M J Kingsford K Heimann C G Alexander A D McKinnon

Coral reefs of the world are surrounded by pelagic blue waters. Inter-reefal waters of the Great Barrier Reef (GBR) are mostly 10 m to 60 m deep on the continental shelf. Beyond the shelf, waters plunge to 4000 m and more. Despite the clarity of the water column a broad diversity of tiny organisms are found in the pelagic environment. Plankton are the ocean's 'wanderers' (from the Greek 'planktos') and are tiny organisms that drift or have limited powers of locomotion. The plants of the sea (phytoplankton, Fig. 14.1) are autotrophic and generate energy, in the form of sugars, by photosynthesis. All other planktonic organisms (bacteria, zooplankton etc., Fig. 14.2) are heterotrophs. Some plankton are 'mes-otrophic', that is, both autotrophic and heterotrophic (e.g. protists, derived from the Greek 'first of all'). Phyto-plankton are the basis of pelagic and benthic food chains that are now known to be complex, and involve many trophic transformations within microbial food webs. Plankton provide important links with reefal and inter-reefal environments as a source of food to suspension feeders (e.g. sponges, clams and corals), fishes and ultimately detrital feeders associated with the substratum (e.g. sea cucumbers).

Plankton that spend all of their lives in the pelagic environment are 'holoplankton' (Fig. 14.1) while others are temporary visitors. Meroplankton are larval forms of reef associated organisms such as corals, crabs, sea urchins and fishes (Fig. 14.4). At times of the year when animals are spawning, larvae can constitute a major component of total plankton; this is especially the case for corals that have 'mass spawning' (also worms, molluscs and echinoderms) and benthic algae that have mass releases of spores into the water column. Larvae range from the small and ciliated planula larvae of jellyfish and corals (about 0.5 mm across) and the transparent brachiolaria larvae of starfish, to large and essentially nektonic (organisms that can swim and move independently of currents) presettlement reef fishes. The pelagic phases of surgeonfish can be over 50 mm Standard Length (SL) at settlement and tank tests have demonstrated they may swim up to 120 km without food. The larvae of many invertebrates are often bizarre in shape and are very large compared to many other plankters. Spindly phyllosoma larvae of lobsters and sinister looking stomotopod larvae are up to 30 times the size of adult copepods and are voracious predators.

Marine snow and other structures

Historically, the pelagic environment was viewed as being occupied by a hierarchy of organisms of different sizes, from protists to jellyfishes. This perception occurred because the majority of samples were collected with nets or water bottles that disrupted or destroyed delicate structures such as are found within gelatinous zooplankton. In addition, flotsam and drifting algae

Figure 14.1 Diversity of phytoplankton. Identification from top left to right: A, Rhizosolenia sp.; B, Thalassionema nitzschoides; C, Dinophysis caudata; D, Odontella sp.; E, Ceratium sp.; F, Chaetoceros sp.; G, Bacteriastrum sp. (intercalary disc); H, Cyanobacterium; I, Pseudoguinardia sp.; J, Bacillaria paxilifer; K, centric diatom in girdle view; L, Bacteriastrum sp. (terminal disc); M, Thalassiosira sp. (Photo: K. Heimann.)

Figure 14.2 Diversity of Zooplankton. A, large calanoid copepod (Labidocera sp.); B, calanoid copepod (Eucalanus sp.); C, calanoid Acartia sp.; D, polychaete worm larva; E, prawn larvae Acetes sp.; F, fish larvae; G, crab larval (zoel stage); H, arrow worm (Chaetognatha). (Photo: K. Heimann.)

and lateral views; D, adult female, dorsal and lateral views, antennae omitted. The cyclopoid Oithona sp.: E, nauplius stage I; F, nauplius stage VI; G, copepodite stage I; H, adult female. (Scale bars = 100 pm.) Most orders of copepods brood their eggs, but the calanoids are almost all free spawners, releasing their eggs directly into the water column (some calanoids, such as Gladioferens and Pseudodiaptomus, are an exception to this rule). A small nauplius emerges from eggs of either type. There are six naupliar stages, and six copepodite stages, the last of which is an adult. Copepods show deterministic growth, and once adult no longer moult. Sexual dimorphism is apparent from the 4th copepodite stage and is most pronounced in the adult. Adults are gonochoristic (have separate sexes) and estimates of maximum age range from 80 to 160 days. (Images: Lawson, Grice, McKinnon, Trujillo-Ortiz and Uchima.)

was thrown out as 'trash' and hence disregarded. In situ observations have revealed that the pelagic environment has an abundance of fine structures composed of mucus, faeces and/or aggregations of phytoplank-ton cells. Mucus and gelatinous fragments are produced by a host of planktonic organisms that include appendicularians, heteropods, salps and jellyfishes.

Mucus fragments coalesce and become a microassemblage of bacteria, protists, phytoplankton and other organisms that graze on them, such as copepods of the family Oncaeidae. This so-called 'marine snow' becomes heavy and slowly leaves the photic zone, providing organic matter to organisms living in deep water and ultimately sinks to the substratum as a 'snow

Figure 14.4 Presettlement reef fish Oxymonacanthus sp.: A, at 3 mm (total length), without a dorsal spine found in juveniles and adults on reefs; B, at 9 mm (TL) with an exaggerated dorsal spine. (Source: J. Leis.)

storm'. This is the raw material of geologic sedimentary successions as foraminifera and other hard-shelled plankton are deposited in the sediment. Other structures that influence the nature of marine assemblages include drifting macroalgae (e.g. Sargassum), rafts of cells (e.g. Oscillatoria), large jellyfish and flotsam. These structures influence the distribution, feeding and survival of many plankters, especially larval forms of crustacea and fishes.

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