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...

The potential for release of ammonium (NH,+) a n d o r urea from the pool of dissolved organic nitrogen (DON) was examined in san~ples taken from Lake l n n e r e t (Israel), the k v e r Charente estuary and coastal water near Ile de Re (French Atlantic coast). After prefiltration through 1.0 or 1.2 pm membranes to remove most of the microbiota with the exception of bacteria, water samples with or without supplements (40 pM) of various organic nitrogen compounds (arginine, glucosamine, guanine, hypoxanthine, lysine, ornithine or thymine) were incubated at in situ temperatures, in the dark. for 7 to 14 d. Concentrations of NH,' and urea were monitored during the incubation period. Increases of NH,' with time were observed in 8 out of 12 experiments with unsupplemented lake samples, and in a single trial with coastal water, but not with Charente estuary water. In some expenments, increases of urea concentrations were also observed. The addition of organic nitrogen compounds almost always led to NH4+ increases in samples from all locations; guanine, hypoxanthine, arginine and, in the case of Charente water, glucosamine gave rise to urea. The addition of nitrification inhibitors (40 pM) at the start of some experiments gave inconsistent results, but in some cases appeared to increase the concentrations of NH,' with time. Taken together, the results of these experiments clearly indicate the potential in natural waters for degradation of DON pool constituents by indigenous bactena a n d o r free dssolved enzymes to NH,+ or urea; these in turn can be effectively exploited by the ambient microbiota. The breakdown of DON with the concomitant release of readily available compounds such as NH,' or urea could be an important process in the nitrogen nutrition of phytoplankton and bacteria.

Fine‐resolution measurements of phytoplankton and physical parameters were made from 31 May to 14 June 2005 in the Ría de Pontevedra (Spain), which is subject to seasonal upwelling. The main objective of this work was to elucidate physical‐biological interactions leading to subsurface aggregations of toxin‐producing microalgae (Pseudo‐nitzschia spp. and Dinophysis spp.). A sequence of upwelling‐relaxation‐upwelling‐downwelling events was recorded with a moored Acoustic Doppler Current Profiler (ADCP). Thin layers (TLs) of Pseudo‐nitzschia spp. and other diatoms (up to 30 µg chlorophyll a L−1) developed and persisted in the pycnocline above cooler (12.5ºC) upwelled water but were vertically displaced and even dispersed during downwelling. The establishment of steep pycnoclines after upwelling pulses and the formation of TLs of Pseudo‐nitzschia spp. and other diatoms suggest that pycnoclines may act as retention areas for these populations. Their vertical displacement during downwelling would explain different patterns observed in the contamination of benthic resources and raft‐mussels. A decimeter‐scale segregation of Pseudo‐nitzschia micans and Dinophysis acuminata populations was observed. The population of D. acuminata, present since March 2005 in Ría de Pontevedra, was never found within the pycnocline, did not perform any significant vertical migration, and was not dispersed during upwelling. Instead, it formed patches (up to 8 × 103 cell L−1) in the warmer (15‐18ºC) surface (0‐4 m) waters associated with a diurnal thermocline, and it spread throughout the ría into a near‐surface layer during relaxation and downwelling. These results demonstrate the importance of considering species‐specific behavior to predict the location of cell maxima.

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Mexandrium minutum (strain AM89BM) was grown in semi-continuous culture (0.2 volume d-l) in N-limiting (N03:P0, = 1.6 and 3.16), in P-Luniting (N P = 160 and 80), and in N-and P-balanced (N:P = 16) me&a. Daily supplies of limiting nutrients were taken up until exhaustion; most residual concentrations before renewal were near the detection limits. Samples were taken during 2 separate periods: Days 5 to 12 and 26 to 36. During both periods. cells grew at the real rates of 0.23 division d-' in the N:P balanced medium, while the growth rate was decreased in all N:P < 16 or > l 6 media during the first period, and only in the N:P = 1.6 medium during the second period. Cells grown in Nlimiting media contained 1/3 less PON and 1/2 less chlorophyll a than cells grown in the N:P balanced condition. The POP content per cell did not vary significantly. In contrast, P0N:POP ratios in cells grown in P-Limiting conditions were over 50. Thus, cells grown in the 2 N:P < l 6 conditions were nitrogen-deficient, whlle cells grown in N:P = 80 and 160 conditions were nitrogen-surfeit. The toxin content.in cells greatly changed according to the N:P regime. Cells grown at N:P balanced conditions showed an average total paralytic shellfish poisoning (PSP) content of 1.24 fmol ceY-' In N-limiting conations, cells contained ca 3 times less toxin, with mean values of 0.41 to 0.45 fmol cell-'. In contrast, cells grown in P-limiting conditions contained on average 3.5 and 7 times more toxins than under balanced N:P conditions; 4.31 fmol cell-' in the N:P = 160 medium and 8.01 fmol cell-' in the N:P = 80 medium.