Abundance, biomass and estimated production of invertebrate fauna associated with seagrass, Heterozostera tasmanica, in Swan Bay and an adjacent area of Port Phillip Bay, Victoria

13 Pages·1999·3.6 MB·

by  Fiona L Bird

ABUNDANCE, BIOMASS AND ESTIMATED PRODUCTION OF
INVERTEBRATE FAUNA ASSOCIATED WITH SEAGRASS,
HETEROZOSTERA TASMANICA, IN SWAN BAY AND AN
ADJACENT AREA OF PORT PHILLIP BAY, VICTORIA
Fiona L. Bird'’* & Gregory P. Jenkins*
'Marine and Freshwater Research Institute, and Zoology Department, University of Melbourne,
PO Box 138, Quecnscliff, Victoria 3225, Australia
^Present address: Crustacea laboratory. Museum of Victoria, 71 Victoria Cresent, Abbotsford,
Victoria 3067, Australia
Bird, F. L. & Jenkins, G. P., 1999:07:31. Abundance, biomass and c.stimated production of
invertebrate fauna associated with seagrass, Heierozostera uisntanica. in Swan Bay
and an adjacent area of Port Phillip Bay. Victoria. Proceedings of the Royal Society
of Victoria 111(1): 1-13. ISSN 0035-9211.
Abundance, biomass and production of invertebrates were compared between Swan Bay,
a seagrass system in a low energy sheltered environment, and an adjacent area of Port Phillip
Bay, which had greater exposure to wave action and tidal currents. Seagrass biomass and
abundances of macrofauna were not significanlly different between the two areas, but organic
content of sediments, biomass and production of macrofauna were significantly higher in
Swan Bay. Faunal composition was markedly different, with Port Phillip Bay sites dominated
by gammaridcan and eaprellidean amphipods, while a variety of often larger macrofaunal
groups characterised Swan Bay sites. In contrast to macrofauna, the epibenihic meiofauna,
dominated by harpaclicoid copepods, was significantly more abundant in Swan Bay. The
results support the contention that sheltered .seagrass areas with elevated organic content of
sediment can provide increased production of food for juvenile fish.
SEAGRASS beds are known to be highly environment with less accumulated debris. This
produetive environments (Hillman et al. 1989), area of Port Phillip Bay is adjacent to strong tidal
supporting diverse and extensive communities of currents associated with exchange of water through
invertebrates. Studies comparing seagrass and un- the narrow entrance at Port Phillip Heads (Black
vegetated habitats have consistently found that ct al. 1993). Shaw & Jenkins (1992) showed that
vegetated areas sustain greater abundance and unvegetated areas of Swan Bay had finer sediments
diversity of benthic invertebrates (Rainer & with higher organic content than unvegetated areas
Fitzhardinge 1981; Poore 1982; Lewis 1984; of the adjacent coast of Port Phillip Bay. Jenkins
Howard ct al. 1989; Edgar 1990a) and fish (Bell et al. (in review) found that fish communities in
& Pollard 1989; Lubbers ct al. 1990; Blaber et al. these scagra.ss beds were dominated by juveniles,
1992). Seagrass is known to trap organic debris and the abundance and biomass of fishes in Swan
and detritus (Norwcll & Jumars 1984), with the Bay was significantly higher than in the adjacent
organic material thought to enhance the secondary coast of Port Phillip Bay. Juvenile fishes at these
production (Pearson & Rosenberg 1978; Mann sites consumed small invertebrates, dominated by
1988; Spies et al. 1988). From this observation it Crustacea (Bird 1990). Edgar & Shaw (1993) found
is hypothesi.sed that exposed habitats accumulate that fish and invertebrate production in Western
less detritus than sheltered habitats and would Australia was much higher in sheltered than in
con.scqucntly have lower overall production levels. exposed sites, and postulated that fishes were
Edgar (1990a) showed by comparing a sheltered attracted to seagrass beds with high production
to an exposed area, that organic enrichment of of invertebrates. We postulated that the more
sediments (by debris and detritus) enhances sheltered environment of Swan Bay would result
invertebrate production. in correspondingly higher organic content of
Swan Bay, Australia, has extensive beds of the sediments and elevated production of invertebrates.
subtidal seagrass, Hetcrozostera lasmaiiica, in a In this paper, we examine this hypothesis by
low energy, sheltered environment with large comparing organic content of sediments, meiofauna
accumulations of debris and drilt algae. The abundance, and macrofaunal abundance, biomass
adjacent coast of Port Phillip Bay has smaller, and production in Swan Bay and an adjacent
more discrete H. lasmanica beds in a higher energy area of Port Phillip Bay.
