Nitrogen, Silicate and Zooplankton Control of the Planktonic Food-web in Spring

The control of silicate, nitrogen and zooplankton was studied with a special emphasis on the planktonic food web on spring bloom on the coastal area of the Baltic Sea in a laboratory experiment. Samples of 150 l of surface seawater were manipulated in transparent plastic laboratory tanks for 3 weeks atin situtemperature (+2 °C) with additions of nitrate and silicate (NO3–N, SiO4–Si), as well as removal of mesozooplankton by filtration through a 100 μ m mesh-sized plankton net. A diatom bloom developed in the experimental tanks within 7 days together with maxima in both picoplanktonic eukaryote and heterotrophic nanoflagellate abundances, during which time inorganic nitrogen and phosphorus were exhausted. Despite the clear response of phytoplankton to the silicate and nitrogen additions, the remainder of the food web was not stimulated by the nutrients. The production and abundance of picoplanktonic algae and bacteria were low and probably controlled by low temperature and bacterial substrates. Maximum growth rates of the organisms varied between 0•1 and 1•2 day−1, being the highest among diatoms and the lowest amongAcartiacopepods. The removal of mesozooplankton through prefiltration enhanced the growth of ciliates with a weak cascading effect through the microbial food web. Most of the biomass and production in the community was concentrated in the net phytoplankton, clearly indicating that in a diatom-dominated spring bloom community in the Baltic sea, only a minor fraction of carbon flow is channelled through the heterotrophic food web. Moreover, increased inputs of silicate and nitrogen to the coastal area before the spring bloom will be channelled to the production of net phytoplankton, therefore not stimulating the heterotrophic food web.