Picoplankton community structure on the Atlantic Meridional Transect: a comparison between seasons

Samples collected from 10 depths at 25 stations in September–October 1996 and 12 depths at 28 stations in April–May 1997 on an Atlantic Meridional Transect between the British Isles and the Falkland Islands were analysed by flow cytometry to determine the numbers and biomass of four categories of picoplankton: Prochlorococcus spp, Synechococcus spp, picoeukaryotic phytoplankton and heterotrophic bacteria. The composition of the picoplankton communities confirmed earlier findings (Zubkov, Sleigh, Tarran, Burkill & Leakey, 1998) about distinctive regions along the transect and indicated that the stations should be grouped into five provinces: northern temperate, northern Atlantic gyre, equatorial, southern Atlantic gyre and southern temperate, with an intrusion of upwelling water off the coast of Mauritania between the northern Atlantic gyre and equatorial waters. Prochlorococcus was the most numerous phototrophic organism in waters of both northern and southern gyres and in the equatorial region, at concentrations in excess of 0.1×10⁶ml⁻¹; it also dominated plant biomass in the gyres, but the biomass of the larger picoeukaryotic algae equalled that of Prochlorococcus in the equatorial region; higher standing stocks of both Prochlorococcus and picoeukaryotes were present in spring than in autumn in waters of both gyres. In temperate waters at both ends of the transect the numbers and biomass of picoeukaryotes and, more locally, of Synechococcus increased, and the Synechococcus, particularly, were more numerous in spring than in autumn. There was a pronounced southward shift of the main populations of both Synechococcus and Prochlorococcus in April–May in comparison to those of September–October, associated with seasonal changes in solar radiation, the abundance of Prochlorococcus dropping sharply near the 17°C contour, while Synechococcus was still present at temperatures below 10°C. Picoeukaryotes were more tolerant of low temperatures and lower light levels, often being more abundant in samples from greater depths, where they contributed to the deep chlorophyll maximum. Heterotrophic bacterial numbers and biomass tended to be highest in those samples where phototrophic biomass was greatest, with peaks in temperate and equatorial waters, which were shifted southwards in April–May compared with September–October.     

基于光合色素化学分类和流式细胞术结合的珠江口自养微微型浮游生物种群时空分布特征

为探究珠江口海域自养微微型浮游生物种群时空分布特征及其与环境之间的关系,于2013年5～11月,运用高液相色谱(HPLC)法和流式细胞术对珠江口海域表层水体中微微型浮游生物进行测定。流式细胞计数结果显示,珠江口海域自养微微型浮游生物由聚球藻(Synechococcus, Syn)和微微型真核生物(Picoeukaryotes,PEUK)组成。聚球藻始终占据总细胞丰度的主导地位。光合色素化学分类法(Chemotaxonomy,CHEMTAX)分析表明,自养微微型浮游生物群落结构具有明显的季节性变化,春季和夏季生物量以聚球藻为主,秋季生物量以青绿藻为主。CHEMTAX分析和流式细胞计数结果的相关性分析表明,在春季和夏季Syn细胞丰度与CHEMTAX生物量(即Syn贡献chla)之间呈现极显著正相关(P<0.01),PEUK细胞丰度与CHEMTAX生物量(即PEUK贡献chla)也存在显著正相关(P<0.05);然而,在秋季则无显著性相关关系(P>0.05)。冗余分析表明,温度和营养盐浓度是影响自养微微型浮游生物群落分布与组成的重要因素。另外,盐度、透明度、悬浮颗粒物对自养...     

