Sargasso Sea Surface Waters Research Articles

Ground-truthing the planktic foraminifer-bound nitrogen isotope paleo-proxy in the Sargasso Sea

We report the nitrogen (N) isotope ratios (δ15N) of planktic foraminifera collected from upper-ocean net tows (surface to 200 m), moored sediment traps, and core-top sediments at the Bermuda Time-series Site in the northern Sargasso Sea between 2009 and 2013. Consistent with previous measurements from low-latitude core-top sediments, the annually-averaged δ15N of organic matter bound within the shells of euphotic zone-dwelling, dinoflagellate symbiont-bearing foraminifera collected in net tows (2.3‰ on average) approximates that of shallow thermocline (∼200 m) nitrate (2.6‰), the dominant source of new N to Sargasso Sea surface waters. Deeper-dwelling foraminifer species without dinoflagellate symbionts tend to have a higher δ15N (3.6‰ on average). We observe no systematic difference between the bulk tissue and shell-bound δ15N in net tow-collected foraminifera. A decline in shell N content is observed from net tows (6.8 nmol/mg) to sediment traps (5.4 nmol/mg) and surface sediment (3.0 nmol/mg). On average, shell-bound δ15N rises from net tows (3.1‰) to sediment traps (3.7‰) but does not change further upon incorporation into the sediments (3.7‰). Together, these observations are consistent with preferential loss of shells or shell portions with lower δ15N and higher N content during sinking through the upper 500 m, followed by a non-isotope fractionating decrease in N content between sinking and burial. Time-series data from sediment traps (and to a lesser extent, surface net tows) exhibit seasonal δ15N variations, with a minimum in early spring, a maximum in late spring and a decline from summer to fall. These variations appear to arise from seasonal changes in the δ15N of total upper-ocean biomass, which are, in turn, driven by early springtime nitrate supply, subsequent nitrate drawdown, and an increase in the relative importance of ammonium recycling into the late summer and early fall. The δ15N connection between total upper ocean biomass and foraminifera indicates that foraminifer-bound δ15N records the δ15N of the annual nitrate supply in oligotrophic (e.g., subtropical) environments but will also be sensitive to the degree of nitrate consumption in high-nutrient regions and possibly to changes in upper-ocean ammonium recycling under some conditions.

Suspended marine particulate proteins in coastal and oligotrophic waters

Metaproteomic analyses were performed on suspended sediments collected in one coastal environment (Washington margin, Pacific Ocean, n=5) and two oligotrophic environments (Atlantic Ocean near BATS, n=5, and Pacific Ocean near HOTS, n=5). Using a database of 2.3 million marine proteins developed using the NCBI database, 443 unique peptides were detected from which 363 unique proteins were identified. Samples from the euphotic zone contained on average 2-3x more identifiable proteins than deeper waters (150-1500m) and these proteins were predominately from photosynthetic organisms. Diatom peptides dominate the spectra of the Washington margin while peptides from cyanobacteria, such as Synechococcus sp. dominated the spectra of both oligotrophic sites. Despite differences in the exact proteins identified at each location, there is good agreement for protein function and cellular location. Proteins in surface waters code for a variety of cellular functions including photosynthesis (24% of detected proteins), energy production (10%), membrane production (9%) and genetic coding and reading (9%), and are split 60-40 between membrane proteins and intracellular cytoplasmic proteins. Sargasso Sea surface waters contain a suite of peptides consistent with proteins involved in circadian rhythms that promote both C and N fixation at night. At depth in the Sargasso Sea, both muscle-derived myosin protein and the muscle-hydrolyzing proteases deseasin MCP-01 and metalloprotease Mcp02 from γ-proteobacteria were observed. Deeper waters contain peptides predominately sourced from γ-proteobacteria (37% of detected proteins) and α-proteobacteria (26%), although peptides from membrane and photosynthetic proteins attributable to phytoplankton were still observed (13%). Relative to surface values, detection frequencies for bacterial membrane proteins and extracellular enzymes rose from 9 to 16 and 2 to 4% respectively below the thermocline and the overall balance between membrane proteins and intracellular proteins grows to an approximate 75-25 split. Unlike the phytoplankton membrane proteins, which are detrital in nature, the bacterial protein suite at depth is consistent with living biomass.

