6 – Seabed Environments, Habitats and Biological Assemblages
57
around the Turtle and Howick Islands in the central
northern GBR (~14.5°S), meadows of dense H. spinulosa
occur (Fig. 6.2). Algae and H. spinulosa habitats also occur over much of the shelf in the Capricorn region (Figs
6.1, 6.2, 6.4O). On the shallow shelf just inside the outer
barrier ribbon reefs in the northern GBR, the signature
of nutrients pumped in by tidal jets can be seen faintly
in Fig. 6.1. These have encouraged the development of
vast banks of Halimeda algae (Figs 6.2, 6.4P) up to 15 m
thick, comprised of the deposited carbonate skeletons
of these algae. Halimeda also occurs elsewhere (Fig. 6.2)
but has not formed such banks. These varied marine
plant communities form substantial areas of habitat for
a plethora of biota, including numerous species of green,
brown and red algae, small fishes, gastropods, sea slugs,
crustaceans, starfish, sea cucumbers, urchins and corals
(Fig. 6.3), which may attain moderate to high biomasses
and, particularly in the case of mixed algal–seagrass
meadows, very high species diversity. The moderately
diverse biota of Halimeda banks includes numerous
species of green algae, small fishes, gastropods, urchins,
corals, sponges and alcyonarians.
General patterns of abundance and diversity
of the major phyla
The GBR seabed biota are represented by more than a
dozen major phyla, which can be quantified in terms of
biomass abundance, frequency of occurrence and numbers of species. Our understanding of these biota depends on the devices used to sample them. Animals
living down in the sediments such as small worms, crustaceans and molluscs (infauna), usually are collected by
a grab or core—a device not used by the Seabed Project.
An epibenthic sled samples sessile biota and slow moving invertebrates (Fig. 6.3) living on the seabed or in the
top few centimetres of the sediment, whereas a research
trawl typically samples the more mobile fauna living
just above the seabed, such as fishes and crustaceans.
Together, the sled and trawl confirmed the dominant biomasses of algae, particularly green algae, in the
areas outlined above (Figs 6.2, 6.3). After algae, sponges
are the next most abundant group; encrusting and massive morphotypes may reach high biomasses in the
higher current areas, stalked and cryptic types are
sparsely distributed in sedimentary areas. Ascidians
are the next most abundant sessile group and have a
similar pattern of distribution, followed by cnidarians,
which tend to be more restricted to the higher current
areas and harder ground, as are bryozoans. Echinoderms are the most abundant and widespread of the
mobile invertebrates, with overall biomass between
that of sponges and ascidians. Molluscs, while widespread in softer sediments, appear to be about half as
abundant. Fishes, better sampled by trawl, are next in
abundance—inshore and muddy areas in particular
tend to have high relative proportions of fishes. Crustaceans were much less abundant. These were followed
by worms, elasmobranchs and minor phyla, none of
which were well sampled by either device.
The ordering of these groups by frequency of species
occurrence differed from that of biomass. Fish species
occurred most frequently, followed by crustaceans,
molluscs, echinoderms, sponges, corals, algae and
ascidians. In terms of numbers of species, sponges were
the richest with more than 1100 taxa, followed by
molluscs (1000), fishes (850), crustaceans (almost
600), echinoderms (500), algae (400), corals (almost
400), bryozoans (300) and ascidians (300). These
statistics indicate a very high diversity for the GBR
seabed, yet the true diversity is much greater, given the
infauna are not included and some phyla were not fully
sorted. More detailed identifications of these samples,
all of which are lodged with the Queensland Museum,
will continue to reveal this diversity.
N HUMAN INFLUENCES ON SEABED
HABITAT AND ASSEMBLAGES
Terrestrial runoff has been reported to be elevated by
human activities and to have implications for coastal
coral reefs (see Chapters 9, 11). Coastal processes influence the composition of seabed biota as outlined above,
but it is unclear whether possible anthropogenic increases in turbidity and sedimentation have caused
any changes to the benthos in recent decades.
