Download Effects of shading on subtidal epibiotic assemblages

Journal of Experimental Marine Biology and Ecology,
234 (1999) 275–290
L
Effects of shading on subtidal epibiotic assemblages
T.M. Glasby*
Centre for Research on Ecological Impacts of Coastal Cities, Marine Ecology Laboratories, A11,
University of Sydney, Sydney, N.S.W. 2006, Australia
Received 12 May 1998; received in revised form 3 August 1998; accepted 17 August 1998
Abstract
Mensurative and manipulative experiments were done to test hypotheses about the effects of
shading on subtidal assemblages of epibiota. Previous studies found large differences in the
composition of epibiotic assemblages on pier pilings (shaded by boats and wharves) and adjacent
rocky reefs at marinas in Sydney, Australia. Different degrees of shading were proposed to explain
the differences between assemblages on these two substrata. Assemblages of epibiota on freestanding, unshaded pilings were sampled and, as predicted, found to be different from those on
shaded pilings and similar to those previously described on rocky reefs. Unshaded pilings were
covered primarily by filamentous and foliose algae and spirorbid polychaetes. Patches on these
unshaded pilings were then experimentally shaded to test the hypothesis that increased shading
would result in the assemblages changing to become like those on permanently shaded pilings.
After shading patches for 9 months, the composition of assemblages changed compared to controls
and taxa such as bryozoans (Fenestrulina mutabilis), serpulid polychaetes, solitary ascidians
(Styela plicata) and sponges became common. Results suggested that different degrees of shading
could explain differences in the cover of many epibiota growing on pier pilings and adjacent rocky
reefs at marinas. Other factors that may be important in structuring subtidal epibiotic assemblages
are also discussed.  1999 Elsevier Science B.V. All rights reserved.
Keywords: Artificial habitats; Environmental impact; Fouling; Light; Marinas; Shade
1. Introduction
Investigations of the effects of shading on marine organisms have often focused on
plants for the obvious reason that rate of photosynthesis is directly influenced by light
intensity. Reduced amounts of light have either been suggested or demonstrated to
influence the growth of various algal species (Moss et al., 1973; Foster, 1975; Santelices
*Tel.: 1 61-2-9351-4282; fax: 1 61-2-9351-6713; e-mail: [email protected]
0022-0981 / 99 / $ – see front matter  1999 Elsevier Science B.V. All rights reserved.
PII: S0022-0981( 98 )00156-7
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T.M. Glasby / J. Exp. Mar. Biol. Ecol. 234 (1999) 275 – 290
and Ojeda, 1984; Reed and Foster, 1984; Kirkman, 1989; Williams, 1994) and
seagrasses (Backman and Barilotti, 1976; Bulthuis, 1983; Dennison and Alberte, 1985;
Fitzpatrick and Kirkman, 1995). Shading has, however, also been shown to influence the
composition of sessile epifaunal assemblages in kelp forests (Kennelly, 1989; Duggins et
al., 1990) and possibly in submarine caves (Cinelli et al., 1977), the growth of sponges
(Wilkinson and Vacelet, 1979) and even the recruitment of fishes (Hair et al., 1994). The
effects of shading on epifauna may either be direct or indirect (the latter often due to
´ and Jansson, 1972).
reduced covers of algae in shaded conditions; Silen
Many early discussions about influences of light on subtidal organisms were based on
comparisons of assemblages at different depths (e.g. Klugh and Martin, 1927; Levring,
1966; Neushul, 1967; Hiscock and Mitchell, 1980; Warner, 1984; Hiscock, 1985). As
noted by Kain et al. (1975) and Jackson (1977), however, many factors other than light
intensity also differ with depth. Thus, controlled manipulative field experiments are
necessary to test properly the effects of different amounts of light (Backman and
Barilotti, 1976). Intensity of light has commonly been manipulated in studies of
seagrasses (e.g. Backman and Barilotti, 1976; Dennison and Alberte, 1985; Fitzpatrick
and Kirkman, 1995), but there has been surprisingly little field experimentation on the
effects of shading on whole assemblages of subtidal epibiota. Most that have been done
have dealt with assemblages on horizontal surfaces under canopies of kelp and have not
continued for longer than 8 weeks (Kennelly, 1989; Duggins et al., 1990). These studies
provided very useful results and made it clear that, on its own, shading is unlikely to
explain fully the observed differences between assemblages under and adjacent to kelp
forests, although it could account for large differences in the abundances of certain
species. Caging experiments by Schmidt and Warner (1984) also suggested that
reductions in light may influence the development of epifaunal assemblages on vertical
surfaces, although light intensity was not measured.
