Active erosion of Undaria pinnatifida Suringar (Laminariales, Phaeophyceae) mass-cultured in Otsuchi Bay in northeastern Japan

Abstract

Production and erosion of the commercially mass-cultured kelp Undaria pinnatifida Suringar f. distans Miyabe et Okamura were investigated between January and April 1998 in Otsuchi Bay, ca. 8 km long and 2 km wide, located on the northeastern coast of Japan. Steady growth in total kelp length was observed from January to March, with rates of 1.1 to 1.8 cm day −1. Erosion rates were consistently low at <0.2 cm day −1 in January and February, but increased to 0.5 cm day −1 in March, and were comparable to the growth rate in April. Biomass erosion represented 30–40% of production in March and over 80% in April. The greater erosion in April was attributed to a low supply of dissolved inorganic nitrogen and aging of the alga. Total net production of the cultured kelp in the bay was 49.5 tonne C, of which, 81% (38.7 tonne C) was harvested while 19% (10.8 tonne C) was lost due to erosion. In terms of nitrogen, 33% of total production (4.68 tonne N) was eroded, while 67% was harvested. Active erosion of the kelp thus appears to be an important source of organic matter for heterotrophic processes in the coastal waters of eastern Asia where kelp cultivation is a major aquaculture industry.