Orthocladiinae VND1 x-x- ns ns
5
Journal of the Royal Society of Western Australia, 88(1), March 2005
Total Relative Abundance
Total Richness
Chironomid Relative Abundance Chironomid Richness
Diversity
Grazer Relative Abundance
10 .
8 -
6
-
'?
CIS CIS CIS
Figure ^ of some community descriptors between treatments. Within each plot, histograms are grouped by stream
°r andalup and Wilson, respectively) and labelled according to treatment (control (C), impact (I) and sediment addition
(S)). Values are means ± 1 S.E.
6
Armstrong, Storey & Davies: Macroinvertebrates of stream cobbles
A Algae
a Diatom matrix
1.5
North Dandalup River o Nostoc
0 Potamogeton
(cid:9734) None
o Ml Control
1 o (cid:9632) Cleared
o O Epiphyte total • Sediment
(cid:9650)
Epiphyte area
0.5
CO O O
(cid:9632)o Velocity
A ©
-0.5
-T
-1.5 l--
-1.5 -1 -0.5 0 0.5 1.5
Dimension 1
A Algae
(cid:9633) Diatom matrix
1.5' Wilson Brook O Nostoc
o Potamogeton
6 None
Pill Control
1 (cid:9733) (cid:9632) Cleared
* Sediment
Conductivity
0.5 (cid:9733) (cid:9733) (cid:9632)
CO (cid:9733)
c
o O Temperature
CC/3 0
a>
E o 0
a AA O
-0.5
O
O A
-1 -0.5 0 0.5 1 1.5
Dimension 1
Figure 3. MDS ordination plots from PATN illustrating the similarity of each cobble sample. Physical variables appear as gradients.
Stress values: Foster: 0.19; North Dandalup: 0.16; Wilson: 0.14.
7
Journal of the Royal Society of Western Australia, 88(1), March 2005
treatment. The chironomid Orthocladiinae V61 showed a made. There was also no significant increase in nutrient:
positive response to catchment clearing, increasing inputs as a result of clearing. Increased nutrient inpuf
significantly in abundance in the impact reaches. has been associated with riparian removal in other
studies with the consequence of marked changes iA
Ordination ecosystem function (see Lake & Marchant 1990). In a
Initial analyses (two-way ANOVAs and ordinations) cleared pasture stream, Mosisch et al. (1999) reported
showed significant differences between streams. increased nutrients (mainly nitrogen) to be more
Therefore, streams were treated in isolation for important in promoting the growth of epilithic algae than
subsequent multivariate analysis. For each stream there light, although filamentous algae have higher light
was a clear separation of impact cobbles from both requirements than unicellular microalgae (Richardson et
control and sediment addition cobbles, indicating al. 1983; Langdon 1998; Hill 1996). The results of the
differences in the composition of macroinvertebrate present study are not surprising, however, given poof
communities following clearing (Figure 3). ANOSIM nutrient status of jarrah forest soils (Bunn & Davies 1990)
testing between treatments was significant in the case of and the short (<1 km) length of the cleared reach.
all three streams (Foster and North Dandalup: P<0.0001;
The sediment addition treatment in this study was
Wilson: PO.Ol). There was some separation of control
designed to mimic elevated turbidity levels in the water
and sediment addition cobbles in Foster Brook, however
column that would be expected during heavy rains in the
this pattern was not observed in the other streams. In
catchment when unconsolidated soils washed into
Foster Brook and North Dandalup River, epiphyte area
streams (pers. obs.). Several elevations of turbidity over a
was positively correlated with uncleared cobble number of days may have a greater effect on cobble
treatments, whereas an increase in the variables communities than the single addition performed for this
conductivity, DO and temperature were associated with
treatment. Other factors such as the long term effect of
cobbles in cleared reaches generally. These patterns
deposited sediment on or around cobbles may also be
suggest that the removal of riparian vegetation resulted
partly responsible for the differences observed between
in a loss of epilithon mass and a slight increase in
impact and control cobble communities. For example, in
conductivity, dissolved oxygen and temperature. The
the long term, decomposition of compounds associated
composition of epilithon was also related to treatment. with materials in the bedload may depress pH and
Cobbles in cleared reaches generally had either a diatom
eliminate acid-sensitive species of Plecoptera and
matrix or no epilithon while cobbles in uncleared reaches
Ephemeroptera (Lemly, 1982).