Shading in the marine environment may not always be a natural phenomenon.
Numerous urban structures have been added to the bays and estuaries around coastal
cities. Marinas are one such development and they have the potential to significantly
increase shading of surrounding marine habitats. Moreover, they may provide many
shaded hard substrata in the form of pilings, pontoons and boats. Assemblages of
epibiota on pilings at marinas and on nearby sandstone rocky reefs have been found to
differ markedly and consistently (Glasby, 1998a). The assemblages on pilings were
dominated by serpulid polychaetes, sponges, solitary ascidians and species of encrusting
bryozoans. Rocks were covered primarily by algae and spirorbid polychaetes (Glasby,
1998a). Many of the taxa found on pilings at marinas also dominate pier pilings in other
places (Karlson, 1978; Kay and Keough, 1981; Kay and Butler, 1983; Butler, 1986,
1991; Butler and Connolly, 1996). These results suggest that either the pilings
themselves or factors associated with them are causing assemblages of epibiota to
develop differently from those on natural hard substrata.
The pilings sampled in the aforementioned studies were made of either wood,
concrete or steel. My observations around Sydney indicate that bryozoans, sponges and
ascidians are also abundant on other artificial surfaces such as rope and plastic. I
suggest, therefore, that factors other than the composition of the substratum are
important in determining the types of fouling organisms that establish. Studies of the
T.M. Glasby / J. Exp. Mar. Biol. Ecol. 234 (1999) 275 – 290
277
effects of different substrata indicate that some species recruit in similar abundances on
quite different surfaces (e.g. Pomerat and Weiss, 1946; Crisp and Ryland, 1960; Caffey,
1982; Anderson and Underwood, 1994) while others do not (e.g. Crisp and Ryland,
1960; Harlin and Lindbergh, 1977; McGuinness, 1989; Anderson and Underwood,
1994).
Other than composition, one of the most obvious differences between many pier
pilings and rocky reefs is that pilings tend to be continuously shaded by wharves and
boats whereas rocky reefs are shaded far less frequently. Thus it is possible that
differences in the amount of light reaching pilings versus rocks could at least partially
explain why different assemblages of organisms have been found on these two surfaces.
The aim of this study was to test hypotheses about the effects of shading on the
composition of subtidal epibiotic assemblages on pilings at marinas in Sydney,
Australia. It was predicted that assemblages on any unshaded pilings should be similar to
those previously described on rocky reefs and different from those on shaded pilings.
Moreover, if unshaded assemblages were experimentally shaded, they should change and
come to resemble those on permanently shaded pilings (i.e. dominated by the same types
of organisms, but not necessarily in the same abundances).
2. Materials and methods
2.1. Epibiota on shaded and unshaded pilings
Assemblages on shaded and unshaded pilings were compared at Mitchell’s Marina in
Pittwater, Broken Bay (30 km north of Sydney; 338409S, 1518159E) in autumn of 1996.
The marina is situated at the end of a large embayment, approximately 8 km from the
open ocean. Further details of the site are given in Glasby (1997). All pilings were
wooden and ranged from 6–9 years old (pilings of each age occurred in the shaded and
unshaded treatments). Twelve replicate 15 3 23 cm photos were taken on shaded and
unshaded wooden pilings at the marina at a depth of 1.5 m below Mean Low Water
Springs (MLWS) using a Nikonos III underwater camera fitted with a 30 cm diopter and
a strobe. Patches on six unshaded and six shaded pilings were photographed by taking
one sample on the east-facing and one on the west-facing side of each piling (it was
assumed that of all the sides, these two were exposed to the most similar amounts of
sunlight). Slides were sampled by projecting the image on to a screen and estimating
percentage covers of taxa using a 15 3 15 cm grid of 64 regularly-spaced points
positioned in the middle of the image.
