Introduction
Large subtidal brown algal species form extensive underwater forests
along the Irish coastline. Dominant species include the orders Fucales
(Himanthalia elongata (Linnaeus) S.F. Gray 1821 andCystoseira spp.), Tilopteridales (Sacchoriza polyschides(Lightfoot) Batters), and Laminariales (Laminaria digitata(Hudson) J.V. Lamouroux, Alaria esculenta (Linnaeus) Greville,Saccharina latissima (Linnaeus) C.E. Lane, C. Mayes, Druehl &
G.W. Saunders, and L. hyperborea (Gunnerus) Foslie). Recently,
non-native species have arrived in these subtidal ecosystems via
poleward distribution shifts (e.g., Laminariales - L. ochroleucaBachelot de la Pylaie; Schoenrock et al., 2019) or from aquaculture
and/or shipping practices (Fucales - Sargassum muticum (Yendo)
Fensholt 1955; Laminariales - Undaria pinnatifida (Harvey)
Suringar; Kraan, 2017). Along semi-exposed rocky coastlines, L.
hyperborea forms dense forests, whereas in the calmer, shallow regions
of tidal loughs and fjords, S. latissima can form small forests
on hard substratum. The paucity of subtidal research makes it difficult
to determine the current and historical scope of these habitats along
Ireland’s shorelines, but the synergistic threats of ocean warming and
increased commercial harvesting present a critical need to better
understand the historical and present day distribution of these
important species in order to protect current and future marine forests.
As ecosystem engineers (Jones et al., 1994), kelp provides structure for
shallow marine habitats as a resource and a habitat for many organisms
(> 300 macrofauna species in L. hyperborea forests
in Ireland throughout the year; K. Schoenrock et al., personal
communication). In other parts of Europe, kelp forests are foraging
habitats for marine birds and fish (Norderhaug et al., 2003; 2005),
provide substrata for diverse marine assemblages (Bengtsson et al.,
2012; H. Christie et al., 2007; H. P. Christie et al., 2003; Norderhaug
et al., 2002; K. Schoenrock et al., personal communication), and are the
basis of multiple food chains from coastal habitats to the deep sea
(Filbee-Dexter et al., 2018), even providing a carbon resource for
phytoplankton (Fredriksen, 2003). These habitats can also dampen wave
energy, protecting coastlines from erosion (Lovas & Tørum, 2001), and
modify parameters of the marine environment, including carbonate
chemistry and light attenuation (Dean, 1985; Hofmann et al., 2011;
Krause-Jensen et al., 2015). In Ireland, observations indicate kelp
forests are seasonal homes to commercial species like the edible crab
(Cancer pagurus Linnaeus), European lobster (Homarus
gammarus Linnaeus) and multiple species of juvenile fish that inhabit
the kelp canopy (Schoenrock et al., personal communication). There are
natural seasonal patterns in these marine communities, but certain
species inhabit kelp forests throughout the year including echinoderms,
such as the common star (< 3 cm in diameter, Asterias
rubens Linnaeus), spiny sea star (Marthasterias glacialisJullien), the common urchin (Echinus esculentus Linnaeus), a full
summary of species is provided in Table 1, (K. Schoenrock et al.,
personal communication). The constant presence of these taxa in kelp
forests over two years of monitoring suggests that they are indicators
of healthy ecosystems in the west of Ireland. Regions to the north,
south, and east have similar communities (K. Schoenrock, personal
communication), but these regions have not been as thoroughly surveyed
as the west of Ireland.
Kelp forest decline has been observed world-wide driven by warming
oceans and heatwave events, anthropogenic inputs (harvesting and
eutrophication), and herbivore pressure (Krumhansl et al., 2016; Reed et
al., 2016; T. Wernberg et al., 2019). In Europe, the distribution of
kelp species has changed over time with climate forcing (from the last
glacial maximum), and species are predicted to continue retracting their
southern range and move northwards with ocean warming (Assis et al.,
2016; Assis, Araújo, et al., 2018; Assis, Serrão, et al., 2018a).
Currently, warmer-water subtidal forests dominated by L.
ochroleuca have retracted from their southern range edges in Morocco to
current limits mid-Portugal (Assis, Serrão, et al., 2018b), while the
cold-water kelp L. hyperborea has retracted its southern range
edge from the Portuguese coastline to the Spanish coastline of the Bay
of Biscay (Assis, Araújo, et al., 2018). Marine forests from the
Mediterranean coast to southern Portugal are poorly understood, though
it seems many species ranges are retracting at their lower latitude
range limits. On the other hand, at higher latitudes abundances of some
species, such as S. latissima and S. polyschides , are
increasing at their northern range edges, while the spread of invasive
species, such as U. pinnatifida, is increasing (Araujo et al.,
2016). In addition, herbivore pressure is devastating L.
hyperborea at its northern ranges (northern Norway), though urchin
populations can fluctuate from year to year, allowing regrowth of some
populations (Hagen, 1995). Harvesting is also a threat to kelp forests
across Europe, particularly France and Norway where the commercial
exploitation of L. hyperborea and L. digitata has been
occurring for decades (Valero et al., 2001). The mechanical removal ofL. hyperborea results in the removal of whole individuals.
Recovery can take greater than five years (Lorentsen et al., 2010), and
populations in the heaviest areas of harvest may be important reservoirs
of genetic diversity (e.g., Brittany, France: Robuchon et al., 2014).
Kelp have been of great interest in the past, in flux with economic,
socio-political, and technical advances beginning during the 1700s.
Therefore, there are many historical accounts of seaweed from sea
captains, fisheries, and naturalists along the Irish coastline from the
late 1700s to the present day. These qualitative historical records
provide perspective on the value of kelp along Ireland’s coastline, but
given the pressures outlined above, we need a better understanding and
documentation of kelp ecosystems. In this review, we focus on the
presence of the subtidal species L. hyperborea from 1700s Ireland
to present day, collating disparate historical records for the first
time. This review significantly contributes to our understanding of kelp
forest function in Ireland at a time when interest in kelp harvesting is
increasing despite the fact we do not understand basic ecological and
evolutionary processes at work in these systems. We show that regular
and systematic monitoring is urgently needed in order to conserve and
inform policy makers to foster resilience, defined as the ability to
recover from a disturbance and maintain ecosystem function, in this
vital coastal resource.