Showing papers by "Woods Hole Oceanographic Institution published in 1987"

Calibration of unsaturation patterns in long-chain ketone compositions for palaeotemperature assessment

Abstract: A series of long-chain (C37, C38, C39), primarily di- and tri-unsaturated methyl and ethyl ketones, first identified in sediments from Walvis Ridge off West Africa and from the Black Sea1, has been found in marine sediments throughout the world2. The marine coccolithophorid Emiliania huxleyi and members of the class Prymnesiophyceae are now the recognized sources of these compounds3,4. Experiments with laboratory cultures of algae showed the degree of unsaturation in the ketone series biosynthesized depends on growth temperature2,5, a physiological response observed for classical membrane lipids6. Brassell and co-workers2,7 thus proposed that systematic fluctuations in the unsaturation of these alkenones noted down-core in sediments from the Kane Gap region of the north-east tropical Atlantic Ocean and correlated with glacial-interglacial cycles provide an organic geochemical measure of past sea-surface water temperatures. Using laboratory cultures of E. huxleyi, we have calibrated changes in the unsaturation pattern of the long-chain ketone series versus growth temperature. The calibration curve is linear and accurately predicts unsaturation patterns observed in natural particulate materials collected from oceanic waters of known temperature. We present evidence supporting the proposed palaeotemperature hypothesis2,7 and suggesting absolute 'sea-surface temperatures' for a given oceanic location can be estimated from an analysis of long-chain ketone compositions preserved in glacial and interglacial horizons of deep-sea sediment cores.

North Atlantic thermohaline circulation during the past 20,000 years linked to high-latitude surface temperature

Abstract: During a surface cooling event 10,000 to 12,000 years ago, higher Cd/Ca and lower 13C/12C ratios are observed in benthic foraminifera shells from rapidly accumulating western North Atlantic sediments. Data from sediment cores show that marked nutrient depletion of intermediate waters occurs in association with reduced glacial North Atlantic Deep Water flux. It is proposed that cold high-latitude sea surface temperatures enhance intermediate-water formation at the expense of deep-water formation.

Regulation of gross growth efficiency and ammonium regeneration in bacteria by substrate C: N ratio1

Abstract: Natural assemblages of marine bacteria were cultured on combinations of C and N sources (amino acids, glucose, and NH,‘) to span a range of substrate C: N ratios from 1.5 : 1 to 10 : 1. Catabolic metabolism of the N component of amino acid substrates led to NH,+ regeneration during exponential growth. The efficiency of this regeneration (RN) and also of the carbon gross growth efficiency (GGE) generally was independent of the sources of C and N, but increased as the C : N ratio of the substrate (C : NJ decrcascd relative to the C : N ratio of the bacterial biomass (C : NJ. The clemental chemical composition (C : N: P ratio) of the bacterial biomass was relatively invariant at about 45 : 9 : 1 and the gross growth efficiency varied from a threshold value of about 40-50% at C : Ns > 6 : 1 up to 94% when C : N, was 1.5 : 1. Hence, R, varied from 00/o when C : N, was 10: 1 up to 86% when C: N, was 1.5 : 1. Inorganic sources of both N and P were taken up only in stoichiometric quantities during this phase of growth. Regeneration of NH,+ during the stationary phase as well as of POd3- occurred, most likely due to endogenous metabolism or cell death, but the magnitude of this regeneration seemed to increase greatly only when C: N, was ~6 : 1. Considering that amino acids frequently do not provide all of the N required and that carbohydrates often are the major C source for growth of marine bacteria, we speculate that C : Ns of available substrates in marine waters is > 10 : 1. Hence, actively growing bacteria may be inefficient remineralizers of N.

