A study on the variations in phytoplankton species composition and biomass in relation to N/P gradients in mesocosms

Abstract

The effects of gradients of inorganic nutrient input on coastal phytoplankton communities were investigated for 2 weeks in June 2002, in the Jangmok marine station of KORDI. This experiment was conducted in 8 mesocosm units consisting of 2.5 m3 polyethlylene bags. The nitrogene (N) input established a N/P ratio gradient among the mesocosms : 2, 4, 8, 16, 24, 32 and 64. The response of phytoplankton biomass and species composition after addition the nutrients in mesocosms was examined in comparision to a mesocosm unit which no nutrient were added. The biomass of the phytoplankton communities increased to a maximum 17.9 ㎍ chlorophyll a l-1 (10-fold above the mean initial value) in the mesocosm unit that N/P ratio gradient was 24/1, on Day 3. The biomass increase was largely attributable to bloom of Raphidophyceae Heterosigma akashiwo in all mesocosms during the early phase of the experiment. The decline of the Heterosigma akashiwo bloom occurred together with the development of flagellates Chroomonas sp. toward the end of the experiment when P was limiting. Consiquently, results from this experiment suggest that the high N/P ratio activated the limitation by another nutrient and induced a shift of phytoplankton species dominance from an initial dominace of nanoplankton to dominace of picoplankton.