Heterotrophic bacterial and <Emphasis Type="Italic">Synechococcus</Emphasis> spp. Growth and mortality along the inshore-offshore in the East China Sea in summer

This study used the dilution method to examine growth and grazing rates of heterotrophic bacteria and an autotrophic picoplankton, Synechococcus spp., from 1 to 11 July 2007 in the East China Sea. The main influence of oceanographic conditions in this aquatic system was the introduction of fresh, high-nutrient water from Changjiang River and the extremely nutrient-poor, high-salinity waters of Kuroshio Water. In these experiments, deviation from linearity in the relationship between dilution factor and net growth rate was significant in a large number of cases. Growth rates for heterotrophic bacteria ranged from 0.024 to 0.24, and for Synechococcus spp. from 0.03 to 0.21 h⁻¹. Grazing rates ranged from 0.02 to 0.19 and 0.01 to 0.13 h⁻¹, respectively. The spatial variations of Synechococcus spp. production to the primary production ratio (SP/PP) were low (<5%) in high Chl a environments and increased exponentially in low Chl a environments, indicating that Synechococcus spp. contributes to a large extent to the photosynthetic biomass in the open sea, especially in the more oligotrophic Kuroshio Water. Furthermore, the results of our dilution experiments suggest that nanoflagellates largely depend on heterotrophic bacteria as an important energy source. On average, heterotrophic bacteria contributes to 76 and 59% of carbon consumed by nanoflagellates within the plume (salinity <31) and outside of it (salinity >31).     

Plankton communities of the South Atlantic anticyclonic gyre

Data collected during cruises of the Former Soviet Union (in 1963–1989) and the British Atlantic meridional transect program (in 1995–1999) were used to analyse macroscale patterns in phyto- and zooplankton biomass, size structure, species diversity, chlorophyll a, and plankton bioluminescence in the macroscale anticyclonic gyre of the South Atlantic Ocean. The spatial pattern of bioluminescence intensity was in good agreement with that of remotely sensed (CZCS) chlorophyll a, phosphate, salinity, and copepod species diversity index distributions especially in terms of geographic inclinations of the isolines, both associated with the north-westward pattern off the South equatorial current. Among the 416 copepod species recorded in samples, 51 species were noted throughout the whole gyre. On the other hand, there were a number of species found only in one of the currents. The mesozooplankton biomass size spectra (calculated in carbon units), exhibited a fairly stable slope of the curve from the eastern periphery of the gyre to its centre. The British Atlantic meridional transect program meridional transect through the western part of the gyre showed mesozooplankton size spectra in greater detail between the equator and 50° S. Although the spectra change slowly along the transect as far as 36° S, there is a general trend toward increasing slopes from the equatorial region to the oligotrophic central gyre. The calculated phyto-to-zooplankton ratio indicated that for the tropical anticyclonic gyres, the mesozooplankton carbon biomass could be represented as the exponential function of the phytoplankton carbon.     

Microbial dynamics in an oligotrophic bay of the western subtropical Pacific: Impact of short-term heavy freshwater runoff and upwelling

This two-year study investigates the possible factors that determine spatial and temporal dynamics of picoplankton (heterotrophic bacteria, autotrophic picoplankton—Synechococcus spp., Prochlorococcus, and picoeukaryotes) and nanoflagellate abundance in the subtropical Ilan Bay, Taiwan, where the inner bay is affected by freshwater run-off from the Lanyang River and the eastern outer bay by the Kuroshio Current. In the inner bay, there was more rain and freshwater discharge in 2005 than in 2004 during the warm season (>24° C, June–September). The abundance of bacteria, Synechococcus spp., Prochlorococcus, and picoeukaryotes and the percentage contributions of pigmented nanoflagellate (PNF %) were two- to eight-fold greater during this period (July in 2005) than for other sampling periods. Relatively low abundance of heterotrophic nanoflagellates (HNF) in the presence of abundant picoplankton prey suggests that top-down control determined HNF abundance in the Ilan Bay, Taiwan.     

