Use of nitrate reductase (NR) activlty as an index of rates of nltrate Incorporation in m a n n e phytoplankton has been complicated by inadequate assays, and the high degree of regulation of the enzyme under different non-steady-state conditions The relationship between NR activity and rates of nitrate incorporation was examined uslng a modified NR assay (which Included bovine serum albumln) under diel p e r~o d~c i t y in irrad~ance, following exhaustion of nitrate, and in the presence of ammonium Laboratory experiments using the diatom Thalassiosira pseudonana showed that on a 14 10 h hght dark cycle, NR activity had 2 d~stinct peaks within 1 cycle, one in the m~d d l e of the light period, and the other towards the end of the dark penod Each of these peaks has been noted In previous work, both in culture and in natural populations, but this is the first descnpt~on of both in 1 cycle Throughout the d~e l cycle, NR activ~ty closely matched rates of nitrate incorporation calculated from increases in part~culate nitrogen In the cultures As n~t r a t e was exhausted in the cultures, NR actlvity declined in step w~t h nitrate incorporation rates (estimated from nitrate depletion, increases in part~culate nltrogen or the product of cell growth rate and cell nitrogen content) In cultures where n~t r a t e was substituted by ammonium, no NR activity was detected, but when ammonium was exhausted, background contamination of the medium with nitrate (approximately 1 PM) was suffic~ent to cause the appearance of NR activity and the uptake of nitrate Dally additions of 2 pM ammonium to cultures growing on nitrate had no effect on the r e l a t~o n s h~p between NR activity and nitrate incorporat~on rate The NR assay was used in a preliminary study of natural assemblages of phytoplankton in Monterey Bay, California, USA, during a post-upwelling diatom bloom Die1 periodicity in NR activity was observed that was virtually identical to that found in laboratory cultures Ammonium inhibition on a time scale simllar to that seen in culture was also recorded These preliminary results are promlslng for the use of NR activity to estimate rates of nltrate incorporat~on In field populat~on of phytoplankton KEY WORDS: N~t r a t e reductase . Enzyme activity . Nitrate incorporatlon . Marine phytoplankton .

In both frontal and stratified water of the Strait of Georgia, changes in dissolved nitrogen concentrations provided evidence for the simultaneous uptake of ammonium, nitrate and urea by a summer phytoplankton community. Chlorophyll a specific uptake rates of ammonium and urea were ca 2 and 2.4 times greater in stratified water than in frontal water, whereas chlorophyll a specific nitrate uptake rates were ca 1.6 times greater in frontal water. Ammonium and urea regeneration rates, calculated using a mass balance approach, were similar in frontal water, but urea regeneration rates were 2 to 5 times greater in the stratified water during the first 12 h of the experiment. Increases in particulate nitrogen could not be accounted for by corresponding decreases in total concentration of dissolved inorganic nitrogen and urea, or by 15N accumulation in the particulates. In frontal water the change in total dissolved inorganic nitrogen and urea consistently overestimated the change in particulate nitrogen, and in stratified water the change in total dissolved inorganic nitrogen and urea consistently underestimated the change in particulate nitrogen. These data suggest that the plankton community In frontal water was losing nitrogen in the form of dissolved organic nitrogen. By contrast, the plankton community in stratified water took up nitrogen compounds which were not measured as part of the total dissolved inorganic and urea nitrogen, but were most likely dissolved organic nitrogen compounds. Results stress the importance of determining uptake rates of all 3 nitrogen substrates (NHfi, N O and urea) using 15N isotopes and by simultaneously measuring the change in concentration of these compounds throughout the incubation period.

ABSTRACT. High discharge rates from the Fraser h v e r create a riverme plume front that moves dally and fortnightly with the tides in the Strait of Georgia. During a spring-neap tidal cycle in July 1987, a study of nutrient and plankton dynamics including vertical profiles of temperature, salinity, fluorescence, nutrients (NOs, NH4, PO4, S i 0 4 and urea), zooplankton, phytoplankton and bacterial biomass and pnmary and heterotrophic productivities was conducted at 3 stations, one situated in the riverine plume and two at the inner and outer estuarine plume. Primary productivity was highest in the outer part of the estuarine plume and lowest in the riverine plume. Bottom to surface daytlme vertical zooplankton hauls revealed no differences in species composition among the 3 stations, but euphausiids were significantly more abundant in the estuarine plume, where phytoplankton abundance was also the highest During post-neap tides, nitrate was undetectable, a subsurface chlorophyll maximum was present and productivity was high in the estuarine plume. During the post-spring tidal period, nutrient concentrations were elevated, maximum chlorophyll concentrations occurred near the surface and primary productivity increased approaching neap tides. Utilizable nitrogen sources (NO, + NH, + urea) and phosphate concentrations in the river were similar to concentrations in the riverine plume, while silicate was significantly higher in the river. Therefore, in late July, nutrient enrichment of the surface waters of the plume, resulting in high primary productivity at the plume boundaries and beyond, appears to be mainly due to entrainment a s the freshwater moves over the seawater.

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