This study aims to investigate the relative effects of urchin removal, non-urchin gastropod herbivores removal, and transplantation on macroalgal forest restoration using an additive manipulation design. A field experiment was conducted in subtidal urchin barren rocks in the eastern coast of South Korea from August 2017 to June 2020 with three experimental treatments: no urchins (NU), NU plus no herbivorous gastropods (NH), NH plus Ecklonia bicyclis transplant (NHT). Six months after experiment initiation, seaweed abundance rapidly increased in all three experimental treatments. The highest peak was found in the spring season of the first year (2018). The year-to-year variations became smaller throughout the survey period. The results of comparing NU and control site indicated that urchin removal had an exclusive effect on algal recovery while additional removal of herbivorous gastropods did not affect algal recovery quantitatively. With the successful establishment of E. bicyclis, the community assemblages of the three treatments became more distinct in the subsequent years, showing diverse dominance in NU, red algal dominance in NH, and dominance of E. bicyclis canopy and understory groups in NHT. This study provides evidence that urchin removal on its own from barren grounds can lead to rapid restoration of seaweed forest in subtidal habitats.

A long-term change of a subtidal macroalgal assemblage has been investigated in Maxwell Bay, King George Island (KGI) of the Antarctic coast by a revisit survey after 30 years. Field surveys were done by SCUBA diving at six sites in 2016–2018 to directly compare with the previous survey conducted in 1988–1993 at the same sites. The total number of macroalgal species was similar between the previous and the present survey, 25 and 27 species respectively. However, the macroalgal assemblage changed substantially with the average similarity of 48.2% between the two surveys. Also, the species-level abundance showed a high variability between surveys. On the other hand, over the 30 years interval there was little overall change at the between-site level hierarchical structure in the subtidal communities of Maxwell Bay. The sites near the penguin rookery consistently showed the highest biodiversity, indicating the importance of land-based nutrients input in Antarctic coastal habitats. A noticeable pattern change over 30 years was the increase of Desmarestia complex and Plocamium cartilagineum and the decrease of Himantothallus grandifolius. Both groups are still dominant, but the shift from Himantothallus to Desmarestia-Plocamium may reflects temperature rise on the Maxwell Bay coast compared to the past.

Intertidal community recovery and resilience were investigated with quantitative and qualitative perspectives as a function of disturbance timing. The study was conducted in a lower intertidal rock bed of the southern coast of South Korea. Six replicates of artificial disturbance of a 50 cm × 50 cm area were made by clearing all visible organisms on the rocky substrate in four seasons. Each of the seasonally cleared plots was monitored until the percent cover data reached the control plot level. There was a significant difference among disturbance timing during the recovery process in terms of speed and community components. After disturbances occurred, Ulva pertusa selectively preoccupied empty spaces quickly (in 2-4 months) and strongly (50-90%) in all plots except for the summer plots where non-Ulva species dominated throughout the recovery period. U. pertusa acted as a very important biological variable that determined the quantitative and qualitative recovery capability of a community. The qualitative recovery of communities was rapid in summer plots where U. pertusa did not recruit and the community recovery rate was the lowest in winter plots where U. pertusa was highly recruited with a long duration of distribution. In this study, U. pertusa was a pioneer species while being a dominant species and acted as a clearly negative element in the process of qualitative recovery after disturbance. However, the negative effect of U. pertusa did not occur in summer plots, indicating that disturbance timing should be considered as a parameter in understanding intertidal community resilience in temperate regions with four distinct seasons.

Kelp forests have declined gradually all over the world. Understanding the trophic structure of such a productive and diverse ecosystem is crucial for its restoration and effective management. Few studies, however, have focused on the trophic structure and functional recovery of kelp forests in the process of restoration. This study was conducted in the eastern coast of Korea where kelp bed restoration was in process with the removal of sea urchins. In addition to quantitative measurement for recovery using common community parameters, we investigated how fast the stability of a food web structure could be established in the newly restored kelp beds with an initially barren condition, using stable isotope analysis, in comparison with a nearby natural bed and barren site. At the restored bed, total algal biomass and diversity reached the levels of the natural bed within 1 year. While the δ¹³C and δ¹⁵N values of macroalgae and organic matter were separated isotopically, they were similar among sites, excluding organic matter at the natural bed in 2019. Most consumers showed similar isotopic values among sites, with overlapping range for δ¹³C of producers. However, some herbivores showed higher δ¹⁵N values as predators/omnivores, particularly at barrens, which could be explained by trophic plasticity depending on the macroalgal structure. In the restored bed, for the first year, higher trophic diversity (CR, CD) and lower trophic redundancy (MNND, SDNND) showed non-overlapping >50% CIs among sites. However, this distinctive stage moved toward the natural bed upon entering the second year as trophic diversity decreased and trophic redundancy increased while natural bed overlapped at<50% CIs. The recovery speed in this system was fast (1 year for quantitative perspectives and 2 years for functional completion). After removing the sea urchins, recruitment of macroalgae quickly increased concomitantly with consumer groups, boosting the diversity and trophic structure of the restored bed. The trophic structure of the first year of restoration was not an intermediate stage toward the completion, but a transient over-shooting state. This might have been triggered by the fast introduction of diverse macroalgae to the urchin-free bare rock space, providing an interesting finding needed to be tested in other temperate marine systems.

Genetic connectivity of kelp populations can provide essential information for estimating their dispersal capability and related perspectives on persistence. We examined population genetic variability to identify connectivity and potential drivers shaping population genetic clusters of Ecklonia cava, a foundation kelp species in the northwest Pacific region. Using microsatellite markers, we analyzed 11 local populations along the Korean coastline from Jeju Island to Ulleungdo. We found that entire study populations were segregated into two genetic groups, inner (south coast populations) and outer (Jeju Island and Ulleungdo) coastal regions, in accordance with current patterns around South Korea and geographical topography. Genetic similarity between the furthest populations (Jeju Island and Ulleungdo) suggest that the dispersal range of E. cava could be largely expanded by the prevalent current. Inter-population genetic structure in the perspectives of isolation-by-distance (IBD) and isolation-by-environment (IBE) revealed that distance and turbidity gradient were both contributors to the gene flow of E. cava. However, different results appeared for a few populations, suggesting that effects of these factors could vary depending on geographic range of populations and surrounding oceanographic conditions. This study suggests that gene flow of E. cava could be driven by multiple seascape factors (ocean currents, coastal topography, distance and sediments) and the interplay among them at a regional scale, which is applicable to other kelp species with diverse scales.