Nitrogen isotopic response of prokaryotic and eukaryotic phytoplankton to nitrate availability in Sargasso Sea surface waters

Taxon‐specific nitrogen (N) isotope data from the summertime Sargasso Sea have previously suggested reliance of prokaryotic phytoplankton on regenerated N but a greater importance of nitrate assimilation for eukaryotic phytoplankton. To investigate this further, particles collected in the summer from ∼ 100 m at the Bermuda Atlantic Time‐series Study site were incubated in particle‐free, unamended surface seawater (from 4 m and 30 m) with measured ambient nitrate and ammonium concentrations. Preincubation and postincubation particles were sorted using flow cytometry, and the δ15N of separated prokaryotic and eukaryotic phytoplankton was analyzed. In July 2009 and 2010, nitrate was undetectable throughout the euphotic zone (the upper ∼ 100 m). The δ15N of prokaryotic biomass was initially low and remained low upon incubation, with one exception out of six. Preincubation eukaryotic δ15N was higher than or similar to prokaryotic δ15N and decreased or remained low after incubation. These data confirm the expectation that, in nitrate‐deplete water, all phytoplankton shift toward a low δ15N characteristic of recycled N assimilation. In June 2010, the euphotic zone nitrate concentration was anomalously high (∼ 1 µmol L−1), and the δ15N of all populations varied as a function of depth, with highest δ15N in the surface where nitrate was lowest. After incubation in surface water, the δ15N of all sorted phytoplankton increased, suggesting nitrate assimilation by all groups, including Prochlorococcus, the nitrate assimilatory capabilities of which have been debated. This study supports the use of the δ15N of sorted phytoplankton as an indicator of the dominant N source fueling their growth.

Assimilation of upwelled nitrate by small eukaryotes in the Sargasso Sea

Phytoplankton growth is potentially limited by the scarcity of biologically available forms of nitrogen such as nitrate and ammonium. In the subtropical ocean gyres, water column stratification impedes the upward flux of nitrate to surface waters. Phytoplankton in these waters are assumed to rely largely on ammonium and other forms of nitrogen recycled during the breakdown of organic matter. Here, we use flow cytometry to separate prokaryotic and eukaryotic phytoplankton collected from Sargasso Sea surface waters in the summers of 2008 and 2009, and to analyse their respective nitrogen isotope ratios. We show that prokaryotes have a uniformly low ratio of 15 N to 14 N, 15 N, consistent with their reliance on recycled nitrogen. In contrast, small eukaryotic phytoplankton, less than 30m in size, have a higher and more variable 15 N, with a mean value similar to that of nitrate in underlying Subtropical Mode Water. For the summertime Sargasso Sea, we estimate that small eukaryotes obtain more than half of their nitrogen from upwelled nitrate. In addition, our data support the view that sinking material derives largely from eukaryotic, not prokaryotic, phytoplankton biomass.

Phosphate and adenosine‐5'‐triphosphate uptake by cyanobacteria and heterotrophic bacteria in the Sargasso Sea

