5 research outputs found
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Uptake of ammonium and urea in the northeast Pacific: comparison between netplankton and nanoplankton
The stable isotope 15N was used to measure nitrogen uptake in the coastal upwelling region off Oregon and Washington, USA, where nitrate concentrations in the surface water ranged from 0.7 to 49.1 μM. Nanoplankton biomass (1 to 10 pm) was relatively more abundant at low-nitrate stations, while netplankton biomass (10 to 200 μm) was dominant at high-nitrate stations. Ammonium was more important than urea as a regenerated nitrogen source for both size classes of plankton, and ambient ammonium concentrations appeared to inhibit urea assimilation. Nanoplankton and netplankton were equally important in use of regenerated nitrogen in low-nitrate waters, while netplankton dominated nitrogen use in high-nitrate waters. In high-nitrate waters, the percent uptake in each size fraction was proportional to the percent particulate nitrogen (PN) and chlorophyll a (chl a) in that fraction. At low-nitrate stations, uptake in each fraction was proportional to PN but not to chl a. Nitrogen-specific (mass N taken up/mass particulate N) uptake rates ranged from 0.2 to 2.6 d-l, and were generally higher for the netplankton than for the nanoplankton fraction. However, these rates are not directly proportional to phytoplankton use as a result of varying amounts of non-phytoplankton N. Phytoplankton N was estimated by assuming a constant chl a/PN ratio for each size class, and used to calculate phytoplankton-specific uptake rates. Phytoplankton growth rates were estimated by extrapolating the short-term uptake rates to daily rates. Since calculated growth rates exceeded the expected maxima for phytoplankton In the low-nitrate coastal water, we postulate heterotrophic utilization of nitrogen
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Nitrogen uptake and phytoplankton growth in coastal upwelling regions
Uptake of nitrogenous nutrients by microplankton off the Washington and Oregon coasts was measured during the 1985 upwelling period. Nitrogen uptake rates in low-NO3- waters (<5 μM) were 0.020–0.258 μmol N liter-1 h-1 and were primarily supported by regenerated nitrogen (71% of total uptake). Nitrogen uptake rates in high-NO3- waters (≥20 μM) were 0.281–1.480 μmol N liter-1 h-1 and new (NO3-) nitrogen supported 83% of total uptake. Phytoplankton N was estimated by assuming a constant Chl a : PN for phytoplankton and was used to calculate phytoplanktonspecific uptake rates.
Despite differences in nutrient concentrations, PN and Chl a at three upwelling sites (Oregon, Benguela, and Peru), NO3- uptake normalized to Chl a, and estimates of NO-3-supported phytoplankton growth rates are remarkably similar. Nitrate uptake supports growth rates on the order of 1–2 d-1. Estimates of phytoplankton growth from rates of NH4+ + NO3- uptake range from 1.36 to 4.79 d-1 and are negatively correlated with concentrations of NO3- and phytoplankton N. The apparent increase in phytoplankton growth rates for waters with lower nitrogen availability may result from heterotrophic bacterial utilization of NH4+ nitrogen
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Ammonium recycling limits nitrate use in the oceanic subarctic Pacific
Seasonal and diel changes in nutrient concentrations and nitrogen assimilation rates were used to assess the effects of NH4+ on NO3- assimilation. Surface-water NO3- concentrations ranged from 6 to 17 μM while NH4+ concentrations ranged from 0 to 0.4 μM. Total N assimilation ranged from 84 to 732 nM d–1 but showed no seasonal trend. NH4+ and urea concentrations were < 1% of total dissolved inorganic N, but use of this “regenerated” N still accounted for 44–89% of total N assimilation. Rates of NO3- assimilation were negatively correlated with ambient NH4+ concentrations, and concentrations of NH4+ between 0.1 and 0.3 μM caused complete inhibition of NO3- assimilation. NO3- was more important as a source of N in spring than in summer. We attribute this pattern to a summer increase in turnover rates for NH4+. Turnover times for the dissolved NH4+ pool were half as long in August as in May. Grazing and recycling in the euphotic zone apparently both play significant roles in preventing depletion of NO3- in the oceanic subarctic Pacific
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Diel periodicity in ammonium uptake and regeneration in the oceanic subarctic Pacific: Implications for interactions in microbial food webs
Diel periodicity in NH4+ uptake and regeneration in the nutrient-rich environment of the oceanic subarctic Pacific was examined. Surface water was incubated in large shipboard microcosms that allowed repeated sampling of the planktonic community for NH4+ cycling rates, bacterial production rates, and population densities of the dominant autotrophs and heterotrophs. Changes in NH4+ concentrations and isotopic enrichments indicated that regeneration took place exclusively at night. Nitrogen uptake rates and bacterial production were maximal during the day. Abundance of <2 μm autotrophic flagellates reached a maximum at the end of day or beginning of night, whereas abundance of zooflagellates in the <2 μm and 1–10 μm size classes showed a twofold variation with maximal abundances occurring at the beginning of day. These contrasting diel patterns suggest short-term, cyclic disequilibrium in production and grazing within the microbial food web. Hence, analysis of relationships among the various components of microbial populations in this type of environment will require sampling on much shorter time scales than has been attempted in past studies