Short-term covariation of dissolved oxygen and phytoplankton photosynthesis in a coastal fish aquaculture site

The influence of phytoplankton photosynthesis (PP) on dissolved oxygen (DO) dynamics was evaluated in a red sea bream (Pagrus major) aquaculture site. The surface PP was monitored continuously by a natural fluorescence sensor for 2e3 weeks during all seasons, except spring. In summer, when the daily PP was low during cloudy weather, DO at the surface markedly decreased below the critical level (5.7 mg O2 L 1), which is needed for normal fish growth. However, on the subsequent clear day, active PP restored the DO back to above the critical level. In fall, under cloudy skies, the surface DO also decreased below the critical level and remained there, even when the subsequent days were clear. In winter, DO remained at high levels throughout the water column due to an active supply from the air and vertical mixing. Besides seasonal changes, surface DO showed a diel cycle with a minimum early in the morning and maximum in the evening. This diel cycle was also regulated by planktonic photosynthesis. This was most obvious and its amplitude was largest in summer, as PP was very high and thermal stratification prevented diffusion of DO from the surface to deeper water. The present study is the first detailed report on the close coupling of DO with PP in coastal waters, even under non-bloom conditions. Cloudy weather and early mornings were found to be the most critical periods in oxygen supply to cultured fish and other pelagic organisms in a fish culture site and other eutrophic coastal area.