We used flow cytometry sorting to quantify assimilation of phosphate and adenosine‐59‐triphosphate (ATP) by heterotrophic bacteria and the cyanobacteria Prochlorococcus and Synechococcus during the fall of 2007 and 2008 and the spring of 2009 in the western Sargasso Sea. Phosphate and ATP uptake rates per cell were 50‐fold and 80‐fold higher, respectively, for Synechococcus than for the other two microbial groups, whereas there was no significant difference between ATP and phosphate uptake per cell by Prochlorococcus and heterotrophic bacteria. Both cyanobacterial groups had higher rates of phosphate uptake per surface area than did the heterotrophic bacteria; Synechococcus had the highest rates per cellular phosphorus (P)‐quota for ATP uptake, and Prochlorococcus usually had lower rates. Heterotrophic bacteria had the lowest uptake rates of ATP and phosphate per cellular P quota. In contrast, total uptake of phosphate and ATP per liter of seawater was dominated by heterotrophic bacteria, while uptake by Prochlorococcus and Synechococcus was less than 10% of the total, as a result of their low abundance (15% and 1.5% of total prokaryotes, respectively). Uptake rates of phosphate and ATP for heterotrophic bacteria and Prochlorococcus were less tightly coupled than for Synechococcus, and phosphate appeared to be favored over ATP for all three groups. Uptake rates of both compounds by Synechococcus were much higher than by the other microbial groups. Cyanobacteria are successful competitors for phosphate and ATP in the Sargasso Sea.

Bioturbation artifacts in zero‐age sediments

Most seafloor sediments are dated with radiocarbon, and the sediment is assumed to be zero‐age (modern) when the signal of atmospheric testing of nuclear weapons is present (Fraction modern (Fm) > 1). Using a simple mass balance, we show that even with Fm > 1, half of the planktonic foraminifera at the seafloor can be centuries old, because of bioturbation. This calculation, and data from four core sites in the western North Atlantic indicate that, first, during some part of the Little Ice Age (LIA) there may have been more Antarctic Bottom Water than today in the deep western North Atlantic. Alternatively, bioturbation may have introduced much older benthic foraminifera into surface sediments. Second, paleo‐based warming of Sargasso Sea surface waters since the LIA must lag the actual warming because of bioturbation of older and colder foraminifera.

Absorption spectral slopes and slope ratios as indicators of molecular weight, source, and photobleaching of chromophoric dissolved organic matter

A new approach for parameterizing dissolved organic matter (DOM) ultraviolet‐visible absorption spectra is presented. Two distinct spectral slope regions (275‐295 nm and 350‐400 nm) within log‐transformed absorption spectra were used to compare DOM from contrasting water types, ranging from wetlands (Great Dismal Swamp and Suwannee River) to photobleached oceanic water (Atlantic Ocean). On the basis of DOM size‐fractionation studies (ultrafiltration and gel filtration chromatography), the slope of the 275‐295‐nm region and the ratio of these slopes (SR; 275‐295‐nm slope : 350‐400‐nm slope) were related to DOM molecular weight (MW) and to photochemically induced shifts in MW. Dark aerobic microbial alteration of chromophoric DOM (CDOM) resulted in spectral slope changes opposite of those caused by photochemistry. Along an axial transect in the Delaware Estuary, large variations in SR were measured, probably due to mixing, photodegradation, and microbial alteration of CDOM as terrestrially derived DOM transited through the estuary. Further, SR varied by over a factor of 13 between DOM‐rich wetland waters and Sargasso Sea surface waters. Currently, there is no consensus on a wavelength range for log‐transformed absorption spectra. We propose that the 275‐295‐nm slope be routinely reported in future DOM studies, as it can be measured with high precision, it facilitates comparison among dissimilar water types including CDOM‐rich wetland and CDOM‐poor marine waters, and it appears to be a good proxy for DOM MW.

Daily variability of dissolved inorganic radiocarbon in Sargasso Sea surface water

Surface water samples were collected daily in June 2000 at a site in the Sargasso Sea to observe variability of Δ 14C values in dissolved inorganic carbon (DIC). Temperature, salinity, DIC concentration, alkalinity, and δ 13C and Δ 14C values of DIC were measured in the samples. Ten Δ 14C measurements averaged 81 ± 8‰ and had a range of 24‰ over the sixteen-day cruise. Δ 14C values were more variable during the latter half of the cruise. Salinity and temperature measurements in the mixed layer throughout the cruise indicate that there were changes in water mass that occurred at our site. We conclude that the daily range of DIC Δ 14C values in the surface ocean at our site is several times greater than the annual change in surface waters in the Sargasso Sea during the last two decades of the 20th century. This points to the importance of obtaining multiple measurements of the surface ocean to adequately define the true variability of DIC Δ 14C measurements.

