Patricia A. Wheeler

Nutrients, organic carbon and organic nitrogen in the upper water column of the Arctic Ocean: implications for the sources of dissolved organic carbon

Abstract Particulate and dissolved organic carbon and nitrogen were analyzed for shelf, slope and basin samples collected during the 1994 Arctic Ocean Section. Concentrations of organic carbon and nitrogen were highest in the surface water and decreased dramatically below 100 m. Over the shelf 40–50% of the organic material was present in the particulate phase, while in the slope and basin waters >-90% of the organic material was present in the dissolved phase. The halocline layer over the Chukchi plateau contained high concentrations of inorganic nutrients but no significant elevation of either organic carbon or organic nitrogen. Highest concentrations of total organic carbon were found in the surface water of the Makarov and Amundsen Basins between 80 and 90° N. The C:N ratio of the dissolved organic material in the upper 100 m ranged from 9 to 25, with highest ratios along the Makarov flank of the Lomonosov Ridge. We suggest that a major portion of the carbon-rich organic material in surface waters is derived from Eurasian shelf riverine inputs, which tend to flow in perimeter currents along the slope and ridges. Our preliminary dissolved organic carbon (DOC) budget suggests that the three major sources of DOC in the central Arctic are in situ production (56%), river run-off (25%), and Pacific water (19%).

AMMONIUN AND NITRATE UPTAKE BY THE MARINE MACROPHYTES HYPNEA MUSVUFORMIS (RHODOPHYTA) AND MACROCYSTIS PYRIFERA (PHAEOPHYTA)1, 2

NH4+ and NO3− uptake were measured by continuous sampling with an autoanalyzer. For Hypnea musciformis (Wulfen) Lamouroux, NO3−up take followed saturable kinetics (K2=4.9 μg‐at N t−1, Vmax= 2.85 μg‐ at N, g(wet)−1. h−1. The ammonium uptake data fit a trucatd hyperbola, i.e., saturation was not reach at the concentrations used. NO3− uptake was reduced one‐half in the presence of NH4+, but presence of NO3− had no effect on NH4+ uptake. Darkness reduced both NO3− and NH4+ uptake by one‐third to one‐half. For Macrocystis pyrufera (L) C. Agardh, NO3− uptake followed saturable kinetices: K2=13.1 μg‐at N. l−1. Vmax=3.05 μg‐at N. g(wet)−1. h−1.NH4+ uptake showed saturable kinetics at concentration below 22 μg‐at N l ‐1 (K2=5.3 μg‐at N.1–1, Vmax= 2.38 μg‐at N G (wet)−1.h−1: at higher concentration uptake increased lincarly with concentrations. NO3−and NH4+ were taken up simulataneously: presence of one form did not affect uptake of the other.

EFFECT OF NITROGEN AND PHOSPHORUS SUPPLY ON GROWTH AND TISSUE COMPOSITION OF ULVA FENESTRATA AND ENTEROMORPHA INTESTINALIS (ULVALES, CHLOROPHYTA)1

The chlorophyte macroalgae Ulva fenestrata (Postels and Ruprecht) and Enteromorpha intestinalis (Linnaeus) Link. were grown under various nutrient regimes in indoor semi‐continuous and batch cultures. Tissue nitrogen contents ranged from 1.3–5.4% N (dry wt), whereas tissue P ranged from 0.21–0.56% P (dry wt). Growth in low nitrogen medium resulted in N:P ratios of 5–8, whereas growth in high nitrogen medium resulted in N:P ratios of 21–44. For U. fenestrata, tissue N:P < 16 was indicative of N‐limitation. Tissue N:P 16–24 was optimal for growth and tissue N:P > 24 was indicative of P‐limitation. Growth of U. fenestrata was hyperbolically related to tissue N but linearly related to tissue P. Phosphorus‐limited U. fenestrata maintained high levels of tissue N, but N‐limited algae became depleted of P. For E. intestinalis, tissue N remained at maximum levels during P‐limitation whereas tissue P decreased to about 85% of maximal levels during N‐limitation. Growth rates for U. fenestrata decreased faster during P‐limitation than during N‐limitation. Simultaneously, tissue P was depleted faster than tissue N. Our results suggest that comparing tissue N and P of macroalage grown in batch cultures is useful for monitoring the nutritional status of macroalgae.

