Nutrient flux into an intense deep chlorophyll layer in a mode-water eddy

An intense deep chlorophyll layer in the Sargasso Sea was reported near the center of an anticyclonic mode-water eddy by McGillicuddy et al. [2007. Eddy–wind interactions stimulate extraordinary mid-ocean plankton blooms, Science, accepted]. The high chlorophyll was associated with anomalously high concentrations of diatoms and with a maximum in the vertical profile of ¹⁴C primary productivity. Here we report tracer measurements of the vertical advection and turbulent diffusion of deep-water nutrients into this chlorophyll layer. Tracer released in the chlorophyll layer revealed upward motion relative to isopycnal surfaces of about 0.4 m/d, due to solar heating and mixing. The density surfaces themselves shoaled by about 0.1 m/d. The upward flux of dissolved inorganic nitrogen, averaged over 36 days, was approximately 0.6 mmol/m²/d due to both upwelling and mixing. This flux is about 40% of the basin wide, annually averaged, nitrogen flux required to drive the annual new production in the Sargasso Sea, estimated from the oxygen cycle in the euphotic zone, the oxygen demand below the euphotic zone, and from the ³He excess in the mixed layer. The observed upwelling of the fluid was consistent with theoretical models [Dewar, W.K., Flierl, G.R., 1987. Some effects of wind on rings. Journal of Physical Oceanography 17, 1653–1667; Martin, A.P., Richards, K.J., 2001. Mechanisms for vertical nutrient transport within a North Atlantic mesoscale eddy. Deep-Sea Research II 48, 757–773] in which eddy surface currents cause spatial variations in surface stress. The diapycnal diffusivity at the base of the euphotic zone was 3.5±0.5×10⁻⁵ m²/s. Diapycnal mixing was probably enhanced over more typical values by the series of storms passing over the eddy during the experiment and may have been enhanced further by the trapping of near-inertial waves generated within the eddy.     

Particle fluxes associated with mesoscale eddies in the Sargasso Sea

We examined the impact of a cyclonic eddy and mode-water eddy on particle flux in the Sargasso Sea. The primary method used to quantify flux was based on measurements of the natural radionuclide, ²³⁴Th, and these flux estimates were compared to results from sediment traps in both eddies, and a ²¹⁰Po/²¹⁰Pb flux method in the mode-water eddy. Particulate organic carbon (POC) fluxes at 150 m ranged 1–4 mmol C m⁻² d⁻¹ and were comparable between methods, especially considering differences in integration times scales of each approach. Our main conclusion is that relative to summer mean conditions at the Bermuda Atlantic Time-series Study (BATS) site, eddy-driven changes in biogeochemistry did not enhance local POC fluxes during this later, more mature stage of the eddy life cycle (>6 months old). The absence of an enhancement in POC flux puts a constraint on the timing of higher POC flux events, which are thought to have caused the local O₂ minima below each eddy, and must have taken place >2 months prior to our arrival. The mode-water eddy did enhance preferentially diatom biomass in its center, where we estimated a factor of three times higher biogenic Si flux than the BATS summer average. An unexpected finding in the highly depth-resolved ²³⁴Th data sets is narrow layers of particle export and remineralization within the eddy. In particular, a strong excess ²³⁴Th signal is seen below the deep chlorophyll maxima, which we attribute to remineralization of ²³⁴Th-bearing particles. At this depth below the euphotic zone, de novo particle production in the euphotic zone has stopped, yet particle remineralization continues via consumption of labile sinking material by bacteria and/or zooplankton. These data suggest that further study of processes in ocean layers is warranted not only within, but below the euphotic zone.     

The carbon dioxide system and net community production within a cyclonic eddy in the lee of Hawaii

