Distribution and importance of autotrophic ultraplankton in a boreal inshore area (Kiel Bight, Western Baltic)

Abstract

From April to October 1986, 4 stations in Kiel Bight and he1 Fjord (Western Baltic) were investigated twice each month for the distribution and importance of autotrophic ultraplankton (< 20 pm) by size-fractionated records of chlorophyll a and primary productivity within the size classes micro- (> 20 pm), nano- (3 to 20 pm) and picoplankton (< 3 pm) Their development was compared with the physical environment in terms of salinity and temperature profiles and nutrient concentrations. Except the diatomdominated spring bloom and the autumn dinoflagellate bloom, ultraplankton contributed 70 to 100 % of phytoplankton biomass and productivity. The change from 'new' production by microplankton in spring to the ammonia-maintained, mainly 'regenerated' production In summer and again to 'new' production by microplanktonic dinoflagellates in autumn was characterized by a nanoflagellate bloom. In May, this bloom was built up by the non-skeletforming form of the silicoflagellate Dictyocha fibula. In September, Prorocentrum minimum bloomed. In between, nanoplankton was the overall dominating size fraction. Picoplankton attained its greatest importance in mid-summer, contributing 8 to 33 % of total phytoplankton chlorophyll and production. Whereas absolute biomass and productivity of picoplankton, as well as of all other size classes, decreased towards the less eutrophic open Kiel Bight, picoplankton contribution to total phytoplankton biomass and production increased in this direction. Nanoplankton contribution increased towards the eutrophic inner Kiel Fjord. Pycnoclines and nutriclines supported both ultraplankton productivity and the structure of their vertical distribution. If the pycnocline was strong, however, picoplankton seemed unable to pass through it. There were indications of pronounced diurnal vertical migration of the nanoflagellates P. rnlnirnun~a nd D. fibula.