Biological CO oxidation in the Sargasso Sea and in Vineyard Sound, Massachusetts

In situ dissolved carbon monoxide (CO) in oligotrophic waters follows a diel cycle varying from 0.3 to 0.5 nmol L™1 before dawn to 2.5 to 3 nmol L™1 in early afternoon, when photo-production of CO exceeds biological CO oxidation and other sinks. Coastal waters may contain up to 15 nmol L™1 [CO] in the daytime. Assays to measure the rate of CO bio-oxidation typically involve the addition of labeled CO to sealed samples, resulting in CO concentrations that are above ambient levels during incubation (up to 9 nmol L™1 CO). We find that biological oxidation of CO obeys first-order kinetics when incubated with up to 4 nmol L™1 [CO] in coastal water samples and up to between 4 and 10.8 nmol L™1 in oligotrophic waters. At higher [CO], kinetic behavior transitions to zero-order or saturation kinetics. CO-oxidation rate coefficients obtained in dark incubations were not representative of the entire diurnal period, as others have assumed. Biological CO-oxidation rate coefficients kco measured in dark incubations of Sargasso Sea surface water in summer were 0.020 ± 0.002 h™1 (mean ± standard deviation) and an order of magnitude greater than those measured in situ during daylight hours (0.002 ± 0.001 h™1). Dark and in situ rate coefficients in early spring were 0.006 ± 0.004 h™1 and 0.003 ± 0.001 h™1, respectively. In dark incubations of Vineyard Sound water, kco was 0.127 ± 0.038 h™1. The apparent half-saturation constant Kapp for CO ranged from 2.04 to 5.44 nmol L™1 CO in both environments.

Lead Isotopes in the Western North Atlantic: Transient Tracers of Pollutant Lead Inputs

In the early 1980s, Patterson and colleagues demonstrated that most lead in oceanic surface waters had an anthropogenic origin. Their discovery occurred during the phasing out of leaded gasoline in North America initiated in the previous decade. The corresponding decrease in anthropogenic lead emissions, verified by Pb/210Pb ratios, accounted for the systematic decline in lead concentrations in surface waters of the western Sargasso Sea. Subsequent changes in anthropogenic lead inputs to the western Sargasso Sea surface waters have been documented by measurements of lead concentrations, isotopic compositions (206Pb/207Pb,208Pb/206Pb), and Pb/210Pb ratios in precipitation and seawater for the period of 1981 to 1994. These data indicate the easterly trade winds are now the primary source of atmospheric lead in Bermuda, and they confirm that the decline of lead concentrations in the North Atlantic is associated with the phasing out of leaded gasoline in North America and western Europe over the past decade. Moreover, temporal variations in the relative contribution of industrial lead inputs from the two sides of the North Atlantic over that period can be quantified based on differences in their isotopic composition. The transient character of those isotopic signatures also allows calculations of pollutant lead penetration rates into the mixed layer and upper thermocline of the western Sargasso Sea.

Isotopic evidence of pollutant lead transport from North America to the subtropical North Atlantic gyre

Lead isotope ratios have been measured in aerosols, seawater, and marine particles collected in 1990–1992 in the subtropical northeastern Atlantic Ocean as part of the JGOFS-EUMELI program. While the atmospheric input has unradiogenic 206Pb207Pb ratios (1.158 ± 0.006), typical of the tradewinds bringing lead from European countries, all the samples collected in the water column have more radiogenic 206Pb207Pb (from 1.170 to 1.196). This demonstrates that lead at the Eumeli sites contains a dominant input from American emissions, that has been circulated across the North Atlantic by the subtropical North Atlantic gyre. Using measurements in Sargasso Sea surface waters as an estimate of the isotopic composition of this input (206Pb207Pb = 1.195 ± 0.004), we calculate a contribution of 42–57% from America in these waters. This demonstrates that American emissions still dominated lead contamination over the North Atlantic in the early 1990s, in spite of the early reduction of leaded gasoline in the USA. These results also give new evidence of the equilibrium between dissolved and particulate phases during scavenging processes.

