The temperatures of Chilean coastal waters vary from north to south, sea surface temperature reaching over 20℃ in the north and below 5℃ in the south (Tala et al., 2016). The coastline is dominated by the Antarctic Circumpolar Current, which breaks into the Humboldt Current flowing up-north and the Cape Horn Current passing by the southernmost tip of Chile (Thiel et al., 2007; Camus, 2001). The dominant Humboldt Current is characterised by continuous upwelling, which brings nutrient-rich waters to northern Chile. Rather more seasonal upwellings occur in southern parts of Thiel et al. (2007). Furthermore, this cold-water current system reduces air temperature, and inhibits precipitation, creating a dry and cold climate with nutrient-enriched coastal waters (Rutllant, 2003). Due to local geographic differences producing localised eddies and current circulations, the Chilean coast is considered highly heterogeneous, creating various niches for a wide variety of species (Thiel et al., 2007). Types of kelps (Order Laminariales) found in Chile are L. nigrescens, L. berteroana, L. spicata, L. trabeculata, and M. pyrifera, and the fucoid Durvillaea antarctica, which is not true kelp, but because of its honeycomb-like inner structure filled with gas, thus positively buoyant, is studied as a species of kelp (Fraser et al., 2012; Vásquez, 2016 and the references therein). In Chilean waters, kelp grow from the intertidal areas up to around 30 m depth (Vásquez, 2016). Only two species are exclusively subtidal, M. pyrifera and L. trabeculata, yet expressing distinct distribution patterns, both along the depth gradient and geographic latitude (Table 1). The first one is the dominant kelp south from 42°S (Chiloé region), and the second one dominates from central up to the northern coast (18°–42°S) and is an endemic species with no other records of distribution outside Chile (Vasquez, 1992; Hoffmann and Santelices, 1997; Vásquez and Buschmann, 1997; Tala et al., 2004; Ramirez et al., 2008; González et al., 2012).

In this subsection, the habitats formed by kelp are divided into three groups: standing kelp forests; surface floating canopy; and detached parts forming rafts (Fig. 2). It is worth noting that kelp wrack provides another niche for various foraging organisms. The macroalgae wrack washed ashore is a direct food, as well as shelter for various upper and lower beach inhabitants (amphipods, isopods, tenebrionids) (Jaramillo et al., 2006; Duarte et al., 2010; Quintanilla-Ahumada et al., 2018). However, it seems that so far only intertidal, or macroalgae wrack of mixed species from intertidal and subtidal habitats has been researched and, to our knowledge, no research focusing entirely on the subtidal kelp wrack and the associated organisms has been published to date. Furthermore, there is little attention paid to benthic microhabitats, (i.e., assemblages of algae and associated organisms formed under the canopy of the attached kelps). It is known that there are macroalgae communities associated and dependent on M. pyrifera abundance, although the species traits (competition vs. habitat provision) still must be explained further (Almanza and Buschmann, 2013). In addition, no research in Chile, to our knowledge, considered kelp holobionts (i.e., assemblages of microorganisms) which are, very likely, even vital for the macroalgae survival, as it has been explained by the study of Michelou et al. (2013) in M. pyrifera specimens from California, USA. Thus, due to the lack of research on these topics in the Southeast Pacific region, the kelp wrack, understorey habitats and kelp holobionts are not described in this review. Environmental heterogeneity provided by kelp is mostly focused on large and medium scales, yet even shifts underpinned in micro-scales can be of utmost importance for meiofauna and small macrofauna-size communities, and therefore should not be overlooked (Shelamoff et al., 2019).

Macrocystis pyrifera is the dominant subtidal kelp in central and southern Chile, with the exception of the Patagonia region (Vásquez, 2016; Mora-Soto et al., 2020). In this area, there are two growth and reproduction patterns of M. pyrifera identified (Buschmann et al., 2004): (1) the kelp population is reproductive throughout the year but only in exposed areas; and (2) in sheltered areas, reproduction starts in late winter, having fertile sporophytes in summer-autumn and completely degrading afterwards. Macrocystis pyrifera, an underwater forest forming kelp, has been studied as an important habitat for many bivalves, annelids, isopods, peracarids, nudibranchs, chitons, gastropods, decapods, amphipods and crustaceans, and their predators such as the asteroid Diplasterias brandti, or the juvenile crabs Lithodes antarctica, as well as spawning grounds for Patagonian squid Doryteuthis gahi (Santelices and Ojeda, 1984; Cárdenas et al., 2007; Hinojosa et al., 2007, 2010; Wichmann et al., 2012; Rosenfeld et al., 2014).

