Structural shift of planktonic communities in seawater enclosures: from autotrophic to heterotrophic communities

Abstract

An outdoor enclosure experiment was carried out near the southern coast of Korea in order to investigate the effect of enclosed environments on planktonic community structure. The experiment was conducted in 6 floating, 220-liter cylindrical enclosures maintained for short periods (17 days). Rapid decreases of inorganic nutrients (N. P, Si), and in the abundance of large diatoms, Coscinodiscus spp., were observed after only one day from the start of the experiment. Decreases in chlorophyll a progressed through the experimental period, although the small diatoms, Chaetoceros spp. and Thalassiosira sp., increased considerably during the initial days. The continuous supply of dissolved organic carbon from crashing stocks of planktonic organisms sustained rapid growth of heterotrophic bacteria. The bacterial increase was followed by increase of heterotrophic nanoflagellates and ciliated protozoa. Noctiluca scintillans flourished as a dominant zooplankter until the end of the experiment, feeding on phytoplankton during the first half of the observations and on the abundant heterotrophic microorganisms during the second half. The results suggested that the enclosed environment provoked a structural shift of the plankton community from autotrophy to heterotrophy and indicated additionally that the trophic status in plankton communities was reflected by an enclosed environment formed by artificial structures in the coastal zone.