Seasonal changes in growth and photosynthesis-light curves of Sargassum horneri (Fucales, Phaeophyta) in Oura Bay on the Pacific coast of central Honshu, Japan

We investigated seasonal changes in the growth, photosynthesis-light curves, and chlorophyll a content of Sargassum horneri (Turner) C. Agardh in Oura Bay on the Pacific coast of central Honshu, Japan, and also characterized the physical environment, including PAR and water temperature. During monthly scuba dives, we tracked the growth of main stems and observed the growth stage of eight to ten individual S. horneri that had been marked when their main stems were > 10 cm until they disappeared from the substratum after maturity. In addition, we measured the rates of net photosynthesis and dark respiration in upper and lower leaves of four individual S. horneri collected monthly with a differential gas volumeter at the monthly mean water temperature, also measuring the weight and chlorophyll a content of the leaves. The seasonal changes in the photosynthetic activity of upper leaves based on wet weight and chlorophyll a had different peaks : the former was positively correlated with the length and growth rate of the main stems, and the latter was negatively correlated with plant age. Moreover, the pattern of seasonal changes in photosynthetic activity based on weight was synchronized with changes in the nutrient content of the seawater.