JGOFS Bermuda Atlantic Time-series Study Research Articles

The spring bloom in the northwestern Sargasso Sea: spatial extent and relationship with winter mixing

Abstract The interannual variability of the spatial extent of the spring bloom in the central Sargasso Sea was quantified by remote-sensing approaches. Proxy measurements employing satellite-derived sea-surface temperature and chlorophyll a concentrations were used to estimate the amount of new production derived from inorganic nitrate supplied by convective winter mixing. Nitrate supply and new productivity were estimated from temperature–nitrate and chlorophyll a –euphotic-zone depth relationships derived from in situ measurements made at the US JGOFS Bermuda Atlantic Time-series Study (BATS) site near Bermuda (31°40′ N, 64°10′ W). Proxy estimates of springtime new production at the BATS latitude fall within the range of previous estimates from geochemical approaches, numerical modeling, and analysis of hydrographic measurements. However, estimates of new production made at BATS are poor predictors of the regional-scale new production due to large interannual differences in the spatial gradients of the controlling factors. Moreover, the BATS latitude is in a region of local maxima of meridional gradients in sea-surface temperature and thereby new production. The BATS site lies in a boundary region between the dynamic, spatially variable and productive northern Sargasso Sea and the spring-bloom-free region of the southern Sargasso. Hence, interpretations of spring bloom-forced regional scale biogeochemical processes from point observations made at BATS should be made with caution.

The spring bloom in the northwestern Sargasso Sea: spatial extent and relationship with winter mixing

The interannual variability of the spatial extent of the spring bloom in the central Sargasso Sea was quantified by remote-sensing approaches. Proxy measurements employing satellite-derived sea-surface temperature and chlorophyll a concentrations were used to estimate the amount of new production derived from inorganic nitrate supplied by convective winter mixing. Nitrate supply and new productivity were estimated from temperature–nitrate and chlorophyll a–euphotic-zone depth relationships derived from in situ measurements made at the US JGOFS Bermuda Atlantic Time-series Study (BATS) site near Bermuda (31°40′ N, 64°10′ W). Proxy estimates of springtime new production at the BATS latitude fall within the range of previous estimates from geochemical approaches, numerical modeling, and analysis of hydrographic measurements. However, estimates of new production made at BATS are poor predictors of the regional-scale new production due to large interannual differences in the spatial gradients of the controlling factors. Moreover, the BATS latitude is in a region of local maxima of meridional gradients in sea-surface temperature and thereby new production. The BATS site lies in a boundary region between the dynamic, spatially variable and productive northern Sargasso Sea and the spring-bloom-free region of the southern Sargasso. Hence, interpretations of spring bloom-forced regional scale biogeochemical processes from point observations made at BATS should be made with caution.

Biogeochemical impacts due to mesoscale eddy activity in the Sargasso Sea as measured at the Bermuda Atlantic Time-series Study (BATS)

Abstract A comparison of monthly biogeochemical measurements made from 1993 to 1995, combined with hydrography and satellite altimetry, was used to assess the impacts of nine eddy events on primary productivity and particle flux in the Sargasso Sea. Measurements of primary production, thorium-234 flux, nitrate+nitrite, and photosynthetic pigments made at the US JGOFS Bermuda Atlantic Time-series Study (BATS) site were used. During the 3 years of this study, four out of six high thorium-234 flux events occurred during the passage of an eddy. Primary production nearly as high as the spring bloom maximum was observed in two mode-water eddies (May 1993 and July 1995). The 1994 spring bloom at BATS was suppressed by the passage of an anticyclone. Distinct phytoplankton community shifts were observed in mode-water eddies, which had an increased percentage of diatoms and dinoflagellates, and in cyclones, which had an increased percentage of Synechococcus . These variations in species composition within mode-water eddies and cyclones may be associated with the ages of the sampled eddies, and/or differences in physical, chemical, and biological factors in these two distinct eddy types. In general, eddies that were 1–2 months old elicited a large biological response; eddies that were 3 months old may show a biological response and were accompanied by high thorium flux; eddies that were 4 months old or older did not show a biological response or high thorium flux. A conceptual model depicting temporal changes during eddy upwelling, maturation, and decay can explain the observations in all seven upwelling eddies present in the time-series investigated herein.

