Gametophytes of two Undaria species, U. pinnatifida and U. undarioides (Laminariales, Phaeophyceae), were studied to determine their water temperature requirements in order to understand their different distributions in Mie Prefecture, Japan. The optimal temperature for growth was 20 ° C for gametophytes of both species, and the upper critical temperature for growth was also the same for both species at 28 ° C. Therefore, the optimal and critical temperatures for growth of the gametophytes are not the main factors determining distribution. The optimal temperature for maturation of U. pinnatifida was approximately 10-15 ° C, whereas it was closer to 20-21 ° C for U. undarioides , a difference between these species of at least 5 ° C. In autumn and early winter, the seawater temperature at the mouth of Ise Bay, where U. pinnatifida is distributed, ranges from 21.6 ° C (October) to 12.7 ° C (December), and off Hamajima, where U. undarioides is found, the range is from 22.7 ° C (October) to 19.1 ° C (December). The seawater temperatures from October to December, which is the maturation season for the gametophytes, agreed well with the optimal temperature requirements for maturation of the gametophytes of both species. Thus the difference in the maturation temperature range of the gametophytes is a major factor determining distribution of these Undaria species along the Japanese coast.

The optimal water temperature in seed germination and the upper critical water temperature in seedling growth were determined for Zostera marina collected from Ise Bay, Japan. The relationship between the seed germination rates and seed storage period (0, 30 and 60 days at 0°C) was also examined. The optimal water temperature for seed germination was in the range from 10 to 15°C regardless of the storage periods, in which germination rates ranged from 35 to 57%. Seedlings grown from seed up to 10 cm in total length were cultured for 1 week under various water temperatures to measure their relative growth rates. The optimal water temperature in growth was in the range from 20 to 25°C; relative growth rates ranged from 2.0 to 2.6%. Seedlings could survive up to a water temperature of 28°C, but most seedlings withered at 29 or 30°C. The optimal water temperatures for seed germination and seedling growth were related to the seasonal changes of water temperature at the sampling site. Although seedlings were rarely observed in the field in summer, they can grow at temperatures as high as 28°C. Therefore, Z. marina may extend its distribution as far as where the summer water temperature is lower than 28°C.

No abstract

The present study was designed to estimate the critical light intensity required for growth of Zostera marina and that which determines its depth limit. Seeds of Z. marina collected at Matsunase, Ise Bay, Mie Prefecture, central Japan were germinated and grown to young plants of 10 cm in length. The young Z. marina were cultured for 1 week in various water temperatures, and their photosynthesis and respiration were measured under various photon irradiances. The daily compensation point was estimated by a mathematical model based on photosynthetic activity and diurnal changes in solar irradiance. The estimated daily compensation point of young Z. marina was 5.7% of sea surface. The depth limit was determined by the Beer–Lambert law concerning the relative solar irradiance on the sea surface and the extinction coefficient. Almost all previous studies report a shallower growing depth of Z. marina than the present result, but the lowest reported data agreed well with the current estimated depth limit. Therefore, the mathematical model in the present study can estimate the production and critical growing depth of Z. marina. The results suggest that the compensation depth is controlled mainly by the solar irradiance reaching the Z. marina beds.

SUMMARY Gametophytes of two Undaria species, U. pinnatifida and U. undarioides (Laminariales, Phaeophyceae), were studied to determine their water temperature requirements in order to understand their different distributions in Mie Prefecture, Japan. The optimal temperature for growth was 20°C for gametophytes of both species, and the upper critical temperature for growth was also the same for both species at 28°C. Therefore, the optimal and critical temperatures for growth of the gametophytes are not the main factors determining distribution. The optimal temperature for maturation of U. pinnatifida was approximately 10–15°C, whereas it was closer to 20–21°C for U. undarioides, a difference between these species of at least 5°C. In autumn and early winter, the seawater temperature at the mouth of Ise Bay, where U. pinnatifida is distributed, ranges from 21.6°C (October) to 12.7°C (December), and off Hamajima, where U. undarioides is found, the range is from 22.7°C (October) to 19.1°C (December). The seawater temperatures from October to December, which is the maturation season for the gametophytes, agreed well with the optimal temperature requirements for maturation of the gametophytes of both species. Thus the difference in the maturation temperature range of the gametophytes is a major factor determining distribution of these Undaria species along the Japanese coast.

