NITROGEN, SILICON AND ZOOPLANKTON CONTROLLING THE BALTIC SPRING BLOOM- AN EXPERIMENTAL-STUDY

The importance of nitrogen, silicon and grazing for the development of the planktonic spring bloom of the transition area between the Baltic Proper and the Gulf of Finland was studied in a laboratory experiment . Water for this experiment was collected prior to the onset of the sp ring bloom in early April 1991. At this time, inorganic nutrient conce ntrations were still at their annual maxim. In the experiment, conduct ed in plastic tanks, the nitrate level was doubled and silicate level tripled by single and combined additions. One tank was filtered with a 100 mu m net to exclude large grazers of phytoplankton. Two unmanipul ated controls were used. The tanks were maintained at +2 degrees C for a 3 week period in 12 h oi. daylight (60 mu E m(-2) s(-1)): during wh ich time their nutrient and phytoplankton dynamics were followed. All added inorganic nitrogen was swiftly taken up inducing rapid phytoplan kton growth; consequently, available phosphorus must have met the need s of growth. The results showed the limiting nature of nitrogen on the maximum primary productivity and chlorophyll a concentration. Silicat e additions affected the structure of the phytoplankton assemblage by promoting specifically the growth of the diatom Chaetoceros holsaticus Schutt, which produced resting spores at later stages of the bloom, w hile Chaetoceros wighamii Brightwell was the dominant diatom species i n other tanks. The absence of large grazers had no effect on phytoplan kton dynamics. (C) 1997 Academic Press Limited.