Zooplankton vertical migration and the active transport of dissolved organic and inorganic nitrogen in the Sargasso Sea

In the past decade a series of studies have shown that the active transport of inorganic carbon and nutrients by diel vertical migration of zooplankton is an important component of the biological pump. Less well known is how excretion of dissolved organic material (DOM) by migrating zooplankton enhances this export by creating a migratory DOM pump. We measured ammonia (NH 4 +) and dissolved organic nitrogen (DON) excretion by common vertically migrating zooplankton at the US JGOFS Bermuda Atlantic Time-series Study (BATS) station in the Sargasso Sea. The inclusion of DON excretion in our study builds on published research on migratory transport and excretion of NH 4 + and dissolved organic carbon (DOC) at BATS. On average, excretion of DON by migrating organisms is 32% (range=15–66%) of the total dissolved nitrogen (TDN) excreted, significantly augmenting previously documented vertical flux of inorganic nitrogen by migrant zooplankton. This process is discussed in the context of the BATS region, where TDN transport by migratory zooplankton is compared to nitrogen export via sinking of PON and to primary production. Estimates of active transport of TDN (NH 4 ++DON) by migrators averaged 13%, 18%, and 27% of the mean monthly sinking PON flux (for the period 1989–1999) at 150, 200, and 300 m, respectively. However, maximum transport of TDN by migrators was from 1.6 to 3.5 times greater than the sinking PON flux at these three depths. On average, the migratory TDN flux was a small proportion (2%) of primary production, but reached a maximum of 19%. Comparison of N excretion data from this study with C and P metabolism data from our other studies in the region indicates that molar remineralization ratios of C, N, and P by migrators differ among species and are not always at the classic Redfield ratio. This may have implications for non-Redfield cycling of nutrients in the western Sargasso Sea.

Natural diets of vertically migrating zooplankton in the Sargasso Sea

The feeding preferences of three common diel vertically migrating zooplankton were investigated from December 1999 to October 2000 at the U.S. JGOFS Bermuda Atlantic Time-Series Study (BATS) station in the Sargasso Sea. Gut content analysis of the copepods Pleuromamma xiphias (Giesbrecht) and Euchirella messinensis (Claus) and of the euphausiid Thysanopoda aequalis (Hansen) indicated that all three species fed on a wide variety of phytoplankton, zooplankton, and detrital material. Diet changes generally reflected seasonal trends in phytoplankton community structure. However, species-specific feeding preferences and differences in feeding selectivity among the three species were evident, and in general agreement with feeding habits predicted from the analysis of mouthpart morphology. The euphausiid T. aequalis fed equally on more different food types compared to both copepod species. The copepod P. xiphias consumed a diverse assemblage of phytoplankton from late winter through the summer (78–93% of gut items, by number, were phytoplankton) and based its diet more strongly on carnivorous feeding in autumn and early winter (31% and 61% of gut items were phytoplankton, respectively). E. messinensis showed the greatest feeding specialization, with a strong preference for pennate diatoms in winter and spring and for coccolithophorids during late summer and fall (constituting 67–93% of gut items by number). All three species consumed diatoms more than other phytoplankton taxa, even though diatoms form only a small fraction of the phytoplankton biomass in the Sargasso Sea. Although the majority of gut items identified were phytoplankton cells, the relative biomass contribution of these small cells may be lower than that of zooplankton and detritus. Zooplankton on which the three species primarily preyed were protozoans and crustaceans, but also included other metazoans such as chaetognaths and cnidarians. Marine snow was also an important component of the diet in all three species, with typically >50% and rarely 4 µm in diameter) found in guts were also likely consumed with marine snow. The species-specific differences in the diets of these three migrating species suggest that an individual species approach is important in determining how feeding habits affect the structure of pelagic food webs and carbon cycling in the sea. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00227-002-0815-8.

