Introduction
Seaweed and seagrass are photosynthetic biological organisms that inhabit coastal areas, typically forming dense, three-dimensional communities in shallow waters. These communities play a crucial ecological role in coastal ecosystems, providing feed, habitat, and nurseries for many associated species (Steneck et al. 2002, Graham 2004, Christie et al. 2009, Harley et al. 2012, Teagle et al. 2017). Additionally, some seaweed species are considered valuable resources for humans and have been harvested by local communities for consumption (Ohno and Largo 1998, Zemke-White and Ohno 1999).
Coastal areas where seaweed and seagrass flourish are considered part of wetlands (Article 1.1, Ramsar Convention on Wetlands of International Importance Especially as Waterfowl Habitat 1971). In Japan, the Ministry of the Environment, Government of Japan (MEGJ), listed 500 important wetlands in 2001 as candidate sites for the Ramsar List of wetlands of international importance (Biodiversity Center of Japan 2001). Field surveys were subsequently conducted in the early 2000s to assess the flora of seaweed and seagrass communities at 129 of these sites (The 7th National Survey on the Natural Environment [Seaweed and seagrass communities], Biodiversity Center of Japan 2008).
While data from a single-year survey provided a snapshot of the status of seaweed and seagrass communities in the region, long-term monitoring is essential to track changes in these communities, particularly in response to exploitation and climate change. To address this need, the “Monitoring Sites 1000” Project was launched in 2003. This initiative, founded by MEGJ, conducts continuous ecological surveys across terrestrial (alpine zones, forests, grasslands) and aquatic ecosystems, including seaweed and seagrass communities, at approximately 1000 sites throughout Japan (Biodiversity Center of Japan 2003).
This project is based on the premise that long-term datasets are critical for detecting trends in the abundance and distribution of “structuring” organisms and monitoring potential changes in natural communities under the influence of climate change. Designed as a 100-year initiative, the project involves close collaboration with scientists from research and academic institutions, as well as non-profit and non-governmental organizations (Biodiversity Center of Japan 2003).
Annual monitoring of coastal areas, including rocky shores, estuaries, seagrass beds, and seaweed communities, began in 2008 following the establishment of a standard methodology discussed in 2007, which could be applied to all these coastal habitats. Quantitative descriptions of these communities, such as species composition, percentage cover, and the vertical distribution of foundation species, were obtained at various sites across Japan (Terada et al. 2021a). Of the 26 coastal sites selected for long-term surveys, 12 were designated for seaweed and seagrass monitoring (six sites each). Annual surveys have been conducted at these sites over the past 15 years (Biodiversity Center of Japan 2019, 2025, Terada et al. 2021a).
During the long-term survey, the 2011 Great East Japan Earthquake (GEJE) and the resulting tsunami disaster occurred along the Sanriku coast, in the northeastern part of Honshu Island facing the Pacific Ocean, causing significant impacts on coastal ecosystems. Since the Monitoring Site 1000 surveys included actual pre-and post-disaster data from this affected area, they contributed to understanding the substantial impacts on coastal biological communities (Noda et al. 2016, Nakaoka et al. 2017, Sakanishi et al. 2018, Biodiversity Center of Japan 2019, 2025, Terada et al. 2021a).
Stable seaweed and seagrass communities have been observed for many years at some sites. In contrast, significant declines and the disappearance of canopy-forming species have been reported at other sites, particularly in the southern part of Japan (Terada et al. 2021a). In Kagoshima Prefecture, located in the southern part of Kyushu Island, two sites were designated: one for seaweed and one for seagrass. A temperate kelp species, Ecklonia radicosa (Kjellman) Okamura (Laminariales), and a seagrass, Zostera marina Linnaeus (Alismatales), have been investigated annually at Nagashima Island and Kagoshima Bay, respectively (Fig. 1). However, both species experienced significant declines and disappeared from their respective sites in the late 2010s (Terada et al. 2021a, 2021b, Shimboku and Terada 2023).
