Primary production and nutrient assimilation in the Iberian upwelling in August 1998

Primary production was measured during two Lagrangian experiments in the Iberian upwelling. The first experiment, in a body of upwelled water, measured day-to-day changes in phytoplankton activity as the water mass moved south along the shelf break. Nutrient concentrations decreased over a five day period, with concomitant increases in phytoplankton biomass. Initially the maximum phytoplankton biomass was in the upper 10m but after four days, a sub-surface chlorophyll maximum was present at 30m. Depth-integrated primary production at the beginning of the experiment was 70mmolC.m⁻².d⁻¹ (838mgC.m⁻².d⁻¹) and reached a maximum of 88mmolC.m⁻².d⁻¹ (1053mgC.m⁻².d⁻¹) on day 3. On day 1, the picoplankton fraction (<2μm) was slightly more productive than larger (>5μm) phytoplankton, but the increase in overall production during the drift experiment was by these larger cells. Nitrate was the dominant nitrogen source. As nutrient concentrations declined, ammonium became increasingly more important as a nitrogen source and the f-ratio decreased from 0.7 to 0.5. Picoplankton cells (<2μm) were responsible for most (65–80%) of the ammonium uptake. The C:N:P uptake ratios were very close to the Redfield ratio for the first four days but as nutrients became depleted high C:N uptake ratios (11 to 43) were measured. Over the period of the experiment, nitrate concentration within the upper 40m decreased by 47.91mmolN.m⁻². In vitro estimates, based on ¹⁵N nitrate uptake, accounted for 56% of the decrease in nitrate concentration observed in the drifting water mass. Ammonium uptake over the same four day period was 16.28mmolN.m⁻², giving a total nitrogen uptake of 43.18mmolN.m⁻².In the second experiment, an offshore filament was the focus and a water mass was sampled as it moved offshore. Nutrient concentrations were very low (nitrate was <10nmol l⁻¹ and ammonium was 20–40nmol l⁻¹). Primary production rate varied between 36mmolC.m⁻².d⁻¹ (436mgC.m⁻².d⁻¹) and 21mmolC.m⁻².d⁻¹ (249mgC.m⁻².d⁻¹). Picophytoplankton was the most productive fraction and was responsible for a constant proportion (ca 0.65) of the total carbon fixation. Uptake rates of both nitrate and ammonium were between 10 and 20% of those measured in the upwelling region. Urea could be a very significant nitrogen source in these waters with much higher uptake rates than nitrate or ammonium; urea turnover times were ca. one day but the source of the urea remains unknown. Urea uptake had a profound effect on calculated f ratios. If only nitrate and ammonium uptake was considered, f ratios were calculated to be 0.42–0.46 but inclusion of urea uptake reduced the f ratio to <0.1. The primary production of this oligotrophic off-shore filament was driven by regenerated nitrogen.     

Spatial patterns of primary production on the shelf, slope and basin of the Western Arctic in 2002

In the spring and summer of 2002 primary production in the Chukchi Sea was measured, using ¹⁴C uptake experiments. Our cruise track encompassed the shelf and continental slope area of the Chukchi and Beaufort Seas progressing into deep water over the Canada Basin. The study area experienced upwards of 90% ice cover during the spring, with ice retreating into the basin during the summer. Production in the spring was light-limited due to ice cover, with average euphotic zone production rates of <0.3 g C m⁻² d⁻¹. Values of 8 g C m⁻² d⁻¹ were observed in association with surface bloom conditions during the initial ice breakup. Considerable nutrient reduction in the surface waters took place between the spring and summer cruise, and although not observed, this was attributed to a spring bloom. Decreased ice cover and increased clarity of surface waters in the summer allowed greater light penetration. The highest rates of production during the second cruise were found at 25–30 m, coincident with the top of the nutricline. Daily euphotic zone productivity in the summer averaged 0.78 g C m⁻² d⁻¹ on the shelf and 0.32 g C m⁻² d⁻¹ on the edge of the Canada basin. These data provide an estimated annual production of 90 g C m⁻² yr⁻¹ in the study area.     

