■ OVERVIEW Generalities
Macroalgae are often referred to as seaweeds, yet they are not actually 'weeds'. Rather, macroalgae is a collective term used for large algae that are macroscopic and generally grow in the sea. They differ from other plants such as seagrasses and mangroves, in that algae lack roots, leafy shoots, flowers, and vascular tissues. In fact, algae are a polyphyletic group with diverse evolutionary histories. Whilst still used, the term 'algae' is no longer recognised as a formal taxon although it remains a useful name when referring to those protists that are photosynthetic.
The macroalgae of the Great Barrier Reef (GBR) are a very diverse and complex group of species and forms. Forms include crusts, foliose, and filamentous thalli (thallus refers to the body of an alga), ranging from simple branching structures to complex forms with highly specialised structures. The specialised structures are adaptations for light capture, reproduction, support, flotation, and/or substrate attachment. The size of coral reef macroalgae ranges from a few millimetres to plants of up to 3-4 m high (such as the brown alga Sargassum). However, in the more nutrient rich temperate regions of the oceans, brown algal kelps may grow to over 50 m in length (e.g. giant bull kelp in California). These organisms are not found on coral reefs.
Tropical macroalgae occupy a variety of habitats, including shallow and deep sections of coral reefs, inter-reefal areas, sandy bottoms, seagrass beds, mangrove roots, rocky intertidal areas, or even within the skeletons of healthy and dead corals, shells and limestone material (endolithic algae). Macroalgae are the major food source for a variety of herbivores, are major reef formers and create habitat for invertebrates and vertebrates of economic interest. They also play critical roles in reef degradation, when coral dominated reefs are replaced by rocky reefs covered in macroalgae.
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