Latitudinal and Vertical Distributions of Phytoplankton Absorption Spectra in the Central North Pacific during Spring 1994

Vertical changes of phytoplankton absorption spectra along 175°E from 48°N to 15°S were examined during spring 1994. The absorption spectra were analyzed using three different approaches; averaging the spectra within same oceanic areas, EOF analysis, and multiple regression analysis. Average spectra showed differences in five oceanic areas; subarctic, Kuroshio, subtropical surface, equatorial surface, and subtropical and equatorial subsurface areas. The distributions of the EOF mode of the variance of absorption spectra and of the pigments estimated by the multiple regression analysis indicated consistent differences of the spectra between those areas. Kuroshio water contains highest chlorophyll a concentrations and low chlorophyll-a-specific absorption spectra, and this may be caused by the package effect with large phytoplankton cell and by low concentrations of photo-protected carotenoids. Subtropical and equatorial subsurface water showed high absorption at 480 nm and indicated the effects of chlorophyll b. Absorption of the subsurface phytoplankton also showed a shift of the blue peak, possibly caused by the presence of divinyl-chlorophyll a. The consistency of the three different analytical methods indicates that the phytoplankton absorption includes significant information on pigment composition along a north-south vertical section of the central North Pacific.     

Volume transports proceeding to the Kuroshio Extension region and recirculating in the Shikoku Basin

The volume transport of the Kuroshio, the western boundary current of the North Pacific subtropical gyre, varies vigorously due to merging of disturbances propagating from the entire North Pacific. Taking into account the recirculation in the Shikoku Basin by the zonal observation line at 30°N to the west of the Izu–Ogasawara Ridge, we estimated the volume transport in the top 1,000 m layer toward the Kuroshio Extension region. The volume transport of the local recirculation gyre in the Shikoku Basin increases associated with the westward extension of the gyre, particularly in the period of the large meandering path of the Kuroshio south of Japan. Meanwhile, most of the transport variations toward the Kuroshio Extension region correspond to those of the Kuroshio transport on the continental slope south of Japan, which vary independently of those of the recirculation gyre.     

Mesoscale eddy effects on temporal variability of surface chlorophyll a in the Kuroshio Extension

We investigated the relationship between chlorophyll a (Chl-a) concentrations estimated from satellite observations and the activity of eddies in the Kuroshio Extension region. High (low) area-averaged Chl-a concentrations were frequently observed in the core of cyclonic (anticyclonic) eddies. Such relationships between Chl-a concentrations and eddy cores were not frequently observed in the southern part of the recirculation gyre, and advection of background meridional gradient of Chl-a by eddy-edge currents accounted for Chl-a spatial variability. Decadal-scale changes of Chl-a concentrations around the Kuroshio Extension were strongly affected by eddy activity and transport but not by large-scale near-surface isopycnal heaving. We also found that decadal changes of nutrient concentrations near the main stream could affect Chl-a concentrations in the southern part of the recirculation gyre via southward transport of eddies and mean flow.     

Association of picophytoplankton distribution with ENSO events in the equatorial Pacific between 145°E and 160°W

Climatological variability of picophytoplankton populations that consisted of >64% of total chlorophyll a concentrations was investigated in the equatorial Pacific. Flow cytometric analysis was conducted along the equator between 145°E and 160°W during three cruises in November–December 1999, January 2001, and January–February 2002. Those cruises were covering the La Niña (1999, 2001) and the pre-El Niño (2002) periods. According to the sea surface temperature (SST) and nitrate concentrations in the surface water, three regions were distinguished spatially, viz., the warm-water region with >28 °C SST and nitrate depletion (<0.1 μmol kg⁻¹), the upwelling region with <28 °C SST and high nitrate (>4 μmol kg⁻¹) water, and the in-between frontal zone with low nitrate (0.1–4 μmol kg⁻¹). Picophytoplankton identified as the groups of Prochlorococcus, Synechococcus and picoeukaryotes showed a distinct spatial heterogeneity in abundance corresponding to the watermass distribution. Prochlorococcus was most abundant in the warm-water region, especially in the nitrate-depleted water with >150×10³ cells ml⁻¹, Synechococcus in the frontal zone with >15×10³ cells ml⁻¹, and picoeukaryotes in the upwelling region with >8×10³ cells ml⁻¹. The warm-water region extended eastward with eastward shift of the frontal zone and the upwelling region during the pre-El Niño period. On the contrary, these regions distributed westward during the La Niña period. These climatological fluctuations of the watermass significantly influenced the distribution of picophytoplankton populations. The most abundant area of Prochlorococcus and Synechococcus extended eastward and picoeukaryotes developed westward during the pre-El Niño period. The spatial heterogeneity of each picophytoplankton group is discussed here in association with spatial variations in nitrate supply, ambient ammonium concentration, and light field.     