Nitrogen cycling in the southern North Sea: consequences for total nitrogen transport

Nitrate and ammonium uptake rates were measured during a series of cruises in the well-mixed region of the southern North Sea from February to September. Water column-integrated uptake rates ranged between 0.01 and 8.7 mmol N m⁻² d⁻¹ and 0.01 and 12.2 mmol N m⁻² d⁻¹ for nitrate and ammonium, respectively, with ammonium uptake dominating after the phytoplankton spring bloom in May. A moored buoy continuously measuring nitrate and chlorophyll a and seabed current meters were also deployed in the central southern North Sea in the region of the East Anglian plume—a permanent physical feature which transports nutrients towards continental Europe. This enabled the flux of water and hence of nutrients across the southern North Sea to be determined and an assessment of the contribution of freshwater nutrients to the flux to be made. A simple box model is developed to relate the phytoplankton uptake of nitrate and ammonium to the transport of nitrate, ammonium and particulate organic matter (POM) across the southern North Sea. This showed the importance of the plume region of the North Sea in the processing of nitrogen, with nitrate dominating total nitrogen transport prior to the spring bloom (10 340×10³ kg N inflow to the plume in March) and transport of nitrogen as ammonium, nitrate and POM in approximately equivalent amounts during summer (2560, 2960 and 2151×10³ kg N inflow to the plume, respectively, in July). The box model also demonstrates more generally the need to assess nitrogen transport as nitrate, ammonium and POM if an improved understanding of the impact of nutrient input in shelf seas is to be achieved.     

Upper ocean export of particulate organic carbon and biogenic silica in the Southern Ocean along 170°W

Upper-ocean fluxes of particulate organic carbon (POC) and biogenic silica (bSi) are calculated from four US JGOFS cruises along 170°W using a thorium-234 based approach. Both POC and bSi fluxes exhibit large variability vs. latitude during the seasonal progression of diatom dominated blooms. POC fluxes at 100 m of up to 50 mmol C m⁻² d⁻¹ are found late in the bloom, and farthest south near the Ross Sea Gyre. Biogenic Si fluxes also peak late in the bloom as high as 15 mmol Si m⁻² d⁻¹, but this flux peak occurs at a different latitude, just south of the Antarctic Polar Front (APF), which is centered around 60°S along this cruise track. The ratios of both POC and bSi export relative to their production rates are large, suggesting an efficient biological pump at these latitudes. The highest relative bSi/POC flux ratios at 100 m are found just south of the APF, coincident with a bSi/POC flux peak seen in 1000 m traps during this same program by Deep-Sea Research II (Honjo et al., Deep-Sea Research II 47, 3521–3548). These data suggest that efficient export at these latitudes can support the high accumulation rates of bSi found in the sediments under and south of the APF, despite the generally low biomass and productivity levels in this region.     

Inorganic and organic nitrogen cycling in the Southern California Bight

On the basis of mass balance calculations performed for nitrogen (N) uptake experiments in the Southern California Bight (SCB), it has been suggested that a significant portion of dissolved inorganic N (DIN) uptake results in the production of dissolved organic N (DON). To investigate this process, the fate of ammonium (NH₄⁺) and nitrate (NO₃⁻) uptake was quantified within the euphotic zone at three coastal stations in the SCB using ¹⁵N tracer techniques. Several trends in the fate of DIN and the production of DON were observed. First, production of particulate N (PN), from both NH₄⁺ and NO₃⁻, was quantitatively more important in near surface waters, while DON release dominated within the nitracline. Second, the percentage of gross N uptake released as DON was generally higher when NO₃⁻, rather than NH₄⁺, was the substrate. Third, the percentage of N released as DON was higher at night, relative to the day. Fourth, rates of DON release were significantly correlated to NH₄⁺ regeneration, suggesting that similar mechanisms are responsible for both processes—presumably grazing. The results of this study indicate that the DON pool is a sink for DIN uptake on the time scale of hours. One implication of this finding is that new production estimates based on ¹⁵NO₃⁻ uptake rates will likely underestimate particle flux out of the surface layer because the rate of NO₃⁻ uptake is underestimated due to loss of DO¹⁵N during the incubation. On time scales of months to years, however, the N that is taken up as NO₃⁻ and released as DON will likely contribute to export flux via incorporation of the dissolved phase during seasonal mixing into sinking particles or transport. The export of DON on these time scales argues for the use of gross uptake rates to calculate f-ratios.     