The effect of amino acids on ammonium utilization and regeneration by heterotrophic bacteria in the subarctic Pacific

Heterotrophic bacteria can assimilate both dissolved organic nitrogen (DON) and ammonium, but they also can excrete ammonium during DON catabolism. The relationships among DON and ammonium uptake and ammonium excretion are not clear. In the subarctic Pacific, our experiments show that the supply of dissolved free amino acids partially controls whether heterotrophic bacteria assimilate or excrete ammonium. Bacteria in these waters apparently preferred to assimilate amino acids rather than ammonium because addition of amino acids significantly inhibited uptake of ammonium. In <0.8 μm size fractions dominated by heterotrophic bacteria, ammonium uptake occurred only when amino acid concentrations were too low to support bacterial growth. When the amino acid supply was adequate, ammonium was regenerated by bacterial assemblages that had been separated from grazers. The C:N ratio of the amino acid pool, which was the predominate nitrogen source for bacteria during initial phases of our experiments, was 2.6. Based on mass balance consideration, ammonium excretion would be expected if bacterial carbon growth efficiency is assumed to be about 50% and the C:N of bacterial biomass is 4. These results suggest that high rates of amino acid turnover should be positively correlated with high rates of ammonium excretion by heterotrophic bacteria. Thus, amino acid cycling increases availability of ammonium to phytoplankton and may affect plankton community structure and rates of new production.

SEASONAL FLUCTUATIONS IN TISSUE NITROGEN, PHOSPHORUS, AND N:P FOR FIVE MACROALGAL SPECIES COMMON TO THE PACIFIC NORTHWEST COAST1

Variations in tissue nitrogen (N) and phosphorus (P) were examined over a complete seasonal cycle for five macroalgae common in Oregon coastal water. Tissue N ranged from 2.0 to 5.5% dry weight (dry wt) in leafy macroalgae (Enteromorpha intestinalis (Linnaeus) Link. Ulva fenestrata Postels et Ruprecht, and Porphyra sp.) and from 0.9 to 2.6% drt wt in branched macroalgae (Codium fragile (Suringar) Hariot and Pelvetiopsis limitata Setchell). Tissue P ranged from 0.32 to 0.86% dry wt in leafy macroalgae and from 0.27 to 0.50% dry wt in branched macroalgae, Ulva fenestrata, C. fragile, and P. limitata appeared to be N limited during part of the year based on tissue N levels. Variations in N:P showed a more distinct seasonal pattern than either tissue N and tissue P. All macroalgae examined appeared to be N limited at least part of the year based on N:P composition, and P limitation occurred for all macroalgae examined except C. fragile. Our results suggest that tissue N:P ratio for macroalgae may be a good index for evaluating in situ nutrient status.

Cold halocline, increased nutrients and higher chlorophyll off Oregon in 2002

Received 24 March 2003; accepted 18 June 2003; published 31 July 2003. [1] Observed changes in the nutrient levels in the halocline of the California Current during 2002 indicated a natural eutrophication that was accompanied by increased chlorophyll and oxygen in surface water. Decreased oxygen in the lower water column over the shelf indicated that much of the phytoplankton production was respired rather than passedontohighertrophiclevels.In2002thehaloclinewater was >1� C colder than usual and 0.5� C colder than any previous observation. Four transect lines off the coast of Oregonshowa50%increaseinnitrate,phosphateandsilicate at 33 psu in 2002 compared to 1998–2001. The increase in nutrients resulted in a 2-fold increase in chlorophyll standing stocks during the summer of 2002 compared with the preceding four years. A significant portion of the increased production was subsequently respired resulting in low oxygen water over the shelf. INDEX TERMS: 4215 Oceanography: General: Climate and interannual variability (3309); 4845 Oceanography: Biological and Chemical: Nutrients and nutrient cycling; 4283 Oceanography: General: Water masses. Citation: Wheeler, P. A., A. Huyer, and J. Fleischbein, Cold halocline, increased nutrients and higher chlorophyll off Oregon in 2002, Geophys. Res. Lett., 30(15), 8021, doi:10.1029/ 2003GL017395, 2003.

