Limiting nutrients of phytoplankton community in Hiroshima Bay, Japan

Ratios of DIN:DIP, indigenous algal assay and nitrogen fixation by phytoplankton were determined to estimate the limiting nutrient for phytoplankton growth in Hiroshima Bay, Seto Inland Sea, Japan. The algal assay was performed using indigenous pico-, nano- and microphytoplankton assemblages fractionated by filtration. Nitrogen fixation was determined using an acetylene reduction method. The DIN:DIP ratio in the study area was above 16:1 (Redfield ratio) in August 1993 and May 1994. However, in other months, the ratio was below 16:1. These values suggest that the growth of phytoplankton community is limited by nitrogen, with an exceptional phosphorus limitation in August 1993 and May 1994. Nutrient enrichment algal assay of size-fractionated phytoplankton community also supported these findings. The growth of picophytoplankton fraction was stimulated by phosphorus addition in August 1993 and May 1994, while nano- and microphytoplankton fractions were stimulated by nitrogen addition in almost all months during the study period. No nitrogen fixation by phytoplankton was detected during the study period. The results suggest that nitrogen rather than phosphorus is the limiting nutrient for the growth of the whole phytoplankton community, but picophytoplankton by itself, tends to be limited by phosphorus compared to other large-size phytoplankton when high DIN:DIP ratio was observed.