The unshaded pilings were free-standing and | 15 m from the shaded pilings
connected to the main wharf of the marina. These free-standing pilings were used for
securing boats, but only at a distance using long ropes. Thus, they were unshaded
because they were distant from boats and the main wharf. Each wooden piling was | 35
cm in diameter (average circumference | 110 cm). The two samples (each 23 cm
high 3 15 cm wide) on a piling thus had approximately 40 cm of substratum between
them and were considered to be independent of each other. Four replicate light intensity
readings were taken next to the shaded and unshaded pilings using a Li-cor (LI-188B)
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integrating quantum photometer. Data were analysed using multivariate (PRIMER
package) and univariate (analysis of variance; ANOVA) techniques.
2.2. Experimental shading of epibiota
The spatial arrangement of pilings in the comparison described above could have
confounded the test for differences due to light intensity with natural differences
between the two areas. Nevertheless, the results of the comparison supported my
hypothesis and a manipulative experiment was designed to retest the hypothesis. The
manipulative shading experiment was also done at Mitchell’s Marina from April
(autumn) to December (summer) in 1996. Shading structures were erected on the six
unshaded, free-standing pilings sampled in the previous mensurative experiment. Two
aluminium beams were clamped on to the pilings at a depth of 1.5 m below MLWS and
were orientated from east to west. The two beams were positioned parallel to each other,
with one beam on either side of a piling. Two pieces of stainless steel threaded rod
(perpendicular to the beams and one on either side of the piling) and nuts were used to
clamp the beams onto the piling. Screws were put through the beams and into the
wooden pilings to stabilize the structure.
Two sides of each piling were used (but never two replicates of the one treatment on a
piling), thus there were twelve sides available for the experiment. To help keep the two
sides independent of one another, I cleared epibiota from a 10 cm wide 3 50 cm long
strip down both sides of each piling (thus limiting the possibility of vegetative growth
from one side to the other). Four replicates were used for each of three treatments,
namely, (1) shade, (2) procedural control (which had all the features of the shaded
treatment, except the shade itself, and so tested for artefacts associated with the shading
structure) and (3) undisturbed control. The position of each replicate was chosen
randomly with the provisos that two replicates of each treatment faced east and two
faced west and that no piling had two patches from the same treatment.
Areas of the pilings were shaded by attaching 3 mm thick black (opaque) perspex in
between the aluminium beams using plastic cable ties (through holes drilled into the
perspex and aluminium). The width of the perspex was equivalent to the diameter of the
pilings ( | 35 cm) and it was 40 cm long. The end of the perspex abutting the pilings was
cut to fit around the piling. The procedural control consisted of 3 mm thick clear perspex
attached in between the aluminium beams in the same manner. Silt, bryozoans and
polychaetes settled on the perspex, so the clear and black perspex was cleaned every 10
days using a scrubbing brush. The undisturbed controls had nothing attached between
the aluminium beams.
A patch on each piling was sampled photographically by placing a frame 5 cm below
the aluminium beams and taking a 15 3 23 cm photo. Slides were sampled using a
15 3 15 cm grid of 64 points as described above. Samples were taken on six occasions,
at the start of the experiment, then after 7, 14, 23, 30 and 37 weeks. Replicate readings
of light intensity (see Section 2.1) were taken next to each area sampled and readings
were taken under the clear perspex before and after cleaning. Percentage cover estimates
for each time of sampling were analysed (using ANOVA and the multivariate package
PRIMER) separately to avoid problems of nonindependence. Univariate data were not
T.M. Glasby / J. Exp. Mar. Biol. Ecol. 234 (1999) 275 – 290
279
transformed unless variances were heterogeneous in which case they were transformed
to ln(x 1 1). Multivariate data were double square root transformed. Formal comparisons
of assemblages on experimentally shaded and permanently shaded pilings were not made
because it was not expected that experimental shading would lead to assemblages on the
two sets of pilings becoming the same, merely that similar species would dominate the
assemblages.
