The effect of enhanced nitrate input on the temporal variation of the plankton community in a mesocosm

Abstract

Temporal variation of the natural planktonic community in the Southern Sea of Korea was investigated by using four floating enclosed bags, (3.2 m deep and 2,500 liter) in order to understand the effect of enriched nitrate on the planktonic community in the spring (March-April) of 2002. Prior to beginning the incubation, the bags were placed in two different concentrations of nitrate, which consisted of control (ambient water) and experimental mesocosms (final concentration of 12,μM). The nitrate concentration in the experimental mesocosms remained significantly higher than those in control mesocosms throughout the study period (ANOVA, p<0.001). Following the addition of nitrate, abundance and chl-a concentration of phytoplankton peaked on Day 1, when diatoms established the peak in the experimental mesocosms. Diatoms consisted mainly of Thalassiosira decipiens, Pseudo-nitzschia pungens, Leptocylindrus danicus, Thalassionema nitzschioides, Chaetoceros pseudocrinitus and Actinoptychus senarius. However, the peak did not lead to the difference in abundance and composition of phytoplankton between control and experimental mesocosms during the study period. The dinoflagellates began to increase soon after the diatoms decreased in all mesocosms. Copepods, as a dominant group in the mesozooplankton community, showed no inimediate peak in relation to the nitrate addition, but only their own developmental process from the eggs to adult stage during the study period. The bottom-up control from enriched nitrate via phytoplankton to adult copepods was not distinguished in terms of the abundance of the planktonic community. This might stem from the relatively low nitrate availability of phytoplankton at no N-limited seawater and the weak coupling between rapidly sunken diatoms and copepods through the water column.