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...