The effects of light and nutrients on rates of ammonium transformation in a eutrophic river

Rates for nitrification, phytoplankton uptake of ammonium, and regeneration of ammonium were measured in the Delaware River as functions of irradiance and nutrient concentrations, using ¹⁵N labeling methods. Phytoplankton uptake increased and nitrification rates declined with increased light intensity. The irradiance level required for maximum uptake by phytoplankton was similar to that for maximal inhibition of nitrification (about 300μEm⁻² s⁻¹). Daily, water-column averaged rates, calculated by integration of the observed rate-intensity relationships, indicate that light plays a key role in regulating the balance between oxidation of NH₄⁺ by bacteria and assimilation by phytoplankton in the Delaware. The results show that uptake of ammonium by phytoplankton in the dark may exceed uptake in the light in optically thick systems.     

Wintertime nutrients in the North Atlantic—new approaches and implications for new production estimates

Observations of wintertime nutrient concentrations in surface waters are scarce in the temperate and subarctic North Atlantic Ocean. Three new methods of their estimation from spring or early summer observations are described and evaluated. The methods make use of a priori knowledge of the vertical distribution of oxygen saturation and empirical relationships between nutrient concentrations and oxygen saturation. A south–north increase in surface water winter nutrient concentration is observed. Winter nitrate concentrations range from very low levels of about 0.5 μmol dm⁻³ at 33°N to about 13.5 μmol dm⁻³ at 60°N. Previous estimates of winter nitrate concentrations have been overestimates by up to 50%. At the Biotrans Site (47°N, 20°W), a typical station in the temperate Northeast Atlantic, a mean winter nitrate concentration of 8 μmol dm⁻³ is estimated, compared to recently published values between 11 and 12.5 μmol dm⁻³. It is shown that most of the difference is due to a contribution of remineralised nitrate that had not been recognized in previous winter nutrient estimates. Mesoscale variation of wintertime nitrate concentrations at Biotrans are moderate (less than ±15% of the regional mean value of about 8 μmol dm⁻³). Interannual variation of the regional mean is small, too. In the available dataset, there was only 1 year with a significantly lower regional mean winter nitrate concentration (7 μmol dm⁻³), presumably due to restricted deep mixing during an atypically warm winter. The significance of winter nitrate estimates for the assessment of spring-bloom new production and the interpretation of bloom dynamics is evaluated. Applying estimates of wintertime nitrate concentrations of this study, it is found that pre-bloom new production (0.275 mol N m⁻²) at Biotrans almost equals spring-bloom new production (0.3 mol N m⁻²). Using previous estimates of wintertime nitrate yields unrealistically high estimates of pre-bloom new production (1.21–1.79 mol N m⁻²) which are inconsistent with observed levels of primary production and the seasonal development of biomass.     

The Southern Antarctic Circumpolar Current Front: physical and biological coupling at South Georgia

The coupling of physics and biology was examined along a 160 km long transect running out from the north coast of South Georgia Island and crossing the Southern Antarctic Circumpolar Current Front (SACCF) during late December 2000. Surface and near surface potential TS properties indicated the presence of three water types: a near-shore group of stations characterised by water which became progressively warmer and fresher closer to South Georgia, an offshore grouping in which sea surface temperatures and those at the winter water level were relatively warm (1.8°C and 0.5°C, respectively), and a third in which surface and winter water temperatures were cooler and reflected the presence of the SACCF. The transect bisected the SACCF twice, revealing that it was flowing in opposite directions, north-westward closest to South Georgia and south-eastwards at its furthest point from the island. The innermost limb was a narrow intense feature located just off the shelf break in 2000–3500 m of water and in which rapid surface baroclinic velocities (up to 35 cm s⁻¹) were encountered. Offshore in the outermost limb, shown subsequently to be a mesoscale eddy that had meandered south from the retroflected limb of the SACCF, flow was broader and slower with peak velocities around 20 cm s⁻¹. Chlorophyll a biomass was generally low (<1 mg m⁻³) over much of the transect but increased dramatically in the region of the innermost limb of the SACCF, where a deepening of the surface mixed layer was coincident with a subsurface chlorophyll maximum (7.4 mg m⁻³) and elevated concentrations down to 100 m. The bloom was coincident with depleted nutrient concentrations, particularly silicate, nitrate and phosphate, and although ammonium concentrations were locally depleted the bloom lay within an elevated band (up to 1.5 mmol m⁻³) associated with the frontal jet. Increased zooplankton abundance, higher copepod body carbon mass and egg production rates all showed a strong spatial integrity with the front. The population structure of the copepods Calanoides acutus and Rhincalanus gigas at stations within the front suggested that rather than simply resulting from entrainment and concentration within the jet, increased copepod abundance was the result of development in situ. Estimates of bloom duration, based on silicate and carbon budget calculations, set the likely duration between 82 and 122 d, a figure supported by the development schedule of the two copepod species. Given this timescale, model outputs from FRAM and OCCAM indicated that particles that occurred on the north side of South Georgia in December would have been in the central-southern Scotia Sea 2–3 months earlier, probably in sea ice affected regions.     