The main kelp grazers found on the Chilean coast are the marine snail Tegula tridentata, the sea urchins Loxechinus albus, Tetrapygus niger and asteroid Heliaster helianthus (Vásquez and Buschmann, 1997; Pérez-Matus et al., 2017). These herbivorous species are identified as keystone species in the Southeast Pacific kelp forests as their abundance affects the resilience of the whole ecosystem (Ortiz et al., 2013; Hermosillo-Núñez, 2020). The closer observations of the effects of sea urchins (one of the most voracious keystone species and kelp grazers), showed very different effect of the dynamics of these echinoderms on kelp habitats along the Chilean coastline as compared to their effect on kelp forest ecosystems along the North American coastline. In California, sea urchins have overgrazed extensive M. pyrifera forests in the past, leaving barren grounds and thus drastically altering the whole coastal ecosystem (Lawrence, 1975; Dayton et al., 1992; Graham, 2004). On the other hand, the investigations conducted along the coastline of South American, mainly Chile, showed only mild effects of kelp grazing by sea urchins (Castilla and Moreno, 1982; Moreno and Sutherland, 1982). The lack of negative impact of the grazing of sea urchins was explained by Vásquez et al. (1984) and Dayton (1985), who collected data from oceanographic observations, sea urchin recruitment patterns, human interference, and experimental studies and suggested that M. pyrifera growth and densities in Chile are affected by multiple factors which differ according to the geographic location. The westwind drift current and exposure to strong wave surges, mostly pronounced in southern Chile, greatly limit the sea urchin larvae recruitment, so the dense areas of M. pyrifera forests can flourish (Dayton, 1985). Moreover, sea urchins are collected by the local fishermen at some locations, which is another factor reducing the population of this echinoderm. Hence, in areas where larvae settlement is easier, sea urchin populations negatively correlate with M. pyrifera densities (Vásquez et al., 1984; Dayton, 1985). Thirty-five years later, Hermosillo-Núñez (2020) brought back the topic of conspicuous echinoderm species relation to kelp forests, investigating in detail the northern Chilean coastal region. The study analysed a broad variety of the main macro invertebrates and their trophic relations, and added more herbivorous and carnivorous echinoderms to the list of keystone species in the kelp forest ecosystem. The author highlighted the importance of the balanced presence of echinoderms in the kelp ecosystem and pointed out the anthropogenic stressors, e.g., intensive kelp harvest, as the most probable cause of altered keystone species complex that may result in kelp ecosystem disruption and, eventually, emerging barren grounds.

The research conducted by Pérez-Matus et al. (2007) on the density of kelp and the understorey canopy diversity (microhabitat) and the associated fish showed that denser kelp community and greater variability of understorey species of seaweed facilitates a more suitable environment for species richness and abundance of fish, including economically valuable species like red cusk-eel, Genypterus chilensis. Furthermore, the kelp habitat that is composed of both, M. pyrifera and L. trabeculata, exhibits higher numbers of fish than ecosystems of solitary kelp species (Pérez-Matus et al., 2007). The latter authors also observed that habitat variability depends on seasonal changes on a local scale, which is not detectable on a regional scale. This suggests that the northern coast of Chile can be characterised as highly heterogeneous in terms of nutrients and primary production. Formerly, Camus and Ojeda (1992) had also observed a high heterogeneity of the L. trabeculata habitat in northern Chile. Vega et al. (2005) described a mixed kelp species habitat of M. integrifolia and L. trabeculata at Caleta Constitución, northern Chile. These authors detected that interspecific competition, as well as morphological and physiological adaptation features, influence the morphology of the sporophytes and the distribution patterns of kelp. Nevertheless, the investigations of microhabitats, as well as the habitats formed by mixed kelp species are purely observational, and, to our knowledge, there is a lack of in situ experimental work.