were dominated by Potamogeton ?crispus and green algae
(Nostoc type and other chlorophytes). Cobbles are not exposed to sediment deposition to the
same extent as the surrounding substrate, however,
increases in bedload transport and sediment deposition
Discussion from the water column have the effect of sealing or
smothering microhabitats of zero water velocity at rock'
Macroinvertebrate communities of cobbles in cleared water interfaces at the edges of, or behind cobbles
reaches had lower macroinvertebrate diversity compared (McClelland & Briisven 1980). The present study was
with uncleared controls, which was likely the result of designed to detect possible changes in the
the removal of riparian vegetation. Contributing to the macroinvertebrate community due to elevated turbidity,
decrease in overall macroinvertebrate diversity was a but might also be used to detect the effects of short-term
significant decrease in the abundance of grazers and sediment deposition on and around cobbles since it was
filterers. This was indicative of a change in the energy observed that sand from the added sediment was
base of cobbles, since the composition and biomass of deposited from the water column in these areas. A
epilithic communities was also altered by clearing. Most greater period of time between sediment addition and
cobbles in uncleared reaches supported crops of algae sampling may have allowed some individuals to either
and the angiosperm Potamogeton, while most cobbles in detach from, or crawl onto the cobble substrate from
cleared reaches were encrusted with a diatomaceous surrounding microhabitats. Gazey (1994) found that the
matrix or had essentially no measurable epilithic layer. ephemeropterans Neboissophlebia occidentalis, Nyungara
No difference in any macroinvertebrate community bunni and Bibulmena kadjina, and the trichopteran
parameter was detected between control and sediment Smicrophylax australica began to drift in significant
treatment cobbles in the uncleared reaches, suggesting numbers 3-24 hours after experimental sediment
that isolated events involving elevated levels of addition due to the effects of deposited sediment rather
suspended sediment have little overall effect on the than elevated turbidity, and therefore, effects in the
diversity of cobble communities in the short term. This present study may be delayed. Campbell and Doeg
was consistent with the study of Lenat et al (1981), who (1989) noted that deposited rather than suspended
observed decreases in density but no significant change sediment may be more important in reorganising
in community structure of rocky substrates. The effects of community structure because the effects may be long
elevated sediment on cobble communities might term if water discharge during spates is not sufficient to
therefore be more important in the longer term and less cleanse the substrate. Deposition of suspended solids
so than changes in other variables such as accumulations occurs in streams where boundary friction slows the
of deposited sediment and others that affect the epilithon. current, however, it is only in low velocity areas that
The magnitude of changes for physical variables such deposits form and elsewhere the sediment is deposited
as conductivity, DO and temperature was generally small and resuspended at greater rates.
and no firm conclusions about the relationship between The results from this study suggest that changes in
the diversity of fauna and physical variables can be the composition of the epilithic community were the
8
Armstrong, Storey & Davies: Macroinvertebrates of stream cobbles
main cause of changes in the composition of the would be indicated by obvious changes in the epilithon
macroinvertebrate community. Most cobble substrates in of cobbles.
uncleared reaches supported a growth of the aquatic
Despite previous studies of northern jarrah forest
plant, Potamogeton ?crispus.t which extended the height
streams that have shown communities within the same
of roughness projection of the cobble and provided a
stream order to be generally homogeneous (Storey et al.
relatively complex microhabitat. The epilithon on cleared
1990), differences in some macroinvertebrate parameters
cobble substrates did not support the same microhabitat.
were found between streams. This was probably due to
The microhabitat amongst the epilithon may have
the differences in epilithon cover between streams. Lush
provided protection in the short-term from elevated
growths of Potamogeton sp. were not present on every
turbidity levels by allowing fauna to avoid any adverse
cobble in uncleared reaches, and although not tested in
effects such as the clogging of respiratory or food
the present study, the variability in epilithic cover in
capturing devices. These fauna may have otherwise
uncleared reaches between streams may have
responded by drifting. Such effects were identified by
contributed to the variation in macroinvertebrate
Lemly (1982) as being the result of long term bedload
communities between streams. Cobbles in jarrah forest
transport on filtering species, and refuge-seeking
streams may therefore be more heterogeneous than
behaviour was one of the strategies employed by some
previously reported (Davies 1994) and this would be a
macroinvertebrate species in a New Zealand braided
consideration when using cobbles as an indicator habitat
stream which was exposed to periodic flood events
for assessing changes in stream ecosystem function.
(Scrimgeour & Winterbourn, 1989).