Implications of ω -oxocarboxylic acids in the remote marine atmosphere for photo-oxidation of unsaturated fatty acids

Abstract: Specific organic compounds have been used as tracers of biological and anthropogenic source inputs to the marine atmosphere1–8. This tracer approach uses the unique molecular signatures or fingerprints for various lipid compound classes of marine and terrestrial plants to identify the sources of the organic substances in aerosols9. However, many of these substances are not stable and undergo transformation reactions. For example, unsaturated fatty acids, which are major constituents of marine and terrestrial plants, have not been frequently detected in remote marine aerosols3,6,7. The mechanism for the transformation of unsaturated fatty acids in the atmosphere has not been elucidated. In this paper, we report the discovery of a homologous series of ω-oxocarboxylic acids (C4–C14; C9 being maximum) in marine aerosols. We propose that these compounds are oxidation products of unsaturated fatty acids in the marine environment. The discovery of ω-oxoacids, along with additional aerosol, sea water, and rain data for mono- and di-carboxylic acids, has led us to postulate a photo-induced oxidative reaction scheme for unsaturated fatty acids in the marine atmosphere and surface seawater.

Experimental petrology of normal MORB near the Kane Fracture Zone: 22°–25° N, mid-Atlantic ridge

Abstract: Melting experiments carried out at 1-atm and at 2 kbar on mid-ocean ridge basalts dredged from the mid-Atlantic ridge near the Kane Fracture Zone (KFZ, 22° to 25° N. latitude) provide a basis for evaluating the role of crystal fractionation in generating compositional variability observed in “normal” mid-ocean ridge basalt. The 1-atm olivine-plagioclase-clinopyroxene saturation boundary for KFZ lavas defines a path in mineral projection schemes and in oxide-oxide diagrams that is displaced from the same experimentally determined boundaries in FAMOUS (Grove and Bryan 1983) and Oceanographer Fracture Zone (Walker et al. 1979) basalts. The glass margins of sparsely phyric KFZ lavas record small amounts of near surface, low pressure fractional crystallization, and their glass and bulk rock compositions are similar. An important signature of low pressure differentiation is recorded in the quenched glass margins of moderately phyric KFZ lavas compared to their bulk rock compositions, and the glass has evolved along low-pressure fractionation paths that are similar to those produced in the 1-atm experiments. Many of the lavas have retained phenocrysts in equilibrium proportions, so that their bulk rock compositions represent liquid compositions. When the effects of near-surface differentiation and crystal accumulation are removed from the Kane data set, and only liquid compositions are considered, a suite of basalt magmas can be identified that forms a trend in mineral component projection schemes parallel to the 1-atm oliv-plag-cpx multiple saturation boundary, but displaced from it toward olivine. These basalts have only olivine and plagioclase as phenocrysts, and are well removed from clinopyroxene saturation at low pressure. The compositional variation can not be generated by mixing any primary liquid composition with a low pressure liquid that has evolved along the oliv-plag-cpx multiple saturation boundary. Major and trace element models of this trend using olivine, plagioclase and clinopyroxene as fractionating phases match the compositional variability. This compositional trend is generated by fractionation at pressures greater than 2 kbar, but within the plagioclase stability field. A review of the data for other normal MORB suites from this part of the mid-Atlantic ridge reveals a similar elevated pressure fractionation signature which persists when the effects of low pressure magma mixing are removed from the data set.

Along-axis variations in seafloor spreading in the MARK area

Abstract: Recent investigations with the manned submersible Alvin and the Angus deep-towed camera sled greatly extended the known range of variations in the style of seafloor spreading along the axis of the Mid-Atlantic Ridge. Five transects of the spreading centre at intervals of10–20 km south of the Kane Fracture Zone at 24° N latitude demonstrate dramatic changes in the style and magnitude of tectonic extension, development of the neovolcanic zone, expression of hydrothermal venting, types oflithologic exposures and morphology of the median valley.

