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Three trans-Atlantic oceanographic surveys (Nova Scotia to Canary Islands) were carried out during fall 1992 and spring 1993 to describe the large-scale variability in hydrographic, chemical and biological properties of the upper water column of the subtropical gyre and adjacent waters. Significant spatial and temporal variability characterized a number of the biological pools and rate processes whereas others were relatively invariant. Systematic patterns were observed in the zonal distribution of some properties. Most notable were increases (eastward) in mixed-layer temperature and salinity, depths of the nitracline and chlorophyll- a maximum, regenerated production (NH 4 uptake) and bacterial production. Dissolved inorganic carbon (DIC) concentrations, phytoplankton biomass, mesozooplankton biomass and new production (NO 3 uptake) decreased (eastward). Bacterial biomass, primary production, and community respiration exhibited no discernible zonal distribution patterns. Seasonal variability was most evident in hydrography (cooler/fresher mixed-layer in spring), and chemistry (mixed-layer DIC concentration higher and nitracline shallower in spring) although primary production and bacterial production were significantly higher in spring than in fall. In general, seasonal variability was greater in the west than in the east; seasonality in most properties was absent west of Canary Islands (∼20°W). The distribution of autotrophs could be reasonably well explained by hydrography and nutrient structure, independent of location or season. Processes underlying the distribution of the microheterophs, however, were less clear. Heterotrophic biomass and metabolism was less variable than autotrophs and appeared to dominate the upper ocean carbon balance of the subtropical North Atlantic in both fall and spring. Geographical patterns in distribution are considered in the light of recent efforts to partition the ocean into distinct “biogeochemical provinces”.

A comparative ecology of chlorophyll, bacteria and picophytoplankton is presented for seven ecological provinces in the North Atlantic. Depth-integrated standing stocks of these biota were measured from boreal polar to subtropical gyral regions. Averaging over all sampling times and locations within each province, it appeared that the integrated biomass of bacteria did not exceed that of phytoplankton in any province. Although this biomass ratio often exceeded unity in surface waters of the subtropical gyral provinces, the ratio for the upper water column as a whole was lowered by the subsurface chlorophyll layer. Bacteria and picophytoplankton, as the potential food resource of micrograzers, appeared to complement each other such that their total biomass did not vary much more than 2-fold amongst the seven provinces. Characteristic parameters of the biotic depth profiles, namely surface concentrations, integrated stocks and depth of maximum, were used to cluster the provinces. The original classification of provinces based on surface chlorophyll fields and characteristic regional physics was reinforced by the inclusion of bacteria and picophytoplankton.

Natural 15 N / 14 N ( δ 15 N) was determined in suspended particulate matter (SPM) and plankton tow samples (20- and 233-μm mesh nets) during extensive surveys along E–W transects from Halifax, Nova Scotia, to the Canary Islands in the fall of 1992 and the spring of 1993. This is the first study reporting δ 15 N measurements along with NO − 3 and NH + 4 uptake rates in the North Atlantic. During both cruises, the δ 15 N of SPM and net-plankton was lower in oligotrophic waters compared with values in eutrophic areas. The lack of correlation between the euphotic zone integrated δ 15 N SPM and NH + 4 concentration and NH + 4 uptake rate at both times suggested that the contrast between regions could not be explained by regenerated production, even though NH + 4 was the preferred N substrate in most regions. In fact, based on a correlation analysis, this study suggested that isotope fractionation during nitrate uptake and the residence time of N in SPM ( τ ) were the two major factors controlling the δ 15 N SPM in 1992. During the fall of 1992, the δ 15 N SPM of the oligotrophic region ( τ of 100–170 d) was consistent with the accumulated product equation in closed systems and a δ 15 N of new nitrate of 4–5‰. In contrast, in eutrophic regions ( τ of 7–40 d), the δ 15 N SPM was consistent with an open system equation. Isotope fractionation ( e ) for growth on nitrate was estimated to be 2.4‰ in the upwelling off Morocco. In 1993, no correlations were observed between δ 15 N SPM and nitrate dynamics, probably because of horizontal advection. Based on the low τ of 5–30 d, an open system equation was used with nitrate as the reactant, and it suggested variations of 2–3‰ in either the δ 15 N NO3 or e .

During the 1989 spring bloom in the western North Atlantic, we estimated the biomass and productivity of bacteria and phytoplankton at two sites (40 and 45°N) representing different water masses. At 40°N, almost all of the phytoplankton carbon could be accounted for by photosynthetic nanoplankton and picoplankton; in contrast, at 45°N, only about half was thus accounted, implying a substantial contribution by photosynthetic microplankton. At both sites, bacterial abundance was quite high (up to 2 × 109 cells l−1), and the rates of bacterial production assessed by incorporation of [3H]thymidine (up to 8 pmol l−1 h−1) and [3H]leucine (up to 240 pmol l−1h−1) were significant. Specific growth rates of bacteria based on [3H]thymidine incorporation were 0.08–0.25 day−1. Taken together, our measurements and assumptions implied a demand for primary production in the order of 16–36% over the euphotic zone or 24–78% over the upper 100 m in the water column. We conclude that ultraphytoplankton and bacteria played significant roles in the flux of carbon during the 1989 North Atlantic spring bloom.

