The influence of the south-west monsoon upon the nutrient biogeochemistry of the Arabian Sea

Variations in the nutrient concentrations were studied during two cruises to the Arabian Sea. The situation towards the end of the southwest monsoon season (September/October 1994) was compared with the inter-monsoonal season during November and December 1994. Underway surface transects showed the influence of an upwelling system during the first cruise with deep, colder, nutrient-rich water being advected into the surface mixed layer. During the southwesterly monsoon there was an area of coastal Ekman upwelling, bringing colder water (24.2°C) into the surface waters of the coastal margin. Further offshore at about 350 km there was an area of Ekman upwelling, as a result of wind-stress curl, north of the Findlater Jet axis; this area also had cooler surface water (24.6°C). Further offshore (>1000 km) the average surface temperatures increased to >27°C. These waters were oligotrophic with no evidence of the upwelling effects observed further inshore. In the upwelling regions nutrient concentrations in the close inshore coastal zone were elevated (NO₃=18 μmol l^-1, PO₄=1.48 μmol l^-1); higher concentrations also were measured at the region of offshore upwelling off the shelf, with a maximum nitrate concentration of 12.5 μmol l^-1 and a maximum phosphate concentration of 1.2 μmol l^-1. Nitrate and phosphate concentrations decreased with increasing distance offshore to the oligotrophic waters beyond 1400 km, where typical nitrate concentrations were 35.0 nmol l^-1 (0.035 μmol l^-1) in the surface mixed layer. A CTD section from the coastal shelf, to 1650 km offshore to the oligotrophic waters, clearly showed that during the monsoon season, upwelling is one of the major influences upon the nutrient concentrations in the surface waters of the Arabian Sea off the coast of Oman. Productivity of the water column was enhanced to a distance of over 800 km offshore. During the intermonsoon period a stable surface mixed layer was established, with a well-defined thermocline and nitracline. Surface temperature was between 26.8 and 27.4°C for the entire transect from the coast to 1650 km offshore. Nitrate concentrations were typically between 2.0 and 0.4 μmol l^-1 for the transect, to about 1200 km where the waters became oligotrophic, and nitrate concentrations were then typically 8–12 nmol l^-1. Ammonia concentrations for the oligotrophic waters were typically 130 nmol l^-1, and are reported for the first time in the Indian Ocean. The nitrogen/phosphorus (N/P) ratios suggest that phytoplankton production was potentially nitrogen-limited in all the surface waters of the Arabian Sea, with the greatest nitrogen limitation during the intermonsoon period.     

Zooplankton associated with the oxygen minimum zone system in the northern upwelling region of Chile during March 2000

Zooplankton in the coastal upwelling region off northern Chile may play a significant biogeochemical role by promoting carbon flux into the subsurface OMZ (oxygen minimum zone). This work identifies the dominant zooplankton species inhabiting the area influenced by the OMZ in March 2000 off Iquique (20°S, northern Chile). Abundance and vertical distribution studies revealed 17 copepod and 9 euphausiid species distributed between the surface and 600 m at four stations sampled both by day and by night. Some abundant species remained in the well-oxygenated upper layer (30 m), with no evidence of diel vertical migration, apparently restricted by a shallow (40–60 m) oxycline. Other species, however, were found closely associated with the OMZ. The large-sized copepod Eucalanus inermis was found below the oxycline and performed diel vertical migrations into the OMZ, whereas the very abundant Euphausia mucronata performed extensive diel vertical migrations between the surface waters and the core of the OMZ (200 m), even crossing it. A complete assessment of copepods and euphausiids revealed that the whole sampled water column (0–600 m) is occupied by distinct species having well-defined habitats, some of them within the OMZ. Ontogenetic migrations were evident in Eucalanidae and E. mucronata. Estimates of species biomass showed a substantial (>75% of total zooplankton biomass) daily exchange of C between the photic layer and the OMZ. Both E. inermis and E. mucronata can actively exchange about 37.8 g C m⁻² d⁻¹ between the upper well-oxygenated (0–60 m) layer and the deeper (60–600 m) OMZ layer. This migrant biomass may contribute about 7.2 g C m⁻² d⁻¹ to the OMZ system through respiration, mortality, and production of fecal pellets within the OMZ. This movement of zooplankton in and out of the OMZ, mainly as a result of the migratory behavior of E. mucronata, suggests a very efficient mechanism for introducing large amounts of freshly produced carbon into the OMZ system and should, therefore, be considered when establishing C budgets for coastal upwelling systems.     