The dynamics of dissolved inorganic carbon (DIC) and processes controlling net community production (NCP) were investigated within a mature cyclonic eddy, Cyclone Opal, which formed in the lee of the main Hawaiian Islands in the subtropical North Pacific Gyre. Within the eddy core, physical and biogeochemical properties suggested that nutrient- and DIC-rich deep waters were uplifted by 80 m relative to surrounding waters, enhancing biological production. A salt budget indicates that the eddy core was a mixture of deep water (68%) and surface water (32%). NCP was estimated from mass balances of DIC, nitrate+nitrite, total organic carbon, and dissolved organic nitrogen, making rational inferences about the unobserved initial conditions at the time of eddy formation. Results consistently suggest that NCP in the center of the eddy was substantially enhanced relative to the surrounding waters, ranging from 14.1±10.6 (0–110 m: within the euphotic zone) to 14.2±9.2 (0–50 m: within the mixed layer) to 18.5±10.7 (0–75 m: within the deep chlorophyll-maximum layer) mmol C m⁻² d⁻¹ depending on the depth of integration. NCP in the ambient waters outside the eddy averaged about 2.37±4.24 mmol C m⁻² d⁻¹ in the mixed layer (0–95 m). Most of the enhanced NCP inside the eddy appears to have accumulated as dissolved organic carbon (DOC) rather than exported as particulate organic carbon (POC) to the mesopelagic. Our results also suggest that the upper euphotic zone (0–75 m) above the deep chlorophyll maximum is characterized by positive NCP, while NCP in the lower layer (>75 m) is close to zero or negative.     

Influence of basin-scale and mesoscale physical processes on biological productivity in the Bay of Bengal during the summer monsoon

Physical forcing plays a major role in determining biological processes in the ocean across the full spectrum of spatial and temporal scales. Variability of biological production in the Bay of Bengal (BoB) based on basin-scale and mesoscale physical processes is presented using hydrographic data collected during the peak summer monsoon in July–August, 2003. Three different and spatially varying physical processes were identified in the upper 300 m: (I) anticyclonic warm gyre offshore in the southern Bay; (II) a cyclonic eddy in the northern Bay; and (III) an upwelling region adjacent to the southern coast. In the warm gyre (>28.8 °C), the low salinity (33.5) surface waters contained low concentrations of nutrients. These warm surface waters extended below the euphotic zone, which resulted in an oligotrophic environment with low surface chlorophyll a (0.12 mg m⁻³), low surface primary production (2.55 mg C m⁻³ day⁻¹) and low zooplankton biovolume (0.14 ml m⁻³). In the cyclonic eddy, the elevated isopycnals raised the nutricline upto the surface (NO₃–N > 8.2 μM, PO₄–P > 0.8 μM, SiO₄–Si > 3.5 μM). Despite the system being highly eutrophic, response in the biological activity was low. In the upwelling zone, although the nutrient concentrations were lower compared to the cyclonic eddy, the surface phytoplankton biomass and production were high (Chl a – 0.25 mg m⁻³, PP – 9.23 mg C m⁻³ day⁻¹), and mesozooplankton biovolume (1.12 ml m⁻³) was rich. Normally in oligotrophic, open ocean ecosystems, primary production is based on ‘regenerated’ nutrients, but during episodic events like eddies the ‘production’ switches over to ‘new production’. The switching over from ‘regenerated production’ to ‘new production’ in the open ocean (cyclonic eddy) and establishment of a new phytoplankton community will take longer than in the coastal system (upwelling). Despite the functioning of a cyclonic eddy and upwelling being divergent (transporting of nutrients from deeper waters to surface), the utilization of nutrients leading to enhanced biological production and its transfer to upper trophic levels in the upwelling region imply that the energy transfer from primary production to secondary production (mesozooplankton) is more efficient than in the cyclonic eddy of the open ocean. The results suggest that basin-scale and mesoscale processes influence the abundance and spatial heterogeneity of plankton populations across a wide spatial scale in the BoB. The multifaceted effects of these physical processes on primary productivity thus play a prominent role in structuring of zooplankton communities and could consecutively affect the recruitment of pelagic fisheries.     

Physical dynamics and biological implications of a mesoscale eddy in the lee of Hawai’i: Cyclone Opal observations during E-Flux III