Changes in chironomid community structure, as
The loss of Potamogeton ?crispus in cleared reaches observed in the present study, is often used as an
might be attributable to increased light availability. indicator of change in the stream ecosystem (Storey &
Limitation in growth and survival of epilithon Edward 1989, Smith & Cranston 1994, Edward et al.
communities after an increase in sediment levels has also 2000). Other species might have some use in this regard.
been documented previously (silt accumulation, Graham A number of the taxa on impact cobbles were unique to
1990; sediment abrasion, Lewis 1973a,b). Generally, it this treatment, although the proportion of unique taxa in
appears that shading limits the growth of larger algal uncleared areas was greater. Species unique to cleared
forms, with algal assemblages in shaded areas being cobble communities included the simuliid Cnephia
dominated by diatoms (Duncan & Blinn 1989). When tonnoiri and the coleopterans Hyphydrus sp. and
light levels are increased, a greater biomass of green and Psychodidae sp., however, they were relatively uncommon
blue-green algae results, with the growth of diatoms and therefore may need to be used in conjunction with
being inhibited by high light conditions (Hill & Knight other indicators such as changes in epilithon. Unique
1988; Steinman et al. 1989; Wellnitz et al. 1996). Loss of taxa may be present on cleared cobbles for one of two
riparian vegetation in other Australian ecosystems also reasons. Firstly, they could be relatively rare and
resulted in a decline in the health of forest streams, and therefore collected by chance alone, or secondly, some
instream primary producers shifted from unicellular species such as the water beetle Sternopriscus marginata
algae to filamentous green algae and macrophytes (Bunn may have preferred other stream meso-habitats prior to
et al. 1999a). However, there are exceptions to this clearing, but increased in numbers in cobble habitats as
pattern. In a subtropical stream surrounded by pasture, they became more favourable in some respect.
diffusion surfaces exposed to full sunlight were
In conclusion, complete removal of riparian vegetation
dominated by diatoms (Mosisch et al. 1999), and a layer
adversely affects biodiversity and composition of
of diatoms and filamentous algae was present on cobbles
macroinvertebrate fauna and epilithic vegetation on
in a braided stream in New Zealand, the outcome of
cobbles. The most important changes are likely to include
scouring and rolling (Scrimgeour & Winterbourn 1989).
reduced bank stability, and increased light and sediment
The cobble epilithon of the jarrah forest streams of the
levels. Short-term increases in suspended sediment levels
present study resembled the latter unshaded streams.
appear to have minimal effect, but the longer term effect,
In Australian tropical and subtropical streams, the coupled with that of sediment deposition could be
epilithic unicellular algae contributes to the majority of greater. Cobble communities as a whole are sensitive to
consumption by macroinvertebrates on cobbles, but in changes in the aquatic ecosystem and are recommended
the northern jarrah forest unicellular algae contributes for use when assessments of ecosystem health are being
little more than 50 % of carbon assimilated by cobble made. The most useful indicator of changes in ecosystem
fauna (Bunn et al. 1999a). Jarrah forest stream ecosystems health may be the change in composition and biomass of
are almost entirely dependent on terrestrial sources of epilithon. In the jarrah forest, clearing causes a general
carbon (Davies 1994; Bunn et al. 1999a) and there is a change from Potamogeton ?crispus to diatomaceous layers.
corresponding low diversity of algal grazers (Bunn & Removal of riparian vegetation is not recommended if
Davies 1990). This suggests that measuring only the aquatic biodiversity and normal ecosystem functioning is
change in abundance of grazers on cobbles does not have to be maintained. This has implications in the south-west
the same utility in the jarrah forest than other stream of Western Australia for logging practices, whereby
ecosystems for detecting effects of disturbances. streamside vegetation should be maintained on all
However, the present study has shown that measures of streamlines.
overall macroinvertebrate diversity on cobbles are useful
for detecting changes in ecosystem health, and grazers,
Acknowledgements: This research was undertaken by the first author in
while making up only part of the community, showed
partial fulfilment of an honours degree, and was supported by the
the same response. Furthermore, a simpler measure of Department of Zoology at The University of Western Australia. Special
the change in macroinvertebrate community structure thanks are extended to J. Angus and D. Stilwell for field assistance; to C.
9
Journal of the Royal Society of Western Australia, 88(1), March 2005
Beurhig for accommodation at North Dandalup; to D. Edward, B. Knott, Gray LJ & Ward JV 1982 Effects of sediment releases from a
J. Prince, D. Cale, B. Wilson and D. Walker for help; the Water Authority reservoir on stream macroinvertebrates. Hydrobiologia 96:
of Western Australia for access to the study site. We thank three 177-184.
anonymous reviewers for their constructive criticism of the manuscript.
Gregory SV, Swanson FJ, McKee WA & Cummins KW 1991 An
ecosystem perspective of riparian zones: focus on links
between land and water. BioScience 41: 540-551.
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