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Dissolved organic nitrogen (DON) produced by the nitrogen-fixer Trichodesmium sp. has the potential to serve as a nitrogen source for the red tide dinoflagellate Karenia brevis. Dis- solved organic matter (DOM) from laboratory cultures of Trichodesmium sp. was isolated, concen- trated and then supplied as a nutrient source to K. brevis cells collected from the Gulf of Mexico. K. brevis abundance increased immediately after Trichodesmium sp. cellular exudate (TCE) addition, allowing the population to double within the first 24 h. There was rapid and complete utilization of the TCE DON as well as ~89% of the TCE dissolved organic phosphorus (DOP). Additionally, ter- minal restriction fragment length polymorphism (TRFLP) was used to assess the bacterial commu- nity response to the addition of TCE . The number of bacterial operational taxonomic units (OTUs) initially increased after the TCE DOM addition, but decreased as K. brevis reached its maximum abundance. Electrospray ionization mass spectrometry (ESI-MS) and Fourier transform ion cy- clotron resonance mass spectrometry (FT-ICR MS) were used to chemically characterize the DOM. Approximately 25% of compounds disappeared within the first 24 h, corresponding to the greatest increase in K. brevis abundance. Using FT-ICR MS, 391 DON and 219 DOP potentially bioavailable compounds were characterized. The bioavailable DON compounds were highly re- duced and 44% had molar ratios indicative of lipid or protein-like compounds. The changes in DON concentration and compound composition show that Tricho desmium sp. provides a sufficient source of nitrogen to directly or indirectly support K. brevis blooms.

Acartia tonsa copepods are not limited to herbivory and can derive up to half their daily ration from predation on heterotrophic ciliates and dinoflagellates. The effects of an omnivorous diet on nutrient regeneration, however, remain unknown. In this study, we fed A. tonsa an exclusively carnivorous diet of either (1a) heterotrophic dinoflagellate Oxyrrhis marina or (1b) Gyrodinium dom- inans, (2) an exclusively herbivorous diet of Thalassiosira weissflogii diatoms, or (3) a mixed omnivo- rous diet. We measured the release rate, composition, and stoichiometry of dissolved organic carbon (DOC), dissolved organic phosphorus (DOP), and nitrogen (urea) in addition to the inorganic nutri- ents, ammonium (NH4 + ) and phosphate (PO4 3- ). Despite similar ingestion rates among treatments, as well as similar C:N ratios of food items, A. tonsa release rates of DOC and NH4 + were highest while feeding on a carnivorous diet and lowest while feeding omnivorously. In contrast, urea, on average, was a higher portion of total nitrogen released in the mixed diet treatment (32 to 59%). DOP release rates were only detectable in diets containing microzooplankton prey. Our results suggest that cope- pod diet plays an important role in determining the quantity and composition of regenerated C, N, and P available to phytoplankton and bacteria. Additionally, the uncoupling of ingestion and nutrient release rates and the variability in released ratios of dissolved C:N:P in our study suggests that stoi- chiometric models based exclusively on predator and prey C:N and N:P ratios may not be adequate in determining stoichiometry of total nutrient release.

In high latitude planktonic ecosystems where the prymnesiophyte alga Phaeocystis pouchetii is often the dominant primary producer, its importance in structuring planktonic food webs is well known. In this study we investigated how the base of the planktonic food web responds to a P. pouchetii colony bloom in controlled mesocosm systems with natural water enclosed in situ in a West Norwegian fjord. Similar large (11 m 3 ) mesocosm studies were conducted in 2 successive years and the dynamics of various components of the planktonic food web from viruses to mesozooplankton investigated. In 2002 (4 to 24 March), 3 mesocosms comprising a control containing only fjord water; another with added nitrate (N) and phosphate (P) in Redfield ratios; and a third with added N, P, and cultured solitary cells of P. pouchetii, were monitored through a spring bloom cycle. In 2003 (27 Feb- ruary to 2 April) a similar set of mesocosms were established, but cultured P. pouchetii was not added. As expected, during both years, addition of N and P without addition of silicate resulted in an initial small diatom bloom followed by a colonial bloom of P. pouchetii (600 to 800 µg C l -1 ). However, the hypothesis that addition of solitary cells of P. pouchetii would enhance subsequent colony blooms was not supported. Interestingly, despite the large production of Phaeocystis colonial material, little if any was transferred to the grazing food web, as evidenced by non-significant effects on the biomass of micro- and mesozooplankton in fertilized mesocoms. Separate experiments utilizing material from the mesocosms showed that colonies formed from solitary cells at rates that required only ca. 1% con- version efficiencies. The results are discussed from the perspective of future research still required to understand the impact of life cycle changes of this enigmatic phytoplankter on surrounding ecosystems.

Factors that influence the release of DON by planktonic assemblages were investigated using size fractionation experiments in the Southern California Bight and in Monterey Bay, California, USA. Incubation experiments, with either 15 NH 4 + or 15 NO 3 - as a tracer, were used to measure rates of net uptake (incorporation of DIN into particulate nitrogen [PN]) and DON release (production of DON during the incubation, through both active and passive mechanisms). DON release varied greatly among experiments and was higher when 15 NO 3 - was the substrate; it accounted for 3 to nearly 100 % of the gross uptake (net uptake plus DON release). Compared with incubations with the

We present a new method for separation of dissolved organic nitrogen (DON) from dissolved inorganic nitrogen (DIN) in marlne samples which permlts the measurement of DO'*N production during DII5N uptake and regeneration experiments. Ion retardat~on resin was used to separate DI1'N from DOI'N and the isolated D0I5N was subsequently analyzed by mass spectrometry Variat~on in D0I5N atom % enrichment in duplicate samples, determined w ~ t h the ion retardation column method, was less than 4"0 . We also separated the low n~olecular weight (LMW) DO"N from total D015N using ultrafiltration and found the ratio of these variables to be a useful index in determining what release processes were likely occurring in a given sample. Examples are presented from 2 types of experiments conducted in Chesapeake Bay waters: first, a 6 h time-course of D015N release from both "NH,' and I5NO3uptake, and second, a series of short-term (0.5 h) release measurements from 15NH,* uptake over the course of a day. In the former, total DO1% release rates resulting from 15NH,* uptake were several-fold higher than those resulting from IsNO3uptake due to extremely low rates of incorporat~on of "NO? into cellular organic material. The release of LMW DOI'N resulting from I5NH,+ uptake decreased from 78 to 21 % of total D0I5N release during the 6 h incubation which suggests that processes other than direct release by phytoplankton (e.g. sloppy feeding, cell lysis) likely became more important as the incubation progressed In the latter expenment, LMW DO'% release was 0 to 16 % of total D0I5N release, which indicates that, in this case, also, processes other than direct release by phytoplankton were dominating the flux of DON from phytoplankton.