Zooplankton metabolism and carbon demand at two seamounts in the NE Atlantic

Zooplankton metabolic rates, determined from electron transfer system (ETS) activity, were studied at two seamounts (Seine: 34°N, 14°W, summit depth ∼170 m; Sedlo: 40°N, 27°W, summit depth ∼750 m) in the northeast (NE) Atlantic during three cruises in November 2003, April 2004 and July 2004. ETS activity and respiratory carbon demand were measured for samples taken at seamount and open-ocean locations in order to probe the hypothesis of locally enhanced seamount productivity. ETS activity and biomass revealed no consistent diel patterns of feeding activity and vertical migration at Seine and Sedlo Seamounts. Spatial differences of biomass-specific ETS activity were observed at both seamounts and coincided with differences in food abundance and quality. At Seine Seamount in April 2004, biomass-specific ETS activity was on average higher at the seamount locations compared to the open ocean, though the enhancement was of a lower magnitude than spatial and temporal variability and had no apparent influence on zooplankton respiratory carbon demand or biomass. A persistent pattern of reduced zooplankton biomass above the summit location at Seine Seamount in April 2004 and July 2004 resulted in a local reduction of respiratory carbon demand. At Sedlo Seamount in November 2003, large spatial differences in biomass-specific ETS activity observed at the seamount locations resulted in a large range of respiratory carbon demand at the seamount, but were not reflected in zooplankton biomass. The depth-integrated (0–150 m) median respiratory carbon demand of the zooplankton community estimated from day and night hauls was 2.1 mg C m⁻² d⁻¹ at Seine Seamount (range: 0.3–6.3) and 2.9 mg C m⁻² d⁻¹ at Sedlo Seamount (range: 1.6–12.0). The sporadic nature and low magnitude of locally higher zooplankton respiration rates at the seamounts, which did not result in locally higher zooplankton standing stock biomass, lead us to reject the hypothesis that locally enhanced seamount productivity provides an autochthonous food supply to the resident faunas at Seine and Sedlo Seamounts. Instead, we conclude that the faunas at both seamounts are more likely supported by advection of food from the surrounding ocean.     

Latitudinal distribution of microbial plankton abundance,production, and respiration in the Equatorial Atlantic in autumn 2000

Phytoplankton and bacterial abundance, size-fractionated phytoplankton chlorophyll-a (Chl-a) and production together with bacterial production, microbial oxygen production and respiration rates were measured along a transect that crossed the Equatorial Atlantic Ocean (10°N–10°S) in September 2000, as part of the Atlantic Meridional Transect 11 (AMT 11) cruise. From 2°N to 5°S, the equatorial divergence resulted in a shallowing of the pycnocline and the presence of relatively high nitrate (>1 μM) concentrations in surface waters. In contrast, a typical tropical structure (TTS) was found near the ends of the transect. Photic zone integrated ¹⁴C primary production ranged from ∼200 mg C m⁻² d⁻¹ in the TTS region to ∼1300 mg C m⁻² d⁻¹ in the equatorial divergence area. In spite of the relatively high primary production rates measured in the equatorial upwelling region, only a moderate rise in phytoplankton biomass was observed as compared to nearby nutrient-depleted areas (22 vs. 18 mg Chl-a m⁻², respectively). Picophytoplankton were the main contributors (>60%) to both Chl-a biomass and primary production throughout the region. The equatorial upwelling did not alter the phytoplankton size structure typically found in the tropical open ocean, which suggests a strong top-down control of primary producers by zooplankton. However, the impact of nutrient supply on net microbial community metabolism, integrated over the euphotic layer, was evidenced by an average net microbial community production within the equatorial divergence (1130 mg C m⁻² d⁻¹) three-fold larger than net production measured in the TTS region (370 mg C m⁻² d⁻¹). The entire region under study showed net autotrophic community metabolism, since respiration accounted on average for 51% of gross primary production integrated over the euphotic layer.     

Biogeography and phenology of satellite-measured phytoplankton seasonality in the California current

Thirteen years (1998–2010) of satellite-measured chlorophyll a are used to establish spatial patterns in climatological phytoplankton biomass seasonality across the California Current System (CCS) and its interannual variability. Multivariate clustering based on the shape of the local climatological seasonal cycle divides the study area into four groups: two with spring-summer maxima representing the northern and southern coastal upwelling zones, one with a summer minimum offshore in mid-latitudes and a fourth with very weak seasonality in between. Multivariate clustering on the seasonal cycles from all 13 years produces the same four seasonal cycle types and provides a view of the interannual variability in seasonal biogeography. Over the study period these seasonal cycles generally appear in similar locations as the climatological clusters. However, considerable interannual variability in the geography of the seasonal cycles is evident across the CCS, the most spatially extensive of which are associated with the 1997–1999 El Niño-Southern Oscillation (ENSO) signal and the 2005 delayed spring transition off the Oregon and northern and central California coasts. We quantify linear trends over the study period in the seasonal timing of the two seasonal cycles that represent the biologically productive coastal upwelling zones using four different metrics of phenology. In the northern upwelling region, the date of the spring maximum is delaying (1.34 days yr⁻¹) and the central tendency of the summer elevated chlorophyll period is advancing (0.63 days yr⁻¹). In the southern coastal upwelling region, both the initiation and cessation of the spring maximum are delaying (1.78 days yr⁻¹ and 2.44 days yr⁻¹, respectively) and the peak is increasing in duration over the study period. Connections between observed interannual shifts in phytoplankton seasonality and physical forcing, expressed as either basin-scale climate signals or local forcing, show phytoplankton seasonality in the CCS to be influenced by changes in the seasonality of the wind mixing power offshore, coastal upwelling in the near-shore regions and basin-scale signals such as ENSO across the study area.     