Nutrient gradients in the western North Atlantic Ocean: Relationship to microbial community structure and comparison to patterns in the Pacific Ocean

Studies of nitrogen and phosphorus dynamics in the oligotrophic surface waters of the western North Atlantic Ocean have been constrained because ambient concentrations are typically at or below the detection limits of standard colorometric methods, except during periods of deep vertical mixing. Here we report the application of high-sensitivity analytical methods—determinations of nitrate plus nitrite (N+N) by chemiluminescence and soluble reactive phosphorus (SRP) by the magnesium induced co-precipitation (MAGIC) protocol—to surface waters along a transect from the Sargasso Sea at 26°N through the Gulf Stream at 37°N, including sampling at the JGOFS Bermuda Atlantic Time-series Study (BATS) station. The results were compared with data from the BATS program, and the HOT station in the Pacific Ocean, permitting cross-ecosystem comparisons. Microbial populations were analyzed along the transect, and an attempt was made to interpret their distributions in the context of the measured nutrient concentrations. Surface concentrations of N+N and SRP during the March 1998 transect separated into 3 distinct regions, with the boundaries corresponding roughly to the locations of the BATS station (∼31°N) and the Gulf Stream (∼37°N). Although N+N and SRP co-varied, the [N+N] : [SRP] molar ratios increased systematically from ∼1 to 10 in the southern segment, remained relatively constant at ∼40–50 between 31°N and 37°N, then decreased again systematically to ratios <10 north of the Gulf Stream. Dissolved organic N (DON) and P (DOP) dominated (⩾90%) the total dissolved N (TDN) and P (TDP) pools except in the northern portion of the transect. The [DON] : [DOP] molar ratios were relatively invariant (∼30–60) across the entire transect. Heterotrophic prokaryotes (operationally defined as “bacteria”), Prochlorococcus, Synechococcus, ultra- and nanophytoplankton, cryptophytes, and coccolithophores were enumerated by flow cytometry. The abundance of bacteria was well correlated with the concentration of SRP, and that of the ultra- and nanophytoplankton was well correlated with the concentration of N+N. The only group whose concentration was correlated with temperature was Prochlorococcus, and its abundance was unrelated to the concentrations of nutrients measured at the surface. We combined our transect results with time-series measurements from the BATS site and data from select depth profiles, and contrasted these North Atlantic data sets with time-series of N and P nutrient measurements from a station in the North Pacific subtropical gyre near Hawaii [Hawaii Ocean Time-series (HOT) site]. Two prominent differences are readily observed from this comparison. The [N+N] : [SRP] molar ratios are much less than 16 : 1 during stratified periods in surface waters at the BATS site, as is the case at the HOT site year round. However, following deep winter mixing, this ratio is much higher than 16 : 1 at BATS. Also, SRP concentrations in the upper 100 m at BATS fall in the range 1–10 nM during stratified periods, which is at least one order of magnitude lower than at the HOT site. That two ecosystems with comparable rates of primary and export production would differ so dramatically in their nutrient dynamics is intriguing, and highlights the need for detailed cross ecosystem comparisons.

