Macroalgae (Seaweeds)

Abstract

Macroalgae is a collective term used for seaweeds and other benthic (attached to the bottom) marine algae that are generally visible to the naked eye. Larger macroalgae are also referred to as seaweeds, although they are not really “weeds”. In this report, macroalgae are treated as marine plants because they are photosynthetic (convert sunlight into food) and have similar ecological roles to other plants. However, macroalgae differ from other marine plants such as seagrasses and mangroves in that macroalgae lack roots, leafy shoots, flowers, and vascular tissues. They are distinguished from microalgae (e.g. diatoms, phytoplankton, and the zooxanthellae that live in coral tissue), which require a microscope to be observed. Macroalgae take a wide range of forms, ranging from simple crusts, foliose (leafy), and filamentous (threadlike) forms with simple branching structures, to more complex forms with highly specialised structures for light capture, reproduction, support, flotation, and attachment to the seafloor. The size of coral reef macroalgae ranges from a few millimetres to plants up to 3-4 m high (e.g. Sargassum, Fig. 1). With few exceptions, macroalgae (seaweeds) grow attached to hard surfaces, such as dead coral or rock; most species can not grow in mud and sand because, in contrast to seagrasses, they lack roots to anchor them to the sediment. Compared to higher (vascular) plants, macroalgae have quite complex life cycles, and a wide variety of modes of reproduction; most algae reproduce by releasing sexually or asexually produced gametes and/or spores (propagules) and by vegetative spread and/or fragmentation (breaking off of plant pieces to produce new individuals). Macroalgae play important roles in the ecology of coral reefs. They are the major food source for a wide variety of herbivores and are the basis of the reef food-web, they are major reef formers, and they create habitat for invertebrates and vertebrates of ecological and economic importance. They also play critical roles in reef degradation, when abundant corals are often replaced by abundant macroalgae. This may result from over-fishing of herbivorous fish, or from pollution by excess nutrients and sediments. In this sense, macroalgae are distinctly different from other groups, such as corals, fishes or seagrasses, where usually “more is better”. Increased macroalgae on a coral reef is often undesirable, indicating reef degradation, although this depends on the type of algae.