3. Results
3.1. Composition of assemblages on shaded and unshaded pilings
Light intensity was very different on shaded and unshaded pilings. At 1.5 m below
MLWS, 8.661.7 mmol m 22 s 21 (mean6S.E.) and 94.266.9 mmol m 22 s 21 of light
reached the shaded and unshaded pilings, respectively. Differences in the covers of taxa
on the two types of pilings were equally striking (Fig. 1). Filamentous algae were
divided into brown / green species (Ectocarpales and Cladophorales) and red species
(Ceramiales). The percentage cover of each group was significantly greater on unshaded
than on shaded pilings (Fig. 1a,b). Conversely, the cover of bryozoans and sponges was
significantly greater on the shaded pilings (Fig. 1c,d). Larger foliose algae (e.g. Dictyota
dichotoma, Sargassum sp.) were relatively uncommon on the unshaded pilings, but they
were not found on the shaded pilings (Fig. 1e). Serpulid polychaetes were most abundant
on shaded pilings (Fig. 1f), whereas spirorbids tended to be more abundant on unshaded
pilings (although there was no significant difference in this regard; Fig. 1g). The cover
of ascidians was quite variable among replicate pilings and no differences were detected
between shaded and unshaded pilings (Fig. 1h). There were significantly fewer taxa on
shaded than on unshaded pilings (Fig. 1i) and also more bare space on the shaded pilings
(not shown, F 5 5.50).
Not surprisingly, the multivariate analysis showed that the overall composition of
assemblages on shaded and unshaded pilings was significantly different (R 5 0.572,
P , 0.001). The non-metric multidimensional scaling (nMDS) ordination shows the
dichotomy quite clearly and also suggests that the variability among assemblages on
unshaded pilings was greater than among shaded pilings (Fig. 2).
3.2. Effects of experimental shading of epibiotic assemblages
There were marked differences in the amount of light reaching the areas in different
treatments. In shaded areas, 5.760.7 mmol m 22 s 21 of light reached the piling compared
to 97.266.3 mmol m 22 s 21 in the unshaded control areas. The intensity of light under
the experimental shades was similar to that recorded for pilings permanently shaded by
pontoons and boats (8.661.7 mmol m 22 s 21 ). The amount of light under the clear
perspex (procedural control) cleaned of sediment etc was 136.8613.4 mmol m 22 s 21
whereas 53.863.6 mmol m 22 s 21 reached pilings under clear perspex that had not been
cleaned for 10 days. The average amount of light under the clear procedural controls
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T.M. Glasby / J. Exp. Mar. Biol. Ecol. 234 (1999) 275 – 290
Fig. 1. Percentage covers ( 1 S.E.) of taxa on unshaded and shaded pilings; n 5 12. Note that y-axis for (i) is
number of taxa, not percentage cover. F-values are from one factor ANOVAs comparing taxa on the two types
of pilings (1, 22 df). * P , 0.05, ** P , 0.01, *** P , 0.001. All variances were homogeneous at P 5 0.05
except for (e) and (g).
was, therefore, 95.3612.7 mmol m 22 s 21 which was very similar to the unshaded
control.
At the start of the shading experiment, all the assemblages were very similar (Table 1;
Fig. 3). They remained similar for the first 7 weeks of the experiment, but after 14
weeks those under the shades appeared to be different from assemblages on the control
pilings (Table 1; Fig. 3). A piling was lost after 10 weeks which meant that for weeks
14–37 there were only three replicates for the shaded and control treatments. R-values
indicated that there were no multivariate differences among the assemblages at any other
time, except perhaps between the shaded and control treatments after 30 weeks (Table
1). The nMDS ordination shows an interesting pattern over time, but note that the stress
value is large and so this plot may not be an accurate representation of the data (Fig. 3).