A time-series assessment of the nitrogen cycle at BATS

Abstract The assimilation of nitrate and ammonium in the euphotic zone of the Sargasso Sea at the US JGOFS Bermuda Atlantic Time-Series Study (BATS) site was measured from February, 1992 through April, 1994. Nitrate concentrations varied from 10 nM d−1 during the blooms in 1993 and 1994, and reaching >150 nM d−1 during 1992. Nitrate assimilation by particles passing a GF/F filter was frequently a significant component of total nitrate use regardless of light level. Ammonium assimilation was more constant than nitrate assimilation over the year and small particles rarely contributed more than a few percent to the total assimilation. Nitrate assimilation was correlated with concentration, both at the highest concentrations during the 1992 bloom and at the lower concentrations during the oligotrophic periods. Integrated nitrate assimilation in 1992 was 0.94 mol N m−2 yr−1 compared to 0.14 mol N m−2 yr−1 in 1993, and was completely dominated by assimilation during the winter bloom. New production during the winter mixing period has not been previously assessed and can be a large contribution to the yearly value. Using the traditional definition of the f-ratio, values ranged from 0 to 0.8 with no pattern for nitrate concentrations between 1 and 50 nM. The constant, low nitrate concentrations in the water column, the lack of variation in the vertical profile and the observed regeneration of nitrate during the incubations all suggest that nitrate assimilation during the oligotrophic period does not contribute to “new” production and nitrate should be considered a regenerated nutrient in the oligotrophic ocean.

Spatial variability of the CO2 sink in the Sargasso Sea

Spatial elds of the partial pressure of carbon dioxide (pCO ) were derived for the entire Sargasso Sea from three-day composite satellite images of sea-surface temperature (SST) and established empirical relationships between surface pCO and SST. Application of the retrieval method from summer of 1994 through fall of 1996 faithfully reproduced the annual cycle of surface pCO at the US JGOFS Bermuda AtlanticTime-Series Study (BATS) site, with a cross-validated root-mean-square (rms) error of 11 }14atm. Estimates of air}sea CO exchange were made using satellite-derived surface pCO and surface wind distributions from global analysis products. Present estimates of air}sea CO exchange at the BATS site agreed well with previous determinations made using in situ data sets. The Sargasso Sea o!shore of the Gulf Stream was found to be a net annual sink for CO (north of 293N at the western end of the study area, and north of &263N at the eastern end). Interannual variability in the net CO exchange was not large ((5%) when integrated over the entire study area, but signicant interannual di!erences existed in the meridional and zonal gradients of CO #ux. These were largely driven by interannual di!erences in seasonal spatial wind distribution. The BATS site was found to be in a region of the strongest spatial gradients of air}sea CO #ux. The present study illustrates the importance of spatial data sets to extrapolating point determinations of the CO #ux to regional scales. 2001 Elsevier Science Ltd. All rights reserved.

Zooplankton vertical migration and the active transport of dissolved organic and inorganic carbon in the Sargasso Sea

The least known component of the “biological pump” is the active transport of carbon and nutrients by diel vertical migration of zooplankton. We measured CO 2 respiration and dissolved organic carbon (DOC) excretion by individual species of common vertically migrating zooplankton at the US JGOFS Bermuda Atlantic Time-series Study (BATS) station. The inclusion of DOC excretion in this study builds on published research on active transport by respiration of inorganic carbon and allows a direct assessment of the role of zooplankton in the production of dissolved organic matter used in midwater microbial processes. On average, excretion of DOC makes up 24% (range=5–42%) of the total C metabolized (excreted+respired) and could represent a significant augmentation to the vertical flux that has already been documented for respiratory CO 2 flux by migrant zooplankton. Migratory fluxes were compared to other transport processes at BATS. Estimates of combined active transport of CO 2 and DOC by migrators at BATS averaged 7.8% and reached 38.6% of mean sinking POC flux at 150 m, and reached 71.4% of mean sinking POC flux at 300 m. DOC export by migrator excretion averaged 1.9% and reached 13.3% of annual DOC export by physical mixing at this site. During most of the year when deep mixing does not occur, diel migration by zooplankton could provide a supply of DOC to the deeper layers that is available for use by the microbial community. A carbon budget comparing migrant zooplankton transport to the balance of fluxes in the 300–600 m depth strata at BATS shows on average that the total migrant flux supplies 37% of the organic carbon remineralized in this layer, and that migrant DOC flux is more than 3 times the DOC flux gradient by diapycnal mixing. New estimates of active transport of both organic and inorganic carbon by migrants may help resolve observed imbalances in the C budget at BATS, but the magnitude is highly dependent on the biomass of the migrating community.