Pérez-Matus et al. (2017) compared the abundance of kelp L. trabeculata with the fishing pressure in upwelling zones vs. adjacent no upwelling zones. The findings concluded that in the upwelling regions, the density, foliage, and recruitment are significantly higher than in the upwelling unaffected areas, regardless of the fishing regulation. This might be due to the fact that the upwelling zones are hard to access for fishermen, and also that the strong wave actions are likely to move the larvae, preventing an abundant settlement of herbivorous species. Furthermore, in upwelling unaffected sites, the abundance of herbivory species is much higher where their predators are removed by the uncontrolled fishery. This leads to a concomitant consequence, which is the overgrazing of the foundation species–kelps. In the areas where fishing is managed, the biomass of predatory fish is significantly greater and the top-down trophic regulation is enhanced, showing a higher abundance of kelp, fish, benthic carnivores and sessile species. The research concludes that the upwelling events along the Chilean coast induce the bottom-up trophic levels, and fishing is the factor significantly affecting a top-down community regulation and, as a consequence, species richness and abundance.

Kelp, including M. pyrifera and L. trabeculata, is known to fuse the holdfast and form a coalescence which undergoes morphological changes at cell level. The coalescent algae exhibit notably larger holdfasts or longer thalli, or a higher number of stripes (Wernberg, 2005; Segovia et al., 2014; González et al., 2015). This fusion can be observed also in non-phylogenetically related species (González et al., 2014, 2015). The holdfast coalescent forms more often in the wave-exposed areas, thus, it is believed that such grafting helps to reduce the detachment of the macroalgae (Wernberg, 2005). Ojeda and Santelices (1984) described species richness and temporal variation of kelp associating benthic invertebrates in southern Chile. The study discovered the community structure and dynamics along with the growing habitat—holdfasts, finding that the young and small holdfasts are rapidly occupied by new species but not the large ones, where the species turnover is close to zero. Furthermore, the arrival of new species along the growing holdfasts does not influence the population of earlier inhabitants. This feature contrasts with the colonisation traits in steady or slow-growing habitats, such as seagrass or coral. Various studies have reported significantly higher numbers of epifaunal species inhabiting kelp holdfasts than the canopy (Coyer, 1984; Adami and Gordillo, 1999; Christie et al., 2003; Winkler et al., 2017). The species richness and abundance of epifauna can be significantly lower in fronds than in holdfast areas. Winkler et al. (2017) compared the composition of epifaunal assemblages between fronds and holdfasts of M. pyrifera at Punta de Talca, central Chile. In their work, they also checked the epifaunal abundance fluctuation seasonality. The fronds of kelp M. pyrifera were found to host 9 different species of epifauna, while 28 species were found among the holdfasts area. The species abundance was also greater (×(8.35±1.65) times) around holdfasts than at fronds areas, even though the total surface area of kelp foliage is substantially larger than the surface area of holdfasts. There was a strong dominance of species Limnoria quadripunctata (Isopoda, 50.6% of relative abundance), Polychaeta (20.7% of relative abundance) and Aora typica (Amphipoda, 10.9% of relative abundance) in holdfasts of M. pyrifera. In the fronds area, the main species were Peramphithoe femorata (Amphipoda, 79.9%) and Amphoroidea typa (Isopoda, 9.4%). In Punta de Talca, a 5℃ difference in water temperature between the austral summer and winter, thermocline in spring and summer up to 10 m depth and upwelling occurrence in spring, cause variations in nutrient availability and, consequently, seasonal variations in kelp growth. Due to the seasonal fluctuations in kelp primary production, which is highest in austral summer and lowest in austral autumn and winter, the highest epifaunal species abundance among kelp fronds was found in summer, while at the same time, the epifauna abundance in holdfast area was the lowest. Former studies suggest that in early winter the epifaunal species shift their habitat, migrating from the canopy down to the holdfasts, as the fronds are more vulnerable to abiotic conditions (increased winds and waves) (Thiel and Vásquez, 2000; Christie et al., 2003; Miranda and Thiel, 2008; Gutow et al., 2009). The overall invertebrate diversity along the Chilean coastal waters increases towards the south, negatively correlating with fish predation, which is strong in north-central Chile but weakens towards the south (Pérez-Matus et al., 2007; Rivadeneira et al., 2011; Navarrete et al., 2014). The seasonal variability in kelp epifaunal community in southern waters is more pronounced than in northern and central Chile, likely because the seasonal differences of water temperature, nutrients and hydrodynamic forces are greater at higher latitudes (Adami and Gordillo, 1999; Winkler et al., 2017; Friedlander et al., 2018).