It can be seen that the assemblages under the shades tracked a different direction from
the other two treatments (Fig. 3). Moreover, assemblages under the unshaded controls
seemed to follow the development of those under the clear procedural control, but were
T.M. Glasby / J. Exp. Mar. Biol. Ecol. 234 (1999) 275 – 290
281
Fig. 2. nMDS ordination comparing assemblages of epibiota on shaded (j) and unshaded (s) pilings; n 5 12.
out of phase by one sampling period (Fig. 3). By the end of the experiment, assemblages
from the two control treatments appeared quite similar whereas those from the shaded
treatment had diverged from the controls. This indicates that the presence of a clear
perspex ‘‘roof’’ had little, if any, effect on the epibiota below.
Brown and green filamentous algae were quite common on the pilings and the cover
of these algae remained similar for the two control treatments throughout the experiment
(Fig. 4a). The cover of filamentous algae on the shaded pilings, however, declined
slowly and by the end of the experiment was significantly less on the shaded pilings than
on pilings of the other two treatments (Fig. 4a; Table 2). Foliose algae (including D.
dichotoma, Sargassum sp., Colpomenia sinuosa) were not particularly abundant on the
Table 1
R-values from multivariate pairwise comparisons involving the three treatments for the shading experiment
sampled six times
Comparison
0 Weeks
7 Weeks
14 Weeks
23 Weeks
30 Weeks
37 Weeks
Shaded vs. clear
Shaded vs. control
Clear vs. control
2 0.250
0.042
0.063
0.219
0.063
2 0.063
0.111
0.741 a
2 0.259
0.241
0.074
2 0.296
0.444
0.593 a
0.426
0.093
0.259
0.037
n 5 4 except for shaded and control samples after 14–37 weeks when n 5 3. The small number of
permutations for these latter times meant that differences could not be detected below P 5 0.1. Those
comparisons which resultled in the smallest possible level of significance are marked a .
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T.M. Glasby / J. Exp. Mar. Biol. Ecol. 234 (1999) 275 – 290
Fig. 3. nMDS ordination showing the relationship among assemblages on shaded pilings (black perspex; j),
procedural control pilings (clear perspex; ) and unshaded control pilings (s) at the six times of sampling
(prior to the commencement of the experiment, then after 7, 14, 23, 30 and 37 weeks). Arrows show the
chronological changes in the assemblages. Points are averages of replicates. Note that the stress value for the
ordination is large.
pilings at the start of the experiment, but they increased in abundance (weeks 7–30, i.e.
winter–spring) on control pilings before declining again after 37 weeks (Fig. 4b). The
cover of foliose algae under the shades declined after just 7 weeks and remained almost
nonexistent for the duration of the experiment (Fig. 4b). After 14 and 30 weeks, the
cover of foliose algae on pilings from the two control treatments was significantly
greater than on the shaded pilings (Table 2).
The cover of red filamentous algae was similar on all pilings for the majority of the
experiment, but after 30 weeks the cover of red filamentous algae on the unshaded
control pilings was significantly less than under the clear and black perspex treatments
(Table 2; Fig. 4c). Serpulid polychaetes (Hydroides) became more abundant on the
shaded pilings as the experiment progressed (Fig. 4d). There were significantly more
serpulids on the shaded pilings compared to the controls after 7 and 23 weeks (Table 2;
Fig. 4d).
The percentage cover of the encrusting bryozoan Schizoporella errata was quite
variable among replicate pilings, especially among the unshaded controls (Fig. 4e).
