5. Seaweed and Seagrass: Ecological Functions
5.1.1. One of the reasons for the importance of seaweeds and seagrasses is their ecological function within ecosystems. These include:
- the ability of some species (kelp and seagrass in particular) to modify their local environment (so-called "ecosystem engineers") by altering sedimentation rates, modifying water flow and wave energy and changing light levels;
- providing habitat and shelter to other species of seaweed, and marine plants and animals, e.g. epiphytes, invertebrates, and fish;
- their role in primary production, which is critical for the productivity and survival of the ecosystem;
- providing a food source, both directly, for grazing species such as sea urchins and indirectly by releasing organic matter into coastal waters; and
- providing spawning grounds and nursery grounds for juveniles, e.g. invertebrates and fish. The latter can include commercial fish species.
5.2. Habitat and Shelter
5.2.1. Seaweeds and seagrasses provide a complex habitat structure for many species of marine algae, plants and animals (Bodkin, 1988; Duggins et al., 1989; Jackson et al., 2001). For example, epiphytic and epizoic organisms may colonise seagrass blades (Nybakken, 2001) and various marine fauna occupy areas on the holdfast, stipe and fronds of seaweeds (Jones et al., 2000). In addition, by altering environmental factors, such as light and water movement, they are able to provide indirect habitat for understorey organisms in the sheltered water column and the rock surface between holdfasts (Sjøtun et al., 2006 in Smale et al., 2016; Wilkinson, 1995), and for infaunal species found within the sediment (Unsworth and Cullen-Unsworth, 2015). Maerl beds, including dead maerl, have a complex open structure formed by interlocking maerl thalli allowing water to circulate, providing suitable habitat for a diverse community of organisms (Lancaster et al., 2014a).
5.2.2. Seaweeds and seagrasses also provide shelter from predation for a number of species (Jackson et al., 2001; Seitz et al., 2013; Kelly et al., 2001; Lancaster et al., 2014b). The level of protection varies, depending on the structural functioning and diversity of the habitat. For example, it has been found that the more diverse a habitat is, the better it is for hiding from predators, whilst maximising foraging opportunities (Jackson et al., 2001).
5.2.3. In addition, seaweed and seagrass provide shelter to marine invertebrates and fish species (see Section 4.7 ), as well as foraging habitat to these species and species of marine mammals (see Section 4.5 ).
5.3.1. The holdfast of larger kelp species is capable of supporting a very large number of species and a diverse range of species assemblages (Edwards, 1980; Christie et al., 2003; Blight & Thompson, 2008; Burrows et al., 2014a). For example, in Norway it was found that, on average, a single kelp plant supports approximately 40 macroinvertebrate species represented by almost 8,000 individuals (Christie, et al., 2003; Burrows et al., 2014a); with increased age, the holdfast habitat volume and diversity increases (Wilkinson, 1995; Christie, et al., 2003). The majority of these fauna include invertebrates such as gastropods, crustaceans and echinoderms (Burrows et al., 2014a).
5.3.2. Different kelp species have different morphologies and life histories and, as such, provide structurally varying habitats. For example, the stipe of Laminaria digitata is shorter and less rigid than that of L. hyperborea. In consequence, the substrate near to L. digitata plants experiences greater physical abrasion by the kelp blades and so fewer species can inhabit the understorey in comparison to L. hyperborea (Kain, 1979). The understorey assemblages associated with L. digitata are thus distinct from those beneath L. hyperborea. Certain species that would otherwise be outcompeted by understorey algae are facilitated by the 'sweeping' by L. digitata, e.g. the limpet Patella ulyssiponensis and the sponge Halichondria panicea. Similarly, subtle differences in morphology ( e.g. holdfast volume and complexity, stipe roughness and susceptibility to epiphyte growth) can have a strong influence on the structure and richness of associated assemblages ( e.g. Blight & Thompson 2008).