A comparative study of M. pyrifera, D. antarctica and L. nigrescens investigated the physiological kelp reaction to temperature and UV exposure. The study examined the ability of different species of kelp to protect from environmental stress and recuperate after the damage, which is an important knowledge for the predictions of the climate change scenarios. The results showed that these species that normally grow in the environment of water temperature below 16℃ have a mechanism to survive a short-term exposure to temperatures up to 30℃. Kelp rapidly induces soluble phlorotannin compounds and delays the induction of non-soluble phlorotannins. The most interesting part is the different limits of each species to the increase of temperature. The species whose upper part of the fronds tend to be exposed to UV even during high tide (M. pyrifera and D. antarctica) had higher limits to endure higher temperatures than the species that are exposed during the low tide but completely submerged during high tide (L. nigrescens) (Cruces et al., 2013). These adaptation differences of some species could be advantageous for surviving in case of temporary unusually high UV penetration because of the changing climate or weather anomalies.

Subtidal kelp populations that form detached free-floating rafts have received extensive attention among ecologists in Chile. Detached kelp free floats with surface currents and thus provides the means of dispersal for many associated species (Waters, 2008; Wichmann et al., 2012). Hinojosa et al. (2010) assessed that detached M. pyrifera can accumulate in rafts reaching 1 500 kg/km² wet biomass during spring season, and can drop down to wet weight 100–200 kg/km² in winter depending on the area, as the floating biomass tends to be higher in outer fjords and areas exposed to the open ocean. Even though detached kelp loses its biomass through time, it continues growing and reproducing while free floating on the surface if the environmental conditions are favourable, (e.g., temperature, solar radiance, and nutrient availability). Experimental studies conducted in the northern, central and southern coast of Chile show the correlation of kelp biomass, water temperature and nutrient availability (Hinojosa et al., 2010, 2011). In the northern coastline of Chile, where the water temperature tends to be higher (13–20℃), the loss of floating biomass was significantly greater than in colder waters (Hinojosa et al., 2010, 2011; Rivadeneira et al., 2011). During the austral summer, rafting kelp shows better persistence in the mid and high latitudes (central and southern coast of Chile), where the water temperature is lower, 9–17℃. Thus, the persistence of floating M. pyrifera along the Chilean coast can be associated with the latitude and seasonality (Macaya et al., 2005; Tala et al., 2016). Great biomass and relatively long survival time (70–125 d; Hobday, 2000; Hernández-Carmona et al., 2006) on the sea surface appears to form an entirely new ecosystem. The community shift is observed shortly after the algae detach: benthic and holdfast epifauna flees and the planktonic organisms inhabit the niche (Dayton, 1985). While the vertical kelp hosts a higher proportion of peracarid species and lower proportion of molluscs, the rafting kelp is inhabited with higher numbers of molluscs than peracarids. Additionally, within the mollusc group, there are changes in species composition. The abundance of gastropods decreases significantly, leaving the bivalves as the dominant group (Wichmann et al., 2012). Besides forming a new ecosystem, rafting kelps are one of the most important vectors of species dispersal and connectivity for both, their own kelp population, and their associated epibiont species. Macaya and colleagues measured the abundance of rafting kelp and the kelp that remained attaching to a substrate, and the proportion of sporophylls (reproductive fronds) in each group (Macaya et al., 2005). Their findings state that rafting kelp is able to release viable zoospores, yet the detachment event reduces the reproduction rate when compared to the abundance of the sporophylls in the attached kelp at a nearby location. Nevertheless, detached kelps remain functionally reproductive long enough to be able to disperse the zoospores while floating with surface currents long distances. In their study, the stalked barnacles (Lepas spp.) were used as bioindicators of the minimum rafting time, which helped to assess the free-floating persistence time and the distance. In fact, some of the rafting sporophytes were driven several hundred kilometres by the Humboldt Current for at least 21 days (Macaya et al., 2005). Kelp-associated communities are also taking advantage of this moving host in order to spread and occupy new territories. The species that are more widespread among biogeographic regions are the ones that are able to persist, directly develop and recruit on the natal floating kelp (i.e., capable of completing the recruitment cycle within the kelp floating time span) (Wichmann et al., 2012). Hence, the dispersal range of rafting-capable organisms depends on the kelp free-floating persistence facilitated by temperature, solar radiation and nutrient availability as well (Helmuth et al., 1994; Wichmann et al., 2012). As an example, the isopod Limnoria quadripunctata is a species that bores into the holdfasts of M. pyrifera for dispersal purposes. As M. pyrifera grows and drifts along the entire coast of Chile, so does this isopod (Haye et al., 2012). However, increasing global water temperature might become a threatening factor for this raft ecosystem. Rothäusler et al. (2011a) experimented with the exposure to a gradient of temperatures and UV rays, mimicking the natural conditions found along the coastline from the north to the south of Chile. The physiological performance of M. pyrifera was recorded to ascertain the rafting persistence. The results showed a significant decrease in carbon content and an increase of carbonic anhydrase enzyme in kelp blades due to increasing water temperature which means the sporophytes are suffering from a thermal shock. As a consequence, the higher water temperatures accelerate the disintegration and sinking of free-floating sporophytes. The experimental conclusions explain the field observations of Macaya et al. (2005), where no free-floating kelp sporophyte was found in the waters >20℃ along the northern coast of Chile (Macaya et al., 2005). Additionally, free-floating kelp is the direct food source for various grazing organisms, and this factor would highly contribute to the group of impacts shortening kelp rafting persistence, although the possible alterations in grazing along with an increasing surface temperature have not been assessed yet (Rothäusler et al., 2009).