Spatial and temporal variation in surfclam (Spisula solidissima) larval supply and settlement on the New Jersey inner shelf during summer upwelling and downwelling

To examine the relationship between near-bottom larval surfclam concentrations and surfclam settlement at an inner continental shelf site off New Jersey (USA), four consecutive sets of settlement experiments were carried out at three stations at the Long-term Ecosystem Observatory (LEO-15) from 14 to 31 July 1997 during upwelling and downwelling. Two inshore stations were on the landward and seaward sides of Beach Haven Ridge at ∼12 m depth, and a third station was 8 km further offshore at ∼20 m depth. In each experiment, four replicate trays of azoic sand from Beach Haven Ridge were placed flush with the seafloor and exposed for 3–7 days. Larval surfclam concentrations were measured every 4 h at 1 m above the bottom (mab) using Moored, Automated, Serial Zooplankton Pumps at the three stations. At all three stations, larval surfclam concentrations (1 mab) were low during upwelling, and higher during and after downwelling. Pulses of highest larval surfclam concentrations coincided with the initial arrival of downwelled warm water. In addition, larval surfclam concentrations were higher at the two inshore stations than at the offshore station. Larval surfclam settlement in the trays was higher during and following downwelling than during upwelling at one inshore station and at the offshore station. At the other inshore station (landward of Beach Haven Ridge), surfclam settlement did not increase during and following downwelling. Overall, surfclam settlement was higher inshore than offshore. The results indicate that spatial and temporal variation in larval surfclam supply was controlled by upwelling and downwelling circulation and that surfclam settlement was influenced by larval supply. Bottom flows across Beach Haven Ridge during a storm may have reduced larval surfclam settlement on the upcurrent side of the ridge, affecting initial densities on a small (∼1 km) scale.     

The structure of zooplankton communities,in the 2 to 2000 μm size range,in the Arabian Sea during and after the SW monsoon, 1994

Zooplankton communities, studied in the surface mixed layer on a 1600 m transect across the Arabian Sea, were found to differ in their temporal and spatial response to seasonal forcing. The transect studied, spanned seasonally eutrophic upwelling, mesotrophic downwelling and aseasonal oligotrophic waters. The nano- and microzooplankton communities constituted a relatively constant compartment in the tropical monsoon ecosystem, whilst the mesozooplankton showed a clear response to both upwelling and season. The heterotrophic nanoflagellates were concentrated in the surface mixed layer, except in the eutrophic upwelling waters of the SW monsoon. They reached maximum cell concentrations of 855 ml^-1 during the SW monsoon and a maximum biomass of 8.4 mg C m^-3 during the intermonsoon. Nanozooplankton standing stocks, in the surface mixed layer, ranged between 7 and 333 mg C m^-2, with highest stocks found during the intermonsoon. The microzooplankton community was dominated by Protozoa, particularly aloricate ciliates and heterotrophic dinoflagellates, which accounted for up to 99% in terms of numbers and up to 71% of the biomass. Sarcodines and metazoan nauplii were recorded in lower numbers (<400 l^-1). The microzooplankton were also concentrated in the surface mixed layer during both periods, except in the eutrophic coastal waters during the SW monsoon, when relatively high biomass values were found below the mixed layer depth. Their standing stocks, in the surface mixed layer, ranged between 50 and 182 mg C m^-2, with the highest concentration found in the mesotrophic offshore waters during the late monsoon period. Total mesozooplankton standing stocks, in the surface 100 m, decreased with distance from the coastal to offshore waters and between seasons, decreasing from 1248 to 238 mg C m^-2 during the late SW monsoon and 656–89 mg C m^-2 during the following intermonsoon. The largest size class, of 1000–2000 μm sized organisms, dominated throughout except at the oligotrophic station during the intermonsoon period, when the smallest class, of 200–500 μm, were more important. The shift in size structure from large to small zooplankton occurred in response to a shift in dominance from large to small phytoplankton cells both spatially, along a eutrophic–oligotrophic gradient, and seasonally. These responses are a result of the physical forcing associated with the monsoon seasons in the Arabian Sea.     

