Resilience and monitoring of Laminaria hyperboreacommunities
Recovery of kelp ecosystems after large disturbances is an important
aspect of resilience in the marine environment. Laminaria
hyperborea is a long lived species, reaching ~15 years
of age in west Ireland (maximum of 18 years in Finnmark, Norway; Sjøtun
et al., 1993) but with an average age of 4 years, where juvenile kelps
reappear throughout the year as canopy is removed by storms,
regenerating the populations annually (Schoenrock et al., personal
communication). There is no destructive grazing in L. hyperboreacommunities in this region. The common urchin, Echinus
esculentus, does not destructively graze gametophytes or juvenile
sporophytes in the sub-canopy, and generally adult kelps are left
untouched (Sjotun et al., 2006). Another urchin species
(Paracentrotus lividus ) was overfished in the
20th century (Barnes & Crook, 2001), and small
populations of the green urchin (Strongylocentrus droebachiensis )
do not pose a threat in Ireland as they do in Norway and urchin
populations generally do in other regions of the world (Estes &
Duggins, 1995; Hagen, 1995; Ling et al., 2015). The blades of L.
hyperborea annually regenerate, starting growth in winter and reaching
maximum length mid-summer, and producing sori from October to March
(Kain & Jones, 1964). The zoospores produced within sori disperse
~ 200 m and settle to develop into gametophytes and
following fertilization, juvenile sporophytes (Fredriksen et al., 1995).
This life cycle may facilitate resilience of kelp populations, allowing
for refuge from environmental and biological stressors as either (i) a
large sporophyte is too large for grazers or (ii) a microscopic stage is
safe from storms or otherwise that would uproot large sporophytes (i.e.,
bet-hedging: Lubchenco & Cubit, 1980). However, our understanding of
the role of kelp gametophytes as a spore bank is limited to only a
handful of studies (e.g., Robuchon, Couceiro, et al., 2014).
Resilience may also be conferred through genetic diversity as genetic
variation is the essential evolutionary mechanism with which species can
respond to environmental stochasticity. Larger, outcrossed populations
tend to be more genetically diverse, than smaller, often inbred,
populations. Studying these patterns in the sea can be challenging as
not all predictions from terrestrial environments necessarily apply
(i.e., chaotic genetic patchiness: Galindo et al., 2006; Selkoe et al.,
2010). Population genetic tools provide a powerful way with which to
study how genetic diversity is partitioned in natural populations, and
by extension, patterns of connectivity and population structure in the
sea (seaweed population genetics reviewed in Krueger-Hadfield & Hoban,
2016; Valero et al., 2011). Myriam Valero et al. (2011) reviewed the
current state of the literature on population genetic patterns in kelp,
with most of the studies centred on kelps in Europe (mainly in France
[Brittany] and Portugal), Australia, Chile, and California.
Interestingly, species that had population genetic data
(Macrocystis pyrifera , Lessonia nigrescens , and L.
digitata ) harboured the highest levels of diversity in areas with
strong harvesting pressure. Population connectivity (with kelp species)
is largely affected by habitat discontinuity (e.g. Billot et al., 2003),
and patterns of isolation by distance are common (e.g. Robuchon, Le
Gall, et al., 2014). Understanding how genetic diversity is partitioned
and how populations are connected to one another is a necessity in order
to determine how populations could recover from harvesting (Robuchon, Le
Gall, et al., 2014) or from disturbances, such as heatwaves seen within
the Pacific Ocean (Wernberg et al., 2019).
Myriam Valero et al. (2011) conclude that “while kelps are economically
and ecologically important, only a few studies have attempted to assess
genetic variation within kelp populations and on small scales.”
