Trends in nitrogen utilization, determined with 15 N-labeled substrates, were related to blooms of distinct phytoplankton groups in the Gulf of Riga, Baltic Sea, during May, June and July 1999. The dominant phytoplankton groups included diatoms, cryptophytes, dinoflagellates, and filamentous cyanobacteria. As the water column became progressively more stratified over the growing season, diatoms comprised a smaller proportion of the total phytoplankton assemblage and almost disappeared by late summer. Their disappearance correlated with undetectable surface-water nitrate concentrations and low nitrate uptake rates (5 to 8% of total nitrogen uptake). Diatoms were the only phytoplankton group significantly associated with the uptake of oxidized nitrogen (nitrate). Cryptophytes, filamentous cyanobacteria and dinoflagellates were significantly associated with uptake of reduced nitrogen including ammonium, urea, dissolved free amino acids and adenine. Our results indicate that uptake of oxidized and reduced forms of nitrogen can be separated in time and space due to association with distinct phytoplankton groups.KEY WORDS: Nitrogen uptake · DON · Nitrate · Diatoms · Cyanobacteria · Cryptophytes · Baltic Sea · Gulf of Riga Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 30: [263][264][265][266][267][268][269][270][271][272][273][274] 2003 (Paerl 1991, Berg et al. 1997, Berman 1997, Carlsson et al. 1998.With an increase in nutrient inputs to a system, there is a tendency for any extra nitrogen to be in the reduced form, and for the ratio of oxidized:reduced nitrogen to decrease (Oviatt et al. 1986). Studies suggest that decreasing the oxidized:reduced supply ratio may contribute disproportionately to the alteration of phytoplankton succession (LaRoche et al. 1997, Glibert & Terlizzi 1999. For example, the Gulf of Riga, a subestuary of the Baltic Sea, has evidenced significant decreases in surface and deep water nitrate concentrations in the last 2 decades but an increase in phytoplankton biomass over the same period (Yurkovskis et al. 1996(Yurkovskis et al. , 1999. Experiments carried out on natural populations and locally isolated algal strains from the Gulf of Riga suggested that cyanobacteria were able to sustain growth by uptake of DON substrates, potentially contributing to cyanobacterial dominance of summer assemblages (Balode et al. 1998, Maestrini et al. 1999. As measured by 15 N-labelled substrates, DON appeared to be a major source of nitrogen to phytoplankton in the river-influenced portion of the Gulf in summer (Berg et al. 2001). We expanded on these studies to identify specific trends in nitrogen utilization related to blooms of distinct phytoplankton groups which could serve as predictors of their occurrence.The majority of nitrogen input to the Gulf of Riga occurs via riverine transport to the southern part, creating a north-south gradient in concentrations of nitrogen and salinity . In addition to riverine input, water column stratification...

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In the Baltic Sea, floating blooms of nitrogen-fixing cyanobacteria occur yearly during late summer. These blooms can sometimes be limited by iron. Due to extensive foresting around the Baltic Sea, iron is entering the Baltic Sea partly bound to dissolved organic material (DOM) via rivers. An experiment was performed in 300 l laboratory mesocosms to test the hypothesis that riverine highmolecular weight dissolved organic matter (HMWDOM), extracted by tangential flow filtration >1000 Da, stimulates the biomass of nitrogen-fixing cyanobacteria, by increasing the availability of iron. The addition of iron/EDTA and of DOM resulted in 5 to 10 times higher biomass of nitrogenfixing cyanobacteria. Accordingly, higher primary production and particulate nitrogen concentration at the end of the experiment were observed in those treatments compared to the control. The removal of mesozooplankton grazers did not have a significant effect on the microphytoplankton biomass and species composition. Nodularia spumigena biomass was highest in the treatments receiving DOM, but addition of iron alone had no significant effect on this. N. spumigena was less positively affected by iron addition than Anabaena cf. inaequalis, suggesting that N. spumigena is a better competitor for iron. Separate microcosms comparing additions of iron, manganese and cobalt showed that iron was limiting for cyanobacterial biomass development. The results strongly suggest that iron bound to DOM can contribute to the iron demands of nitrogen-fixing cyanobacteria in the Baltic Sea.

The importance of dissolved silicate (DSi), dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON), phosphate and dissolved organic phosphorus (DOP) on algal growth is analysed for the Gulf of Riga and the adjacent open Baltic Sea. The results of three cruises (May, June, and July, 1999) along a transect across the Gulf of Riga from the entrance to the Daugava River to the open Baltic are presented. Nutrient-limitation was identified on the basis of available nutrient concentrations and stoichiometric analysis. In spring, phosphate appeared to be the algal-growth-potential-limiting nutrient at the entrance of the Daugava River, DSi in the central Gulf, and DIN at the open Baltic station. There was no correlation between limiting nutrient and spring phytoplankton community structure. Both the DIN and phosphate pools of the upper mixed layer were exhausted by mid-May, except at the river mouth. In summer there was a good correlation between phytoplankton biomass and DOP along the transect. Contrary to the situation in the open Baltic, the lower layer DIN/phosphate ratio in the Gulf of Riga significantly exceeds the Redfield ratio, and upwelling likely does not favour nitrogen-fixing species. Therefore, the upper layer DOP pool should be regarded as potentially the main source of phosphorus for nitrogen-fixing cyanobacteria in the Gulf of Riga.