New production in the subarctic Pacific Ocean: Net changes in nitrate concentrations, rates of nitrate assimilation and accumulation of particulate nitrogen

Changes in water column nitrate and particulate nitrogen (PN) concentrations and rates of nitrate assimilation at 50°N 145°W were measured over a four-month interval for 1984, 1987 and 1988. Rates of nitrate depletion in the upper 80m of the water column averaged 12mg N m⁻²d⁻¹, but most of the net depletion occurred during May when rates were high (75mg N m⁻²d⁻¹) compared to later in the year. Particulate nitrogen (collected on GF/F filters) increased 2- to 3-fold during the month of May and accounted for 30–60% of the net nitrate depletion for May. Mean rates of PN accumulation for the 4-month intervals were 2.4mg N m⁻²d⁻¹ and accounted for about 20% of the net nitrate depletion. Rates of nitrate assimilation (measured in incubation bottles with ¹⁵N) averaged 45.0±4.5mg N m⁻²d⁻¹ (mean±SD), and appeared to decrease between May and September. A good correspondence between in situ and incubation estimates of nitrate assimilation was found for the 4-month comparison, but not for the month of May when net changes in nitrate concentrations were greatest. Vertical and horizontal inputs of nitrate are about the same order of magnitude as biological removal, thus the high inout of nitrate into the euphotic zone contributed to the continuously high nitrate concentrations in this region. Seasonal changes in nitrate and PN were significant and need to be considered in comparisons of new and export production.     

Benthic metabolism and the fate of dissolved inorganic nitrogen in intertidal sediments

We determined patterns of benthic metabolism and examined the relative importance of denitrification (DNF) and dissimilatory nitrate reduction to ammonium (DNRA) as sinks for nitrate (NO₃⁻) in intertidal sediments in the presence and absence of benthic microalgal (BMA) activity. By influencing the activity of BMA, light regulated the metabolic status of the sediments, and, in turn, exerted strong control on sediment nitrogen dynamics and the fate of inorganic nitrogen. A pulsed addition of ¹⁵N-labeled NO₃⁻ tracked the effect and fate of dissolved inorganic nitrogen (DIN) in the system. Under illuminated conditions, BMA communities influenced benthic fluxes directly, via DIN uptake, and indirectly, by altering the oxygen penetration depth. Under dark hypoxic and anoxic conditions, the fate of water column NO₃⁻ was determined largely by three competing dissimilatory reductive processes; DNF, DNRA, and, on one occasion, anaerobic ammonium oxidation (anammox). Mass balance of the added ¹⁵N tracer illustrated that DNF accounted for a maximum of 48.2% of the ¹⁵NO₃⁻ reduced while DNRA (a minimum of 11.4%) and anammox (a minimum of 2.2%) accounted for much less. A slurry experiment was employed to further examine the partitioning between DNF and DNRA. High sulfide concentrations negatively impacted rates of both processes, while high DOC:NO₃⁻ ratios favored DNRA over DNF.     

Eutrophication and the rate of net nitrification in a coastal marine ecosystem

Rates of net nitrification were calculated for four large (13 m³) estuarine-based microcosms that had been subjected to inorganic nutrient enrichment. Calculated rates were based on two years of weekly nitrate and nitrite measurements and ranged from a maximum of 0·55 μmol NO₂₊₃⁻ produced l⁻¹ day⁻¹ in the control tank (no enrichment) to over 13 μmol NO₂₊₃⁻ produced l⁻¹ day⁻¹ in the most enriched tank (receiving 18·6 μmol NH₄ l⁻¹ day⁻¹). Almost all NO₂₊₃⁻ production was pelagic, little was benthic. Net NO₃⁻ production or net NO₂⁻ production dominated the net nitrification rates during different seasons. Good correlations were found between various oxidation rates and substrate concentrations. The calculated net nitrite production rates were 10 to 1000 times higher than previously reported rates for open ocean systems, demonstrating the potential importance of nitrification to estuarine systems.     