Natural abundance-level measurement of the nitrogen isotopic composition of oceanic nitrate: an adaptation of the ammonia diffusion method

Abstract We have adapted the “ammonia diffusion” method of nitrate extraction for natural-abundance level nitrogen isotopic measurement of oceanic nitrate. The method involves: (1) sample concentration (by boiling or evaporation); (2) conversion of nitrate to ammonia using Devarda's alloy; and (3) the gas-phase diffusion of ammonia onto an acidified glass fiber disk which is sandwiched between two porous Teflon membranes. We have investigated the conditions necessary to effect complete ammonia recovery from natural seawater samples and the use of Devarda's alloy under these conditions. In addition, we have characterized the blanks in this method and designed a protocol to minimize them. Here, we report our protocol for nitrate extraction from seawater and provide an explanation of the protocol based on our method development work. To demonstrate the performance of the method, we present nitrate nitrogen isotopic data from nitrate standard additions to Sargasso Sea surface water and from several Southern Ocean depth profiles. The nitrate extraction method gives highly reproducible, complete recovery of nitrate and a standard deviation for isotopic analysis of The blanks inherent in the extraction procedure are from Devarda's alloy and seawater dissolved organic nitrogen (“DON”). The N blank of the Devarda's alloy reagent depends on brand and lot number. The Devarda's alloy which we are currently using results in a blank of ~ 0.4 nmol N per 100 ml of seawater (effectively 0.4 μM). An isotopic correction is made for this blank. For standard incubation conditions, stored Woods Hole seawater (with ~ 10 μM DON) gives a ~ 0.6 μM DON blank, while stored Sargasso Sea (with ~ 6 μm DON) surface water gives a DON blank of 0.3–0.5 μM. The DON blank appears to cause the ≤ 0.3%. difference between the measured and actual isotopic composition of nitrate added to Sargasso Sea surface water at the 5 μM nitrate level. We discuss several ways to lower the DON blank for samples in which the DON concentration is high relative to the nitrate concentration. The nitrogen isotopic data from several Southern Ocean profiles, in conjunction with the other results presented in this paper, demonstrate the consistency of the data produced by the ammonia diffusion method. The ammonia diffusion-based protocol is more reliable and allows for better precision than the nitrate reduction/ammonia distillation method ( Cline and Kaplan, 1975 ) in our hands. While the samples have an incubation time of 4 days or longer, we find that the diffusion method allows for higher throughput than the distillation method because samples can be run conveniently in large batches.

An Empirical Eigenfunction Analysis of Sea Surface Temperatures in the Western North Atlantic

The empirical orthogonal function decomposition is used to analyze time records of AVHRR sea surface temperature observations of the western North Atlantic from 32.9° to 43.6°N, 62.7° to 76.3°W. A manually declouded dataset covering the spring of 1985 is analyzed. The majority (80%) of the variance about the mean is accounted for by an empirical eigenfunction, which is identified with seasonal warming. This eigenfunction shows that the shelf water, excluding Georges Bank, warms the most rapidly; the surface water of the Gulf of Maine warms a little less rapidly and the Gulf Stream and Sargasso Sea surface water warm the least rapidly. The SST of the Gulf Stream is also shown to behave more like that at 30°N than like Sargasso Sea water immediately to its south (∼35°N). The second EOF is found to be a small correction to the general warming rate described by the first EOE The third and fourth EOFs are determined primarily by meander propagation. Observatiods with partial cloud cover from the period 1985 to 1991 are also analyzed. Again, the dominant effect is identified as seasonal warming.