Uptake of ammonium and nitrate by heterotrophic bacteria and phytoplankton in the sub-Arctic Pacific

Heterotrophic bacteria are known to account frequently for a large fraction of NH+4 uptake in many oceans, but NO-3 uptake by bacteria is not thought to be as important. As part of the SUPER program, NH+4 and NO-3 uptake by heterotrophic bacteria in the sub-Arctic Pacific was examined and compared with use of dissolved free amino acids (DFAA). Heterotrophic bacteria accounted for 31% (range of <5–40%) and 32% (5–60%) of total uptake of NH+4 and NO-3, respectively, over 4 months of study. Although the average uptake of NO-3 by bacteria was equal to that of NH+4, bacteria actually assimilated relatively more NO-3 than NH+4 in those samples when the two processes were measured simultaneously. Uptake of NH+4 or NO-3 alone was about equal to bacterial biomass production, whereas uptake of dissolved free amino acids (DFAA) was only 40% of bacterial production, suggesting that the uptake of other dissolved organic nitrogen sources is relatively small. Uptake of NH+4 by bacteria correlated significantly with bacterial production, and both NH+4 uptake and bacterial production correlated significantly with DFAA turnover. In contrast, there was no significant correlation between any microbial parameter and NO-3 uptake. Biomass production of heterotrophic bacteria appears to be supported largely by DIN uptake and, to a lesser extent, uptake of DFAA in the sub-Arctic Pacific. Uptake of NO-3 is more important in supporting bacterial growth than previously hypothesized, although it remains the least preferred N source.

Organic carbon and nitrogen in the northern California current system: comparison of offshore, river plume, and coastally upwelled waters

Abstract During the first year of the Northeast Pacific GLOBEC program we examined the spatial distributions of dissolved and particulate organic carbon and nitrogen in the surface waters off the Oregon and Washington coasts of North America. Eleven east–west transects were sampled from nearshore waters to 190 km offshore. Hydrographic data and the distribution of inorganic nutrients were used to characterize three distinct water sources: oligotrophic offshore water, the Columbia River plume, and the coastal upwelling region inshore of the California Current. Warm, high salinity offshore water had very low levels of inorganic nutrients, particulate organic carbon (POC) and dissolved organic carbon (DOC). Warm, low salinity water in the Columbia River plume was relatively low in nitrate, but showed a strong negative correlation between salinity and silicate. The river plume water had the highest levels of total organic carbon (TOC) (up to 180 μM) and DOC (up to 150 μM) observed anywhere in the sampling area. Cold, high salinity coastal waters had high nutrient levels, moderate to high levels of POC and particulate organic nitrogen (PON), and low to moderate levels of DOC and dissolved organic nitrogen (DON). Each of these regions has characteristic C:N ratios for particulate and dissolved organic material. The results are compared to concentrations and partitioning of particulate and dissolved organic carbon and nitrogen in other regions of the North Pacific and North Atlantic Oceans.