Longitudinal patterns of ecosystem processes and community structure in a subarctic river continuum

Abstract: Ecosystem processes and community structure in running waters of the boreal forests of Quebec, Canada, are strongly influenced by climate and channel geomorphology. Here we present an overview of a project examining longitudinal trends as small streams gradually coalesce into large rivers, summarizing our results in a series of budgets and predictive equations describing changes in organic carbon dynamics and community structure. There were significant trends with stream order for 70% of the 73 components, processes, and ratios examined. Of 46 independent components examined, 63% showed a significant trend with stream order. As stream size increased from 1st to 9th order there was a decrease in total carbon inputs (i.e., precipitation, throughfall, primary production, and allochthonous materials) followed by a gradual increase due to greater primary production in streams >6th order. The standing stock of carbon decreased exponentially downstream, and total carbon outputs (i.e., respiration, leaching, methane evasion, and insect emergence) increased slightly downstream. Nevertheless, some ecosystem—level processes, as well as community structure, showed equivocal trends, which were apparently due to the hierarchical scale of examination and the relative degree of physicochemical vs. biological control, of the processes and communities. The data, when placed in a watershed perspective, showed that total carbon inputs were evenly distributed by stream order throughout the 19 871—km2 Moisie River drainage network. Most carbon was stored in the small 1st to 3rd order streams, whereas the majority of organic carbon was metabolized in the 7th to 9th order rivers. Fluvial transport of organic carbon to the Gulf of St. Lawrence was nearly three times that of the measured total annual input, suggesting that inputs of dissolved organic carbon in groundwater were more important than previously expected. Ecosystem—level measurements of carbon retention and utilization also showed significant trends with stream order. The spiraling length for carbon increased exponentially from 8—15 km in small streams to 426 km in the 9th order river. There was a concomitant decrease in reach retention with stream order, while the rate coefficient of respiration and rate of downstream movement increased with order. The stream metabolism index, a measure of ecosystem efficiency, increased from 1st to 7th order, thereafter decreasing as streams became larger. These trends with stream order were related to physical gradients in channel dimensions, hydrology, riparian influences, and sunlight. We conclude that these subarctic lotic ecosystems have numerous strong relationships with stream order and that the dynamics can be described by a relatively small set of predictive equations.

Macrofaunal colonization of disturbed deep-sea environments and the structure of deep-sea benthic communities

Abstract: Trays of azoic sediment and organic additions were used to determine the responses of deep-sea species to disturbance at two sites and south of New England: Deep Ocean Station One (DOS 1) at 1800-m depth and Deep Ocen Station Two (DOS 2) at 3600-m depth. Species diversity was higher in the natural community at the deeper site. Rates of colonization were similar at the two sites, but slow relative to those in similar experiments conducted in shallow water. Trays of azoic sediment resting on the bottom for 5 years did not achieve the density of individuals or species found in the natural community. Species of polychaete worms in the families Spionidae, Capitellidae, and Sigalionidae were the most consistent colonists regardless of the kind of disturbance. Highest densities were achieved in patches of organic material. At the deeper DOS 2 site, densities of the sibling species of Capitella were 829 per m2 in the vicinity of wood blocks, and the dorvilleid polychaete, Ophryothrocha sp. A, occurred dat 1274 per m2 in natural, patchy accumulations of decomposing seaweed, Sargassum. The response to disturbance represented by the sediment trays was much more variable at DOS 2 than at DOS 1, with several taxa achieving high densities in only one tray. Pholoe anoculata, Ophelina cylindricaudata, and Hesionidae spp. were consistent colonists of the sediment trays at DOS 1. A few taxa such as Capitella spp., Myriotrochus spp. and Ophiura ljungmani increased greatly under screens. Predators excluded by the screens in these experiments may normally prevent juveniles of these species from becoming abundant. Occasional escape from predation may explain the rare instances of dense populations of these species. The responses of deep-sea populations to Sargassum, wood and azoic sediments indicate that a temporal mosaic of small-scale patches of organic enrichment and disturbance are very important in structuring deep-sea communities. The high diversity of species in the deep sea is maintained by: (1) patchiness of organic input against a background of low productivity; (2) sporadic, small-scale, discrete disturbance events occurring against a background of relative constancy; (3) the lack of barriers to dispersal among populations distributed over an enormous area.