Structure of potential upwelling areas in the Philippines

The three-dimensional structure of two potential mesoscale upwelling areas that are located in the external waters of the Philippine archipelago (i.e. northwest of Luzon and east of Mindanao) were constructed by analysis of historical data. A unique characteristic of both upwelling sites is that they can be identified by their anomalously cold subsurface temperatures rather than sea surface temperature distributions. As such, they cannot be observed in sea surface temperature fields derived by satellite imagery. The data used in the analysis were obtained from the National Oceanographic Data Center hydrographic database. Objective analysis was performed to produce monthly temperature fields at several standard depths within the upper 500 m of the region 0–30°N and 100–140°E with a horizontal grid resolution of 0.5°. The extent and timing of these upwelling areas are described. A review of existing hypothesis on the mechanisms for their evolution and seasonal modulation are presented. The change in heat content during the upwelling season is greater than 300 W m⁻² in both areas. Based on the excursion of isotherms, vertical velocities of 83 cm day⁻¹ and 26 cm day⁻¹ were obtained for upwelling northwest of Luzon and east of Mindanao, respectively.     

Diatom biomass and productivity in oceanic and plume-influenced waters of the western tropical Atlantic ocean

Whereas diatoms (class Bacillariophyceae) often dominate phytoplankton taxa in the Amazon estuary and shelf, their contribution to phytoplankton dynamics and impacts on regional biogeochemistry are poorly understood further offshore in the western tropical Atlantic Ocean (WTAO). Thus, relative contribution of diatoms to phytoplankton biomass and primary production rates and associated environmental conditions were quantified during three month-long cruises in January–February 2001, July–August 2001, and April–May 2003. The upper water column was sampled at 6 light depths (100%, 50%, 25%, 10%, 1% and 0.1% of surface irradiance) at 64 stations between 3° and 14°N latitude and 41° and 58°W longitude. Each station was categorized as ‘oceanic’ or ‘plumewater’, based on principal component analysis of eight physical, chemical and biological variables. All stations were within the North Brazil Current, and plumewater stations were characterized by shallower mixed layers with lower surface salinities and higher dissolved silicon (dSi) concentrations than oceanic stations. The major finding was a much greater role of diatoms in phytoplankton biomass and productivity at plumewater stations relative to oceanic stations. Mean depth-integrated bSi concentrations at the plumewater and oceanic stations were 14.2 and 3.7 mmol m⁻², respectively. Mean depth-integrated SiP rates at the plumewater and oceanic stations were 0.17 and 0.02 mmol m⁻² h⁻¹, respectively. Based on ratios of SiP and PP rates, and typical Si:C ratios, diatoms contributed on average 29% of primary productivity at plumewater stations and only 3% of primary productivity at oceanic stations. In contrast, phytoplankton biomass (as chlorophyll a concentrations) and primary production (PP) rates (as ¹⁴C uptake rates) integrated over the euphotic zone were not significantly different at plumewater and oceanic stations. Chlorophyll a concentrations ranged from 8.5 to 42.4 mg m⁻² and 4.0 to 38.0 mg m⁻² and PP rates ranged from 2.2 to 11.2 mmol m⁻² h⁻² and 1.8 to 10.8 mmol m⁻² h⁻² at plumewater and oceanic stations, respectively. A conservative estimate of annual integrated SiP in offshore waters of Amazon plume between April and August is 0.59 Tmol Si, based on mean SiP rates in plumewaters and satellite-derived estimates of the area of the Amazon plume. In conclusion, river plumewaters dramatically alter the silicon dynamics of the WTAO, forming extensive diatom-dominated phytoplankton blooms that may contribute significantly to the global Si budget as well as contributing to energy and matter flow off of the continental shelf.     

Phytoplankton dynamics in the NE subarctic Pacific

Ocean Station Papa (OSP, 50°N 145°W) in the NE subarctic Pacific is characterised as high nitrate low chlorophyll (HNLC). However, little is known about the spatial extent of these HNLC waters or the phytoplankton dynamics on the basin scale. Algal biomass, production and size-structure data are presented from winter, spring and summer between 1992 and 1997 for five stations ranging from coastal to open-ocean conditions. The inshore stations (P04–P16) are characterised by the classical seasonal cycle of spring and late summer blooms (production >3 g C m⁻² d⁻¹), diatoms are not Fe-stressed, and growth rate is probably controlled by macronutrient supply. The fate of the phytoplankton is likely sedimentation by diatom-dominated spring blooms, with a pelagic recycling system predominating at other times. The offshore stations (P20/OSP) display low seasonality in biomass and production (OSP, mean winter production 0.3 g C m⁻² d⁻¹, mean spring/summer production 0.85 g C m⁻² d⁻¹), and are dominated by small algal cells. Low Fe availability prevents the occurrence of diatom blooms observed inshore. The main fate of phytoplankton is probably recycling through the microbial food web, with relatively low sedimentation compared to inshore. However, the supply of macro- and micro-nutrients to the coastal and open ocean, respectively, may vary between years. Variability in macro-nutrient supply to the coastal ocean may result in decreased winter reserve nitrate, summer nitrate limitation, subsequent floristic shifts towards small cells, and reduced primary production. Offshore, higher diatom abundances are occasionally observed, perhaps indicating episodic Fe supply. The two distinct oceanic regimes have different phytoplankton dynamics resulting in different seasonality, community structure and fate of algal carbon. These differences will strongly influence the biogeochemical signatures of the coastal and open-oceanic NE subarctic Pacific.     

