EVALUATION OF EFFECTS OF ALLOCHTHONOUS WATER INPUTS ON PRIMARY PRODUCTION IN AN INNER BAY USING FATTY ACID COMPOSITION ANALYSIS

Abstract

 We assessed effects of allochthonous water inputs on community structures of primary producers and their productivity in Shizugawa Bay. We collected subsurface sea water from the bay and also waters potentially inflowing into the bay (i.e., river waters, groundwaters, offshore and bottom waters) in September 2017. In the laboratory, each of the allochthonous waters were mixed with the subsurface bay water to mimic the possible mixing of water in the real environment, and the concentration of fatty acid markers of bacteria, greenalgae, diatom and dinofllagelate after a 14-day incubation. As a result, it was found that the response to the treatment differed between the producer groups, and also that there is a competition for nitrogen between the groups. Specifically, bacteria had a higher productivity under conditions of relatively higher nitrate concentration, while productions of algae such as diatom and dinoflagellate were greater under conditions of high phosphate concentration. In addition, community structures of primary producers changed with N/P ratio. Our results further suggest that the composition of fatty acids produced in the inner bay may change significantly due to the change of the quality of the influent waters, so it is necessary to comprehensively manage the bay and surrounding watersheds to protect ecosystems and achieve sustainable fishery production in inner bay systems.