Radium isotopes in coastal and open ocean surface waters of the Western North Pacific

Using manganese-impregnated fiber extraction and high-efficiency gamma counting techniques, we measured the distribution of ²²⁸Ra and ²²⁶Ra in surface waters near the coast of Japan and in the western North Pacific. There is no evidence in our data that any significant amount of ²²⁸Ra is added to open ocean surface waters from the coastal waters around Tokyo Bay. High ²²⁸Ra concentrations (> 10 dpm/10³ kg), were observed along the Kuroshio Current as compared to < 2.5 dpm/10³ kg between 10° and 30°N of the central gyre, and hence the major source of ²²⁸Ra in the surface water is likely to be the East Asian continental shelf zones. A simple one-dimensional eddy diffusion and advection model is used to explain the observed decrease of ²²⁸Ra from coast to the open ocean. The model results indicate two mixing regimes across the Kuroshio Current System with apparent eddy diffusion coefficients of K_y = 4 × 10⁵ cm² s⁻¹ at distance y < 200 km from the coast, and K_y = 4 × 10⁷ cm² s⁻¹ at y > 200 km. Along 40°N where an eastward flow of the ‘Kuroshio Extension’ prevails, an advective flow of > 0.1 knot is consistent with the observation of nearly constant ²²⁸Ra along the track.The geographical distribution pattern of ²²⁸Ra is clearly different from that of atmospherically derived ²¹⁰Pb. Thus the ²²⁸Ra in surface water serves as a useful tracer that accompanies fluvially and coastally derived elements during their subsequent lateral transport toward the central gyre.     

Surface phytoplankton pigment distributions in the Atlantic Ocean: an assessment of basin scale variability between 50°N and 50°S

We present an overview of the spatial distributions of phytoplankton pigments along transects between the UK and the Falkland Islands. These studies, undertaken as a component of the UK Atlantic Meridional Transect (AMT) programme, provided the first post-launch validation data for the NASA SeaWiFS satellite. Pigment data are used to characterise basin-scale variations in phytoplankton biomass and community composition over 100° of latitude, and to compliment the definition of hydrographic oceanic provinces. A summary of the key pigment characteristics of each province is presented.Concentrations of total chlorophyll a (totCHLa = chlorophyll a, CHLa + divinyl CHLa, dvCHLa) were greatest in high latitude temperate waters (>37°N and >35°S), and in the Canary Current Upwelling system. In these regions, the total carotenoid (totCAR) budget was dominated by photosynthetic carotenoids (PSCs). High accessory pigment diversity was observed of which fucoxanthin (FUC), 19'–hexanoyloxyfucoxanthin (HEX), and diadinoxanthin (DIAD) were most abundant, indicating proliferation of large eukaryotes and nanoflagellates. In contrast, tropical and sub-tropical waters exhibited concentrations of totCHLa below 500 ng l⁻¹, with the North Atlantic Sub-tropical East gyre (NASE, 26.7–35°N), South Equatorial Current (SeqC, 7–14.6°S) and South Atlantic tropical Gyre (SATG, 14.6–26°S) characterised by totCHLa of <100 ng⁻¹. These waters exhibited relatively limited pigment diversity, and the totCAR budget was dominated by photoprotecting pigments (PPCs) of which zeaxanthin (ZEA), a marker of prokaryotes (cyanobacteria and prochlorophytes), was most abundant. DvCHLa, a marker of prochlorophytes was detected in waters at temperatures >15°C, and between the extremes of 48°N and 42°S. DvCHLa accounted for up to two-thirds of totCHLa in oligotrophic provinces demonstrating the importance of prochlorophytes to oceanic biomass.Overall, HEX was the dominant PSC, contributing up to 75% of totCAR. HEX always represented >2% of totCAR and was the only truly ubiquitous carotenoid. Since HEX is a chemotaxonomic marker of prymnesiophytes, this observation reflects the truly cosmopolitan distribution of this algal class. ZEA was found to be the most abundant PPC contributing more than one third of the total carotenoid budget in each transect.Greatest seasonality was observed in highly productive waters at high latitudes and in shallow continental shelf waters and attributed to proliferation of large eukaryotes during spring. Concentrations of the prokaryote pigments (ZEA + dvCHLa) also exhibited some seasonality, with elevated concentrations throughout most of the transect during Northern Hemisphere spring.     

