Effect of water flow velocity on the growth of the kelp Saccharina japonica under unilateral flow

Abstract

Growth rate of macroalgae is affected by water flow. However, few experimental works have been conducted for the species distributed in Japan. In this study, we set up a siphon-pipe system which maintains water flow velocity, to examine the effect of water flow on the growth rate of the kelp Saccharina japonica. Seawater, which was taken from the sea and filtered with sand, flowed only once through the experimental pipe. The seawater was not adjusted for temperature and nutrients so that the nitrate concentration of the seawater remained below the detection limit (less than 0.1 mg L^-1). Flow rates in the experiments were set at three steps in a range of 0.2 to 8.1 cm s^-1 and the growth rate of the kelp significantly increased with water flow. For the experiments set at five steps in a range of 2.0 to 20.4cm s^-1, the growth rate of the kelp increased with water flow below 13-14cm s^-1 and it was relatively stable in water flow higher than 13-14 cm s^-1. For the experiments with 0.2 to 29.4 cm s^-1, the growth rate increased with water flow below 10.2 cm s^-1 and it was not significantly different between 10.2 cm s^-1 and 29.4 cm s^-1. Photosynthetic oxygen evolution rate of the kelp, which was measured at three steps of the water flow of 0.6-3.0 cm s^-1, increased in higher flow rates. These results suggest that the growth rate of the kelp could be dependent on water flow velocity below 10 cm s^-1.