5.3.3. Physical factors, such as wave exposure, substrate ( e.g. unstable boulders, solid bedrock) and location on the infralittoral fringe, influence not only the distribution of different kelp species ( Section 3.3 ) but also their associated understorey assemblages (Wilkinson & Wood, 2003). The nature of inter-specific and regional-scale variability in kelps as habitat formers in the UK (and the wider implications for biodiversity) is poorly understood and remains an important knowledge gap in the field of kelp bed ecology.
5.4.1. Maerl provides an attachment site for animals such as feather stars, hydroids and bryozoans (Lancaster et al., 2014a). The loose structure provides shelter for small gastropods, crustaceans, bivalves and juvenile fish, and the fauna that live within the substrate (infauna) include many bivalves such as Mya truncata and Dosinia exoleta. Fauna that live on the surface (epifauna) include small crustacea. Red seaweeds, sea firs and scallops may also colonise the surface of maerl. Many species have a high specificity to maerl beds, including certain polychaetes ( e.g. Glycera lapidum, Sphaerodorum gracilis and Polygordius lacteus) and amphipods ( e.g. Parametaphoxus fultoni, Atylus vedlomensis and Animoceradocus semiserratus). Several species of algae are almost entirely restricted to calcareous habitats and are characteristically found in maerl beds ( e.g. Halymenia latifolia, Scinaia turgida and Gelidiella calcicola).
5.5.1. Seagrasses provide a stable, sheltered and permanent habitat for many fish and invertebrate species (Jackson et al., 2001). The structural habitat of seagrasses reduces flow velocities (Bos et al., 2007), which in turn alters the surrounding environment, leading to increased sedimentation and a reduction in sediment grain size (Bos et al., 2007, van Katwijk et al., 2010). Seagrasses also provide protection from predators and support a wide range of species during different life -history stages (Jackson et al., 2001).
5.6.1. Particularly during the autumn and winter months, various kelps can be washed up on the shores as a result of wave action during storms (Kelly, 2005; Orr, 2013). Beach-cast kelps are sometimes referred to as drift weed. Before the drift weed is washed ashore, it acts as a floating shelter for many organisms such as crustaceans and juvenile fish (Kelly, 2005). As the drift weed rots on the shore, it provides shelter for invertebrates, including amphipods, polychaetes, coleoptera and diptera larvae (Kelly, 2005; Orr, 2013). Large accumulations of beach-cast seaweed in the intertidal and supralittoral zones also benefit fauna because they maintain relatively stable micro-climatic conditions, and therefore shelter macroinvertebrates from extreme temperatures and protect them from desiccation when the tide recedes (Orr, 2013). Fauna may also be attracted to drifting seaweed in the surf/swash zone because it partially protects them from rapidly moving water by reducing local current velocities (Orr, 2013).
5.7. Primary and Secondary Production
Primary Production 
5.7.1. Seaweeds and seagrasses contribute considerably to the total production of inshore waters. Kelp primary production per unit area is amongst the highest known in aquatic ecosystems (Birkett et al., 1998 cited in Kelly, 2005). Mann (1973) reports productivity levels in kelp forests ranging from 800 g C/m 2 (in California) to as much as 2,000 g C/m 2 in the Indian Ocean. In Scottish waters it has been estimated that an area of 2,900 km 2 has a typical production rate of 1,300 g C/m 2 /yr (Dayton, 1985; Burrows et al., 2014b). However, production rates have been found to vary widely between kelp species and depth, with Laminaria spp. achieving 1225 g C/m 2 /yr at its most favourable depth in south-west England (Bellamy et al., 1968) and 1750 g C/m 2 /yr at the Canadian Atlantic coast (Mann, 1972). Low production values have been recorded in Saccharina latissima in Scotland at only 120 g C/m 2 /yr (Johnston et al., 1977), which was attributed to nutrient limitation in a low flow site (Mann, 1982).
5.7.2. Intertidal fucoids have been estimated as having slightly lower, although still high, levels of primary production (Kelly, 2005). Seagrass beds are densely populated with plants and also have high rates of productivity. A study by McRoy & McMillan (1977), for example, estimated that temperate seagrass beds have a productivity rate of 500 to 1,000 g C/m 2/yr.