Species assemblages of leptocephali in the Sargasso Sea and Florida Current

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 121:11-26 (1995) - doi:10.3354/meps121011 Species assemblages of leptocephali in the Sargasso Sea and Florida Current Miller MJ Regional assemblages of leptocephali of 5 families of shelf eels (Chlopsidae, Congridae, Moringuidae, Muraenidae and Ophichthidae) from the Sargasso Sea and Florida Current were compared with hydrographic features and adult distributions. There were 2 major patterns in the distributions of the &GT37 species of leptocephali that were collected. First, more species and greater abundances were found at or south of fronts in the Subtropical Convergence Zone (STCZ) of the Sargasso Sea, with the most diverse assemblages at stations closest to fronts in the west. Second, the smallest leptocephali of all species were located close to the Bahama Banks, in the Florida Current and in stations close to southerly fronts in the western STCZ. The most distinct discontinuities in numbers of species occurred at fronts at the boundary between southern Sargasso Sea surface water and mixed convergence zone water. Impoverished assemblages were found north of these fronts and in the eastern STCZ. Species richness was highest in the Florida Current and at the westernmost frontal station in the STCZ. Anticyclonic circulation northeast of the northern Bahamas may facilitate entrainment of leptocephali from the Bahamas and Florida Current into fronts in the STCZ, which appear to transport leptocephali eastward. The circulation patterns of the region are hypothesized to influence both the regional assemblage structure and the diversity of life history strategies that may be used by eels inhabiting the region. Leptocephali . Anguilliformes . Distribution . Species assemblages . Sargasso Sea . Florida Current . Fronts . Bahamas . Life history . Spawning . Larval transport Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 121. Publication date: May 25, 1995 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1995 Inter-Research.

Gulf Stream warm surges: Thermal anomalies resulting from cold core ring interactions

Observations from thermal satellite imagery reveal a thermal anomaly pattern in the sea surface temperature (SST) field of the Gulf Stream downstream of Cape Hatteras which is a direct result of Gulf Stream‐eddy interactions upstream of Cape Hatteras. The general sequence of events observed in the images is as follows: (1) Detrainment of Gulf Stream surface water occurs as a result of an interaction with a cold core ring; (2) downstream of the interaction the Gulf Stream's SST decreases several degrees over a few days owing to the loss of the upstream surface flow; (3) a visible “jet” of warm surface water is advected down the major axis of the Gulf Stream after the stream‐ring interaction subsides and the detrainment diminishes. The jet maintains a 1–3°C temperature difference with the surrounding Gulf Stream water as it propagates downstream. This paired cooling/warming process is labeled a “warm surge event” because the sharp leading edge of the warm jet (i.e., the surge) is easily detectable in the satellite images, while the anomalous surface cooling preceding the passage of the warm jet is difficult to observe. Vertical profiles of temperature, downstream velocity, and cross‐stream velocity all indicate that the upper 100–150 m of the Gulf Stream are significantly altered during an event. Satellite and in situ data also indicate that just downstream of the interaction there is enhanced entrainment of Sargasso Sea surface waters into the Gulf Stream to replenish the detrained Gulf Stream water.

Species assemblages of leptocephali in the Subtropical Convergence Zone of the Sargasso Sea

Species assemblages of leptocephali are described in relation to density in the Subtropical Convergence Zone (STCZ) of the Sargasso Sea. Seven transects across fronts were made during four cruises in the late winter or early spring of 1983, 1985 and 1989. About 50 species from 13 families of eels were collected at 66 stations, but fewer than 10 species were abundant in all transects. Four oceanic species, two Anguilla species and Conger oceanicus appeared to be spawning in the STCZ. Leptocephali of most species whose adults inhabit the continental shelf were consistently large in size and were more abundant at or south of fronts and in the western transects. Leptocephali of the two most common oceanic species, Nemichthys scolopaceus and Serrivomer beanii, and the most common shelf species, Ariosoma baleaticum, were also more abundant in the western transects, but were abundant at some stations on both sides of fronts. Discontinuities in the assemblages of Anguilla and most shelf species occurred at the location of fronts that formed at the northernmost extent of southern Sargasso Sea surface water (defined as sigma-t < 25.6 kg m-j). These species were rare or absent in mixed convergence zone water (defined as sigma-t 25.2-25.6) north of the fronts. Cluster analysis and ordination of assemblages at 31 night stations reflected the greater species richness and abundance in the west and in the southern water mass. Patterns of assemblage structure within and among transects suggest that convergence of surface water toward fronts in the STCZ may concentrate leptocephali close to fronts and that frontal jets may transport leptocephali eastward.