There were no significant differences in the cover of this bryozoan among treatments at
any time of sampling (Table 2). The pattern for another bryozoan, Fenestrulina
T.M. Glasby / J. Exp. Mar. Biol. Ecol. 234 (1999) 275 – 290
283
Fig. 4. Percentage covers (6S.E.) of taxa on shaded (j), procedural control ( ) and control (s) pilings at
different stages of the shading experiment. Note that y-axis for (l) is number of taxa. * Significant difference
among treatments at that time. n 5 4 For weeks 0 and 7, n 5 3 for weeks 14–37.
mutabilis was quite different (Fig. 4f), increasing steadily on the shaded pilings and
remaining relatively constant on the other pilings (Fig. 4f). Only after 30 weeks,
however, was the cover of Fenestrulina on shaded pilings significantly greater than on
the other pilings (Table 2). Watersipora subtorquata was not as common as the other
bryozoans, but its percentage cover increased on the shaded pilings over time (Fig. 4g).
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Table 2
F-values from one factor analyses of variance for the manipulative shading experiment.
Taxon
0 Weeks
7 Weeks
14 Weeks
23 Weeks
30 Weeks
37 Weeks
Br / gr filamentous
Foliose algae
Red filamentous
Serpulids
Schizoporella
Fenestrulina
Watersipora
Sponge
Styela
Spirorbids
Bare space
No. of taxa
0.34
1.00
0.39
0.54
1.34
0.18
–
0.02
–
1.50
1.80
0.66
0.06
3.12
0.19
5.33 *
1.08
0.54
1.00
1.60
–
–
1.59
1.85
0.96
13.33 **
0.02
2.67
0.35
2.07
0.95
1.02
2.60
0.50
–
1.09
0.06
1.45
0.55
9.06 *
0.81
0.50
3.39
1.91
3.95
–
1.00
0.90
1.65
12.95 **
12.40 **
3.29
1.30
28.80 ***
0.31
1.46
34.76 ***
3.56
0.81
0.20
6.04 *
1.00
10.05 *
3.14
1.96
3.53
0.66
0.33
3.17
0.50
0.29
1.91
Three treatments were compared: shaded, clear procedural control and undisturbed control at each of six times
(0, 7, 14, 23, 30 and 37 weeks). For samples after 0 and 7 weeks, n 5 4 (i.e. df for test are 2, 9) and n 5 3 for
the remaining samples (i.e. df for test are 2, 6). A dash indicates that taxon was not present in any treatment.
All variances were homogeneous at P 5 0.05 except for Watersipora (7, 23 wks), foliose algae (37 wks), Styela
(23, 30, 37 wks), spirorbids (30 wks) and bare space (23 wks).
There was, however, a great deal of variability among replicates and no significant
differences were detected among treatments (Table 2).
The cover of sponges generally decreased during the experiment, but after 30 weeks
the cover on shaded pilings tended to be greater than on control pilings (Fig. 4h). There
were, however, no significant differences among treatments at any time of sampling
(Table 2). The percentage cover of the solitary ascidian Styela plicata increased on
shaded pilings throughout the experiment while it remained very small on the other
pilings (Fig. 4i). Because of the variability among shaded pilings, a significant
difference among treatments was detected only for the sample after 30 weeks (Table 2;
Fig. 4i). Spirorbid polychaetes were quite rare on the pilings and no significant
differences were detected among treatments at any time of sampling (Table 2; Fig. 4j).
The amount of bare space on pilings fluctuated throughout the experiment (Fig. 4k), but
was always similar for each treatment (Table 2). The total number of taxa did not differ
among treatments (Table 2) and did not vary greatly over time (Fig. 4l).
4. Discussion
This study provided clear evidence that shading can affect the cover of many taxa on
pilings and has the potential to influence the composition of whole assemblages of
subtidal epibiota. After 9 months, experimental shading of long-established assemblages
led to changes in the cover of taxa such as filamentous algae, serpulid polychaetes, the
bryozoan F. mutabilis and the solitary ascidian S. plicata. The cover of each taxon,
except filamentous algae, was increased by reductions in light. The marked differences
between shaded and unshaded assemblages of epibiota were remarkably consistent with
previously described differences between (relatively shaded) pilings and (less shaded)
T.M. Glasby / J. Exp. Mar. Biol. Ecol. 234 (1999) 275 – 290
285
rocks (Glasby, 1998a). In the present study, unshaded pilings (like rocks; Glasby, 1998a)
were generally covered by spirorbid polychaetes and filamentous and foliose algae,
whereas shaded pilings were dominated by serpulid polychaetes, bryozoans, sponges and
solitary ascidians. Different degrees of shading could explain many, but not all, of the
previously documented differences between assemblages on pilings and rocky reefs.
