Effects of nutrient enrichment on the growth rates and community structure of summer phytoplankton from the Gulf of Riga, Baltic Sea

Sea water collected in mid-July was spiked with different combinations of nutrients. Dissolved inorganic nitrogen (DIN) became exhausted, while significant amounts of P-PO4 and Si-SiO3 remained available. NH4 was taken up before NO3, but, in some cultures, NO3 uptake appears to have started in the presence of similar to 7.5 mu M N NH4. In the original assemblage, 29 species were found, belonging to five algal divisions. Cryptophytes and chlorophytes dominated the numbers of individuals, while diatoms and cyanobacteria contributed most to the total biovolume. The toxic dinoflagellate Dinophysis acuminata contributed 5% of the total biovolume. The diatom Skeletonema costatum and the cyanobacterium Microcystis reinboldii were the fastest growers during the first days of culturing, with growth rates up to 1.7 divisions (div.) day(-1). Growth rates of the chlorophyte Oocystis borgei and of the cyanobacteria Microcystis wesenbergii and Snowella lacustris were highest during the last days of the culture period, with >1 div. day(-1). At the end of the experiment, 17 out of the original 29 species had disappeared, including most dinoflagellates and diatoms, species numbers of both chlorophytes and cryptophytes halved, while only one cyanobacterium out of five disappeared. The chlorophyte O.borgei increased its biovolume from 7.7% to 51.0% of the total, while the cyanobacteria M.reinboldii and S. lacustris increased theirs from 0.1% to 10.8%, and from 12.4% to 63.5%, respectively. Five species not found in the original assemblage were recorded at the end of the experiment. Overall, the cryptophytes were a large or the largest component of the standing stock after 3 days, while diatoms contributed most to the total biovolume; nonetheless, the contribution by other taxa was still significant. At the end of the experiment, chlorophytes and cyanobacteria were the dominant taxa, while contributions by other taxa to both the standing stock and the biovolume were negligible, except in iron- and (Co + Mn + Mo)-supplemented cultures. In cultures to which iron had been added, the nitrogen yield index was 2.4 times higher than for similar nutrient conditions but without iron. The ability to sustain growth from uptake of dissolved organic matter (DOM) and N-2 fixation, combined with production of allelopathic substances, is assumed to have enabled the cyanobacteria to dominate the assemblages by the end of the culture period.