An investigation of the spring bloom was carried out in the western North Atlantic (40–50°W) as one component of the multi-nation Joint Global Ocean Flux Study (JGOFS) North Atlantic Bloom Experiment (NABE). The cruise track included an extended hydrographic section from 32 to 47°N and process studies at two week-long time-series stations at 40 and 45°N. Biological and chemical data collected along the transect indicated that the time-series stations were located in regions where the spring bloom was well developed; algal biomass was high and surface nutrient concentrations were reduced from maximum wintertime levels. Despite similarities in the vertical structure and magnitude of phytoplankton biomass and productivity, the two stations clearly differed in physical, chemical and other biological characteristics. Detailed depth profiles of the major autotrophic and heterotrophic microplankton groups (bacteria, phytoplankton, microzooplankton) revealed a strong vertical coherence in distribution at both sites, with maximum concentrations in the upper 50 m being typical of the spring bloom. Ultraplankton (< 10 μm) were an important component of the primary producers at 40°N, whereas larger netplankton (diatoms, dinoflagellates) were more important at 45°N. Silicate depletion was clearly evident in surface waters at 45°N, where diatoms were most abundant. Despite the relative importance of diatoms at 45°N, dinoflagellates dominated the biomass of the netplankton at both sites; however, much of this community may have been heterotrophic. Bacterial biomass and production were high at both stations relative to phytoplankton levels, particularly at 45°N, and may have contributed to the unexpectedly high residual ammonium concentrations observed below the chlorophyll maximum layer at both stations. Microzooplankton grazing dominated phytoplankton losses at both stations, with consumption as high as 88% of the daily primary production. Grazing losses to the mesozooplankton, on the other hand, were small (

Euphotic zone concentrations and fluxes of nitrate, ammonium, particulate and dissolved organic nitrogen were measured over an 18 month period at the VERTEX time-series site in the oligotrophic northeast Pacific (33°N, 139°W). Variations in all N-forms were significant but not clearly linked to the temporal hydrographic cycle. Inorganic-N uptake (nitrate + ammonium) from 15 N tracer experiments generally paralleled primary productivity variations, peaking in summer; ammonium accounted for most of the uptake (∼90%) and temporal variability. Comparisons of 15 N results with estimates of autotropic N-uptake from 14 C incorporation into protein suggest that as much as 40% of the annual inorganic-N uptake was due to microheterotrophs; peak heterotrophic N-uptake occurred in summer when heterotrophic biomass was at its maximum. Nitrate uptake (new production) was less variable than ammonium uptake and annually equivalent to particulate nitrogen export from sediment traps. Dissolved organic-N (DON) represented the largest and most variable N-pool, accounting for 80–90% of the total nitrogen in the euphotic zone. Vertical DON gradients, however, were small and temporally invariant, implying little contribution to the biogenic nitrogen export from the euphotic zone.

Particulat organic carbon and nitrogen fluxes measured with free-floating seediment traps deployed six times over an 18-month period were combined with 14C primary production and 15N uptake measurements in order to obtain annual estimates of new production (NP) and associted f-ratios at the VERTEX time-series site. The site, located in the northeast Pacific Ocean at 33°N, 139°W, was occuped at trimonthly intervals to conduct water column studies and to recover/redeploy the sediment traps. The upper 250m of the VERTEX site exhibited considerable variability in some biological properties over seasonal time scales. While integrated photoautotrophic biomass remained relatively constant (0.57 ± 0.1 g C m−2) during the 18-month period, both integrated primary production and particulate ATP varied approximately 2.5-fold, ranging from 220 to 550 mg C m−2d−1 and 0.6–1.5 g C m−2, respectively. There was also considerable variation in both NP and f-ratios over the 18-month sampling period, although most of the NP variability was associated with short-term (i.e

Interzonal diel migrant plankton and nekton obtain organic carbon by feeding at night above the main pycnoline of subtropical and tropical oceans, and respire part of it by day in the interior of the ocean below the pycnocline. Using data from seven oceanic stations, and conservative models to compute respiration at depth, we show that this flux of respiratory carbon ranged from 20 to 430 mg C m−2 d−1 or 13–58% of computed particulate sinking flux across the pycnocline. If this flux occurs consistently between 50°N and 50°S, it will add about 5–20% (depending on method of calculation) to current estimates of global sinking flux of organic carbon across the pycnocline.