Plankton dynamics and carbon flux in an area of upwelling off the coast of Morocco

A carbon flux study was carried out off the coast of Morocco, at 31°N, in a region characterized by the presence of a persistent cyclonic eddy. Two short-term (4 and 3 day) deployments of free-floating sediment traps were combined with water column sampling and rate process measurements as the ship followed the traps. For a period of 36 h between trap deployments, a hydrographic section was run along 31°30'N as part of a larger scale survey being carried out simultaneously on the R.V. A. von Humboldt. The first trap deployment was near the eastern margin of the eddy and the traps moved to the north and west in a frontal jet associated with its northern boundary. After the second deployment, which was at the recovery point of the first, the traps moved to the west and then to the southwest. Throughout the study, chlorophyll concentrations varied between 27 and 125 mg m⁻² (0–100 m), with highest concentrations in the upwelled water nearest the coast and in upwelled water generated within the cyclonic eddy. Particulate organic carbon (POC) and particulate organic nitrogen (PON) concentrations were relatively uniform (13.6±1.8 and 1.63±28 g m⁻² with phytoplankton carbon accounting for 16–85% of total POC. Bacterial carbon was 5% of total POC and mesozooplankton carbon concentrations were equivalent to 9% of total POC. Microzooplankton biomass was not assessed but POC:PON ratios in the water column were often high, suggesting there was sometimes a large detrital component in the POC. Primary production rates varied between 1.0 and 2.5 g C m⁻² day⁻¹. Bacterial consumption accounted for 50% of primary production. Metabolic rates suggested that copepods were ingesting more than 0.4 g C m⁻² day⁻¹. while filtration rates suggested that ingestion of phytoplankton carbon was only 0.2 g C m⁻²day⁻¹, even when phytoplankton constituted 85% of the POC. f-ratios (based on uptake rates for ¹⁵N-nitrate and ammonia) were between 0.1 and 0.4, and excretion by mesozooplankton could account for 40% of the daily ammonium uptake by phytoplankton. HPLC pigment analysis showed that when chlorophyll biomass was high, diatoms were dominant, whereas when it was low, small prymnesiophytes, chlorophytes and diatoms were all important. The composition of the fluoresecent pigments in material in the sediment traps indicated that intact phytoplankton and copepod faecal pellets were the main sources but the relative rates of sedimentation of pigment, POC and PON for the two trapping periods did not reflect differences that were observed in the overlying water column. This was likely to be the result of spatial heterogeneity and strong horizontal currents heterogeneity and strong horizontal currents within the euphotic zone. Thus, material collected at 100 m probably did not originate in the water column immediately overlying the traps and trapping efficiencies might also have been variable.     

The production of biogenic silica in the Weddell and Scotia Seas

During the EPOS leg 2 cruise (European Polarstern Study, November 1988–January 1989), the production rate of biogenic silica in the euphotic zone was measured by the ³⁰Si method at stations in the Scotia and Weddell Seas.The highest integrated production rates were observed in the Scotia Sea (range: 11.2–20.6 mmol Si m⁻² day⁻¹), the marginal ice zone of the Weddell Sea exhibiting somewhat lower values (range: 6.0–20.0 mmol Si m⁻²day⁻¹).Our results demonstrate that as far as biogenic silica production is concerned the marginal ice zone of the Weddell Sea is considerably less productive than that of the Ross Sea. Our results also indicate that the water of the Antarctic Circumpolar Current (ACC) could be more productive in late spring and early summer than at the beginning of spring. Possible reasons for the differences among the three subsystems (Ross Sea, Weddell Sea and ACC) are discussed.     

Benthic primary production, respiration and remineralisation: in situ measurements in the soft-bottom Abra alba community of the western English Channel (North Brittany)

In situ measurements of ammonium and carbon dioxide fluxes were performed using benthic chambers at the end of spring and the end of summer in two soft-bottom Abra alba communities of the western English Channel (North Brittany): the muddy sand community (5 m, about 10% of surface irradiance) and the fine-sand community (19 m, about 1% of surface irradiance). High rates of ammonium regeneration were measured in the two communities at the end of summer (296.03±40.07 and 201.7±62.74 μmolN m⁻² h⁻¹, respectively) as well as high respiration rates (2.60±0.94 and 2.23±0.59 mmolC m⁻² h⁻¹, respectively). Significant benthic gross primary production (up to 6.11 mmolC m⁻² h⁻¹) was measured in the muddy sand community but no benthic primary production was measured in the fine-sand community. It suggests that microphytobenthic production values used in simulations previously published for these two communities were overestimated while values of community respiration were underestimated. The study confirms that this benthic system is heterotrophic and strengthens the idea that an important pelagic-benthic coupling is required for the functioning in such coastal ecosystems.     

