Furuya, K.*, T. Yoshikawa*, Neelam Ramaiah*, H. Otobe**, I. Takeuchi**, S. Kaga***, M. Takagi***, K. Sekiguchi***, and M. J. Kishi****

* Graduate School of Agricultural and Life Sciences, the University of Tokyo
** Otsuchi Marine Research Center, Ocean Research Institute, the University of Tokyo
*** Iwate Fisheries Technology Center
****Graduate School of Fisheries Sciences, Hokkaido University

Seasonal variations in primary production of phytoplankton was examined in Otsuchi Bay by continuous monitoring of natural fluorescence. Since the aquacultured macroalgae, particularly wakame (Undaria pinnatifida; Laminariales, Phaeophyceae) is a dominant macroalga species in this bay, we compared phytoplankton production with that of wakame during the spring.

The fluorometrically-derived production was frequently validated with a 14C uptake rate that yielded significant correlations. The highest phytoplankton production was observed during the spring-time with a mean rate of 1434 ± 1256 mgC m-2 d-1, followed by summer rates of 974 ± 950 mgC m-2 d-1. Production of cultured U. pinnatifida throughout the bay increased steadily in January and February, and reached its maximum in March. The average total biomass for the entire bay from January to April was 3.1 g C m-2. Wakame released dissolved and particulate matters into the water by erosion, viz removal of the aged part of the thallus. The erosion began in early March, and peaked in mid March, when biomass erosion represented 30-40% of production.

Phytoplankton primary production consistently exceeded that of wakame by >20 times. Although phytoplankton were of primary importance as a primary producer, macroalgae contributed to organic matter production for planktonic heterotrophs through the erosion and dissolved organic matter exudation.