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 spring bloom in early April 1991. At this time, inorganic nutrient concentrations were still at their annual maxim. In the experiment, conducted in plastic tanks, the nitrate level was doubled and silicate level tripled by single and combined additions. One tank was filtered with a 100 μm net to exclude large grazers of phytoplankton. Two unmanipulated controls were used. The tanks were maintained at +2 °C for a 3 week period in 12 h of daylight (60 μE m −2 s −1 ), during which time their nutrient and phytoplankton dynamics were followed. All added inorganic nitrogen was swiftly taken up inducing rapid phytoplankton growth; consequently, available phosphorus must have met the needs of growth. The results showed the limiting nature of nitrogen on the maximum primary productivity and chlorophyll a concentration. Silicate 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, while Chaetoceros wighamii Brightwell was the dominant diatom species in other tanks. The absence of large grazers had no effect on phytoplankton dynamics.