Carbon-13 constraints on the seasonal inorganic carbon budget at the BATS site in the northwestern Sargasso Sea

The seasonal budget of dissolved inorganic carbon ( C) in the mixed layer at the U.S. JGOFS Bermuda Atlantic Time-series Study (BATS) site is assessed on the basis of a 4-yr time series (1991–1994) of high-precision C, alkalinity and the 13C/ 12C ratio of C. Compared to previous studies, our budget is constrained by observed changes in 13C/ 12C , which permit calculation of the net community production. We are thus able to quantitatively separate all processes that contribute to the observed seasonal cycle in the mixed layer. Uncertainties in the C budget are determined using a Monte Carlo method. We found that net community production is mainly responsible in generating the observed C drawdown of about 26 μmol kg -1 between April and October by removing on the average −40±4 μmol kg -1 from the mixed layer. This net community production occurs in the absence of measurable nitrate and phosphate concentrations. We hypothezise that N 2 fixation in combination with vertical migration may be large enough to meet the biological nutrient demand in the mixed layer during this summer/fall period. Physical processes do not explain the observed summer/fall drawdown except when very improbably large horizontal advective velocities are assumed. Annual net community production of 2.3±0.9 mol m -2 is not well constrained by 13C/ 12C , but if tentatively extrapolated to the whole euphotic layer, it is well within the range of estimates based on a wide variety of methods. The region around BATS is computed to be a moderate sink for atmospheric CO 2 with an annual uptake of 1.8±0.5 mol m -2, with about 25% of this influx associated with the uptake of anthropogenic CO 2.

Seasonal dynamics of colored dissolved material in the Sargasso Sea

Observations from the Sargasso Sea have shown that the light attenuation spectrum is a function of both phytoplankton pigments and a detrital-like component that varies independently. Here we examine the nature and dynamics of these detrital-like variations by analyzing a time-depth series of visible and ultraviolet light absorption spectra for colored (chromophoric) dissolved materials [CDOM; ag(λ)] and detrital particulates [ad(λ)] collected at the US JGOFS Bermuda Atlantic time-series study (BATS) site. At 440nm, CDOM absorption, ag(440), made up of on an average more than one-half of the total non-water absorption coefficient, while detrital particulate absorption, ad(440), was generally a minor constituent. The vertically integrated stock of CDOM (0–140m) increased from the beginning of spring until the end of summer in 1994 and 1995, although near-surface values (⩽40m) were strongly depressed near the surface at the peak of summer. This summertime reduction of the mixed layer ag(λ) is likely due to photooxidation of CDOM to optically inactive forms. Further, values of CDOM absorption were unrelated directly to indices of total water column dissolved organic carbon (DOC) stocks measured at BATS. We hypothesize that CDOM is produced as a by-product of the microbial breakdown of DOC and is destroyed due to photooxidation. Summertime CDOM dynamics at this site can be quantified with these two processes, as mixing below the summertime mixed layer can be assumed to be small. Our results imply, for the first time in the blue ocean, a link between microbial community activity and CDOM dynamics and clearly provide an explanation for the nature of non-chlorophyll light attenuation observed in the Sargasso Sea.