FIONA L. BIRD AND GREGORY P. JENKINS
2
INVERTEBRATE FAUNA ASSOCIATED WITH SEAGRASS, HETEROZOSTERA TASMANICA 3
METHODS for 48 hours at 60°C. Organic content of the
sediment was estimated from the small-core
Study area
samples. Total contents of each sample was
The study area included Swan Bay and an dried for 48 hours at 60°C and then burned in
adjacent area in Port Phillip Bay, situated on a combustion oven for two hours at 500°C.
the Bellarine Peninsula, Victoria (Fig. 1). Tidal The change in weight due to the burning was
currents in Swan Bay are less than 0.05 ms"' recorded, and this change was considered pro¬
compared with currents between 0.1 and 0.5 m.s”' portional to the organic content.
on the adjacent coast of Port Phillip Bay (Black
et al. 1993). Moreover, maximum wind fetch is
Data analysis
less than 10 km in Swan Bay, but greater than
40 km on the adjacent coast of Port Phillip Bay Analysis of macrofauna was restricted to in¬
(Fig. 1). Two sites were sampled in Swan Bay; vertebrates passing through the 5.6 mm sieve
Tin Can and North Jetty, and two sites were because larger animals were not consumed by
sampled in Port Phillip Bay; St Leonards and juvenile fish in the study area (Bird 1990), and
Beacon. Samples were taken .seasonally over two in the case of mobile animals, would have a
days beginning on 1 May, 1 August, 29 October greater ability to avoid the corcr. Core data were
1990 and 20 January 1991. One additional site initially checked for homogeneity of variances
in each area. Mid Jetty and Edwards Point, was using Cochran’s test, and if necessary log(x-^l)
included in October for sampling of organic or square-root transformed. The data were then
content of sediment and meiofauna abundance analysed using an analysis of variance with two
(Fig- 1). bays, two sites nested within each bay, five
replicates at each site, on each of four dates.
Time and bay were treated as fixed factors, while
Field methods
sites were considered to be random factors.
The invertebrate community was sampled using The biomass of macrofaunal groups was esti¬
two sizes of corers. A 90 mm diameter PVC mated from abundance and previously determined
core, was used to take five haphazardly placed mean weights of these groups in individual
replicates, from each of the four sites on all sieve-size classes (Edgar 1990b). The production
four dates, to estimate abundance, biomass and of a specific size-class and type of cpifaunal
production of macrofauna. A 32 mm diameter invertebrate was calculated using the equation
corcr was used to obtain eight replicate samples developed and tested by Edgar (1990b) which
at each of the six sites in October. Five replicates relates daily macrobenlhic production (pg/d) to
from each site were used for analysis of organic ash-free dry weight (pg) and water temperature
content and the remaining three for analysis of (°C). The estimated production of a specific macro¬
meiofauna. Both of the corers were pushed into faunal group in a given size class was multiplied
the sediment to a depth of 10 cm to extract a by the number in that size class and summed for
constant volume of sediment. The samples were all size classes. Total daily production was
preserved in 99% ethanol mixed with Rose Bengal estimated from the combined produetion values
to make the animals more visible while sorting. for each macrofaunal category.
Laboratory methods
RE,SULTS
Each large-core sample was washed through a
series of nested sieves of 5.6, 4.0, 2.8, 2.0, 1.4,
1.0, 0.71 and 0.5 mm mesh. .Small cores were Bay and site characteristics
washed through a sieve series of 0.5, 0.355, 0.25, Water temperatures tended to greater extremes in
0.18, 0.125 and 0.063 mm mesh. Each category Swan Bay than in Port Phillip Bay (Table 1).
of invertebrate in each size class was counted. Temperatures conform to those found by Jenkins
Seagrass biomass was estimated by drying the (1986) except in October when we encountered
seagrass material remaining in the largest sieve an unseasonally warm sampling day. When
Fig. 1. Location of the sampling sites in Swan Bay and an adjacent area of Port Phillip Bay.
4 FIONA L. BIRD AND GREGORY P. JENKINS
calculating production estimates for that date,
Source Mean square DF F-ratio P
the temperature 170°C (from Jenkins 1986) was
used for all sites. No significant difference in
Date 18.727 3 1.681 0.269
seagrass biomass was found between bays Bay 18.075 1 3.342 0.209
(Table 2). The seagrass biomass values ranged Date*Bay 9.902 3 0.889 0.499