Picophytoplankton dynamics and production in the Arabian Sea during the 1995 Southwest Monsoon

Phytoplankton community structure is expected to shift to larger cells (e.g., diatoms) with monsoonal forcing in the Arabian Sea, but recent studies suggest that small primary producers remain active and important, even in areas strongly influenced by coastal upwelling. To better understand the role of smaller phytoplankton in such systems, we investigated growth and grazing rates of picophytoplankton populations and their contributions to phytoplankton community biomass and primary productivity during the 1995 Southwest Monsoon (August–September). Environmental conditions at six study stations varied broadly from open-ocean oligotrophic to coastal eutrophic, with mixed-layer nitrate and chlorophyll concentrations ranging from 0.01 to 11.5 μM NO₃ and 0.16 to 1.5 μg Chl a. Picophytoplankton comprised up to 92% of phytoplankton carbon at the oceanic stations, 35% in the diatom-dominated coastal zone, and 26% in a declining Phaeocystis bloom. Concurrent in situ dilution and ¹⁴C-uptake experiments gave comparable ranges of community growth rates (0.53–1.05 d⁻¹ and 0.44–1.17 d⁻¹, to the 1% light level), but uncertainties in C:Chl a confounded agreement at individual stations. Microzooplankton grazing utilized 81% of community phytoplankton growth at the oligotrophic stations and 54% at high-nutrient coastal stations. Prochlorococcus (PRO) was present at two oligotrophic stations, where its maximum growth approached 1.4 d⁻¹ (two doublings per day) and depth-integrated growth varied from 0.2 to 0.8 d⁻¹. Synechococcus (SYN) growth ranged from 0.5 to 1.1 d⁻¹ at offshore stations and 0.6 to 0.7 d⁻¹ at coastal sites. Except for the most oligotrophic stations, growth rates of picoeukaryotic algae (PEUK) exceeded PRO and SYN, reaching 1.3 d⁻¹ offshore and decreasing to 0.8 d⁻¹ at the most coastal station. Microzooplankton grazing impact averaged 90, 70, and 86% of growth for PRO, SYN, and PEUK, respectively. Picoplankton as a group accounted for 64% of estimated gross carbon production for all stations, and 50% at high-nutrient, upwelling stations. Prokaryotes (PRO and SYN) contributed disproportionately to production relative to biomass at the most oligotrophic station, while PEUK were more important at the coastal stations. Even during intense monsoonal forcing in the Arabian Sea, picoeukaryotic algae appear to account for a large portion of primary production in the coastal upwelling regions, supporting an active community of protistan grazers and a high rate of carbon cycling in these areas.     

Plankton net community production and dark respiration in the Arabian Sea during September 1994

Plankton community net and gross production and dark respiration were determined from in vitro changes in dissolved inorganic carbon and dissolved oxygen during September 1994 along a southeast offshore transect in the Arabian Sea. Surface rates of gross production decreased from 17±0.7 mmol C m^-3 d^-1 at a coastal upwelling station to 3±0.8 mmol C m^-3 d^-1 at the most offshore station. The euphotic zone at the time of sampling was predominantly heterotrophic, with integrated net community production values ranging from 15±7 mmol C m^-2 d^-1 inshore to −253±32 mmol C m^-2 d^-1 offshore. Calculations of the respiration attributable to the major plankton groups could account for 61–87% of the dark community respiration measured at the inshore stations, but only 15–26% of the community respiration determined offshore. Comparison of the fluxes of dissolved inorganic carbon and oxygen revealed a tendency for higher respiratory quotients than those calculated for organic metabolism prevailing at the offshore stations.     

CO2 outgassing off central Chile (31–30°S) and northern Chile (24–23°S) during austral summer 1997: the effect of wind intensity on the upwelling and ventilation of CO2-rich waters

The distribution of pH and alkalinity has been used to calculate the distribution of total inorganic carbon (TC) and fugacity of carbon dioxide (fCO₂) in the upper 200 m of the water column in coastal upwelling areas off northern Chile (23–24°S, near Antofagasta) and central Chile (30–31°S, near Coquimbo) during austral summer 1997. In these upwelling areas, colder surface waters were oxygen poor and strongly CO₂ supersaturated (100% near Antofagasta and 200% near Coquimbo), although below the pycnocline the CO₂ supersaturation invariably exceeded 200% in both areas. The larger surface CO₂ supersaturation and outgassing at 30°S were associated with stronger winds that promoted the upwelling of denser water (richer in CO₂) as well as a higher air–sea CO₂ transfer velocity. The consistent decrease in intensity of the southerly winds (as derived from NSCAT scatterometer data) from 30–31°S to 23–24°S suggests a corresponding decline in the intensity of the CO₂ outgassing due to upwelling. Additionally, we suggest here that the intensity of the local upwelling forcing (i.e. alongshore–equatorward winds) plays a role in determining the water mass composition and phytoplankton biomass of the coastal waters. Thus, while deep upwelling of salty and cold water resulted in high fCO₂ (up to 1000 μatm) and very low phytoplankton biomass (chlorophyll a concentration lower than 0.5 mg m⁻³), the shallow upwelling of less salty (e.g. salinity <34.5) and less CO₂-supersaturated water resulted in a higher phytoplankton biomass, which further reduced surface water fCO₂ by photosynthesis.     