The experimental reduction of light had significant effects on the cover of many
species. Algae were negatively affected by shading. This is consistent with results from
other studies which have demonstrated that the cover of various algal species is reduced
in shaded conditions (e.g. Reed and Foster, 1984; Kennelly, 1989; Fitzpatrick and
Kirkman, 1995). Filamentous algae were common on all pilings, but after 23 weeks,
began to decrease in cover on the shaded pilings. Conversely, larger foliose algae were
uncommon on all pilings at the start of the shading experiment, but later increased in
cover on the control pilings. The shading appeared to inhibit any short-term increase in
the cover of foliose algae. Shading probably directly affected the cover of these algae by
decreasing their rate of photosynthesis (Levring, 1966). Semishaded conditions may also
favour competitive exclusion of algae by sessile invertebrates—removal of invertebrates
(by fish) in these conditions led to an increased cover of algae (Foster, 1972).
The abundance of serpulid polychaetes was enhanced on shaded pilings. This
conforms with the findings of Miura and Kajihara (1984) who described the photonegative behaviour of serpulid larvae just prior to settlement. Furthermore, the authors
suggested that serpulids may settle preferentially on shaded, artificial structures.
Marsden (1988) warned, however, that responses to light by larvae may vary among
species of serpulids. Certainly it has been observed that the larvae of some species of
Hydroides tend to settle in greatest numbers in illuminated areas (Zeleny, 1905; Wisely,
1958). The results of the present study support those of other experimental field studies
(Miura and Kajihara, 1984; O’Donnell, 1984; Duggins et al., 1990) which have reported
serpulids in greatest numbers in shaded areas.
Although the cover of sponges did not differ significantly between shaded and
unshaded treatments (due to large variability among replicates in the shaded treatment),
there was a tendency for the cover to be greatest in shaded areas. Given more time, a
significant difference may have developed between the shaded and control treatments.
There is evidence that some sponges are very abundant in shaded conditions under piers
(Wells et al., 1964; Sutherland and Karlson, 1977; Kay and Butler, 1983) and in
submarine caves (e.g. Cinelli et al., 1977; Bibiloni et al., 1989), but may only be
abundant at certain times of the year (Sutherland and Karlson, 1977; Osman, 1977).
Moreover, they can take a long time to start to dominate epibiotic assemblages (Osman,
1977; Kay and Keough, 1981; Russ, 1982). Unlike sponges, the growth of the solitary
ascidian S. plicata in shaded conditions was quite rapid. Results from another study
indicate that the abundance of this ascidian may also be influenced by proximity to the
seafloor; they were most abundant many metres from the bottom (unpublished data).
It has been suggested that shading may indirectly affect the recruitment of bryozoans
´ and Jansson, 1972;
by limiting the growth of competitively dominant microalgae (Silen
Duggins et al., 1990). The current study did not enable indirect and direct effects to be
distinguished, but some bryozoans were clearly influenced in some way by shading. The
cover of F. mutabilis increased in shaded conditions. Many other studies have found
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T.M. Glasby / J. Exp. Mar. Biol. Ecol. 234 (1999) 275 – 290
´ and
bryozoans to be more abundant in shaded than in unshaded conditions (e.g. Silen
Jansson, 1972; Todd and Turner, 1986; Duggins et al., 1990; Fitzpatrick and Kirkman,
1995). In the present study, another encrusting bryozoan, S. errata, did not respond
positively to shading. Perhaps shade does not affect the growth of this bryozoan, but it is
also possible that it was either inhibited by other species or merely slow to respond to
the shading. S. plicata (and other epifaunal species) have been shown to exclude
Schizoporella (Sutherland, 1978) and in the present study Styela were very abundant in
shaded treatments. Furthermore, Schizoporella is a very sturdy encrusting bryozoan
(unlike Fenestrulina which is far more delicate) and may, therefore, grow much slower
than Fenestrulina (Osman, 1977). Thus, differential growth rates may have also
contributed to the differences in response to shading over the 9 month period. The
relatively short duration of the experiment could have also influenced the results for
sponges and foliose macroalgae which tend to take longer to establish than many other
sessile epibiota (e.g. Sousa, 1979; Kay and Keough, 1981; Russ, 1982; Hirata, 1986,
1987).