In response to their disappearance, broader distributional surveys were conducted at these sites and their adjacent waters: in 2019 for E. radicosa and in 2021 for Z. marina (Terada et al. 2021b, Shimboku and Terada 2023). This review summarizes the long-term change of these two species in Kagoshima based on field surveys in the present study and previous reports.
Ecklonia radicosa in Nagashima Island
Along the coast of Nagashima Island and the western coast of the Satsuma Peninsula, Kagoshima Prefecture, Kyushu Island, dense populations of the temperate kelp E. radicosa have been found (Fig. 1; Terada et al. 2016, Terada 2024). As this species is regarded as the alga that distributes to the lowest latitudes of Japan (31° N–35° N) among temperate Japanese Ecklonia/ Eisenia species, with Kagoshima Prefecture marking the southern limit of its distribution (Terada et al. 2016, Akita et al. 2020), a site named the “Satsuma-Nagashima Site” was designated on Nagashima Island, facing the East China Sea (32°08’34.7” N 130°06’41.8” E; Fig. 2) as part of the Monitoring Site 1000 survey, based on these distributional characteristics. The survey is conducted using the line-transect method and permanent quadrats to monitor long-term changes in the coverage and species composition (Terada et al. 2021a, Biodiversity Center of Japan 2025).
Fig. 2.
Occurrence (black dots, coverage greater than 20%) of the brown alga, Ecklonia radicosa in Nagashima Island and adjacent waters, Kagoshima Prefecture, Japan, as determined by surveys conducted in 2005 (A) and 2019 (B). The yellow dots represent Sites A and B, indicating the long-term monitoring sites, with Ecklonia radicosa having disappeared in the Site A in 2017.
In 2016, E. radicosa communities at the study site suddenly disappeared from the substrata, except for a few juvenile individuals that lost their blades due to herbivory. Since 2017, this alga has completely disappeared from the site and has not yet recovered. In response to this disappearance, we conducted broader distributional surveys at the site and in adjacent waters on Nagashima Island in 2019 (Biodiversity Center of Japan 2019, 2020, 2025, Terada et al. 2021b). Before the disappearance of E. radicosa, its occurrence levels varied slightly depending on the habitat environment (e.g., substrate type, sheltered areas, or ocean-facing sites); however, communities of this alga were broadly observed. However, the disappearance of E. radicosa communities occurred widely, not only at the annual survey site but also along the entire west coast of Nagashima Island, facing the East China Sea (Fig. 2; Terada et al. 2021b). This alga was also reported to have disappeared from the coast of the Satsuma Peninsula, located further south, also facing the East China Sea. Similarly, several species of Sargassum (e.g., Sargassum fusiforme (Harvey) Setchell, S. patens C. Agardh, etc., Fucales), which form dense Fucoid communities, also completely disappeared from the west coast of the island.
In a past distributional survey of seaweed communities on Nagashima Island conducted in 2005, E. radicosa and Sargassum spp. were found across the entire island, including the east coast, which is located in the sheltered area of Yatsushiro Bay. In fact, during the 2019 survey, these species, with large communities, were confirmed in Yatsushiro Bay, suggesting that the seaweed communities were now restricted to sheltered areas, away from the open sea (Terada et al. 2021b). Based on these findings in 2019, we established an additional monitoring site (Fig. 2; Satsuma-Nagashima Site B; 32°14’23.1” N 130°09’30.9” E) in Yatsushiro Bay in 2021, near the Kagoshima University Marine Station, and have continued annual surveys at both the original site (Satsuma-Nagashima Site A) and the new site (Satsuma-Nagashima Site B; Biodiversity Center of Japan 2025).
Zostera marina in Kagoshima Bay
The seagrass Z. marina (Fig. 1) is a cosmopolitan species widely distributed across temperate and subarctic zones in the Northern Hemisphere. In Japan, this species is found throughout the country, except for the subtropical Ryukyu (Nansei Islands), Izu, and Ogasawara (Bonin) Islands (Ohba and Miyata 2020). Since Z. marina can be found in Kagoshima Bay, in the southern part of Kyushu Island, it is considered the southernmost limit of its distribution in Japan and the western Pacific (Kawano et al. 2012a, 2012b, Ohba and Miyata 2020).