The 20‐Hz signals of finback whales (Balaenoptera physalus)

Abstract: The 20‐Hz signals of finback whales (Balaenoptera physalus) were analyzed from more than 25 years of recordings at a variety of geographic locations on near‐surface hydrophones close to whales and on deep hydrophone systems. These signals were composed of 1‐s pulses of sinusoidal waveform with downward sweeping frequency from approximately 23 to 18 Hz at variable source levels up to 186 dB (re: 1 μPa at 1 m), usually with slightly lower levels for the pulses at the beginning and end of sequences. These ‘‘20‐Hz’’ pulses were produced in signal bouts (separated by more than 2 h) lasting as long as 32.5 h. Bouts were composed of regularly repeated pulses at intervals of 7–26 s (typically), either at one nominal pulse rate or at two alternating (doublet) pulse intervals. Signal bouts were interrupted by rests of 1–20 min at roughly 15‐min intervals and by irregular gaps lasting between 20 and 120 min. The distribution of these signals throughout the year and their temporal sequence were analyzed from the cont...

Measuring the flux and sinking speed of marine snow aggregates

Abstract: The flux and concentration of large amorphous aggregates (marine snow) were measured using a combination camera and sediment trap deployed at 380 m (390 m water depth) in the Panama Basin in April 1985. Results at this site indicate that essentially all settling particles arrive at the sea floor as components of large aggregates, with insignificant contributions of solitary fecal pellets. The observed flux of any particle divided by its concentration gives an estimate of a model parameter which can be interpreted as particle settling speed. Application of this method to marine snow indicates that, on average, larger aggregates (4–5 mm) settle more slowly (1 m day−1) than smaller aggregates (1–2.5 mm, 36 m day-1). These sinking speed estimates represent the average sinking speed of all aggregates photographed in the water column at the depth of the sediment trap. As such, these numbers may reflect the relative abundances of fast and slow-sinking aggregates rather than the absolute sinking speed of material entering the trap. Particles were observed to change positions or disappear between photographs, indicating water movement which may have been associated with the activity of grazing organisms within the sediment trap.

Hydroxide decomposition of dimethylsulfoniopropionate to form dimethylsulfide

Abstract: The kinetics of DMS production resulting from reaction of OH(-) with DMSP were investigated as a function of hydroxide concentration and temperature. The reaction was first-order with respect to DMSP and OH(-). The second order rate constant at 20+/-1 C is 0.0044/M/sec. The activation energy for this reaction is 14.4 kcal/mode. The investigation indicates that the rate of reaction of DMSP with OH(-) is very slow at the pH of seawater, suggesting that DMSP, which may be a major precursor of DMS in seawater, decomposes in the ocean by other mechanisms. A bacterium which produces DMS from DMSP quantitatively at rates many orders of magnitude higher than indicated by OH(-1) decomposition has been cultured, suggesting that enzymatic processes accelerate the production of DMS from DMSP in seawater.

Near-surface turbulence in a grid-stirred tank

Abstract: In order to elucidate the turbulent structure below a shear-free gas-liquid interface, turbulence measurements were made in a 50 cm square by 40 cm deep tank stirred by a vertically oscillating grid well below the surface, using a split-film anemometer probe rotating in a horizontal circle. This instrument is able to measure both vertical and horizontal velocity fluctuations to within 0.4 mm of the surface, from which spatial spectra and profiles of r.m.s. velocity fluctuations and integral lengthscales can be calculated. The turbulent structure is affected by the presence of the surface within a ‘surface-influenced layer’ roughly one integral scale, or ten per cent of the distance from the surface to the centre of the grid stroke, in thickness. The shapes of the spectra and profiles within the surface-influenced layer are predicted to a good first approximation by the source theory of Hunt & Graham (1978), which treats the turbulent structure as the superposition of homogeneous turbulence with an irrotational velocity field driven by a source distribution at the surface which cancels the vertical velocity fluctuations there. The magnitudes (as opposed to the shapes) of the profiles scale according to the values that would otherwise occur in the vicinity of the surface-influenced layer were the surface not present. These magnitudes are adequately predicted by the bulk relations determined by Hopfinger & Toly (1976) and Thompson & Turner (1975), with no apparent dependence on turbulent Reynolds number. There are some minor discrepancies between the measured profiles and those of Hunt & Graham. A thin layer of reduced velocity fluctuations below what would be expected from the theory was observed near the surface. Also, anisotropy in the velocity spectra at depths within the surface-influenced layer extended well into the inertial subrange, whereas the Hunt & Graham theory predicts no anisotropy at high wavenumbers.