E-Flux III (March 10–28, 2005) was the third and last field experiment of the E-Flux project. The main goal of the project was to investigate the physical, biological and chemical characteristics of mesoscale eddies that form in the lee of Maui and the Island of Hawai’i, focusing on the physical–biogeochemical interactions. The primary focus of E-Flux III was the cyclonic cold-core eddy Opal, which first appeared in the NOAA GOES sea-surface temperature (SST) imagery during the second half of February 2005. During the experiment, Cyclone Opal moved over 160 km, generally southward. Thus, the sampling design had to be constantly adjusted in order to obtain quasi-synoptic observations of the eddy. Analyses of ship transect-depth profiles of CTD, optical and acoustic Doppler current profiler (ADCP) data revealed a well-developed feature characterized by a fairly symmetric circular shape with a radius of about 80 km. Depth profiles of temperature, salinity and density were characterized by an intense doming of isothermal, isohaline and isopycnal surfaces. Isopleths of nutrient concentrations were roughly parallel to isopycnals, indicating the upwelling of deep nutrient-rich water. The deep chlorophyll maximum layer (DCML) shoaled from a depth of about 130 m in the outer regions of the eddy to about 60 m in the center. Chlorophyll concentrations reached their maximum values in Opal's core region (about 40 km in diameter), where nutrients were upwelled into the euphotic layer. ADCP velocity data clearly showed the cyclonic circulation associated with Opal. Vertical sections of tangential velocities were characterized by values that increased linearly with radial distance from near zero close to the center to a maximum of about at roughly 25 km from the center, and then slowly decayed. The vertical extent of the cyclonic circulation was primarily limited to the upper mixed layer, as tangential velocities decayed quite rapidly within a depth range of 90–130 m. Potential vorticity analysis suggests that only a relatively small (about 50 km in diameter) and shallow (to a depth of approximately 70 m) portion of the eddy is isolated from the surrounding waters. Radial movements of water can occur between the center of the eddy and the outer regions along density surfaces within an isopycnal range of σ–t_23.6 () and σ–t_24.4 (). Thus the biogeochemistry of the system might have been greatly influenced by these lateral exchanges of water at depth, especially during Opal's southward migration. While the eddy was translating, deep water in front of the eddy might have been upwelled into the core region, leading to an additional injection of nutrients into the euphotic zone. At the same time, part of the chlorophyll-rich waters in the core region might have remained behind the translating eddy and, thus contributed to the formation of an eddy wake characterized by relatively high chlorophyll concentrations.     

TIMS measurements of Ra and Ra in the Gulf of Lion, an attempt to quantify submarine groundwater discharge

Submarine groundwater discharge (SGD) is now recognized as an important pathway for water and chemical species fluxes to the coastal ocean. In order to determinate SGD to the Gulf of Lion (France), we measured the activities of ²²⁶Ra and ²²⁸Ra by thermal ionization mass spectrometry (TIMS) in coastal waters and in the deep aquifer waters of the Rhone deltaic plain after pre-concentration of radium by MnO₂. Compared to conventional counting techniques, TIMS requires lower quantities of water for the analyses, and leads to higher analytical precision. Radium isotopes were thus measured on 0.25–2 L water samples containing as little as 20 fg of ²²⁶Ra and 0.2–0.4 fg of ²²⁸Ra with precision equal to 2%. We demonstrate that coastal surface waters samples are enriched in ²²⁶Ra and ²²⁸Ra compared to the samples further offshore. The high precision radium measurements display a small but significant ²²⁶Ra and ²²⁸Ra enrichment within a strip of circa 30 km from the coast. Radium activities decrease beyond this region, entrained in the northern current along the shelf break or controlled by eddy diffusion. The radium excess in the first 30 km cannot be accounted for by the river nor by the early diagenesis. The primary source of the radium enrichment must therefore be ascribed to the discharge of submarine groundwater. Using a mass-balance model, we estimated the advective fluxes of ²²⁶Ra and ²²⁸Ra through SGD to be 5.2 × 10¹⁰ and 21 × 10¹⁰ dpm/d respectively. The ²²⁶Ra activities measured in the groundwater from the Rhone deltaic plain aquifer are comparable to those from other coastal groundwater studies throughout the world. By contrast, ²²⁸Ra activities are higher by up to one order of magnitude. Taking those groundwater radium activities as typical of the submarine groundwater end-member, a minimum volume of 0.24–4.5 × 10¹⁰ l/d is required to support the excess radium isotopes on the inner shelf. This has to be compared with the average rivers water runoff of 15.4 × 10¹⁰ l/d during the study period (1.6 to 29% of the river flow).     