from 4 to 10 g per core (628 to 1572 g.m~^) SB sites 10.816 1 1.405 0.240
(Fig. 2). More detritus, seagrass debris and drift PPB sites 0.001 1 0.000 0.993
algae was observed inside Swan Bay than at SB silcs*Dalc 5.164 3 0.671 0.573
the sites in Port Phillip Bay. Swan Bay sediment PPB sites*Date 17.123 3 2.225 0,094
was shown to have a greater organic content Error 7.696 64
than sediment in Port Phillip Bay (Table 3).
Table 2. Nested analysis of variance comparing the
A difference in organic content was also found
estimated seagrass biomass in Swan Bay (SB) and an
between the Swan Bay sites (Fig. 3). adjacent area of Port Phillip Bay (PPB). DF—degrees
of freedom; P—probability.
Sites Date
May August October January Source Mean square DF F-ratio P
1990 1990 1990 1991
Bay 0.228 1 8.545 0.043
North Jetty 15.5 12.1 25.0 22.3 SB sites 0.051 2 7,385 0.005
Tin Can 14.7 10.9 25.0 21.4 PPB sites 0.002 2 0.306 0.741
St Leonards 16.4 11.5 18.9 20.2 Error 0.007 16
Beacon 17.0 11.5 18.2 19.8
Table 3. Nested analysis of variance comparing the
Table 1. Water temperature measurements taken at four organic content of sediments in Swan Bay (SB) and an
sites in Swan Bay and an adjacent area of Port Phillip adjacent area of Port Phillip Bay (PPB). DF—degrees
Bay (°C). of freedom; P—probability.
Fig. 2. Mean seagrass biomass at
sites in Swan Bay and Port Phillip Bay
on four sampling dates. Solid, 30 May
1990; open, 31 August 1990; diagonal,
30 October 1990; stipples, 31 January
1991. Error bars are standard error.
North Jetty Tincan St Leonards Beacon
Swan Bay | Port Phillip Bay
Site
Macrofaunal communities
No significant difference in macrofaunal abun¬ biomass of macrofauna was highcsl for May
dance was found between bays (Table 4, Fig. 4). and August samples in Swan Bay (Fig. 4). There
Swan Bay had a significantly higher estimated was also a significant interaction between
biomass of maerofauna than Port Phillip Bay abundances at sites in Swan Bay with date
(Table 5). Macrofauna biomass also varied signi¬ (Table 5). Estimated production of macrofauna for
ficantly amongst dates (Table 5). In general. Swan Bay was also significantly higher than for
INVERTEBRATE FAUNA ASSOCIATED WITH SEAGRASS, HETEROZOSTERA TASMANICA 5
Fig. 3. Mean percentage of organic
content in sediments at sites in
Swan Bay and Port Phillip Bay.
North J—North Jetty; Mid J—Mid
Jetty; St Leo—St Leonards. Error
bars are standard error.
North J Tincan Midi St Leo Beacon Edwards Pt
Swan Bay Port Phillip Bay
Site
Source Mean square DF F-ratio P Source Mean square DF F-ratio P
Date 19173.7 3 2.338 0.173 Date 0.046 3 1.578 0.290
Bay 52173.1 1 5.600 0.142 Bay 2.126 1 55.900 0.017
Date* Bay 25052.4 3 3.055 0.113 Date* Bay 0.093 3 3.204 0.105
SB sites 270.4 1 0.041 0.839 SB sites 0.044 1 1.972 0.165
PPB sites 18361.2 1 2.814 0.098 PPB sites 0.032 1 1,435 0.235
SB sites*Date 14458.5 3 2.214 0.095 SB sites*Date 0.048 3 2.160 0.101
PPB sitcs*Date 1944.0 3 0.298 0.827 PPB sites*Date 0.010 3 0.432 0.731
Error 65.30.4 64 Error 0.022 64
Table 4. Nested analysis of variance comparing the Table 6. Nested analysis of variance comparing the
abundance of macrofauna in Swan Bay (SB) and an estimated production of macrofauna in Swan Bay (SB)
adjacent area of Port Phillip Bay (PPB). DF—degrees and an adjacent area of Port Phillip Bay (PPB). DF—
of freedom; P —probability. degrees of freedom; P—probability.
Port Phillip Bay (Table 6). The highest estimated
Source Mean square DF F-ralio P
production of approximately 170 mg.m"^.d“' was
recorded at Tin Can in May (Fig. 4). Production
Date 9525.3 3 6.459 0.026
Bay 64412.6 1 104.515 0.009 estimates were also significantly different between
Date*Bay 3381.5 3 2.293 0.178 sites in Swan Bay, however, this varied with date
SB sites 703.9 1 1.006 0.320 (Table 6). Annual production in g.m’^.yr' at
PPB sites 528.7 1 0.756 0.388 each site was estimated to be 45.5 for North
SB sites*Date 2640.9 3 3.774 0.015 Jetty, 39.7 for Tin Can, 18.6 for vSt Leonards and
PPB sites*Dale 308.7 3 0.441 0.724 21.2 for Beacon.
Error 699.7 64
Macrofaunal communities in seagrass beds were
distinctly different between the two bays (Fig. 5).
Table 5. Nested analysis of variance comparing the
biomass of macrofauna in Swan Bay (SB) and an Amphipods dominated in Port Phillip Bay sites;
adjacent area of Port Phillip Bay (PPB). DF—degrees caprellid amphipods were found only in Port
of freedom; P—probability. Phillip Bay, and gammaridean amphipods were at
6 FIONA L. BIRD AND GREGORY P. JENKINS
e
r
o
c
r
e
p
a
n
u
a
f
o
r
c
a
m
f
o
e
c
n
a
d
n
u
b
a
n
a
e
M