The Ecology of Deep-Sea Hydrothermal Vent Communities

Abstract: Publisher Summary Hydrothermal vents, first discovered in 1977 at 2500m depth on the Galapagos Rift, provide an example of such an alternative ecosystem supported by chemosynthetic primary production (Jannasch and Mottl, 1985). Hydrothermal fluid pouring from the cracks, crevices, and chimneys in the sea floor supports large numbers and kinds of bacteria capable of deriving energy from reduced compounds, such as hydrogen sulfide. This life-supporting fluid is the result of sea water circulating deep within porous basalts and reacting at high temperatures with the rock from magma chambers along ridge axes, where new sea floor is being formed. Large clusters of red-plumed tube-worms emerging from 2- to 3-m-long, thick, white tubes give another-worldly appearance to the rich community of animals sustained by the bacteria. These oases contrast sharply with the surrounding relatively barren rock surfaces on the mid-ocean ridge. Hydrothermal vent populations are of particular interest since these dense populations of large, fast-growing animals flourish in the dark at high pressures and low temperatures, the usual environment of the deep sea.

Wind-driven ocean currents and ekman transport.

Abstract: Oceanographic Engineering of the Woods Hole Oceanographic Institution and the Massachusetts Institute of Technology, Woods Hole, MA 02543. Oceanographers have long sought to verify the theoretical Ekman transport relation, which predicts that a steady wind stress acting together with the Coriolis force will produce a transport of water to the right of the wind. In situ measurements of wind and ocean currents provide a detailed view of this phenomenon. By separating the wind-driven current from the measured total current and by averaging over a long record, it is found that the observed transport is consistent with theoretical Ekman transport to within about 10 percent. In this case the wind-driven transport is strongly surface trapped, with 95 percent occurring in the upper 25 meters as a result of fair summer weather.

Large proportion of marine planktonic ciliates found to contain functional chloroplasts

Abstract: Planktonic ciliates, most importantly tintinnids and oligotrichs (subclass Choreotrichia), are important components of the micro-plankton in coastal waters1. At times, the biomass of ciliates is equivalent to the biomass of larger zooplankters2. The role of ciliates as grazers has been recognized (see ref. 3 for review), but their role as chloroplast-bearing cells has not been widely appreciated4. Many planktonic oligotrichs are pigmented because they sequester chloroplasts derived from a variety of chromophytic algae5–7. We report the first quantitative data on the frequency of chloroplast retention among planktonic oligotrichs and tintinnids. In surface waters during the spring and summer, approximately 42% of the planktonic ciliates have chloroplasts. During the autumn and winter, chloroplast-retaining species are less abundant, but still constitute at least 10% of the combined tintinnid and oligotrich fauna. Water samples containing large numbers of pigmented ciliates have a high chlorophyll content8 and fix carbon9. Although it has been speculated that sequestered chloroplasts in ciliates are functional5–7, we have demonstrated photosynthesis by chloroplast-containing ciliates experimentally. The trophic position of ciliates in planktonic food webs needs to be reconsidered.

3H and 3He in the Beta Triangle: Observations of Gyre Ventilation and Oxygen Utilization Rates

Abstract: Isopycnal maps of 3H–3He age (τ) with about 100 km resolution have been obtained for a 1000-km scale area in the eastern subtropical North Atlantic. The midscale texture of the maps is consistent with isopycnal diffusivities of order 500 m−2 s−1. Analysis of the distributions within the context of advection-diffusion equations reveals that the larger scale 3H–3He age gradients observed within the area are not seriously affected by mixing on those surfaces that outcrop in the region of Ekman downwelling. Thus, isopycnal velocities can be estimated from 1/∇τ, but record only the velocity component normal to the outcrop. The velocities thus obtained agree well with geostrophic estimates, although the comparison is flawed by the fundamental mismatch in timescales between the two techniques. Backward extrapolation of the maps to zero-age outcrop positions indicate that the contribution of unventilated, older, recirculated water to the gyre flow above 600 m depth is small in this area (no more than 20%...