Ba, Si, U, Al, Sc, La, Th, C and C/C in a sediment core in the western subarctic Pacific as proxies of past biological production

A sediment core covering the last 145 kyrs was collected in the western subarctic Pacific (WSAP), and analyzed for Ba, U, Al, Sc, La, Yb, Th, biogenic opal (Opal) and organic carbon (C_org) as well as its isotopic ratio (δ¹³C). This study examined the change of past biological production in WSAP with multiple proxies, together with understanding the relation between Loess from the Asian continent and the biological production. The Loess content was estimated from the metal components, Al, Sc, La, Yb and Th. In this high latitude core (50°N), the Loess content was generally high during the glacial periods, but it was also high even in some interglacial periods. The excess amount of Ba relative to the detrital material composition, Ba_ex, showed the best correlation with the Vostok δD (r = 0.72, p < 0.001), indicating that the biological production was lower in the glacial periods than in the interglacials. This corroborates the pervasive correlation between Ba_ex in the polar region, WSAP and the Antarctic Sea, and Antarctic temperature, combined with previous research. This correlation might be explained by the stratification caused by cooling. In addition, the time variations of Ba_ex in WSAP were similar to those of Ba_ex in the Okhotsk Sea and of other proxies (C_org and Opal) in both the Okhotsk and the Bering Sea, indicating the spatial homogeneity of Ba_ex in WSAP including proximal marginal seas. The Opal content was more weakly correlated with the Vostok δD (r = 0.46, p < 0.001) than Ba_ex, reflecting that Opal in WSAP including proximal marginal seas was spatially heterogeneous compared to Ba_ex. While both the C_org content and U_ex, the excess amount of U relative to the detritus composition, were not positively correlated with the Vostok δD, they behaved similarly in the sediments. The positive correlation between δ¹³C and the Vostok δD (r = 0.42, p < 0.001), between δ¹³C and Ba_ex (r = 0.60, p < 0.001) and between δ¹³C and Opal (r = 0.36, p < 0.01) indicates that δ¹³C in WSAP may give some information on the phytoplankton growth rate. There was not a significant correlation between the spatially homogeneous Ba_ex in WSAP and Loess (r = − 0.16, p > 0.01), suggesting that the increase of biological production with the increase of Loess supply during the glacial periods did not occur.     

Nutrient distributions in baroclinic eddies of the oligotrophic North Atlantic and inferred impacts on biology

High-sensitivity (nanomolar) techniques for nitrate and phosphate were applied to study nutrient patterns in the euphotic zone of mesoscale eddies in the Sargasso Sea during the EDDIES project. Surface concentrations of nitrate plus nitrite (DNN) and phosphate (DIP) were found in the range of 1–20 nM with substantial spatial variability in the eddies, with resulting mean N:P molar ratios of 2.1. Chlorophyll biomass was well correlated with DNN but not DIP in the upper euphotic zone, suggesting N-limitation of marine phytoplankton at this time of year. Within the upper 140 m, the water column experienced a transition from a P-enriched (relative to Redfield ratio) shallow layer to a N-enriched deep layer, which may suggest downward transport and subsequent remineralization of high N/P biogenic products presumably originating from N₂ fixation. Chlorophyll biomass in the deep chlorophyll maximum of eddies was found to be tightly related to eddy–induced variability in major nutrients (N, P, Si) and nutrient stoichiometry, suggesting that the impact of eddies on biology is through control of nutrient availability. Because the eddies were likely to be in various phases of development (different degrees of both biological and physical maturity), full interpretation of eddy data and dynamics will require better coverage of a full eddy life cycle.     

N2O Production, Nitrification and Denitrification in an Estuarine Sediment

The mechanisms regulating N₂O production in an estuarine sediment (Tama Estuary, Japan) were studied by comparing the change in N₂O production with those in nitrification and denitrification using an experimental continuous-flow sediment–water system with¹⁵N tracer (¹⁵N-NO−3 addition). From Feburary to May, both nitrification and denitrification in the sediment increased (246 to 716 μmol N m⁻² h⁻¹and 214 to 1260 μmol N m⁻² h⁻¹, respectively), while benthic N₂O evolution decreased slightly (1560 to 1250 nmol N m⁻² h⁻¹). Apparent diffusion coefficients of inorganic nitrogen compounds and O₂at the sediment–water interface, calculated from the respective concentration gradients and benthic fluxes, were close to the molecular diffusion coefficients (0·68–2·0 times) in February. However, they increased to 8·8–52 times in May except for that of NO−2, suggesting that the enhanced NO−3 and O₂supply from the overlying water by benthic irrigation likely stimulated nitrification and denitrification. Since the progress of anoxic condition by the rise of temperature from February to May (9 to 16 °C) presumably accelerated N₂O production through nitrification, the observed decrease in sedimentary N₂O production seems to be attributed to the decrease in N₂O production/occurrence of its consumption by denitrification. In addition to the activities of both nitrification and denitrification, the change in N₂O metabolism during denitrification by the balance between total demand of the electron acceptor and supply of NO−3+NO−2 can be an important factor regulating N₂O production in nearshore sediments.     