)
g
m
(
e
r
o
c
r
e
p
a F/g. 4. Mean abundance,
n
u biomass, and estimated
a
of production of macrofauna
cr at sites in Swan Bay and
a
m Port Phillip Bay on four
f sampling dates:
o
s (A) abundance;
as (B) biomass;
m
(C) production.
o
bi Key to dates in caption
n for Fig. 2. Error bars
a
e are standard error.
M

d
^
m
g.
m
(
a
n
u
a
f
o
r
c
a
m
f
o
n
o
cti
u
d
o
r
p
n
a
e
M North Jetty Tin Can St Leonards Beacon
Swan Bay Port Phillip Bay
Site
INVERTEBRATE FAUNA ASSOCIATED WITH SEAGRASS, HETEROZOSTERA TASMANICA 1
I
U
• a>
o cu
cu O
y.
a
B

p
hilli
P

ort
P

d
n
a
o cu y
o a
B
can B wan
Tin S
!¥ in
s
e
sit

at

s
e
g
a
bl
m
e
s
s
a

al
n
u
a
of
cr
a
m

of

n
o
siti
o
p
m
o
c

nt
e
c
etty Per
J
h
rt
o
N
uouisodiuoo oSbjuoojoj
FIONA L. BIRD AND GREGORY P. JENKINS
40 North Jetty
30
20
10
0
40 Tincan
30
20
10
Fig. 6. Per cent frequency of
0 macrofauna in sieve-size classes
at sites in Swan Bay and Port
40 St Leonards Phillip Bay.
30
20
10
0
40
30
20 llh^
10
0
0.5 0.71 1 1.4 2 2.8 4
Sieve sizes (mm)
INVERTEBRATE FAUNA ASSOCIATED WITH SEAGRASS, HETEROZOSTERA TASMANICA 9
least three times more abundant. Cumaceans were In contrast to macrofauna, abundance of
also more abundant in Port Phillip Bay. Other meiofauna was significantly higher in Swan Bay
invertebrates such as tanaids, gastropods and (Table 7). On average, meiofaunal abundance
bivalves were more important at sites in Swan in Swan Bay was approximately four-fold that
Bay. The January samples from Beacon were on the adjacent coast of Port Phillip Bay (Fig. 7).
unusual in the high representation of tanaidaceans The major difference in the meiofaunal abundances
and isopods (Fig. 5). The October samples from between the two areas occurred in the 0.063 to
Tin Can had unusually high numbers of ophiuroids 0.18 mm sieve-size range (Fig. 8). This was due
(Fig. 5). to the presence of high abundances of epibenthic
The size structure of the macrofaunal assem¬ harpacticoid copepods in this size range in Swan
blages showed some variation amongst sites Bay (Fig. 8).
(Fig. 6). Swan Bay sites generally had a higher
proportion of larger invertebrates (greater than DISCUSSION
1.4 mm sieve size). Port Phillip Bay sites appeared
Scagrass biomass and organic matter arc two
to have a higher proportion of invertebrates in the
physical parameters known to be correlated with
1 mm sieve-size class.
invertebrate abundance and diversity in sca¬
grass beds. Heck & Whetstone (1977) found
that invertebrate species number and abundance
increased with plant biomass. Mann (1988)
suggested that organic matter trapped in a sea-
Source Mean square DF F-ratio P grass bed would provide food for detritovores
and Edgar (1990a) showed that organic enrichment
Bay 101.2 1 21.463 0.010 of sediments increased invertebrate production.
SB sites 6.0 2 2.006 0.142 Edgar & Shaw (1993) found organic matter to be
PPB sites 3.4 2 1.289 0.311 greater in sheltered scagrass beds than unsheltered
