Ecological study of Sargassum macrocarpum C. Agardh (Fucales, Phaeophyta)

Sargassum macrocarpum C. Agardh (Fucales, Phaeophyta) is a perennial brown alga and is highly differentiated into the hold fast, stem, main branches, vesicles, receptacles and so on. This species forms dense populations called Sargassum beds, which play a major role in the primary production in coastal ecosystems, and are important as nursery grounds for coastal fishes and other animals. The present study focused on the biological characteristics of S. macrocarpum with reference to its production ecology. Special attention was paid to the relationship between the structure and the function of the S. macrocarpum population on the flat rock area in Fukawa Bay, facing the Sea of Japan, Yamaguchi Prefecture, Japan. The following characteristics were examined, 1) growth and maturation from embryos to adult thalli using methods of tagging, clipping, transplanting in a field study, and tank culture in a laboratory study, 2) seasonal and yearly changes in standing crop, density, productive structure, age composition, dispersion map and the analysis of the regeneration process, 3) critical light intensity for growth in the population and daily compensation point of juvenile thalli, 4) estimations of net production by the summation method and a mathematical model based on the rate of photosynthesis and light intensity under water and in the population. Eggs and embryos on female receptacles were observed in June. After attachment and germination on the sea bed, embryos grew to 2 - 5mm in total length by October to December. Growth of the juvenile thalli was very slow and reached only 10cm or less in total length after approximately one year from germination. This slow growth pattern was observed in the field and the indoor tank culture. Thereafter, the thalli began to grow from fall to winter and reached about 15cm in total length after 1.5 years from germination. In June, after 2 years, only two thalli of about 50cm or more in total length bore receptacles and matured. The majority of the other thalli, however, did not bear receptacles and grew to 10 - 26cm in total length. Thereafter, the length of the main branches of these thalli grew rapidly. Therefore, it was clarified that it took at least two years of growth by S. macrocarpum to reach maturation. Additionally, receptacles were evident in the majority of adult thalli, which were collected in the field, with three or more growth rings in the hold fast. During the period from June to August, all of the old branches withered and new branches had already developed on the perennial stems of the adult thalli. New branches grew rapidly from fall to winter, and bore receptacles from March to June. The main branches of the adult thalli deteriorated until spring through the winter, approximately one year after development of the main branches. Therefore, it is thought that this periodical growth pattern of the main branches plays an important role in the stability of the population. Growth rings observed in the hold fast were confirmed as annual rings which formed periodically once a year. There was an adequate correlation between stem diameter and the numbers of annual rings. Therefore, the stem diameter was used as the standard characteristic of age. Additionally, this characteristic was readily measurable underwater in the natural population. The maximum life span of S. macrocarpum was considered to be nine years or more, according to the age analysis using the stem diameter and the number of annual rings. The regeneration process of the population corresponded well to typical "gap regeneration" in terrestrial climax forests. Three phases of gap, building and maturation were distinguished with reference to the structural and dynamic features of the population, and the turnover time of the canopy layer was 4-5 years. Juvenile thalli on the population floor play an important role as major constituents of the coming generation The estimated daily compensation point of the juvenile thalli was 1.3% of the sea surface light intensity. This value agreed well with the observed critical light intensity for growth of the juvenile thalli on the population floor. Results indicated that juvenile thalli are strongly influenced by the change in light distribution on the population floor according to yearly changes in the population density. Annual net production of newly sprouting branches in June from 1993 to 1994 was about 1.6kg dry wt.mE-2yearE-1 based on the summation method, which was calculated from the monthly change in the productive structure. The relative value of the annual net production to the maximum standing crop in May was about 1.4. Daily net production reached a maximum of about 7g dry wt.mE-2dayE-1 from February to March. These high production rates of the S macrocarpum. population represents one of the main primary producers in coastal ecosystems. For estimating the production of the S. macrocarpum population, a mathematical model was developed using factors of the photosynthesis-light curves of the upper and lower leaves the attenuation coefficient of sea water, the extinction of light in the population and the daily change in solar irradiance. The estimated daily production agreed well with the measured daily production based on the summation method. This model suggests that in the deeper population, the production decreased rapidly by the high attenuation coefficient caused by the turbidity of the sea water. In the present study, it was clarified that the S. macrocarpum population was characterized by a perennial growth pattern, and was maintained and stabilized by gap regeneration and high productivity over many years. This study provides fundamental methods and data on which to base marine afforestation, which is important for the enhancement of nursery grounds and for the preservation of environmental conditions in coastal ecosystems. Additionally, it is thought that the methods used in this study are applicable for the study of the production ecology of the other macroalgal populations.