The Kuroshio Extension System: Its Large-Scale Variability and Role in the Midlatitude Ocean-Atmosphere Interaction

The Kuroshio Extension and its recirculation gyre form an interconnected dynamic system. The system is located at a crossroads where the meso-scale and large-scale oceanic variability are highest, and where the ocean-atmosphere interaction is most active in the Pacific Ocean outside of the tropics. Following a brief review of the mean flow and meso-scale eddy variability, this study describes in detail the large-scale structural change (an oscillation between an elongated and a contracted state) observed in the Kuroshio Extension system. Causes for this structural change are explored next, and it is argued that the basin-wide external wind forcing and the nonlinear dynamics associated with the inertial recirculation gyre are both important factors. Data analysis results are reviewed and presented, emphasizing that the surface Kuroshio Extension is not simply a well-mixed layer passively responding to heat flux anomalies imposed by the atmosphere. It is argued that large-scale changes in the Kuroshio Extension system influence the surface ocean heat balance and generate wintertime sea surface temperature (SST) anomalies through both horizontal geostrophic heat advection and re-emergence to the surface mixed layer of sequestered mode water temperature anomalies. This revised version was published online in August 2006 with corrections to the Cover Date.     

Influence of Particle Mixing on Vertical Profiles of Chlorophyll a and Bacterial Biomass in Sediments of the German Bight, Oyster Ground and Dogger Bank (North Sea)

In May and September 1999 11 stations were sampled in the southern and central North Sea, located in the German Bight, eastern Oyster Ground and Dogger Bank. The study focused on the influence of particle mixing on transport of chlorophyll a to deeper sediment layers and vertical bacterial distribution (max. DEPTH=10 cm). The sampling stations were chosen to reflect a gradient in environmental conditions in the North Sea. The sampling stations differed in respect to redox potential (eH up to −243 mV in the German Bight and up to 274 mV in the offshore regions), silt content (up to 54% in the German Bight and 0·34% at the northern Dogger Bank) and different proportion of fresh organic material on total organic matter content (C/N ratios ranging from 9·27 in the German Bight up to 1·72 in the offshore sediments). Although bacterial densities (8·55×10⁹ g⁻¹in the German Bight up to 0·35×10⁹ g⁻¹in offshore sediments) were significantly correlated to chlorophyll a content in the sediment (P<0·01), inconsistencies in the temporal pattern of both variables in the surficial sediment layer suggested, that the dynamics of bacterial densities is generally controlled by food supply but also by other variables. The chlorophyll a content in the surficial sediments of the German Bight (up to 1·84 μg g⁻¹) was significantly higher than in the Oyster Ground (up to 0·58 μg g⁻¹) and the Dogger Bank area (up to 0·68 μg g⁻¹). With increasing chlorophyll a input to the benthic realm a subsequent enhanced burial of this compound into deeper sediment layers was expected either by biological (bioturbation) or by physical sediment mixing. However, the vertical profile of chlorophyll a decreased steeply in the sediments of the German Bight. Contrary, subsurface peaks were measured in the offshore areas. It was concluded from these results, that the vertical distribution of organic matter in sediments is less limited by the quantitative input from the water column but concomitant with particle mixing itself. The extent and possible mechanisms of particle mixing in the different study areas in relation to specific environmental factors is discussed.     