Based on these distributional characteristics, a study site named the “Ibusuki Site” was designated in 2009 for the long-term monitoring of Z. marina in Ibusuki City, located at the southern end and mouth of Kagoshima Bay (Fig. 3; 31°10’05.1” N, 130°35’25.2” E; Biodiversity Center of Japan 2019), as part of the Monitoring Sites 1000 survey. Coverage and species composition have been investigated annually as part of long-term monitoring (Biodiversity Center of Japan 2019, 2025). Since Z. marina in Kagoshima Bay is known to exhibit an annual ecotype, all plants, including their underground rhizomes, wither by summer after flowering and seeding. As the community relies entirely on new recruits emerging in late autumn, the area of the seagrass bed, as well as its coverage and density observed during the annual surveys in April, appeared unstable each year (Kawano et al. 2012b). Notably, coverage has been gradually declining since 2011, and the community completely disappeared from the study site in 2018 (Biodiversity Center of Japan 2020).
In response to this disappearance, broader distributional surveys were conducted at the site and in adjacent waters of Kagoshima Bay in 2021 (Shimboku and Terada 2023). The results revealed that the disappearance of the seagrass beds was widespread, not only at the annual survey site but also along the entire coast of Kagoshima Bay (Fig. 3; Tanaka et al. 2013, Shimboku and Terada 2023).
Kagoshima Bay hosts four species of seagrass: Z. marina, Zostera japonica Ascheron et Graebner, Halophila nipponica J. Kuo, and Halophila major (Zollinger) Miquel (Alismatales; Kawano et al. 2012a, Shimboku and Terada 2023). However, large-scale seagrass beds are primarily composed of Z. marina. In the 2021 survey, while all four species were found in the bay, seagrass beds larger than 100 m² were exceedingly rare, with the total area of seagrass beds estimated at just 4.39 hectares. This represented a dramatic decline compared to previous surveys: only 11.6% of the area recorded in 2006 and a mere 2.4% of the area observed in 1978 (Fig. 4; Shimboku and Terada 2023). Dense seagrass beds remained at some sites in the sheltered area of Kagoshima City, located in the interior part of Kagoshima Bay; however, seagrasses were mostly sparse at other sites.
Fig. 4.
Long-term change in the estimated area of seagrass beds in Kagoshima Bay based on surveys conducted in 1978, 2004, 2006 (Tanaka et al. 2013), and 2021 (Shimboku and Terada 2023).
Discussion
In the temperate regions of Japan, temperate kelp species (i.e., Ecklonia and Eisenia) often form dense and expansive kelp forests (Terada et al. 2021a). Of these temperate kelp species, E. radicosa is known as the alga that occurs at the lowest latitudes of Japan (31° N–35° N), with Kagoshima Prefecture marking the southern limit of its distribution (Terada et al. 2016, Akita et al. 2020). In addition, since the southern distributional limits of the other three temperate kelp species, Ecklonia cava Kjellman (including subsp. kurome and subsp. stolonifera), Eisenia bicyclis (Kjellman) Setchell, and Eisenia nipponica H. Kawai, S. Akita, K. Hashimoto et Hanyuda, are located in more northern parts of Japan, E. radicosa is the only temperate kelp species that can be found in Kagoshima Prefecture in southern Japan (Terada et al. 2016, 2021a). This indicates that no kelp forests exist in the subtropical Ryukyu Islands of Japan (Tanaka et al. 2013).
Kagoshima Prefecture is regarded as a boundary region between temperate and tropical marine algae in Japan (Terada et al. 2021a). Consequently, many temperate marine algae species reach their southern distributional limit in this region (e.g., Undaria pinnatifida (Harvey) Suringar, Watanabe et al. 2014). The study site at the Satsuma-Nagashima Site is considered one of the largest and densest E. radicosa communities near the species’ southern distributional limit (Terada 2024).