At a station (NFLUX), occupied for 10 days in the northern Sargasso Sea, we assembled data to test the hypothesis that diel migrant biota in the open ocean induce a downward flux of dissolved inorganic nitrogen that is significant in relation to the passive flux under gravity of particulate organic nitrogen at the base on the photic zone. We founf taht NH4+ excretion rates by day at depths below the photic zone were approximately 8% of particulate nitrogen flux obtained by a sediment trap array at the same depth. This figure is in the lower part of the range of values calculated previously for 10 published station data sets; we attribute this to the quite high rate of passive flux relative to primary production at the NFLUX station.

In Baffin Bay, phytoplankton populations from the 1% light level were susceptible to photoinhibition whereas populations from the 50% light level were not. Both the initial slope of the light saturation curve and the assimilation number were smaller for the deep samples than for the shallow ones. The optimal irradiance for photosynthesis by the near-surface populations was comparable to the maximum irradiance to which they might be exposed in situ , but that for the populations near the bottom of the photic zone was higher than the populations could experience in situ . The general magnitudes of the photosynthesis parameters for these late summer arctic populations fell towards the low end of the range observed in temperate waters.

Properties of the light saturation curve of photosynthesis and ribulose-1,5-bisphosphate carboxylase (RuBPC) activity are shown to change qualitatively in a natural population of marine phytoplankton during a spring bloom. Evidence is presented to show that these changes constitute photoadapative responses to increasing irradiance. As irradiance increased during the bloom, both the level of light-saturated photosynthesis ( P m ) and the initial slope of the light saturation curve ( α = photosynthetic efficiency) increased whether those parameters were normalized to chlorophyll a concentration ( P m b , α b ) or to cell numbers ( P m c , α c ). The magnitudes of these changes were such that I k (= P m / α , the photoadaptation parameter) did not change, but I m , the light intensity at which photosynthesis becomes saturated, increased. RuBPC activity, both chlorophyll a (RuBPC b ) and cell number normalized (RuBPC c ), also increased during the bloom. We suggest that these adaptations were achieved by simultaneously increasing the number of photosynthetic units, proportionately decreasing the photosynthetic unit size, and increasing both the concentrations of the enzymes of the dark reactions and possibly also of photosynthetic electron transport components. We also observed diminished levels of photoinhibition in the high light adapted cells late in the bloom and have suggested that this was a consequence of the same suite of physiological changes. In situ carbon fixation per cell increased during the bloom whereas no change occurred in this parameter when normalized to chlorophyll a concentration. Although these photoadaptive responses thus permitted carbon to be fixed in situ more rapidly per cell, at a constant efficiency with respect to investment of energy in the photosynthetic apparatus, they did not result in a change in growth rate. Based on consideratios of the role of time scale in physiological adaptation, however, it is suggested that the observed alterations in photosynthesis with increasing irradiance might permit a cell to more rapidly fill an energy quota for division, possibly an advantage in a mixing environment in which energy is patchily distributed, both spatially and temporalyy. Phosphoenolpyruvate carboxylase activity when normalized to chlorophyll a (PEPC b ) did not change during the bloom while chlorophyll a normalized dark carbon fixation decreased sharply and was quantitatively small compared to PEPC b . On this basis and considering that RuBPC b increased during the bloom, it is suggested that, although PEPC may be involved in dark carbon fixation, its most important quantitative role is probably an indirect one in light dependent photosynthesis. We have also considered the relevance of laboratory results on photoadaptation to interpretations of field studies and have suggested that batch culture studies must be treated with caution but that turbidistat and semi-continuous methods provide reasonable simulations of natural conditions.

A comparison of nitrogen assimilation rates based on uptake of 15 NH 4 + and incorporation of 14 CO 2 into protein at three stations near the Hawaiian Islands indicates that heterotrophic processes accounted for at least 50 to 75% of the NH 4 + uptake. These results suggest that bacteria may compete effectively with phytoplankton for inorganic nutrients in the sea.

Urea concentrations, uptake, and excretion were measured at various locations in northern Baffin Bay and surrounding waters during the summer of 1980. Concentrations were variable ( 2.00 mg-at. N m−3) but followed patterns of decreasing concentration with depth in the euphotic zone and with distance from land. Urea accounted for > 50% of the total dissolved nitrogen in the upper mixed layer at most stations. Urea uptake rates showed generally the same distributional patterns as did concentrations and on the average accounted for 32% of the total nitrogen (NO3− + NH4+ + urea) productivity in the eupholic zone. Ammonium, and frequently NO3−, were utilized in preference to urea. Dual isotope (14C and 15N-urea) labelling experiments suggested that most urea-C was respired as CO2 while 50 to 80% of the urea-N was incorporated by the phytoplankton. Excretion measurements suggested that the four dominant macrozooplankton species (Calanus hyperboreus, C. finmarchicus, C. glacialis, and Metridia sp.) supplied only −3% of the urea-N but –40% of the NH4+-N requirements of the primary producers.