CHANGES IN NITROGEN POOLS IN ULVA FENESTRATA (CHLOROPHYTA) AND GRACILARIA PACIFICA (RHODOPHYTA) UNDER NITRATE AND AMMONIUM ENRICHMENT

The accumulation of nitrogen in different cellular pools by the macroalgae Ulva fenestrata (Postels and Ruprecht) (Chlorophyta) and Gracilaria pacifica (Abbott) (Rhodophyta) was studied in a laboratory experiment. After 8 or 9 days of nitrogen enrichment, nitrate, ammonium, free amino acid (FAA), protein, chlorophyll (chl), phycoerythrin (PE), and insoluble nitrogen pools were extracted and analyzed, and their relative contribution to total nitrogen (TN) was assessed. In U. fenestrata, the nitrate and ammonium enrichments resulted in a significant increase of TN from 2.41% dry weight (dw) to 4.19% and 4.71% dw, respectively. All the extracted N pools increased significantly. In G. pacifica, TN increased more under ammonium enrichment than under nitrate enrichment. In both macroalgae, proteins and FAA were the most important N storage pools. Protein‐N ranged from 700 to 2300 μmol N·g dw−1 (43%–66% of TN) and contributed the most to TN increase (41%–89%). The FAA pool was always larger in G. pacifica than in U. fenestrata. In both species, the FAA pool accounted for 4%–17% of TN (70–600 μmol N·g dw−1). In U. fenestrata, nitrate can represent a temporary storage pool: it accumulated up to 200 μmol N·g dw−1 (7% of TN) and contributed more than FAA to overall increase in cellular nitrogen. In contrast, G. pacifica had a small nitrate pool. The PE pool in G. pacifica increased with TN but was never more than 9% of total protein‐N or 6% of TN, and it was less important than FAA as a storage pool. All TN was recovered in the extracted and insoluble N pools at the end of the experiment in U. fenestrata. In G. pacifica, the extracted and insoluble N pools accounted on average for 83%–90% of TN.

15N measurements of ammonium and nitrate uptake by Ulva fenestrata (Chlorophyta) and Gracilaria pacifica (Rhodophyta): Comparison of net nutrient disappearance, release of ammonium and nitrate, and 15N accumulation in algal tissue1

Ammonium and nitrate uptake rates in the macroalgae Ulva fenestrata (Postels and Ruprecht) (Chlorophyta) and Gracilaria pacifica (Abbott) (Rhodophyta) were determined by 15N accumulation in algal tissue and by disappearance of nutrient from the medium in long‐term (4–13 days) incubations. Nitrogen‐rich algae (total nitrogen> 4% dry weight [dw]) were used to detect isotope dilution by release of inorganic unlabeled N from algal thalli. Uptake of NH4+ was similar for the two macroalgae, and the highest rates were observed on the first day of incubation (45 μmol N·g dw−1·h−1 in U. fenestrata and 32 μmol N·g dw−1·h−1 in G. pacifica). A significant isotope dilution (from 10 to 7.9 atom % enrichment) occurred in U. fenestrata cultures during the first day, corresponding to a NH4+ release rate of 11 μmol N·g dw−1·h−1. Little isotope dilution occurred in the other algal cultures. Concurrently to net NH4+ uptake, we observed a transient free amino acid (FAA) release on the first day in both macroalgal cultures. The uptake rates estimated by NH4+ disappearance and 15N incorporation in algal tissue compare well (82% agreement, defined as the percentage ratio of the lower to the higher rate) at high NH4+ concentrations, provided that isotope dilution is taken into account. On average, 96% of added 15NH4+ was recovered from the medium and algal tissue at the end of the incubation. Negligible uptake of NO3− was observed during the first 2–3 days in both macroalgae. The lag of uptake may have resulted from the need for either some N deprivation (use of NO3− pools) or physiological/metabolic changes required before the uptake of NO3−. During the subsequent days, NO3− uptake rates were similar for the two macroalgae but much lower than NH4+ uptake rates (1.97–3.19 μmol N·g dw−1·h−1). Very little isotope dilution and FAA release were observed. The agreement between rates calculated with the two different methods averaged 91% in U. fenestrata and 95% in G. pacifica. Recovery of added 15NO3− was virtually complete (99%). These tracer incubations show that isotope dilution can be significant in NH4+ uptake experiments conducted with N‐rich macroalgae and that determination of 15N atom % enrichment of the dissolved NH4+ is recommended to avoid poor isotope recovery and underestimation of uptake rates.