Nutrient and phytoplankton dynamics off the west coast of Vancouver Island during the 1997/98 ENSO event

Six research cruises were conducted off the west coast of Vancouver Island between April and October of 1997 and 1998 as part of the Canadian GLOBEC project to compare nutrient and phytoplankton dynamics between ENSO (1997) and non-ENSO (1998) years. Limited sampling also was conducted during three cruises in 1999. During the 1997 ENSO period, there was a shallow thermocline (∼10 m) that resulted in a shallower mixed layer, lower salinity and density, and stronger summer stratification. In general on the shelf, the 1997 growing season was characterized by higher nitrate (7.5 μM) and silicic acid (17 μM) concentrations, lower total chlorophyll (∼76 mg m⁻²), lower phytoplankton carbon biomass (0.2 mg C L⁻¹), and lower diatom abundance and biomass than in 1998. Phytoplankton assemblages were dominated by nanoplankton in 1997 and by diatoms in 1998. These results suggest that the 1997 ENSO was responsible for a reduction in the growth and biomass of larger phytoplankton cells. In mid-1998, the hydrographic characteristics off the west coast of Vancouver Island changed suddenly. The 1997 poleward transport of warm water reversed to an equatorward transport of coastal water in July 1998, which was accompanied by normal summer upwelling. During 1998, a large diatom bloom (mainly dominated by Chaetoceros debilis, Leptocylindrus danicus and to a lesser extent by Skeletomema and Pseudo-nitzschia sp.) was observed in July over the continental shelf. This large bloom resulted in chlorophyll concentrations of up to 400 mg m⁻², primary productivity of up to 11 g C m⁻² d⁻¹, and near undetectable dissolved nitrogen concentrations at some of the shelf stations in 1998. In contrast, during 1997, the sub-tropical waters that were advected over the slope, resulted in low chlorophyll a and primary productivity (generally <1 g C m⁻² d⁻¹). Therefore, there was a sharp contrast between the very high primary productivity on the shelf in July 1998, due to normal nutrient replenishment from summer upwelling and outflow from the Strait of Juan de Fuca, and the lower primary productivity during the 1997 ENSO year. During 1998, non-ENSO conditions resulted in phytoplankton biomass that was twice as high on the shelf as that measured in regions beyond the continental shelf of the west coast of Vancouver Island.     

Observations on the Rim Current structure,CIW formation and transport in the western Black Sea

CTD and ADCP measurements together with a sequence of satellite images indicate pronounced current meandering and eddy activity in the western Black Sea during April 1993. The Rim Current is identified as a well-defined meandering jet stream confined over the steepest topographic slope and associated cyclonic–anticyclonic eddy pairs located on both its sides. It has a form of highly energetic and unstable flow system, which, as it propagates cyclonically along the periphery of the basin, is modified in character. It possesses a two-layer vertical structure with uniform upper layer speed in excess of 50 cm/s (maximum value ∼100 cm/s), followed by a relatively sharp change across the pycnocline (between 100 and 200 m) and the uniform sub-pycnocline currents of 20 cm/s (maximum value ∼40 cm/s) observed up to the depth of ∼350 dbar, being the approximate limit of ADCP measurements. The cross-stream velocity structure exhibits a narrow core region (∼30 km), flanked by a narrow zone of anticyclonic shear on its coastal side and a broader region of cyclonic shear on its offshore side. The northwestern shelf circulation is generally decoupled from the influence of the basinwide circulation and is characterized by much weaker currents, less than 10 cm/s. The southward coastal flow associated with the Danube and Dinepr Rivers is weak during the measurement period and is restricted to a very narrow coastal zone.The data suggest the presence of temperature-induced overturning prior to the measurements, and subsequent formation of the Cold Intermediate Water mass (CIW) within the Northwestern Shelf (NWS) and interior of the western basin. The newly formed shelf CIW is transported in part along the shelf by the coastal current system, and in part it flows downslope across the shelf and intrudes into the Rim Current convergence zone. A major part of the cold water mass, however, seems to be trapped within the northwestern shelf. The CIW mass, injected into the Rim Current zone from the shelf and the interior region, is then circulated around the basin.     