Unlike other species in the Japanese Ecklonia/Eisenia groups, which thrive as perennials, E. radicosa is an annual species observed only from winter through early summer (Akita et al. 2014, Terada et al. 2021b). After releasing zoospores in summer, the sporophytes disappear from the habitat by autumn. Since monitoring began in 2010, the E. radicosa community at this site had shown instability from 2010 to 2015. This instability is attributed to the fact that all individuals in the community rely on new recruits from the fertilized microscopic gametophyte stage.
In 2016, remaining juvenile individuals lost their blades due to herbivory, suggesting that herbivory had a significant impact before the algae could mature (Biodiversity Center of Japan 2019, 2020, 2025, Terada et al. 2021a). Consequently, insufficient zoospore supply may have prevented successful recruitment in the habitat. The potential loss of the microscopic gametophyte stage likely led to the complete disappearance of these communities from the site after 2017. By 2019, surveys on the western coast of the island found no evidence of E. radicosa, a finding corroborated by recent annual surveys. Given the prolonged absence of E. radicosa and the loss of its microscopic stage in the habitat, recovery of the community appears to be highly unlikely.
The differing statuses of E. radicosa and Sargassum species on the west coast (facing the East China Sea) and the east coast (Yatsushiro Bay) of Nagashima Island may be influenced by variations in seawater temperature. Notably, the seawater temperature in the sheltered Yatsushiro Bay is consistently 1–3°C lower than that of the west coast, a difference that is especially pronounced in winter. This temperature variation is believed to result from the contrasting environmental conditions of sheltered versus open marine habitats (Terada et al. 2016, 2021a).
Herbivory by fish generally decreases as seawater temperatures drop during late autumn and winter (Vejříková et al. 2016). However, due to climate change, seawater temperatures in autumn to winter have likely remained relatively high, potentially intensifying herbivory pressure during the critical recruitment period for new algal growth. Consequently, the differences in algal presence between the west and east coasts may be attributed to variations in herbivory intensity driven by these temperature changes. Elevated herbivory pressure caused by climate change undoubtedly contributes to the degradation of temperate seaweed ecosystems (Vergés et al. 2014, 2016). Understanding these dynamics is essential for developing effective conservation and management strategies aimed at preserving seaweed and seagrass communities in the face of climate change (Zarzyczny et al. 2022).
The difference between annual and perennial ecotypes may also have contributed to the decline of seagrass beds in Kagoshima Bay. Generally, Z. marina is known to be perennial, with communities persisting on the substrata year-round. This ecological trait facilitates the formation of stable communities and ecosystems, as networks of underground rhizomes help stabilize sandy and muddy bottoms, even under strong wave action. However, in Kagoshima Bay, Z. marina exhibits an annual ecotype, where all the meadows transition into generative stocks each year. By summer, following flowering and seeding, all plants including the underground rhizomes wither and die (Kawano et al. 2012b).
Typhoons frequently occur during the summer on Kyushu Island and may have a significant impact on the Ibusuki Site. During the summer to autumn period (July – October), when no plants are present, the substrata of sandy and muddy bottoms are often displaced by wave action from typhoons. Additionally, the communities tend to remain unstable as new recruits are vulnerable to herbivory and other environmental stressors (Kawano et al. 2012b, Biodiversity Center of Japan 2025). Herbivory of seagrasses by fish has also often been observed in Kagoshima Bay (Mr. Kawabata per. com.), and this impact may be a serious concern preventing the thriving of seagrass communities (Valentine and Heck 1999). As a result, the area covered by the communities and the density of seagrass gradually declined. Ultimately, the population was unable to produce sufficient new recruits and is considered to have disappeared.
Degradation of seaweed and seagrass communities, as well as shifts to subtropical assemblages, have been reported not only in the southern part of Kyushu Island but also in the warm temperate regions of Japan, including Shikoku and northern Kyushu Islands (Tanaka et al. 2012, Kiyomoto et al. 2021). Long-term monitoring surveys are essential for promptly detecting these shifts and changes, as well as for developing a better understanding to inform the restoration and conservation of marine ecosystems.