Eddy activities of the surface layer in the western North Pacific detected by satellite altimeter and radiometer

Geosat radar altimeter data during the first year (from November 1986 to November 1987) of its Exact Repeat Mission are analyzed to estimate the eddy kinetic energy and propagation characteristics of anomalies of sea surface dynamic topography (SSDT) for the western North Pacific. SSDT anomalies are compared with anomalies of sea surface temperature (SST) derived from NOAA satellite radiometer data. The eddy kinetic energy (K _e ) is large in the Kuroshio stationary meander region and Kuroshio Extension region. In the downstream region of the Kuroshio Extension,K _e is especially large on the upstream and downstream sides of prominent bathymetric features. In the interior region of the subtropical gyre is found a zonal tongue of largeK _e at around 20–20°N. Westward propagation is dominant in the SSDT and SST anomaly field at mid-latitudes. Longitude-time lag correlation diagrams reveal the coincidence of SSDT and SST anomalies statistically, which fact suggests the baroclinic nature of the anomalies. Zonal phase speeds of SSDT anomalies are approximately equal to the theoretical speeds of baroclinic first-mode long Rossby waves, but the meridional variation of observed phase speeds does not follow the simple theoretical variation of decreasing speeds monotonously with increasing latitudes.     

Distribution of deep near-inertial waves observed in the Kuroshio Extension

The distribution of deep near-inertial waves (NIWs) is investigated using data mainly from an array of 46 near-bottom acoustic current meter sensors spanning a 600 km × 600 km region as part of the Kuroshio Extension System Study during 2004–2006. The deep NIW distribution is interpreted in the context of both upper-layer and near-bottom mapped circulations. The wintertime-mean mixed-layer NIW energy input, modeled from observed wind stress, has the same range of values north and south of the Kuroshio Extension in this region. Yet, the wintertime-mean deep NIW energy distribution reveals a sharp factor-of-5 decrease from north to south of the Kuroshio jet. This direct observational evidence shows that the Kuroshio Extension blocks the equatorward propagation of NIWs. The NIW energy that does reach the sea floor within the subset of wintertime observations in the subtropical gyre arrives with patchy spatial and temporal distribution. Elevated NIW energy in deep water is associated with anticyclones in the deep barotropic flow and unassociated with upper layer eddies.     

Matiere en suspension et phosphore en particules dans l'atlantique tropical sud le long de 4°W en Novembre 1971

Particles and particulate phosphorus (Pp) were studied along 4°W in the tropical south Atlantic, between Ivory Coast and 24°S. Seston concentrations in the upper layers (0–150 m) are about 0.350 mg/l; they decrease in the intermediate layers (0.252 mg/l between 150 and 500 m) and lower layers (0.215 mg/l below 500 m). Along 4°W it is possible to define some geographical zones; thus, the South equatorial counter current waters are shown to be characterized by very weak concentrations of particulate phosphorus (< 0.020 μg-at/l). Ratios between particulate phosphorus, suspended matter and chlorophyll a have been computed: Part/Pp is about 2; the relationship Chl-a-Pp is linear for Chl-a concentrations lower than 1.5 μg/l, increasing exponentially for Chl-a values increasing from 1.5 μg/l to 35 μg/l.