SUMMARY “Monitoring Sites 1000” – Japan's long‐term monitoring survey was established in 2003, based on the Japanese Government policy for the conservation of biodiversity. Ecological surveys have been conducted on various types of ecosystems at approximately 1000 sites in Japan for 15 years now and are planned to be carried out for 100 years. Since 2008, seaweed communities had been monitored at six sites, featuring the kelp (e.g. Saccharina and Ecklonia; Laminariales) and Sargassum (Fucales) communities in the subarctic and temperate regions of Japan. Annual surveys were carried out during the season when these canopy‐forming seaweeds are most abundant. A non‐destructive quadrat sampling method, with permanent quadrats placed along transects perpendicular to the shoreline, was used to determine species composition, coverage, and vertical distribution of seaweeds at these sites; while destructive sampling was done every 5 years to determine biomass. The occurrence of canopy‐forming species Saccharina japonica (var. japonica) and Ecklonia cava have appeared to be stable at the Muroran (southwestern part of Hokkaido Island) and Shimoda (Pacific coast of middle Honshu Island) sites, respectively; whereas the coverage of Ecklonia radicosa (= Eckloniopsis radicosa) at the Satsuma‐Nagashima site in southern part of Kyushu Island was highly variable until its sudden disappearance from the habitat in 2016. Thalli of E. radicosa lost most of their blades through browsing by herbivorous fish, and thus, this may be one of the causes of the decline. A shift in the community structure related to environmental changes had also been observed at some other sites. Pre‐ and post‐disaster data revealed the impact of the 2011 earthquake and tsunami disasters, including a shift in the vertical distribution of Ecklonia bicyclis (= Eisenia bicyclis) to shallower depths at the Shizugawa site in the Pacific coast of northern Honshu Island, due to seafloor subsidence.

No abstract

SUMMARY We monitored an Eisenia bicyclis kelp bed during a survey of the rocky coast subtidal zone of Shizugawa Bay, the Sanriku Coast, northeastern Honshu, Japan, from 23 July 2008, to detail the biodiversity, which was subsequently directly impacted by the 2011 Great East Japan Earthquake (GEJE). To assess temporal changes in abundance of the dominant canopy forming kelp E. bicyclis and in the distribution patterns of macroalgae along a water depth gradient, percent coverage of macroalgae has been observed in permanent quadrats set near the lower limit of the Eisenia bed and in quadrats set along a water depth gradient. The GEJE, which induced huge tsunami waves and coseismic seafloor subsidence, occurred during the monitoring survey period and also affected the coastal communities in Shizugawa Bay. After the GEJE, the cover of E. bicyclis within the permanent quadrats near the lower limit of E. bicyclis gradually declined, and reached zero by July 2014. Also in the line transect survey, the offshore (deep) edge of the Eisenia bed showed a tendency to shift shoreward (upward) after the GEJE; the Eisenia bed near the pre‐earthquake offshore (deep) edge declined and finally disappeared after the GEJE. Combined with results of the permanent quadrat and line transect surveys, the post‐earthquake gradual decline and subsequent complete disappearance of the Eisenia bed within the permanent quadrats probably indicates an upward shift of the deep edge of the subsided kelp bed. Gradual change in the E. bicyclis bed over 2 years after the GEJE is a unique opportunity to document the response of a kelp bed to coseismic subsidence, demonstrating the slow and prolonged recovery process of E. bicyclis to subsidence caused by the mega‐earthquake to the pre‐earthquake depth zone.