Likewise, few studies have included temporal scales in monitoring
efforts for genetic diversity. This is even more apparent along the
coast of Ireland where until recently there were no systematic studies
of the population genetics of kelp species. Schoenrock et al. (2019)
found that genetic diversity in the non-native L. ochroleuca was
comparable to the southern range edge of this species rather than closer
populations in France. Moreover, the excess of heterozygotes at Scots
Port in Bellmullet was interpreted as the result of recent admixture
following a founder event (Schoenrock et al., 2019). In addition,
glacial refugia, or areas of long-term persistence during glacial
maxima, have been predicted for L. hyperborea along the southern
coastline of Ireland (Assis et al., 2016), suggesting these areas may
harbour unique genetic diversity. Schoenrock et al. (2020) confirmed
that the highest levels of allelic diversity and heterozygosity were
found in the L. hyperborea population at Lough Hyne in the
southwest of Ireland. They genotyped seven other population along the
west coast from County Cork to County Donegal and found patterns of
decreasing diversity as well as isolation by distance. However, only
eight sites spread over much of the west coast of Ireland were included
in this study, rendering it difficult to study smaller scale patterns in
genetic structure. On-going analysis of forty-two sites along the entire
coastline of Ireland should help investigate this further (K. Schoenrock
et al., personal communication), with temporal sampling to provide
insight into the genetic stability of L. hyperborea in Ireland
(sensu Valero et al., 2011). Continued monitoring of these
genetic resources, as well as expanding the number of taxa included
(other canopy species like S. polyschides or S.
latissima ), will be important moving forward.
Monitoring kelp forest habitats in Ireland is a difficult task, as the
reticulated coastline is highly exposed to the dynamic North Atlantic
Ocean. A recent survey indicates that healthy kelp ecosystems can be
quantified through density and height of the kelp bed, which was
quantified using single beam sonar with video validation of species ID
(Biosonics; Scally et al., 2020). This technology is incredibly helpful
when creating a mapping tool for sub-surface forests, however population
surveys from the west of Ireland indicate that density and height of
stable kelp forests have huge fluctuations throughout the year (peak in
summer) with an average of 20.21 individuals m-2, few
of which are canopy forming; greatest kelp height is observed in shallow
habitats (~ 2 m depth, LAT; K. Schoenrock, personal
communication), although forests reach ~ 15 m depth on
islands off Ireland’s coasts (C. Maggs, personal communication). A
better monitoring scheme should be put in place and could include the
use of sonar (see Blight et al., 2011; Mac Craith & Hardy, 2015) or
satellite platforms to map these ecosystems. Although more typically
used to monitor blooms in estuarine and coastal habitats (Ulvaspp.: Mora-soto et al., 2020), satellite data is also useful in mapping
kelp species that span the water column, and was pioneered in the east
Pacific (M. pyrifera: Mora-soto et al., 2020; Cavanaugh et al.,
2010). Simms & Dubois (2010) created a method for submerged kelp beds
in the north western Atlantic, which could potentially be used on the
subtidal L. hyperborea forests in Ireland.
A recent review by Duffy et al. (2019) classifies marine macroalgae and
seagrass monitoring as an ‘emerging priority’ globally for ocean and
coastal management. Tiered observation systems are proposed to monitor
broadscale patterns at wider intervals, using remote-sensing coupled
with underwater observations, but detailed in situ sampling
annually at selected sites is also advised, to capture information such
as taxonomic associations to bolster data and understanding of ecosystem
function (Duffy et al., 2019). Ireland and other small countries are
unlikely to devote substantial resources to regular kelp forest
monitoring without more apparent delivery of ecosystem services. A
foreseeable way to monitor status and trends in these habitats would be
to ground-truth remote sensing technology and supplement this effort
with citizen scientist observations. Seasearch Ireland provides a scheme
for CFT divers to ‘adopt a site’ and kelp forests could be targeted in
their region, documenting the habitats kelp species are found in or
form. The presence of associated faunal species abundance, particularly
large mobile species that are easier to see (see indicator species in
Table 1), would help to create a data set where fluctuations in species
assemblages within kelp forests could be monitored, filling key
information gaps on the ecosystem services provided by these ecosystems
(e.g. Bertocci et al., 2015). National governments should be committed
to monitor kelp ecosystems under European Union (EU) environmental
legislation (EU Marine Strategy Framework Directive, MSFD (European
Commission, 2008), and the EU Water Framework Directive (European
Commission, 2000) because ecosystem based management (EBM) is central to
the legislations objectives (Berg et al., 2015), including healthy
commercial fish and shellfish stocks (MSFD descriptor 3) and healthy
marine food webs (MSFD descriptor 4).