The effects of darkness and anaerobiosis on dinoflagellate cyst germination1,2

Abstract: The effects of light and anaerobiosis on germination of marine dinoflagellate resting cysts were examined. Germination of all species was completely inhibited during 7 weeks of anaerobic incubation, although the cysts remained viable. Light requirements were more variable. Germination rates in the light varied 20-fold between species; dark rates varied by a similar factor but were generally much lower. One species (Gonyaulax polyedra) required light to germinate, three germinated faster in the light than in the dark (Gonyaulax tamarensis, Scrippsiella sp., and Gonyaulax verior), and one germinated at comparable rates in the light and in the dark (Gonyaulax rugosum). Brief exposure to light at the beginning of the experiments made it impossible to say whether germination is possible in constant darkness. It is clear, however, that prolonged light exposure is a requirement only for G. polyedra, although it can significantly accelerate germination of the other species. Germination inhibition by darkness and anaerobiosis helps to explain the subsurface accumulation of dinoflagellate cysts in marine sediments and the persistence of benthic cyst populations at times when temperatures favor germination. These constraints are sufficiently common that many cysts will never germinate once they reach bottom sediments.

An endogenous annual clock in the toxic marine dinoflagellate Gonyaulax tamarensis

Abstract: Blooms of the toxic dinoflagellate Gonyaulax tamarensis (synonyms Protogonyaulax tamarensis1 and Alexandrium tamarense2) cause outbreaks of paralytic shellfish poisoning (PSP) in coastal waters throughout the world. In the Gulf of Maine, episodes occur between April and November, a seasonally due in part to life-cycle alternations between motile, vegetative cells and resting cysts which overwinter in bottom sediments3,4. Newly formed cysts have a mandatory 2–6 month dormancy period during which germination is not possible5, but once mature, the resting state will continue if temperatures are unfavourable5 or oxygen is unavailable6. We now report another factor controlling germination of cysts of G. tamarensis from deep coastal waters—an endogenous annual clock that can override an otherwise favourable environment for germination. Similar annual variability in germination has not been observed for cysts of this species from shallow estuaries. These results represent the first conclusive demonstration of an endogenous circannual rhythm in a marine plant. They are evolutioiiarily and ecologically significant because an endogenous annual clock can lead to the release of motile cells into deep and relatively invariant bottom waters at those times when temperature and light at the surface are suitable for growth. In shallow waters where seasonal variability is large and extends to bottom sediments, a strategy similar to that of the seeds of terrestrial plants would be more appropriate, namely a direct coupling between germination and the external environment.

Testing for Climate Change: An Application of the Two-Phase Regression Model

Abstract: A statistical test for detecting a change in the behavior of an annual temperature series is presented The test is based on the two-phase regression model By trading the hypothesized time of change as an unknown parameter, the approach allows an inference to be made about the time of change The approach also avoids a serious problem, called data-dredging, that can arise in testing for change occurring at a specified time The test is applied to a series of Southern Hemisphere temperatures, and the hypothesis of no change cannot be rejected

Factors controlling emission of dimethylsulphide from salt marshes

Abstract: Salt marshes are presently identified as systems exhibiting high area-specific sulfur emission in the form of dimethylsulfide (DMS) and H2S, with the former predominating in vegetated areas of the marshes. Attention is presently given to the distribution of DMS in salt marshes; it is found that this compound primarily arises from physiological processes in the leaves of higher plants, especially the grass species Spartina alterniflora. Uncertainties associated with DMS emission measurements are considered.