Résumé

L'étude de la répartition du seston et du phosphore particulaire (Pp) a été faite sur la radiale 4°W, dans l'Atlantique tropical, entre la Côte d'Ivoire et 24°S. Les concentrations de seston dans la couche superficielle (0–150 m) sont voisines de 0,350 mg/l; elles décroissent dans les eaux intermédiaires (0,252 mg/l de 150 à 500 m) et profondes (0,215 mg/l de 500 à 700 m). Le long du méridien 4°W, le Pp permet de caractériser des zones géographiques; le contre-courant sud équatorial est ainsi mis en évidence par des eaux particulièrement pauvres en Pp (< 0,020 μatg/l). Des relations entre le phosphore particulaire, la matière en suspension, et la chlorophylle ont été établies: le rapport Part/Pp est voisin de 2; la relation Chl-a-Pp, linéaire pour les concentrations en Chl-a inférieures à 1,5 μg/l, devient exponentielle pour des valeurs de Chl-a supérieures allant de 1,5 μg/l à 35 μg/l.     

Benthic microalgal biomass in sediments of Onslow Bay, North Carolina

Sediment samples were collected at stations along cross-shelf transects in Onslow Bay, North Carolina, during two cruises in 1984 and 1985. Station depths ranged from 11 to 285 m. Sediment chlorophyll a concentrations ranged from 0·06 to 1·87 μg g⁻¹ sediment (mean, 0·55), or 2·6–62·0 mg m². Areal sediment chlorophyll a exceeded water column chlorophyll a a at 16 of 17 stations, especially at inshore and mid-shelf stations. Sediment ATP concentrations ranged from 0 to 0·67 μg g⁻¹ sediment (mean, 0·28). Values for both biomass indicators were lowest in the depth range including the shelf break (50–99 m). Organic carbon contents of the sediments were uniformly low across the shelf, averaging 0·159% by weight. Photography of the sediments revealed extensive patches of microalgae on the sediment surface.Our data suggest that viable benthic microalgae occur across the North Carolina continental shelf. The distribution of benthic macroflora on the North Carolina shelf indicates that sufficient light and nutrients are available to support primary production out to the shelf break. Frequent storm-induced perturbations do not favour settling of phytoplankton, an alternative explanation for the presence of microalgal pigments in the sediments. Therefore, we propose that a distinct, productive benthic microflora exists across the North Carolina continental shelf.     

Role of environment and hydrography in determining the picoplankton community structure of Sagami Bay,Japan

Seasonal variations in the picoplankton community were investigated from June 2002 to March 2004 within the photic zone of Sagami Bay, Japan. The study area was mostly dominated by coastal waters during the warm period (mixed layer water temperature ≥ 18°C). During the cold period (mixed layer water temperature ≤ 18°C), the water mass was characterized by low temperature and high saline waters indicative of the North Pacific Subtropical Mode Water (NPSTMW). Occasionally, a third type of water mass characterized by high temperature and low saline properties was observed, which could be evidence of the intrusion of warm Kuroshio waters. Synechococcus was the dominant picophytoplankton (5−28 × 10¹¹ cells m⁻²) followed by Prochlorococcus (1−5 × 10¹¹ cells m⁻²) and picoeukaryotes during the warm period. Heterotrophic bacteria dominated the picoplankton community throughout the year, especially in the warm period. During the Kuroshio Current advection, cyanobacterial abundance was high whereas that of picoeukaryotes and heterotrophic bacteria was low. During the cold period, homogeneously distributed, lower picophytoplankton cell densities were observed. The dominance of Synechococcus in the warm period reflects the importance of high temperature, low salinity and high Photosynthetically Active Radiation (PAR) on its distribution. Cyanobacterial and heterotrophic bacterial abundance showed a positive correlation with temperature. Prochlorococcus and picoeukaryotes showed a positive correlation with nutrients. Picoeukaryotes were the major contributors to the picophytoplankton carbon biomass. The annual picophytoplankton contribution to the photosynthetic biomass was 32 ± 4%. These observations suggest that the environmental conditions, combined with the seasonal variability in the source of the water mass, determines the community structure of picoplankton, which contributes substantially to the phytoplankton biomass and can play a very important role in the food web dynamics of Sagami Bay.     