Helium Isotopes in Pacific Waters from Adjacent Region of Honshu, Japan

We have measured helium isotopic ratios of thirty-seven Pacific water samples from various depths collected in adjacent regions of Honshu, Japan. The ³He/⁴He ratios vary significantly from 0.989 R _atm to 1.208 R _atm where R _atm is the atmospheric ratio of 1.39 × 10⁻⁶. The mid-depth (750–1500 m) profile of ³He/⁴He ratios at ST-1 located Northwestern Pacific Ocean east of Japan (Off Joban; 37°00′ N, 142°40′ E) is significantly different from that at ST-2 of the Northern Philippine Sea south of Japan (Nankai Trough; 33°07′ N, 139°59′ E), suggesting that these waters were separated by a topographic barrier, the Izu-Ogasawara Ridge. Taking ³He/⁴He data of the Geosecs expeditions in the western North Pacific, an extensive plume of 15% excess ³He relative to the air may be traced at ST-1 over 12,000 kilometers to the northwest of the East Pacific Rise where the mantle helium may originate. The 20% excess found at ST-2 may be attributable to the additional source of the subduction-type mantle helium in the Okinawa Trough. A 15% excess of ³He has also been discovered at a depth of about 1000∼1500 m at ST-3 adjacent to Miyakejima Island (33°57′ N, 139°22′ E) and ST-4 of Sagami Bay (35°00′ N, 139°22′ E). It is confirmed that mid-depth all over the western North Pacific water is affected by the mantle helium with a high ³He/⁴He ratio. This revised version was published online in July 2006 with corrections to the Cover Date.     

Plankton dynamics and carbon flux in an area of upwelling off the coast of Morocco

A carbon flux study was carried out off the coast of Morocco, at 31°N, in a region characterized by the presence of a persistent cyclonic eddy. Two short-term (4 and 3 day) deployments of free-floating sediment traps were combined with water column sampling and rate process measurements as the ship followed the traps. For a period of 36 h between trap deployments, a hydrographic section was run along 31°30'N as part of a larger scale survey being carried out simultaneously on the R.V. A. von Humboldt. The first trap deployment was near the eastern margin of the eddy and the traps moved to the north and west in a frontal jet associated with its northern boundary. After the second deployment, which was at the recovery point of the first, the traps moved to the west and then to the southwest. Throughout the study, chlorophyll concentrations varied between 27 and 125 mg m⁻² (0–100 m), with highest concentrations in the upwelled water nearest the coast and in upwelled water generated within the cyclonic eddy. Particulate organic carbon (POC) and particulate organic nitrogen (PON) concentrations were relatively uniform (13.6±1.8 and 1.63±28 g m⁻² with phytoplankton carbon accounting for 16–85% of total POC. Bacterial carbon was 5% of total POC and mesozooplankton carbon concentrations were equivalent to 9% of total POC. Microzooplankton biomass was not assessed but POC:PON ratios in the water column were often high, suggesting there was sometimes a large detrital component in the POC. Primary production rates varied between 1.0 and 2.5 g C m⁻² day⁻¹. Bacterial consumption accounted for 50% of primary production. Metabolic rates suggested that copepods were ingesting more than 0.4 g C m⁻² day⁻¹. while filtration rates suggested that ingestion of phytoplankton carbon was only 0.2 g C m⁻²day⁻¹, even when phytoplankton constituted 85% of the POC. f-ratios (based on uptake rates for ¹⁵N-nitrate and ammonia) were between 0.1 and 0.4, and excretion by mesozooplankton could account for 40% of the daily ammonium uptake by phytoplankton. HPLC pigment analysis showed that when chlorophyll biomass was high, diatoms were dominant, whereas when it was low, small prymnesiophytes, chlorophytes and diatoms were all important. The composition of the fluoresecent pigments in material in the sediment traps indicated that intact phytoplankton and copepod faecal pellets were the main sources but the relative rates of sedimentation of pigment, POC and PON for the two trapping periods did not reflect differences that were observed in the overlying water column. This was likely to be the result of spatial heterogeneity and strong horizontal currents heterogeneity and strong horizontal currents within the euphotic zone. Thus, material collected at 100 m probably did not originate in the water column immediately overlying the traps and trapping efficiencies might also have been variable.     