Basin-scale variability of phytoplankton biomass,production and growth in the Atlantic Ocean

The latitudinal distributions of phytoplankton biomass, composition and production in the Atlantic Ocean were determined along a 10,000-km transect from 50°N to 50°S in October 1995, May 1996 and October 1996. Highest levels of euphotic layer-integrated chlorophyll a (Chl a) concentration (75–125 mg Chl m⁻²) were found in North Atlantic temperate waters and in the upwelling region off NW Africa, whereas typical Chl a concentrations in oligotrophic waters ranged from 20 to 40 mg Chl m⁻². The estimated concentration of surface phytoplankton carbon (C) biomass was 5–15 mg C m⁻² in the oligotrophic regions and increased over 40 mg C m⁻² in richer areas. The deep chlorophyll maximum did not seem to constitute a biomass or productivity maximum, but resulted mainly from an increase in the Chl a to C ratio and represented a relatively small contribution to total integrated productivity. Primary production rates varied from 50 mg C m⁻² d⁻¹ at the central gyres to 500–1000 mg C m⁻² d⁻¹ in upwelling and higher latitude regions, where faster growth rates (μ) of phytoplankton (>0.5 d⁻¹) were also measured. In oligotrophic waters, microalgal growth was consistently slow [surface μ averaged 0.21±0.02 d⁻¹ (mean±SE)], representing <20% of maximum expected growth. These results argue against the view that the subtropical gyres are characterized by high phytoplankton turnover rates. The latitudinal variations in μ were inversely correlated to the changes in the depth of the nitracline and positively correlated to those of the integrated nitrate concentration, supporting the case for the role of nutrients in controlling the large-scale distribution of phytoplankton growth rates. We observed a large degree of temporal variability in the phytoplankton dynamics in the oligotrophic regions: productivity and growth rates varied in excess of 8-fold, whereas microalgal biomass remained relatively constant. The observed spatial and temporal variability in the biomass specific rate of photosynthesis is at least three times larger than currently assumed in most satellite-based models of global productivity.     

Volume transport and distribution of deep circulation at 165°W in the North Pacific

We conducted full-depth hydrographic observations between 8°50′ and 44°30′N at 165°W in 2003 and analyzed the data together with those from the World Ocean Circulation Experiment and the World Ocean Database, clarifying the water characteristics and deep circulation in the Central and Northeast Pacific Basins. The deep-water characteristics at depths greater than approximately 2000 dbar at 165°W differ among three regions demarcated by the Hawaiian Ridge at around 24°N and the Mendocino Fracture Zone at 37°N: the southern region (10–24°N), central region (24–37°N), and northern region (north of 37°N). Deep water at temperatures below 1.15 °C and depths greater than 4000 dbar is highly stratified in the southern region, weakly stratified in the central region, and largely uniform in the northern region. Among the three regions, near-bottom water immediately east of Clarion Passage in the southern region is coldest (θ<0.90 °C), most saline (S>34.70), highest in dissolved oxygen (O₂>4.2 ml l^?1), and lowest in silica (Si<135 μmol kg^?1). These characteristics of the deep water reflect transport of Lower Circumpolar Deep Water (LCDW) due to a branch current south of the Wake–Necker Ridge that is separated from the eastern branch current of the deep circulation immediately north of 10°N in the Central Pacific Basin. The branch current south of the Wake–Necker Ridge carries LCDW of θ<1.05 °C with a volume transport of 3.7 Sv (1 Sv=10⁶ m³ s^?1) into the Northeast Pacific Basin through Horizon and Clarion Passages, mainly through the latter (～3.1 Sv). A small amount of the LCDW flows northward at the western boundary of the Northeast Pacific Basin, joins the branch of deep circulation from the Main Gap of the Emperor Seamounts Chain, and forms an eastward current along the Mendocino Fracture Zone with volume transport of nearly 1 Sv. If this volume transport is typical, a major portion of the LCDW (～3 Sv) carried by the branch current south of the Wake–Necker and Hawaiian Ridges may spread in the southern part of the Northeast Pacific Basin. In the northern region at 165°W, silica maxima are found near the bottom and at 2200 dbar; the minimum between the double maxima occurs at a depth of approximately 4000 dbar (θ～1.15 °C). The geostrophic current north of 39°N in the upper deep layer between 1.15 and 2.2 °C, with reference to the 1.15 °C isotherm, has a westward volume transport of 1.6 Sv at 39–44°30′N, carrying silica-rich North Pacific Deep Water from the northeastern region of the Northeast Pacific Basin to the Northwest Pacific Basin.     