Comparison of factors controlling phytoplankton productivity in the NE and NW subarctic Pacific gyres

The subarctic North Pacific is one of the three major high nitrate low chlorophyll (HNLC) regions of the world. The two gyres, the NE and the NW subarctic Pacific gyres dominate this region; the NE subarctic Pacific gyre is also known as the Alaska Gyre. The NE subarctic Pacific has one of the longest time series of any open ocean station, primarily as a result of the biological sampling that began in 1956 on the weathership stationed at Stn P (50°N, 145°W; also known as Ocean Station Papa (OSP)). Sampling along Line P, a transect from the coast (south end of Vancouver Island) out to Stn P has provided valuable information on how various parameters change along this coastal to open ocean gradient. The NW subarctic Pacific gyre has been less well studied than the NE gyre. This review focuses mainly on the NE gyre because of the large and long term data set available, but makes a brief comparison with the NW gyre. The NE gyre has saturating NO₃ concentrations all year (winter = about 16 μM and summer = about 8 μM), constantly very low chlorophyll (chl) (usually <0.5 mg m⁻³) which is dominated by small cells (<5 μm). Primary productivity is low (about 300–600 mg C m⁻² d⁻¹ and varies little (2 times) seasonally. Annual primary productivity is 3 to 4 times higher than earlier estimates ranging from 140 to 215 g C m⁻² y⁻¹. Iron limits the utilization of nitrate and hence the primary productivity of large cells (especially diatoms) except in the winter when iron and light may be co-limiting. There are observations of episodic increases in chl above 1 mg m⁻³, suggesting episodic iron inputs, most likely from Asian dust in the spring/early summer, but possibly from horizontal advection from the Alaskan Gyre in summer/early fall. The small cells normally dominate the phytoplankton biomass and productivity, and utilize the ammonium produced by the micrograzers. They do not appear to be Fe-limited, but are controlled by microzooplankton grazers. The NW Subarctic Gyre has higher nutrient concentrations and a shallower summer mixed depth and photic zone than Stn P in the NE gyre. Chl concentrations tend to be higher (0.5 to 1.5 μg L⁻¹) than Stn P, but primary productivity in the summer is similar to Stn P (600 mg C m⁻² d⁻¹). There are no seasonal data from this gyre. Iron enrichment experiments in October, resulted in an increase in chl (mainly the centric diatom Thalassiosira sp.) and a draw down of nitrate, suggesting that large phytoplankton are Fe-limited, similar to Stn P.     

HPLC analysis of algal pigments: a comparison exercise among laboratories and recommendations for improved analytical performance

Pure individual and mixed pigment standards were distributed among Joint Global Ocean Flux Study (JGOFS) pigment analysts to estimate the variability of their spectrophotometric and chromatographic systems. To monitor the integrity of the pigments during the comparison exercise, chlorophyll and carotenoid standards were archived and periodically analyzed by high-performance liquid chromatography (HPLC). Pigment standards stored in the dark under nitrogen at − 20 °C were found to be stable for periods of at least one year. Results from three separate intercalibration exercises document a better agreement for spectrophotometric analyses than for HPLC. For the spectrophotometric comparisons, 90% of the pigments analyzed by participant laboratories were within ± 6% of the mean “consensus” values. By contrast, 65 and 85% of the laboratories agreed to within ± 10 and ± 20%, respectively, when chromatographic analyses were compared. Chlorophyll absorption measurements obtained with a diode array-type spectrophotometer were 6–9% lower than those obtained with monochromator-type spectrophotometers. These underestimates probably result from chlorophyll fluorescence contamination associated with the optical configuration of the diode array spectrophotometer. It was also determined that HPLC methods which are not capable of separating monovinyl chlorophyll a from divinyl chlorophyll a can produce 15–25% overestimates of total chlorophyll a concentration in Prochlorococcus-dominated oceanic waters. A simple dichromatic approach is described for eliminating this variable source of error caused by co-elution of these structurally-related pigments. The use of internal standards and periodic calibration checks with external standards is highly recommended for improving analytical performance.