Egg production of Calanus finmarchicus—A basin-scale study

Egg production of Calanus finmarchicus was studied during joint basin-scale surveys in April–June 2003 in the Norwegian Sea. Surveys covered the whole Norwegian Sea and were conducted from Norwegian, Icelandic and Faroese research vessels. Stations were classified as being in pre-bloom, bloom or post-bloom phase according to levels of chlorophyll a and nitrate. Individual egg production rates and population egg production rates were calculated and compared between areas. Both individual egg production rates (eggs female⁻¹ day⁻¹) and population egg production rates (eggs m⁻² day⁻¹) were significantly higher in bloom areas compared with pre-bloom and post-bloom areas. However, when integrated over an estimated duration of the three phases, the time-integrated egg production (eggs m⁻²) in most years was highest in the pre-bloom phase, and this was explained by the longer duration of this phase compared with the two other phases.     

High concentrations of exopolymeric substances in Arctic winter sea ice: implications for the polar ocean carbon cycle and cryoprotection of diatoms

Exopolymeric substances (EPS) produced by microorganisms play important roles in various aquatic, porous, and extreme environments. Only recently has their occurrence in sea ice been considered. We used macroscopic and microscopic approaches to study the content and possible ecological role of EPS in wintertime fast ice near Barrow, Alaska (71°20′ N, 156°40′ W). Using Alcian blue staining of melted ice samples, we observed high concentrations of EPS in all samples examined, ranging from 0.79 to 7.71 mg xanthan gum equivalents (XGEQV) l⁻¹. Areal conversions to carbon equivalents yielded 1.5−1.9 g C m⁻² ice in March and 3.3−4.0 g C m⁻² in May (when the ice was thicker). Although EPS did not correlate with macronutrient or pigment data, the latter analyses indicated ongoing or recent biological activity in the ice within temperature horizons of −11°C to −9°C and warmer. EPS correlated positively with bacterial abundance (although no functional relationship could be deduced) and with dissolved organic carbon (DOC) concentrations. Ratios of EPS/DOC decreased at colder temperatures within the core, arguing against physical conversion of DOC to EPS during freezing. When sea-ice segments were maintained at representative winter temperatures (−5°C,−15°C and −25°C) for 3−14 months, the total EPS content increased significantly at rates of 5−47 μg XGEQV l⁻¹ d⁻¹, similar to published rates of EPS production by diatoms. Microscopic images of ice-core sections at these very cold temperatures, using a recently developed non-invasive method, revealed diatoms sequestered in spacious brine pockets, intact autofluorescent chloroplasts in 47% of the (pennate) diatoms observed, and indications of mucus in diatom-containing pores. The high concentrations of EPS detected in these winter ice cores represent a previously unrecognized form of organic matter that may contribute significantly to polar ocean carbon cycles, not only within the ice but after springtime release into the water column. The EPS present in very high concentrations in the brine of these microhabitats appear to play important buffering and cryoprotectant roles for microorganisms, especially diatoms, against harsh winter conditions of high salinity and potential ice-crystal damage.     

Rates of nitrification and nitrate reduction in nearshore marine sediments at near ambient substrate concentrations

Nitrification and nitrate reduction were measured simultaneously by a ¹⁵N-isotope dilution technique in the top 2 cm of sandy sediments in Great South Bay, Long Island, New York. Experiments were done at three times, under three different sets of environmental conditions. Nitrification rates remained between 0.010 and 0.015 μg-at N (g dry wt)⁻¹ (24 h)⁻¹ despite decreasing temperature. Nitrate reduction ranged from 0.02 to 0.11 μg-at N (g dry wt)⁻¹ (24 h)⁻¹. Nitrate reduction exceeded nitrification in two experiments. In the third, at low temperature and apparently high oxygen levels, rates of nitrification and nitrate reduction were comparable. We conclude that there is not a constant relationship between nitrification and nitrate reduction in this environment. Attempts to measure rates of nitrification by using the inhibitor chlorate were not successful.     

