Sinking as a factor affecting phytoplankton species succession: The use of selective loss semi-continuous cultures

Abstract

The interactions between sinking rate, nutrient-limitation and species succession of natural phytoplankton assemblages were studied in outdoor semi-continuous cultures under four treatments. Two cultures were NH 4-limited and two were Si-limited. One NH 4- and one Si-limited culture were subjected to selective loss (due to sinking) and the other two were subjected to random loss due to daily aliquot removals following mixing of the cultures. Under NH 4-limitation in the random loss cultures, centric and pennate diatoms dominated while in the selective loss cultures, flagellates dominated by the end of the 25-day experiment. Under Si-limitation a mixture of flagellates, pennate and centric diatoms occurred in the random loss cultures, while the selective loss cultures consisted of flagellates and a very small centric diatom, Chaetoceros gracilis Lauder. Skeletonema costatum (Grev.) Cleve was very sensitive to Si-limitation and sank more quickly than Chaetoceros spp. Pennate diatoms had lower sinking rates than centrics and formed a background group in flagellate dominated communities in the selective loss cultures. Phytoplankton species competition and succession are influenced by growth and loss processes. Sinking rate is an important selective loss process in temperate coastal areas which can be simulated by using this selective loss, semi-continuous culturing technique. The advantages of using this technique to study the effects of sinking rate and its interaction with nutrients on succession in natural phytoplankton assemblages are discussed.