Effects of a Chrysochromulina polylepis subsurface bloom on the planktonic community

During spring (May-June) 1988 an extensive subsurface bloom of the haptophycean flagellate Chrysochromulina polylepis developed in Scandinavian waters (the KattegatSkagerrak area). Here we report on the vertical distribution of bacteria, heterotrophic flagellates, ciliates and copepods at a permanent station in the Southern Kattegat during the bloom. At the height of the bloom, the density of C. polylepis reached 60 to 70 X 10' cells 1-' in the pycnocline. At this time no potential grazers were present in the subsurface bloom and bacterial production was extremely low. Field and laboratory experiments showed that C. polylepis inhibited the activity of planktonic bacteria, ciliates and copepods. During the decay of the bloom, algae were colonized by bacteria and the pycnocline was subsequently re-lnvaded by heterotrophic flagellates, small aloricate ciliates and copepods. Two weeks after the height of the bloom, the normal pelagic food web structure was re-established.