Shear instability in a highly stratified estuary

Abstract: Shear instability is found to be the principal mechanism of vertical exchange within the pycnocline of a salt wedge estuary. A field program involving high-resolution velocity and density measurements, as well as high-frequency acoustic imagery, allowed direct comparison of instantaneous Richardson number distributions to the occurrence of shear instability. The theoretical stability threshold of 0.25 is consistent with the measurements, based on estimates of gradients that contain the mean as well as fluctuations due to internal waves. An effective stability threshold based on mean gradients is found to be approximately one-third, reflecting a significant contribution of internal wave shear. The integral effect of the mixing process is to homogenize the gradients of velocity and density, producing linear profiles of these quantities across the pycnocline. A turbulent Prandtl number of unity is suggested by the vertical distributions of velocity and density during periods of active vertical mixin...

C-SALT: An investigation of the thermohaline staircase in the western tropical North Atlantic

Abstract: The Caribbean-Sheets and Layers Transect (C-SALT) field program focused on the “staircase” layering found in the main thermicline when temperature and salinity distributions are favorable for the formation of salt fingers Large- and small-scale surveys, microstructure studies and temporal monitoring of the layers were carried out in spring and autumn of 1985 in an area east of Barbados in the tropical Atlantic The staircase investigated contained about 10 well-mixed layers, 5–30 m thick, separated by thinner interfaces with sharp changes in temperature and salinity The depth range of the layers coincided with a minimum density ratio The area of strong steps was 05× 10 6 km 2 in the spring and 06 × 10 6 km 2 in the autumn Individual layers were laterally coherent over scales of 200–400 km and retained their identity for the eight months between surveys Layers became warmer, saltier and denser to the north and west, with a heat/salt density ratio of 085 This ratio is close to that expected for a vertically divergent salt finger flux This direct indication of water mass conversion provides striking evidence that salt fingers maintain the staircase and make a major contribution to mixing in the thermocline of the region

Ocean Response to a Hurricane. Part I: Observations

Abstract: The response of the ocean to hurricanes was investigated using aircraft-deployable expendable current profilers (AXCP). The goals were to observe and separate the surface wave and surface mixed layer (SML) velocities under the storms and to map the across-track and along-track velocity and temperature response in the mixed layer and thermocline. Custom instrumentation was prepared, including slower failing AXCPs, and the AXCP equipment was installed on NOAA WP-3D aircraft. Research flights were made into two 1984 hurricanes: Norbert, in the eastern Pacific off Baja California (19°N, 109°W), and Josephine, off the east coast of the United States (29°N, 72°W). Thirty-one probes were deployed in each hurricane, and about half the AXCPs provided temperature and velocity profiles. Most velocity profiles exhibited strong surface wave contributions, slablike velocities in the SML, strong shears beneath the SML, and only weak flows in the upper thermocline. Separation of the surface gravity wave velociti...

Grazing of Attached Bacteria by Heterotrophic Microflagellates

Abstract: Four species of heterotrophic microflagellates were examined for their ability to graze attached and unattached bacteria. The species tested displayed pronounced differences in their ability to graze the bacteriumPseudomonas halodurans attached to chitin particles. Two species of microflagellates (Monas andCryptobia sp.) efficiently grazed unattached bacteria but showed little or no ability to graze attached or aggregated cells. In contrast,Rhynchomonas nasuta andBodo sp. showed marked preferences for attached and aggregated bacteria and a limited ability to graze unattached cells. The density of attached bacteria was reduced by an order of magnitude due to grazing byBodo andR. nasuta, even though the density of unattached bacteria was ∼5–90× the density of attached cells. The maximum densities attained by microflagellates in the cultures were related to the density of unattached bacteria forMonas andCryptobia but not forBodo andR. nasuta. Growth of the latter two species appeared to be related to the density of attached or aggregated bacteria. Based on the results of these experiments, it is concluded that the pelagic existence of microflagellates that graze attached bacteria may be strongly linked to the distribution of suspended particles and their associated bacteria. In addition, the removal of attached bacteria by microflagellates can significantly affect the density of bacteria attached to particles in the plankton. This activity may have important implications for the controversy concerning the relative importance of attached and free-living bacteria in the plankton.