Variability in plankton community structure, metabolism, and vertical carbon fluxes along an upwelling filament (Cape Juby, NW Africa)

The variability in dissolved and particulate organic matter, plankton biomass, community structure and metabolism, and vertical carbon fluxes were studied at four stations (D₁–D₄), placed along a coastal-offshore gradient of an upwelling filament developed near Cape Juby (NW Africa). The filament was revealed as a complex and variable system in terms of its hydrological structure and distribution of biological properties. An offshore shift from large to small phytoplankton cells, as well as from higher to lower autotrophic biomass, was not paralleled by a similar gradient in particulate (POC) or dissolved (DOC) organic carbon. Rather, stations in the central part of the filament (D₂ and D₃) presented the highest organic matter concentrations. Autotrophic carbon (POC_Chl) accounted for 53% (onshore station, D₁) to 27% (offshore station, D₄) of total POC (assuming a carbon to chlorophyll ratio of 50), from which nano- and pico-phytoplankton biomasses (POC_{A < 10 μm}) represented 14% (D₁) to 79% (D₄) of POC_Chl. The biomass of small hetrotrophs (POC_{H < 10 μm}) was equivalent to POC_{A < 10 μm}, except at D₁, where small autotrophs were less abundant. Dark community respiration (R_d) in the euphotic zone was in general high, almost equivalent to gross production (P_g), but decreasing offshore (D₁–D₄, from 108 to 41 mmol C m⁻² d⁻¹). POC sedimentation rates (POC_sed) below the euphotic zone ranged from 17 to 6 mmol C m⁻² d⁻¹. Only at D₄ was a positive carbon balance observed: P_g−(R_d + POC_sed) = 42 mmol C m⁻² d⁻¹. Compared to other filament studies from the NE Atlantic coast, the Cape Juby filament presented lower sedimentation rates and higher respiration rates with respect to gross production. We suggest that this is caused by the recirculation of the filament water, induced by the presence of an associated cyclonic eddy, acting as a trapping mechanism for organic matter. The export capacity of the Cape Juby filament therefore would be constrained to the frequency of the interactions of the filament with island-induced eddies.     

Impact of a cyclonic eddy on phytoplankton community structure and photosynthetic competency in the subtropical North Pacific Ocean

A synoptic spatial examination of the eddy Haulani (17–20 November 2000) revealed a structure typical of Hawaiian cyclonic eddies with divergent surface flow forcing the upward displacement of deep waters. Hydrographic surveys revealed that surface water in the eddy center was ca. 3.5°C cooler, 0.5 saltier, and 1.4 kg m⁻³ denser than surface waters outside the eddy. Vertically integrated concentrations of nitrate+nitrite, phosphate and silicate were enhanced over out-eddy values by about 2-fold, and nitrate+nitrite concentrations were ca. 8× greater within the euphotic zone inside the eddy than outside. Si:N ratios were lower within the upper mixed layer of the eddy, indicating an enhanced Si uptake relative to nitrate+nitrite. Chlorophyll a concentrations were higher within the eddy compared to control stations outside, when integrated over the upper 150 m, but were not significantly different when integrated over the depth of the euphotic zone. Photosynthetic competency, assessed using fast repetition-rate fluorometry, varied with the doming of the isopycnals and the supply of macro-nutrients to the euphotic zone. The physical and chemical environment of the eddy selected for the accumulation of larger phytoplankton species. Photosynthetic bacteria (Prochlorococcus and Synechococcus) and small (<3 μm diameter) photosynthetic eukaryotes were 3.6-fold more numerically abundant outside the eddy as compared to inside. Large photosynthetic eukaryotes (>3 μm diameter) were more abundant inside the eddy than outside. Diatoms of the genera Rhizosolenia and Hemiaulus outside the eddy contained diazotrophic endosymbiontic cyanobacteria, but these endosymbionts were absent from the cells of these species inside the eddy. The increase in cell numbers of large photosynthetic eukaryotes with hard silica or calcite cell walls is likely to have a profound impact on the proportion of the organic carbon production that is exported to deep water by sinking of senescent cells and cells grazed by herbivorous zooplankton and repackaged as large fecal pellets.     