Due to atmospheric pressure changes, the dry South American coast can experience unusually high precipitation levels, and the warm water flow from Southeast Asia. This phenomenon is called ENSO and it occurs every 3–10 years in the southeastern Pacific Ocean (Gelpke, 2017).

Vega et al. (2005) looked into the effect of ENSO—abnormally higher seawater temperature—on two kelp species, L. trabeculata and M. integrifolia. The warmer water temperature period was followed by an abnormally cold-water period—La Niña. The two kelp species reacted differently to oscillating temperatures (Fig. 3). During ENSO, the adult sporophytes of M. integrifolia showed an increase of abundances, although the juvenile sporophytes did not exhibit any significant changes in growth. When the temperature dropped significantly (1999–2000 La Niña period), both adult and juvenile sporophytes drastically decreased in abundance. However, L. trabeculata responded differently. ENSO had a negative effect on the abundance of L. trabeculata, whereas La Niña temperatures favoured the growth of the kelp. The effect of ENSO seems to be significantly greater for juvenile sporophytes than for adult kelp. Nevertheless, the authors stated that the abundance patterns of M. integrifolia and L. trabeculata did not change drastically during the ENSO event in 1997–1998. The mild effect of ENSO in the study area could be explained by the constant upwelling, which cools surface waters and brings the nutrients up. Furthermore, the upwelling locations act as a source of kelp propagules for dispersing and recolonizing the locations without upwelling where the environmental conditions are suitable. The kelp abundance pattern was rather affected by the following water temperature drop during 1999–2000 (La Niña). The increase of sea urchin populations, especially T. niger, during La Niña period, added a significantly negative factor of overgrazing into the change of growth patterns and abundance of kelp (Vega et al., 2005). In contrast, in northern Chile, Peru, and California, where there were no upwelling events, the locations severely suffered the extinctions of Lessonia and Macrocystis populations during the same ENSO event (Fernández et al., 1999; Ladah et al., 1999; Lleellish et al., 2001; Edwards, 2004).