5.7.3. In addition to the seaweeds and seagrasses themselves, primary production is provided by the microbial and macroalgal communities living on the fronds and leaves of the individual plants (Wilkinson & Wood, 2003).
Growth of Kelps - contribution to primary production
5.7.4. As well as being very productive, kelps have very fast growth rates (North, 1971; Nybakken, 2001). Irradiation intensity and temperature are the primary factors influencing growth rate and the maximum biomass of both Laminaria digitata and L. hyperborea (Werner & Kraan, 2004) has been recorded in early autumn (Kelly, 2005).
5.7.5. The main growing point of kelps is the meristem at the junction of the stipe and frond. As new tissue is formed at the base of the frond, old tissue is lost at the distal end by decay and damage, so that the production rate is much greater than that indicated by change in frond size (Wilkinson, 1995).
5.7.6. L. hyperborea are perennial; each year this kelp species renews its entire blade or lamina and the stipe increases in size (Kelly, 2005). The blade or lamina primarily sheds in the late spring and early summer; although this can occur at any time of the year and older plants can also be completely removed during winter storms. Regeneration time varies depending upon the time of the year the removal occurs. If the blade is removed prior to the growth peak then regeneration can be 5 months (Kelly, 2005). However, if it occurs at the beginning of or during the low growth phase, then regeneration can take 10 months. The time taken to recover is also a function of the age of the plant.
5.7.7. Other kelp species retain the frond but keep adding to it from the base while the older parts at the distal tip erode. After a few years the distal loss and the new growth balance so that the frond has an approximately fixed size but it is still very productive in terms of inputting detritus and dissolved organic matter ( DOM) into the ecosystem (see following section on Secondary Production).
Secondary Production 
5.7.8. Seaweeds and seagrasses contribute to secondary production in the ecosystem in two ways. The first is by being grazed directly, which is discussed in more detail in Section 5.4. The second is through their entry into the food chain in the form of detritus and/or dissolved organic matter ( DOM) (Wilkinson, 1995). Detritus and dissolved organic matter may be processed through the microbial loop or consumed by a wide range of detritivores before entering the food web (Krumhansl & Scheibling, 2012 cited in Smale et al., 2016).
5.7.9. Detritus ranges in size from small fragments to whole plants (see Growth of Kelps above, for examples) (Nybakken, 2001). Davison (1998 cited in Wilkinson & Wood, 2003) suggests that Zostera beds produce approximately 1 tonne of detrital material per km 2.
5.7.10. Dissolved organic matter from kelp supports infaunal communities beyond the kelp bed itself (Stamp & Hiscock, 2015) by increasing levels of dissolved organic matter within the sediment (Stamp & Hiscock, 2015); this provides valuable nutrition to potentially low productive habitats (Smale et al., 2013). Seagrasses also contribute to secondary production by providing an important source of dissolved organic matter for surrounding coastal habitats (Lancaster et al., 2014b).
5.7.11. In addition Chapman (1984) reports that one L. digitata plant may produce 6 thousand million spores per year (Kelly, 2005). These spores only last a few days and are very transient, but it is possible that they are an important food source for species within the immediate area of the kelp (Kelly, 2005).
5.7.12. Beach-cast seaweed and associated particulate organic matter play a central role in sandy beach food webs (Orr, 2013). As beach-cast seaweed rots on the shore, it provides food in the form of organic matter for invertebrates, including amphipods, hypoxic/anoxic-tolerant infaunal opportunistic polychaetes such as Malacoceros sp. and Capitella spp., and coleoptera and diptera larvae (Kelly, 2005; Orr, 2013). These invertebrates are subsequently a food resource for various arthropods and birds (see Section 5.4). Drifting seaweed will break down into finer particles which is then consumed by suspension feeders ( e.g. mysids and bivalves) and deposit feeders ( e.g. polychaetes and benthic amphipods) in the inshore beach environment (Orr, 2013).