Hydrography and circulation of the West Antarctic Peninsula Continental Shelf

The water mass structure and circulation of the continental shelf waters west of the Antarctic Peninsula are described from hydrographic observations made in March–May 1993. The observations cover an area that extends 900 km alongshore and 200 km offshore and represent the most extensive hydrographic data set currently available for this region. Waters above 100–150 m are composed of Antarctic Surface Water and its end member Winter Water. Below the permanent pycnocline is a modified version of Circumpolar Deep Water, which is a cooled and freshened version of Upper Circumpolar Deep Water. The distinctive signature of cold and salty water from the Bransfield Strait is found at some inshore locations, but there is little indication of significant exchange between Bransfield Strait and the west Antarctic Peninsula shelf. Dynamic topography at 200 m relative to 400 m indicates that the baroclinic circulation on the shelf is composed of a large, weak, cyclonic gyre, with sub-gyres at the northeastern and southwestern ends of the shelf. The total transport of the shelf gyre is 0.15 Sv, with geostrophic currents of order 0.01 m s^-1. A simple model that balances across-shelf diffusion of heat and salt from offshore Upper Circumpolar Deep Water with vertical diffusion of heat and salt across the permanent pycnocline into Winter Water is used to explain the formation of the modified Circumpolar Deep Water that is found on the shelf. Model results show that the observed thermohaline distributions across the shelf can be maintained with a coefficient of vertical diffusion of 10^-4 m² s^-1 and horizontal diffusion coefficients for heat and salt of 200 and 1200 m² s^-1, respectively. When the effects of double diffusion are included in the model, the required horizontal diffusion coefficients for heat and salt are 200 and 400 m² s^-1, respectively.     

Monsoonal influence on the distribution of phytoplankton pigments in the Arabian Sea

Variations in the distribution of chemotaxonomic pigments were monitored in the Arabian Sea and the Gulf of Oman at the end of the SW monsoon in September 1994 and during the inter-monsoon period in November/December 1994 to determine the seasonal changes in phytoplankton composition. The Gulf of Oman was characterized by sub-surface chlorophyll maxima at 20-40 m during both seasons, and low levels of divinyl chlorophyll a indicated that prochlorophytes did not contribute significantly to the total chlorophyll a. Prymnesiophytes (19′-hexanoyloxyfucoxanthin), diatoms (fucoxanthin) and chlorophyll b containing organisms accounted for most of the phytoplankton biomass in September, while prymnesiophytes dominated in November/December. In the Arabian Sea in September, high total chlorophyll a concentrations up to 1742 ng l^-1 were measured in the coastal upwelling region and a progressive decline was monitored along the 1670 km offshore transect to oligotrophic waters at 8°N. Divinyl chlorophyll a was not detected along this transect except at the two most southerly stations where prochlorophytes were estimated to contribute 25–30% to the total chlorophyll a. Inshore, the dominance of fucoxanthin and/or hexanoyloxyfucoxanthin indicated that diatoms and prymnesiophytes generally dominated the patchy phytoplankton community, with zeaxanthin-containing Synechococcus also being important, especially in surface waters. At the southern oligotrophic localities, Synechococcus and prochlorophytes dominated the upper 40 m and prymnesiophytes were the most prominent at the deep chlorophyll maximum. During the inter-monsoon season, total chlorophyll a concentrations were generally half those measured in September and highest levels were found on the shelf (1170 ng l^-1). Divinyl chlorophyll a was detected at all stations along the Arabian Sea transect, and we estimated that prochlorophytes contributed between 3 and 28% to the total chlorophyll a, while at the two oligotrophic stations this proportion increased to 51–52%. While procaryotes were more important in November/December than September, eucaryotes still accounted for >50% of the total chlorophyll a. Pigment/total chlorophyll a ratios indicated that 19′-hexanoyloxyfucoxanthin-containing prymnesiophytes were the dominant group, although procaryotes accounted for 65% at the two southerly oligotrophic stations.     

Distribution of bacterial abundance and cell-specific nucleic acid content in the Northeast Pacific Ocean

We tested the idea that bacterial cells with high nucleic acid content (HNA cells) are the active component of marine bacterioplankton assemblages, while bacteria with low nucleic acid content (LNA cells) are inactive, with a large data set (>1700 discrete samples) based on flow cytometric analysis of bacterioplankton in the Northeast Pacific Ocean off the coast of Oregon and northern California, USA. Samples were collected in the upper 150 m of the water column from the coast to 250 km offshore during 14 cruises from March 2001 to September 2003. During this period, a wide range of trophic states was encountered, from dense diatom blooms (chlorophyll-a concentrations up to 43 μg l⁻¹) at shelf stations during upwelling season (March–September) to lower chlorophyll-a concentrations (0.1–5 μg l⁻¹) during winter (November–February) and at basin stations (>1700 m depth). We found only weakly positive relations of log total bacterial abundance to log chlorophyll-a concentration (as a proxy for availability of organic substrate), and of HNA bacteria as a fraction of total bacteria to log chlorophyll-a. Abundance of HNA and LNA bacteria co-varied positively in all regions, although HNA bacteria were more responsive to high phytoplankton biomass in shelf waters than in slope and basin waters. Since LNA cell abundance in general showed responses similar to those of HNA cell abundance to changes in phytoplankton biomass, our data do not support the hypothesis that HNA cells are the sole active component of marine bacterioplankton.