Error 2.6 16
beds, and this correlated with a higher production
of invertebrates in the sheltered environments.
Table 7. Nested analysis of variance comparing the
Scagrass biomass in Swan Bay and Port Phillip
abundance of meiofauna in Swan Bay (SB) and an
adjacent area of Port Phillip Bay (PPB). DF—degrees Bay did not significantly differ, but the organic
of freedom; P—probability. content of sediment was higher in Swan Bay.
a
8 150
oO .
C
3
o 100
us
o
Fij’. 7. Mean abundance of
meiofaunal organisms at sites
in Swan Bay and Port Phillip
Bay on 29 October 1990.
Error bars arc standard error.
North J Tin Can MidJ St Leo Beacon Edwards Pt
Swan Bay Port Phillip Bay
Site
10 FIONA L. BIRD AND GREGORY P. JENKINS
2
o
o
>-¥
Q<
a
c
:3
.<4a^ fig. S. Mean abundance of
o nieiofauna of different sieve-
*6S size components in Swan
<*- Bay and Port Phillip Bay.
o
0) Solid region denotes mean
CJ abundance of cpibcnthic
§
harpacticoids. Error bars
•a
c are standard error.
3
I
Sieve sizes (mm)
Drift algae was seen to accumulate and decompose protect it from the direct effect of wave and
in Swan Bay and this would enhance organic current stress.
content of the sediment. Macrofaunal abundance gave little indication of
The macrofaunal communities of the two the differences between the bays, but Swan Bay
bays were distinctly different. The macrofaunal had a significantly higher biomass and estimated
assemblages at Port Phillip Bay sites were production of macrofauna than the adjacent area
dominated by amphipods, whereas the Swan Bay of Port Phillip Bay. This difference was probably
sites supported a wider range of invertebrate because larger invertebrates, especially tanaids
types. The dominance of amphipods in Port Phillip and molluscs, were more abundant inside Swan
Bay may have been due to the greater exposure Bay than Port Phillip Bay, thus contributing more
to wave action and currents compared with Swan to the total biomass and production. Temperature
Bay. Fenwick (1976) found that fauna in an algal would appear not to have been an important
community was influenced by wave exposure; factor infiucncing production. The highest pro¬
with a high energy environment characterised by duction estimates for Swan Bay were recorded in
low species diversity and very high densities of May when temperatures were lower than in Port
amphipods. This distribution is consistent with Phillip Bay. High temperatures in Swan Bay over
the suggestion that animals with strong grasping summer may have had a negative effect on cpi-
appendages, such as amphipods, often dominate faunal production by causing migration to deeper
algal communities at exposed sites (Takeuchi el al. water. Differences in macrofaunal production
1987; Hagerman 1966). The secure hold an estimated here may be underestimates if there is
individual has on a blade of seagrass would a ditlerence in the available food resources in