5.7.13. Large accumulations of beach-cast seaweeds and seagrasses can occur along the coastline creating a source of recycled nutrients and detrital material, which is of great benefit to the local ecology of nearshore habitats (Kirkman & Kendrick, 1997). In addition, kelps have the potential to provide a rich food source to assemblages many kilometres from its source (Vanderklift & Wernberg, 2008) as well as enriching offshore deep sediments at depths of 900 m or more (Vetter & Dayton, 1998). This is highly dependent on the site specific hydrodynamics of an area, and typically kelps are consumed and decomposed near to where they grow.
5.7.14. The decomposition of leaves and stems of the seagrass Ruppia spp. has also been found to support benthic communities and be a source of primary production to deeper water and drift line communities (Verhoeven & van Vierssen, 1978; Zieman et al., 1984; Kantrud, 1991 cited in Wilkinson & Wood, 2003).
Food Web Dynamics
5.7.15. There are a few species which feed directly on seaweeds, primarily grazers such as sea urchins. It has been observed by Jones and Kain (1967), for example, that sea urchins such as Echinus esculentus graze on L. hyperborea and remove sporelings and juveniles. Although the level of grazing by urchins is not high, it has been found to control the depth of distribution of L. hyperborea and reduce the understorey community abundance and diversity (Stamp & Hiscock, 2015).
5.7.16. Seagrasses have high rates of primary production which support a range of diverse fauna (Wilkinson & Wood, 2003). They provide a food source for many fish and invertebrate species (Jackson et al., 2001). Some bird species feed directly on seagrass, and seagrass beds (including Ruppia spp.) are thus an important food source for them. Zostera beds are heavily grazed by overwintering wildfowl and, in particular, are an important food source for Brent Geese and Canada Geese (Lancaster et al., 2014b). Canada Geese favour intertidal and shallow subtidal Z. marina (Tyler-Walters & Wilding, 2008b), while Mute and Whooper Swans favour intertidal Z. noltii (Nacken & Reise, 2000). The importance of seagrass habitats to these species has been highlighted in Tubbs & Tubbs (1983 cited in Wilkinson & Wood, 2003) who report that Brent Geese grazing reduced Zostera cover from 60 -100% in September to 5 -10% between mid-October and mid-January.
5.7.17. Seaweed and seagrass habitats also provide a food resource and foraging habitat for higher trophic levels such as fish, birds and marine mammals. Diving seabirds and sea ducks, which typically eat large invertebrates, shellfish, fish eggs or fish, are known to feed within kelp forests due to the high biomass and biodiversity associated with kelp and the subsequent food availability (Kelly, 2005). For example, a study in Norway found that Common Eiders selected kelp forest as foraging grounds throughout the winter months. Black Guillemots also feed on fish ( e.g. butterfish) in kelp habitats ( SNH, pers. comm.). The infralittoral fringe of kelp forests is also an important area for feeding birds, in particular wading birds, as prey items such as crustaceans, bivalves and amphipods are exposed as the tide ebbs (Kelly, 2005).
5.7.18. Seals and otters will also forage in kelp forests and seagrass beds due to the high biomass and diversity supported by the habitat. Seals feed on fish species, such as wrasse, that occur in kelp forests (Tollit et al., 1998; Wilkinson & Wood, 2003). Coastal otters are also likely to utilise the productive inshore waters where seaweed and seagrass habitats are present, as they support high levels of fish and crustacean prey species ( SNH, 2015a). Further information on the foraging ranges of seals and otters is provided in Section 4.4 .
5.7.19. The inshore fauna around drifting fragments of seaweed are consumed by fish and predatory crustaceans such as shrimp (Orr, 2013). Migratory shorebirds stop over in the Outer Hebrides and feed prolifically on invertebrates within beach-cast seaweed, as do breeding waders in the summer, and hence the seaweed is a rich feeding ground for birds at various stages of their life cycle (Orr, 2013).