ABSTRACT
Marine macroalgae belong to a polyphyletic group of living organisms divided into three main groups, the greens (Phylum: Chlorophyta), browns (Phylum: Ochrophyta) and the reds (Phylum: Rhodophyta) (see Chapter 1 for more information). While these are generally multi-cellular, some have microscopic phases, as part of an alternation of generations. These important marine primary producers, able to colonize a large latitudinal spectrum on worldwide seas, are morphologically highly varied. Some are large and robust, constituted mostly by a distinct stipe and lamina (or blade); others range from branched or unbranched monofi laments to dense parenchymatous thalli and calcareous and non-calcareous encrusting algae. All need to be submerged (at least temporarily) in saline water, to have a substratum to attach to and to have some nutrient supply to feed on. While most attach to hard substrata, some are epiphytic, and others may
occasionally grow unattached under specifi c conditions. Owing to their sessile nature, marine macroalgae are organisms infl uenced by the specifi c environmental conditions of the water which bathes them; water quality and local physicochemical conditions will determine species occurrence and community composition. Macroalgae are at the base of productive food webs (Harley et al. 2012), including some with recognized economic value (Graham 2004, Norderhaug et al. 2005). Marine macroalgae are linked to human culture and economic systems, providing ecosystem goods and services; they can be directly used as food or medicine (see Chapters 7-10), and may confer physical protection to coastal areas (Rönnbäck et al. 2007). Large brown macroalgae such as Laminaria hyperborea may form dense kelp forests on subtidal reefs which absorb wave energy, and may therefore be locally important in coastal protection (e.g., Angus 2014, Angus and Rennie 2014).
