Establishment Of Seaweed Beds Of Sargassum Thunbergii In Intertidal Zone

Seaweed beds on rocky bottoms have been recognized as a coastal marine ecosystem, consisting of macroalge and other marine organism communities, and they play an important role in ecological and biological function. In recent years, however, many naturally occurring macroalgal beds worldwide have been severely degraded by various anthropogenic perturbations. The marine intertidal zone is considered among the most stressful of all environments, because daily exposure at low tide results in daily fluctuation in a variety of physical factors such as temperature, humidity, salinity, wave exposure and light. Consequently, in comparison to subtidal habitats, restoration of the intertidal macroalgal beds may be more challenging in the stressful environment. In this study, we developed a new Sargassum thunbergii restoration method involving creating intertidal habitat, seeding with artificially collected germlings and covering with a double-layer shading net, in order to establish seaweed beds of S. thunbergii at the target restoration site. The main results showed as following.1. Spatio-temporal distribution of intertidal benthic macroalgae and their diversity around the Xiaoheishan IslandAn ecological demographic study was conducted to investigate to the spatio-temporal distribution of intertidal benthic macroalgae and their diversity in32sections around the Xiaoheishan Island in spring, summer, autumn2011and winter2012. The results revealed that a total of46species of benthic macroalgae were identified during the study, including33Rhodophyta,9Phaeophyta, and4Clorophyta. Flora characteristic of benthic macroalgae distribution was obvious in the intertidal zone of Xiaoheishan Island, and it also changed obviously with seasons in species composition of benthic algae. More species were recorded in summer (39species) than in spring (19species), autumn (21species) and winter (21species), among which only11species existed in all four seasons. There was something in common for the dominant species in four seasons in terms of dominance; however, their functions in the macroalge community changed greatly with seasons. For instance, S. thunbergii was the first dominant species in spring, autumn and winter, but it was the third in summer. The mean values of Shannon-Weaver diversity index (H’), Margalef s richness index (R) and Pielou’s evenness index (J’) were1.64±0.27,0.60±0.36and0.60±0.22in spring;2.07±0.33,0.90±0.21and 0.67±0.22in summer;1.81±0.15,0.74±0.25and0.63±0.30in autumn;1.88±0.27,0.76±0.15and0.59±0.1in winter, respectively. There was great difference in biodiversity of benthic macroalgae among sections, which was related to the stability and complexity of intertidal benthic macroalge community. In comparison with historical data, the succession of community structure and diversity in intertidal seaweed beds of S. thunbergii have been severely degraded around the Xiaoheishan Island.2. Diurnal changes of photosynthetic quantum yield in the intertidal macroalga Sargassum thunbergii under simulated tidal emersion conditionsIn this study, a three-way factorial experimental design was used to investigate the diurnal changes of photosynthetic activity of the intertidal macroalga S. thunbergii in response to temperature, tidal pattern and desiccation during a simulated diurnal light cycle. The maximum (Fv/Fm) and effective (ΦPSⅡ) quantum yield of photosystem Ⅱ (PSⅡ) were estimated by chlorophyll fluorescence using a pulse amplitude modulated fluorometer. Results showed that this species exhibited sun-adapted characteristics, as evidenced by the daily variation of Fv/Fm and ΦPSⅡ. Both yield values decreased with increasing irradiance towards noon and recovered rapidly in the afternoon suggesting a dynamic photoinhibition. The photosynthetic quantum yield of S. thunbergii thalli varied significantly with temperature, tidal pattern and desiccation. Thalli were more susceptible to light-induced damage at high temperature of25℃and showed complete recovery of photosynthetic activity only when exposed to8℃. In contrast with the mid-morning low tide period, although there was an initial increase in photosynthetic yield during emersion, thalli showed a greater degree of decline at the end of emersion and remained less able to recover when low tide occurred at mid-afternoon. Short-term air exposure of2h did not significantly influence the photosynthesis. However, when exposed to moderate conditions (4h desiccation at15℃or6h desiccation at8℃), a significant inhibition of photosynthesis was followed by partial or complete recovery upon re-immersion in late afternoon. Only extreme conditions (4h desiccation at25℃or6h desiccation at15℃or25℃) resulted in the complete inhibition, with little indication of recovery until the following morning, implying the occurrence of chronic PSII damage. Based on the magnitude of effect, desiccation was the predominant negative factor affecting the photosynthesis under the simulated daytime irradiance period. Our results may explain the distribution pattern of this species in natural habitats, where it is generally restricted to tide pools in the intertidal zone of wave-swept rocky shores. These results suggest that artificially created habitats to avoid or mitigate desiccation stress at low tide may be prerequisite for the restoration of damaged S. thunbergii beds in intertidal zone.3. Establishment of intertidal seaweed beds of Sargassum thunbergii through habitat creation and germling seedingMany naturally occurring macroalgal beds on the coastal areas of China have been severely degraded by various anthropogenic perturbations. Few attempts have been made to develop restoration techniques for seaweed beds in intertidal ecosystems, owing to the complex and dynamic variations in physical conditions of the habitat. We developed a new S. thunbergii restoration method involving creating intertidal habitat and seeding with artificially collected germlings. In June2010, artificial rectangular pools constructed of a4:5:1rate of high-strength cement, sand and water on a rocky intertidal platform were seeded with S. thunbergii germlings released from fertile thalli during the low tide. Artificial pools were covered with a double-layer shading net until the next tidal cycle to prevent germlings from being dislodged by water motion, resulting in a majority of young germlings successfully attaching to the pool bottom by rapid development of rhizoids. Two months after seeding, juvenile sporophytes attained a length of15-20mm. After one year following seeding, S. thunbergii in the restored bed reached a density of118.5±13.2(mean±SE) thalli m-2, covered32.7±0.1%of the artificial substrate, and attained an average length of34.2±1.6cm with7.3±0.6laterals per thalllus. The proportion of fertile laterals in restored population was73.6±3.0%, indicating that these fertile thalli may serve as a source of new recruits to enhance the recovery of algal population. Furthermore, restored S. thunbergii beds facilitated the presence of seven other species of macroalgae with species richness (R), diversity(H’) and evenness (J’) reaching0.65±0.04,1.06±0.09and0.67±0.05, respectively. Therefore, the construction of artificial pools, germlings seeding coupled with shading net covering in natural habitat may be an effective approach for the restoration of S. thunbergii, and potentially other seaweeds in rocky intertidal habitats.