A new coupled, one-dimensional biological-physical model for the upper ocean: Applications to the JGOFS Bermuda Atlantic Time-series Study (BATS) site

This paper presents a new coupled, one dimensional biological-physical model applied to the subtropical region near Bermuda. The physical component of the model, which is driven by smooth climatological forcing, successfully reproduces the long-term seasonal cycles of upper ocean temperature, salinity and boundary layer depth from Hydrostation S. The nitrogen-based biological model, which includes the effects of photoadaptation, phytoplankton aggregation, and particle remineralization in the aphotic zone, shows significant skill in capturing the major features of the annual chlorophyll field (e.g. spring bloom, deep chlorophyll maximum) and depth-integrated chlorophyll and primary production as exhibited by the U.S. JGOFS Bermuda Atlantic Time-series Study (BATS) data. The introduction of variable phytoplankton chlorophyll-to-nitrogen ratios is found to be important for simulating the subsurface chlorophyll maximum, and the model solutions show a realistic deep nitracline in the summer and a low annual average f-ratio of ∼0.21 compared to previous modeling work. The performance of the model solutions are weakest during the late summer, when the model can not supply enough nutrients to support the high production observed in the stratified near-surface waters. The coupled model has large winter production values, leading to a substantial export of organic material from the euphotic zone via downward turbulent mixing. The model predicts a total export production from the euphotic zone of 0.24 mol N m −2 year −1, approximately equally partitioned between particle sinking and suspended matter detrainment. The bulk of the export production is remineralized in the shallow aphotic zone, and only a small fraction is transported below the depth of the maximum winter mixed layer and thus contributes to “biological pump”.

Stocks and dynamics of bacterioplankton in the northwestern Sargasso Sea

We examined seasonal variations of bacterioplankton stocks and distributions in the upper 250 m in the Sargasso Sea near Hydrostation S and the U.S. JGOFS Bermuda Atlantic Time-series Study (BATS) site during 1987–1988 and 1991–1994. Mean vertical profiles of bacterial abundance, cell volume, and 3H-thymidine and 3H-leucine incorporation rates varied seasonally, and distribution patterns were correlated with physical mixing in the fall and winter. Conversion factors for 3H-thymidine and 3H-leucine incorporation were determined empirically to be 1.63 and 0.078 x 10 18 cells mol −1, respectively. Integrated bacterial biomass and production within the euphotic zone were low compared to other oceanic sites and ranged between 241–411 mg C m −2 and 11–36 mg C m −2 day −1, respectively. Seasonal variation in bacterial biomass and production was observed; however, the range of variation was less than two-fold despite a five-fold range in primary production. Bacterial biomass (BB): phytoplankton biomass (PB) ratios remained high during the summer and fall, with bacterial biomass dominating the chl a-C estimates at times, and BB:PB ratios decreased in the winter and spring due to increased phytoplankton production. Low bacterial production (BP):phytoplankton production (PP) ratios were observed for all seasons. Although BP:PP ratios were low, growth efficiencies observed in this region indicate that carbon flux through seasonal BP could account for 17-> 100% of seasonal PP. The small response of bacterial production during and after a phytoplankton bloom may indicate that the majority of dissolved organic carbon (DOC) that accumulates in post bloom conditions is of semi-labile quality, resulting in slow bacterial oxidation of DOC.

Chronic substrate limitation of silicic acid uptake rates in the western Sargasso Sea