Fluorescence assessment of the maximum quantum efficiency of photosynthesis in the western North Atlantic

The maximum quantum efficiency (ø II) of phytoplankton photosystem II photochemistry was assessed using a pump and probe fluorometer on an offshore-onshore transect from the oligotrophic blue waters of the western Sargasso Sea to the eutrophic waters of lower Delaware Bay. Low values of the ratio of variable to maximum fluorescence ( F v/ F m) of dark adapted samples provided evidence for reduced quantum efficiences surface waters ( z = 10 m) of the Sargasso Sea. In contrast, higher values of F v/ F m in estuarine samples indicated near maximal ø II in lower Delaware Bay, with intermediate values of F v/ F m observed over the continental shelf. Variations of F v/ F m in surface samples (5 or 10 m depth)_were associated with changes in hydrographic features that were consistent with the hypothesis that reduced values of ø II in Sargasso Sea surface waters were due to nutrient limitation of phytoplankton photosynthetic energy conversion. In contrast, F v/ F m of samples collected from the chlorophyll maximum layer were independent of distance from shore and water mass characteristics.

Antarctic intermediate water intrusion into South Atlantic Bight shelf waters

Surface seawater samples were collected from seven stations on the coastline bordering the South Atlantic Bight and the southern Mid-Atlantic Bight in March 1986 and analysed for radiocarbon. Depletion in radiocarbon activity was observed in shelf water along the South Carolina coast at Myrtle Beach and Isle of Palms. If Sargasso Sea surface water withΔ 14C = 168‰ and Antarctic Intermediate Water (AAIW) withΔ 14C = −90‰ are two end members which supply water to this coastal region, a contribution of about 20–25% AAIW is required to produce theΔ 14C values observed.

Distribution and Identification of Luminous Bacteria from the Sargasso Sea

Vibrio fischeri and Lucibacterium harveyi constituted 75 of the 83 luminous bacteria isolated from Sargasso Sea surface waters. Photobacterium leiognathi and Photobacterium phosphoreum constituted the remainder of the isolates. Luminescent bacteria were recovered at concentrations of 1 to 63 cells per 100 ml from water samples collected at depths of 160 to 320 m. Two water samples collected at the thermocline yielded larger numbers of viable, aerobic heterotrophic and luminous bacteria. Luminescent bacteria were not recovered from surface microlayer samples. The species distribution of the luminous bacteria reflected previously recognized growth patterns; i.e., L. harveyi and V. fischeri were predominant in the upper, warm waters (only one isolate of P. phosphoreum was obtained from surface tropical waters).

The transport and budget of sterols in the western North Atlantic Ocean1,2

A general correlation exists between total free sterols, particulate organic carbon, particulate organic nitrogen, and chlorophyll a in the upper 300 m of the water column in the western North Atlantic Ocean. High values are found in shelf waters and in the subsurface chlorophyll a maximum in the Sargasso Sea, lower values at a Gulf Stream meander or ring fringe station.A diatom sterol, 24‐methylcholesta‐5,22‐dienol, appears to be produced only within the euphotic zone. Below this, consumption or removal processes dominate and the concentration of this compound decreases with depth. On the other hand, a zooplankton sterol, cholesterol, occurs throughout the water column, exhibiting maxima at the base of the mixed layer and at depths where particles tend to accumulate because of favorable chemical or density gradients.Flux calculations show that at most 0.05–0.3% of the sterols produced by phytoplankton in Sargasso Sea surface waters is deposited to the ocean floor. The sterol residence time (the average lifetime of a sterol molecule before it is metabolized) in the euphotic zone was calculated to be about 1 month, whereas the deep water residence time is 20–150 years.