Spatio-temporal phytoplankton variability and nutrient addition bioassays as indicators of nutrient limitation of phytoplankton growth in Gwangyang Bay, Korea

Abstract

To assess the effect of nutrient limitation on phytoplankton growth, and its influence on seasonal variation in phytoplankton community structure, we investigated abiotic and biotic factors in surface and bottom waters at 20 stations in inner and offshore areas of Gwangyang Bay, Korea. Algal bioassay experiments were also conducted using surface water, to assess the effects of nutrient addition on the phytoplankton assemblages. The fate of major nutrients in the bay was strongly dependent on the discharge of freshwater from the Seomjin River. River flow during the rainy season provides a high nitrogen (N) influx, pushing the system toward stoichiometric phosphorus (P) limitation. However, at some times during the rainy season there was insufficient N to maintain phytoplankton growth because it was rapidly consumed through nutrient uptake by phytoplankton under stratified environmental conditions. Diatoms made a relatively large contribution to total phytoplankton biomass. The dominant diatom, particularly in winter and summer, was a Skeletonema costatum while Eucampia zodiacus and the cryptophyte Cryptomonas spp. dominated in spring and autumn, respectively, comprising more than 75% of the community at most stations. In the bioassay experiments the phytoplankton biomass increased by 30&#8211 600% in the +N (added nitrogen) and +NP (added nitrogen and phosphorus) treatments relative to the control and the +P (added pher and offshore areas of Gwangyang Bay, Korea. Algal bioassay experiments were also conducted using surface water, to assess the effects of nutrient addition on the phytoplankton assemblages. The fate of major nutrients in the bay was strongly dependent on the discharge of freshwater from the Seomjin River. River flow during the rainy season provides a high nitrogen (N) influx, pushing the system toward stoichiometric phosphorus (P) limitation. However, at some times during the rainy season there was insufficient N to maintain phytoplankton growth because it was rapidly consumed through nutrient uptake by phytoplankton under stratified environmental conditions. Diatoms made a relatively large contribution to total phytoplankton biomass. The dominant diatom, particularly in winter and summer, was a Skeletonema costatum while Eucampia zodiacus and the cryptophyte Cryptomonas spp. dominated in spring and autumn, respectively, comprising more than 75% of the community at most stations. In the bioassay experiments the phytoplankton biomass increased by 30&#8211 600% in the +N (added nitrogen) and +NP (added nitrogen and phosphorus) treatments relative to the control and the +P (added ph