Some of the differences between shaded and unshaded assemblages described here
were similar to those reported between assemblages on pilings at certain marinas and
controls (Glasby, 1997). Pilings at those marinas may have been more shaded (by large
numbers of boats) than pilings at the controls (small private jetties) and I suggest that
this was the cause for some of the differences between assemblages at the two types of
places. Moreover, I propose that assemblages typically described on settlement (or
fouling) plates would often be very different from those on natural hard substrata
because of differences in the degree of shading (and possibly distance from the seafloor).
The classic method of deploying settlement plates is to suspend them (often face down)
from some sort of floating pontoon (e.g. Wisely, 1959; Dean and Hurd, 1980; Russ,
1977; Withers and Thorp, 1977; Osman, 1977; Sutherland and Karlson, 1977; Russ,
1982; Greene and Schoener, 1982; Todd and Keough, 1994). Thus, the plates are shaded
at least partially and generally positioned a few metres from the seafloor. The
assemblages that develop on these plates are often characterised by bryozoans, serpulids,
sponges, ascidians and barnacles (see Refs. above) and quite different from the
depauperate assemblages (dominated, like natural rocks, by filamentous algae and
spirorbids) that have been reported on settlement plates attached directly to rocky reef
(Glasby, 1998b). No doubt there are numerous factors that may influence the development of assemblages on settlement plates, but I suggest that the effects of shading have
not been fully realised.
5. Conclusion
Differences in assemblages growing on pilings and rocky reefs at marinas that were
described previously (Glasby, 1998a) were probably due largely to differences in
shading. There was strong evidence to suggest that the cover of the bryozoan F.
mutabilis, the solitary ascidian S. plicata, serpulid polychaetes, algae and possibly
sponges on pilings was dependent upon the degree to which the pilings were shaded.
Algae occurred most commonly in unshaded areas, whereas the other taxa were more
T.M. Glasby / J. Exp. Mar. Biol. Ecol. 234 (1999) 275 – 290
287
abundant in shaded areas. The distribution of other taxa in shaded and unshaded
conditions was not consistent with previously described differences between assemblages on pilings and rocky reefs, so clearly other factors (or combinations of factors)
must influence their distributions. For example, shade and proximity to the bottom
(unpublished data) appear to affect the abundance of Styela. Flow of water may also
influence in some way the abundance of certain taxa. It has been demonstrated that the
hydrodynamic forces around structures such as pilings may be substantially greater than
those around larger, more complex surfaces such as rocky reefs (e.g. Abelson and
Denny, 1997). It seems most likely that a combination of shade, position in the water
column and flow (which may or may not be related to position) may strongly influence
the composition of subtidal assemblages of epibiota.
Acknowledgements
This work was funded by an Australian Postgraduate Award, the Institute of Marine
Ecology and, during the preparation of the manuscript, the Centre for Research on
Ecological Impacts of Coastal Cities (University of Sydney). Thank you to Prof. A.J.
Underwood who gave advice throughout all stages of this work. I thank P. Barnes, P.
Gibson, G. Housefield and V. Mathews for assistance in the field. Drs G. Rouse and D.
Gordon identified polychaetes and bryozoans, respectively, and Prof. Tony Larkum
helped identify algae. M.G. Chapman, G. Housefield and A.J. Underwood provided
useful discussion. The manuscript was improved greatly by comments from Drs L.
Airoldi, M.S. Foster and an anonymous referee.
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