A widespread activity on the seabed in the GBR is
trawling for prawns. Previous research showed that
trawling can have direct impacts, particularly on easily
removed and/or slow to recover biota, but stressed the
importance of assessing the results in the context of the
57
around the Turtle and Howick Islands in the central
northern GBR (~14.5°S), meadows of dense H. spinulosa
occur (Fig. 6.2). Algae and H. spinulosa habitats also occur over much of the shelf in the Capricorn region (Figs
6.1, 6.2, 6.4O). On the shallow shelf just inside the outer
barrier ribbon reefs in the northern GBR, the signature
of nutrients pumped in by tidal jets can be seen faintly
in Fig. 6.1. These have encouraged the development of
vast banks of Halimeda algae (Figs 6.2, 6.4P) up to 15 m
thick, comprised of the deposited carbonate skeletons
of these algae. Halimeda also occurs elsewhere (Fig. 6.2)
but has not formed such banks. These varied marine
plant communities form substantial areas of habitat for
a plethora of biota, including numerous species of green,
brown and red algae, small fishes, gastropods, sea slugs,
crustaceans, starfish, sea cucumbers, urchins and corals
(Fig. 6.3), which may attain moderate to high biomasses
and, particularly in the case of mixed algal–seagrass
meadows, very high species diversity. The moderately
diverse biota of Halimeda banks includes numerous
species of green algae, small fishes, gastropods, urchins,
corals, sponges and alcyonarians.
General patterns of abundance and diversity
of the major phyla
The GBR seabed biota are represented by more than a
dozen major phyla, which can be quantified in terms of
biomass abundance, frequency of occurrence and numbers of species. Our understanding of these biota depends on the devices used to sample them. Animals
living down in the sediments such as small worms, crustaceans and molluscs (infauna), usually are collected by
a grab or core—a device not used by the Seabed Project.
An epibenthic sled samples sessile biota and slow moving invertebrates (Fig. 6.3) living on the seabed or in the
top few centimetres of the sediment, whereas a research
trawl typically samples the more mobile fauna living
just above the seabed, such as fishes and crustaceans.
Together, the sled and trawl confirmed the dominant biomasses of algae, particularly green algae, in the
areas outlined above (Figs 6.2, 6.3). After algae, sponges
are the next most abundant group; encrusting and massive morphotypes may reach high biomasses in the
higher current areas, stalked and cryptic types are
sparsely distributed in sedimentary areas. Ascidians
are the next most abundant sessile group and have a
similar pattern of distribution, followed by cnidarians,
which tend to be more restricted to the higher current
areas and harder ground, as are bryozoans. Echinoderms are the most abundant and widespread of the
mobile invertebrates, with overall biomass between
that of sponges and ascidians. Molluscs, while widespread in softer sediments, appear to be about half as
abundant. Fishes, better sampled by trawl, are next in
abundance—inshore and muddy areas in particular
tend to have high relative proportions of fishes. Crustaceans were much less abundant. These were followed
by worms, elasmobranchs and minor phyla, none of
which were well sampled by either device.
The ordering of these groups by frequency of species
occurrence differed from that of biomass. Fish species
occurred most frequently, followed by crustaceans,
molluscs, echinoderms, sponges, corals, algae and
ascidians. In terms of numbers of species, sponges were
the richest with more than 1100 taxa, followed by
molluscs (1000), fishes (850), crustaceans (almost
600), echinoderms (500), algae (400), corals (almost
400), bryozoans (300) and ascidians (300). These
statistics indicate a very high diversity for the GBR
seabed, yet the true diversity is much greater, given the
infauna are not included and some phyla were not fully
sorted. More detailed identifications of these samples,
all of which are lodged with the Queensland Museum,
will continue to reveal this diversity.
N HUMAN INFLUENCES ON SEABED
HABITAT AND ASSEMBLAGES
Terrestrial runoff has been reported to be elevated by
human activities and to have implications for coastal
coral reefs (see Chapters 9, 11). Coastal processes influence the composition of seabed biota as outlined above,
but it is unclear whether possible anthropogenic increases in turbidity and sedimentation have caused
any changes to the benthos in recent decades.
A widespread activity on the seabed in the GBR is
trawling for prawns. Previous research showed that
trawling can have direct impacts, particularly on easily
removed and/or slow to recover biota, but stressed the
importance of assessing the results in the context of the