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).