The kinetics of Si uptake at the JGOFS Bermuda Atlantic Time-series Study (BATS) Site in the western Sargasso Sea (31°50′N, 64°10′W) were examined on nine cruises between November 1991 and November 1993. Si uptake rates were severely substrate-limited on every cruise during all seasons. Surprisingly, the observed uptake kinetics were among the most inefficient examined to date despite persistently low (<0.9 μM) ambient silicic acid concentrations ([Si(OH) 4 ]) throughout the upper 100 m. Uptake kinetics did not usually conform to the Michaelis-Menten function. Rather, rates increased approximately linearly with increasing substrate concentration to ca 5.5 μM, the highest concentration tested, on seven of the nine cruises. Half saturation constants on the remaining two cruises when some degree of hyperbolic response to increasing [Si(OH) 4] was observed were 1.6 ± 0.55 and 2.6 ± 1.5 μM (s.e.). The slopes of the linear kinetic curves ranged from 1.5 × 10 −3 to 8.3 × 10 −3 h −1 μM −1 with no clear seasonal trend. Ambient [Si(OH) 4] restricted in situ uptake rates to < 12–16% of those measured at the highest concentration employed, ca 5.5 μM. These results indicate that substrate limitation of Si uptake is both chronic and severe in this region of the Sargasso Sea. The observed severity of uptake rate limitation strongly suggests Si limitation of diatom growth rates. Comparison of estimated in situ Si uptake rates with silica dissolution rates within sediment traps deployed at the base of the euphotic zone suggest that diatom assemblages with the observed uptake kinetics would require [Si(OH) 4] greater than ca 0.6 μM to support Si uptake rates sufficient to balance losses due to dissolution. This may explain why Si(OH) 4 is never depleted to the nanomolar concentrations observed for NO −3 and HPO 4 2− at this site. However, essentially all Si(OH) 4 in excess of the threshold concentration of ca 0.6 μM is utilized by diatoms, suggesting that Si limits the yield of siliceous biomass. Limitation of the yield of diatoms by Si is consistent with the estimated mole ratio of N:Si:P supplied annually to the base of the surface layer from depth. That ratio, 20:5.2:1, demonstrates a clear deficit of Si(OH) 4 in the upward flux of those nutrients compared to Redfield proportions.

Seasonal and interannual variability of oceanic carbon dioxide species at the U.S. JGOFS Bermuda Atlantic Time-series Study (BATS) site

The seasonal and interannual dynamics of the oceanic carbon cycle and the strength of air-sea exchange of carbon dioxide are poorly known in the North Atlantic subtropical gyre. Between October 1988 and December 1993, a time series of oceanic measurements of total carbon dioxide (TCO 2), alkalinity (TA) and calculated pCO 2 was obtained at the Bermuda Atlantic Time-series Study (BATS) site (31°50′N, 64°10′W) in the Sargasso Sea. These measurements constitute the most extensive set of CO 2 species data collected in the oligotrophic North Atlantic. Seasonal changes in surface and water-column CO 2 species were ∼40–50 μmol kg −1 in TCO 2, ∼20 μmol kg −1 in TA, and ∼90–100 μatm in calculated pCO 2. These large changes were driven principally by deep convective winter mixing, temperature forcing and biological activity. TA was well correlated with salinity (with the exception of a 15–25 μmol kg −1 drawdown of TA on one cruise resulting from open-ocean calcification). TCO 2 and pCO 2 were well correlated with seasonal temperature changes (8–9°C). Other underlying processes, such as biological production, advection, gas exchange of CO 2 and vertical entrainment, were important modulators of the carbon cycle, and their importance varied seasonally. Each spring-to-summer, despite the absence of measurable nutrients in the euphotic zone, a 35–40 μmol kg −1 decrease in TCO 2 was attributed primarily to the biological uptake of TCO 2 (evaporation/precipitation balance, gas exchange, and advection were also important). An increase in TCO 2 during the fall months was associated primarily with entrainment of higher TCO 2 subsurface waters. These seasonal patterns require a reassessment of the modelling of the carbon cycle using nutrient tracers and Redfield stoichiometries. Overall, the region is a weak sink (0.22–0.83 mol C m −2 year −1) for atmospheric CO 2. Upper ocean TCO 2 levels increased between 1988 and 1993, at a rate of ∼ 1.7 μmol kg −1 year −1. This increase appears to be in response to the uptake of atmospheric CO 2 through gas exchange or natural variability of the subtropical gyre.