Long-term trends and interannual variability of temperature in Drake Passage

Temperature data collected over the last 36 years (1969–2004) in Drake Passage are used to examine interannual temperature variation and long-term trends in the upper ocean. To reduce the effect of variation from different sampling locations and temporal variability introduced by meridional shifts in the Polar Front (PF), the data were divided into two sub-regions north (3800 temperature profiles) and south (3400) of the PF. Temperature anomalies were formed by removing a temporal mean field for each profile in each sub-region at 100 m depth intervals from the surface to 700 m. North of the PF, statistically significant warming trends of 0.02 °C yr⁻¹ were observed that were largely depth-independent between 100 and 700 m. A statistically significant cooling trend of −0.07 °C yr⁻¹ was observed at the surface south of the PF, which was smaller (−0.04 °C yr⁻¹) but still statistically significant when possible seasonal sampling biases were accounted for. The observed cooling at the surface and warming at depth is largely consistent with a poleward shift of the PF due to enhancement of westerly winds in the Southern Ocean, as recently suggested by models and observations. The observed annual temperature anomalies in the upper 400 m north of the PF and in the upper 100 m south of the PF are highly correlated to variability in sea ice, and also to climate indices of the Antarctic Oscillation and the El Niño Southern Oscillation. Variability in sea ice and temperature anomalies lag El Niño variability in the Pacific, with a phasing consistent with the observed cyclical patterns of sea ice and sea surface temperature associated with the Antarctic Circumpolar Wave or Antarctic Dipole Mode in the Southern Ocean. In contrast, the sea ice variability and temperature anomalies at all depths north of the PF and at 0–100 m depth south of the PF were primarily coincident with, or led the Antarctic Oscillation Index. No significant correlations were found with the large-scale climate variability indices in southern Drake Passage below 100 m depth, which is occupied by upper Circumpolar Deep Water (uCDW). This water mass is not formed locally, is largely isolated from the surface, and exhibits vertical and lateral homogeneity. Hence changes may be difficult to detect in the available measurements, and climate variation in the source water regions of uCDW may take a long time to reach Drake Passage.     

Ammonium uptake kinetics and interactions between nitrate and ammonium uptake inChattonella antiqua

Ammonium uptake kinetics and interactions between nitrate and ammonium uptake were examined inChattonella antiqua. After the addition of ammonium to the culture ofC. antiqua, the ammonium concentration decreased linearly with time. The ammonium uptake rate as a function of ammonium concentration followed the Michaelis-Menten equation; the maximal uptake rate was 2.0 pmol cell^–1hr^–1 and the half saturation constant, 2.2M. Although the ammonium uptake was not affected by nitrate, uptake of nitrate was rapidly (15min) suppressed by ammonium and a 50% reduction in nitrate uptake was observed at an ammonium concentration ofca. 2M.     

Temporal and spatial variability in phytoplankton ammonium and nitrate uptake in the Delaware Estuary

Phytoplankton NH₄⁺ and NO₃⁻ uptake was examined along the longitudinal salinity gradient of the Delaware Estuary over several seasonal cycles using ¹⁵N-tracer techniques. Saturated nitrogen uptake rates increased directly with water temperature and reached a maximum of 380 nmol Nl⁻¹h⁻¹ during summer. This temperature dependence was related primarily to changes in the rate of maximum chlorophyll specific uptake, which varied exponentially between 2 and 70 nmol N [μg Chl h]⁻¹ over a temperature range of 2–28°C. Despite these high uptake rates, balanced growth (C:N7:1) could be maintained over the diel light cycle only by highly efficient nitrogen uptake at low light intensities and dark uptake below the photic zone and at night (dark UPTAKE=25% maximum uptake).Ammonium fulfilled 82% of the annual phytoplankton nitrogen demand in the estuary despite dominance of NO₃⁻ in the ambient dissolved inorganic nitrogen pool. The predominance of NH₄⁺ uptake occurred because of the general suppression of NO₃⁻ assimilation at NH₄⁺ concentrations in excess of 2 μ . This suppression, however, was not as universal as has been reported for other systems, and it is suggested that the extremely high NO₃⁻ concentrations found in the estuary contribute to this pattern. Nitrate was a significant source of nitrogen only during periods of high phytoplankton production in summer, and when NH₄⁺ concentrations were low towards the end of the spring bloom.