MEPS 517:51-60 (2014)  -  DOI: https://doi.org/10.3354/meps11014

ABSTRACT: In the Ariake Sea, Japan, the pennate diatom Asteroplanus karianus has formed massive blooms each winter since 2008. During the peak of the bloom, seaweeds of the genus Pyropia, which are cultivated to make nori products, are discolored due to nutrient deficiency. We investigated the nutrient dynamics associated with A. karianus blooms in the Ariake Sea and measured the uptake of nitrate and phosphate by an axenic strain of A. karianus. Dissolved inorganic nitrogen and dissolved inorganic phosphorus in the coastal waters were significantly lower in areas where A. karianus cells were proliferating (r ≤ -0.955, p < 0.01), and these nutrients were severely depleted during the period of blooming. In the laboratory, the maximum uptake rate and half-saturation constant of A. karianus cultures were 27.3 fmol N cell^-1 h^-1 and 7.44 µmol N l^-1, respectively, for nitrate and 22.6 fmol P cell^-1 h^-1 and 3.61 µmol P l^-1, respectively, for phosphate. This study demonstrates that A. karianus blooms significantly deplete nutrients in the water column. The winter blooms presumably deplete dissolved inorganic nitrogen and dissolved inorganic phosphorus from the water column and may indirectly cause Pyropia nutrient deficiency.

KEY WORDS: Ariake Sea · Asteroplanus karianus · Nutrient depletion · Pyropia · Winter bloom