Major biological patterns and important physical factors

The primary factor influencing biological assemblages is mud (MD vector, Fig. 6.1). The terrestrial muddy i nshore areas along much of the length of the GBR (Fig. 6.4A) are often also very turbid (K4) and/or have high levels of chlorophyll (CH; green areas Fig. 6.1). These inshore (green-blue) areas tend to be devoid of visible biological habitat attached to the seabed (Fig. 6.2 (white), Fig. 6.4A), but typically are about 60-80% bio-turbated (Fig. 6.2 (grey), Fig. 6.4B) indicating the presence of burrowing animals such as worms, shrimps and bivalves. The mobile fauna has low diversity and low biomass (Fig. 6.3) but still includes many small fishes, prawns, crabs, mantis shrimps, gastropods and heart urchins—many of which feed on the deposits. A few sessile filter feeders, such as sea pens, survive in

Figure 6.2 Map of the distribution of broad biological seabed habitat types observed during towed video camera transects. (Image: CSIRO.)

these soft sediments. Further offshore where the turbidity is lower the muddy sediments (blue areas, Fig. 6.1) are of carbonate, indicating their biological origin. Typically these habitats are about 40% biotur-bated (Fig. 6.2) and have a fauna similar to the inshore muds, but with some stalked and cryptic sponges and surface dwelling bryozoans (lace corals) (Fig. 6.3). In the north, carbonate muddy habitats extend across most of the shelf (Fig. 6.1) given the protection of the ribbon reefs. In the deeper Capricorn Channel, the carbonate muddy habitats are among the most barren (Fig. 6.2), with about 20% bioturbation, and have the lowest diversity and biomass (Fig. 6.3) comprising a few crabs and fishes. Typically, with greater distance across the shelf, the substratum is sandier or even coarser (Fig. 6.4C), comprising entirely of biogenic carbonate and may support a variety of biohabitats (Fig. 6.2), some times even bioturbating animals (Fig. 6.4D), though more commonly surface biota such as starfish, crabs, gastropods, fish, algae, and filter-feeding alcyonarians and sponges—sometimes in great abundance (Fig. 6.3).

At the opposite extreme to mud, benthic stress (BS) is one of the strongest biophysical forces (red-orange areas, Fig. 6.1). The inshore vicinity of Broad Sound and Shoalwater Bay (orange) has the largest tidal range in the GBR and is accompanied by extreme currents (lower left orange branch of key) that scour sediments away, exposing rubble, stones and rock. Offshore, these tidal forces also cause extreme currents of clearer water (lower right red branch of key) to surge through the narrow channels of the Pompeys hard-line (red), again scouring the seabed to the limestone base (Fig. 6.4E). Strong benthic stress also occurs in the far northern GBR and Torres Strait, and in some

Figure 6.3 Map of the distribution of biomass of the major biological groups sampled by an epibenthic sled. (Image: CSIRO.

local areas such as the Whitsunday Passage. In many of these areas, bare seabed is often interspersed with gardens of colourful filter-feeding sea fans, whips and sponges (Figs 6.2, 6.4F, G), which benefit from the stable hard seabed and suspended food particles—in the offshore passages, encrusting bryozoans form extensive biogenic rubble habitat (Fig. 6.4H). The fauna has very high diversity and moderate biomass (Fig. 6.3) and includes many species of bryozoans, sponges, and gorgonians, as well as fishes, crustaceans, bivalves, starfish, urchins and corals. The scoured sediments typically are deposited in ripples, waves and dunes (Fig. 6.4!) on the fringes of these high stress (red) areas. In offshore sandy areas with medium currents, crinoid feather stars may be extremely abundant on the seabed (Fig. 6.4/); also, relic coralline outcrops and shoals with a rich sessile biota, including living hard corals, may occur in deep areas between emergent coral reefs (e.g. Fig. 6.4K).

The deeper clear waters near the outer edge of the continental shelf may be influenced by upwelled nutrients (e.g. PO, indigo areas, Fig. 6.1). Despite the depth, algae (including crustose coralline algae (Fig. 6.4L) that are adapted to low-light conditions) may be prolific in these clear nutrient rich waters (Fig. 6.2) and contribute to habitats of moderate diversity and biomass (Figs 6.3, 6.4M). Areas where upwellings intrude onto the shelf can also be seen in Fig. 6.1 (faint indigo on grey), particularly offshore from Townsville. Here, along the outer margins of the inshore turbid/muddy areas, where the water is clear enough to allow sufficient light to reach the seabed, a >200 km long band of mixed algae (including Caulerpa, Fig. 6.4N) and patchy seagrass (primarily Halophila spinulosa) proliferates (Fig. 6.2). Similarly,

Figure 6.4 Photographs of some example seabed habitat types observed during towed video camera transects. A, turbid muddy inshore seabed, with filefish; B, bioturbated silty inner shelf seabed; C, coarse outer shelf sediment with sparse biota; D, large bioturbation mounds in offshore sand; E, scoured rocky seabed in extreme current area; F, soft corals in strong current channel; G, gorgonian garden on hard ground; H, bryozoan rubble in strong current channel; I, rippled sand in inshore high current area; J, crinoids on sand in offshore strong current area; K, shoal ground in deep water; L, Ulva growing on patches of coralline algae at shelf edge; M, diverse algae and coral near shelf edge; N, dense algal bed (Caulerpa); O, sea-grass (Halophila spinulosa) bed; P, Halimeda algae bank. (Photos: CSIRO.)

Figure 6.4 Photographs of some example seabed habitat types observed during towed video camera transects. A, turbid muddy inshore seabed, with filefish; B, bioturbated silty inner shelf seabed; C, coarse outer shelf sediment with sparse biota; D, large bioturbation mounds in offshore sand; E, scoured rocky seabed in extreme current area; F, soft corals in strong current channel; G, gorgonian garden on hard ground; H, bryozoan rubble in strong current channel; I, rippled sand in inshore high current area; J, crinoids on sand in offshore strong current area; K, shoal ground in deep water; L, Ulva growing on patches of coralline algae at shelf edge; M, diverse algae and coral near shelf edge; N, dense algal bed (Caulerpa); O, sea-grass (Halophila spinulosa) bed; P, Halimeda algae bank. (Photos: CSIRO.)

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, 6.40). 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.

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