Seasonal patterns of ocean biogeochemistry at the U.S. JGOFS Bermuda Atlantic time-series study site

Seasonal patterns in hydrography, oxygen, nutrients, particulate carbon and nitrogen and pigments were measured on monthly cruises at the Bermuda Atlantic Time-series Study site, 80 km southeast of Bermuda. Between October 1988 and September 1990, the annual cycle was defined by the creation of 160–230 m-deep mixed layers in February of each year and a transition to strong thermal stratification in summer and fall. The 230 m mixed layer in February 1989 resulted in mixed-layer nitrate concentrations of 0.5–1.0 μmole kg −1, carbon fixation rates over 800 mg C m −2 day −1, and a phytoplankton bloom with chlorophyll concentrations over 0.4 mg m −3. Chlorophyll a, particulate organic matter, inorganic nutrients and primary production had returned to prebloom levels the following month with the exception of a chlorophyll maximum layer at 100 m. Particle fluxes at 150 m in February 1989 reached 56 mg C m −2 day −1 and 11 mg N m −2 day −1 (0.77 mmole N m −2 day −1). Estimates of new production during the bloom period calculated from changes in oxygen and nitrate profiles ranged from 100 to 240 mmoles N m −2, significantly higher than the sediment trap fluxes and approaching the measured total production rates. In spring of 1990, mixed layer depths did not exceed 160 m, nitrate was rarely detectable in the upper euphotic zone, chlorophyll a concentrations were similar to 1989, and particulate organic matter concentrations were lower. The period of elevated biomass lasted for 3 months in 1990, and phytoplankton pigment composition varied between cruises. The average rates of primary production and particle flux were higher in 1990 than those measured in the spring of 1989, despite the differences in mixed layer depth. Throughout both years, NO 3 : PO 4 ratios in the upper thermocline exceeded Redfield ratios. The maintenance of this pattern requires a net uptake of PO 4 between 150 and 250 m, a depth range usually associated with net remineralization. The exact mechanism that maintains elevated PO 4 uptake and its implication for the nutrient supply to the euphotic zone remain unknown.

Seasonal variability in primary production and particle flux in the northwestern Sargasso Sea: U.S. JGOFS Bermuda Atlantic time-series study

The relationship between primary production and sediment trap-derived downward flux of particulate organic matter was characterized over a 2 year period at the U.S. JGOFS Bermuda Atlantic Time-series Study (BATS) site to evaluate the importance of temporal variations in upper ocean biogeochemical processes. Water column-integrated primary production (∫ PP), determined once each cruise using 14C incubations ( in situ dawn-to-dusk), peaked in late winter/early spring of both 1989 and 1990. Smaller increases in ∫ PP also occured in July 1989 and October–December 1990. Annual ∫ PP was 9.2 mol C m −2 y −1 in 1989 and 12 mol C m −2 y −1 in 1990. This was higher than the 1959–1963 annual average (6.8 mol C m −2 y −1) determined at Station “S” located approximately 50 km northwest of the BATS site. Fluxes associated with sinking of total particulate mass, particulate organic carbon (POC) and particulate organic nitrogen (PON) were measured at 150, 200, 300 and 400 m using a free-floating sediment trap array generally deployed once each cruise for 72 h. Fluxes varied seasonally, and within our ability to resolve differences (i.e. monthly sampling), there was no distinguishable time offset between peaks in ∫ PP and corresponding peaks in elemental flux. Fluxes generally decreased with increasing depth, and fluxes of POC and PON were positively correlated with particulate mass flux at all depths. POC/PON (C/N) ratios at 150 m during periods of high ∫ PP were generally characteristics of live planktonic biomass. Higher C/N ratios in material collected by the deeper traps were consistent with more rapid losses of PON than POC from sinking particles. POC and PON fluxes at 150 m, nominally the base of the euphotic zone, were positively correlated with ∫ PP. The fraction of ∫ PP leaving the euphotic zone in the form of sinking particles (i.e. collected in traps) varied seasonally and was inversely proportionato ∫ PP. Surface export of organic matter estimated by sediment traps at 150m was 0.78 mol C m −2y −1 (0.10 mol N m −2y −1) in 1989 and 0.77 mol C m −2y −1 (0.11 mol N m −2y −1) in 1990.