Downward export of carbon by diel migrant mesozooplankton in the central equatorial Pacific

Using data collected during cruises of the JGOFS equatorial Pacific Study in March/April and October of 1992 at the equator (140°W), we examine the downward transport of carbon by three size classes of die] migrant mesozooplankton (200–500 gm, 500–1000 μm and 1000–2000 gm). In addition to respiratory carbon flux, we consider the flux due to mortality of migrators below the euphotic zone. Diel migrant mesozooplankton biomass was estimated from the difference between nighttime and daytime biomass within the euphotic zone. Except for a four-day period early in the March/April cruise, mesozooplankton nighttime biomass was significantly larger than daytime biomass within the euphotic zone during both cruises. We estimate that the downward flux of carbon from the euphotic zone due to mesozooplankton die] vertical migrators was an average of 0.6 mmol Cm⁻² d⁻¹ and 1.1 mmol C m⁻² d⁻¹ during the March/April and October cruises, respectively. Addition of this flux to the gravitational particle sinking flux estimated from²³⁴Th measurements during the same period results in a 31 % increase in the carbon export flux from the euphotic zone in the equatorial Pacific during the March/April cruise and a 44% increase in the October cruise. The migratory flux is strongly dependent on whether feeding takes place below the euphoric zone, the length of time migrators spend in the deep waters, and the mortality rate of migrators.     

A nitrate and silicate budget in the equatorial Pacific Ocean: a coupled physical–biological model study

A coupled physical–biological model was developed to simulate the low-silicate, high-nitrate, and low-chlorophyll (LSHNLC) conditions in the equatorial Pacific Ocean and used to compute a detailed budget in the Wyrtki box (5°N–5°S, 180–90°W) for the major sources and cycling of nitrogen and silicon in the equatorial Pacific. With the incorporation of biogenic silicon dissolution, NH₄ regeneration from organic nitrogen and nitrification of ammonia in the model, we show that silicon recycling in the upper ocean is less efficient than nitrogen. As the major source of nutrients to the equatorial Pacific, the Equatorial Undercurrent provides slightly less Si(OH)₄ than NO₃ to the upwelling zone, which is defined as 2.5°N–2.5°S. As a result, the equatorial upwelling supplies less Si(OH)₄ than NO₃ into the euphotic zone in the Wyrtki box, having a Si/N supply ratio of about 0.85 (2.5 vs. 2.96 mmolm⁻² day⁻¹). More Si(OH)₄ than NO₃ is taken up with a Si/N ratio of 1.17 (2.72 vs. 2.33 mmolm⁻² day⁻¹) within the euphotic zone. The difference between upwelling supply and biological uptake is balanced by nutrient regeneration and horizontal advection. Excluding regeneration, the net silicate and nitrate uptakes are nearly equal (1.76 vs. 1.84 mmolm⁻² day⁻¹). However, biogenic silica export production is slightly higher than organic nitrogen (1.74 vs. 1.59 mmolm⁻² day⁻¹) following a 1.1 Si/N ratio. In the central equatorial Pacific, low silicate concentrations limit diatom growth; therefore non-diatom new production accounts for most of the new production. Higher silicate supply in the east maintains elevated diatom growth rates and new production associated with diatoms dominate upwelling zone. In contrast, the new production associated with small phytoplankton is nearly constant or decreases eastward along the equator. The total new production has a higher rate in the east than in the west, following the pattern of surface silicate. This suggests that silicate regulates the diatom production, total new production, and thereby carbon cycle in this area. The modeled mean primary production is 48.4 mmolCm⁻² day⁻¹, representing the lower end of direct field measurements, while new production is 15.0 mmolCm⁻² day⁻¹, which compares well with previous estimates.     

The influence of a mature cyclonic eddy on particle export in the lee of Hawaii

Mesoscale eddies may enhance primary production (PP) in the open ocean by bringing nutrient-rich deep waters into the euphotic zone, potentially leading to increased transport of particles to depth. This hypothesis remains controversial, however, due to a paucity of direct particle export measurements. In this study, we investigated particle dynamics using ²³⁴Th–²³⁸U disequilibria within a mesoscale cold-core eddy, Cyclone Opal, which formed in the lee of the Hawaiian Islands. ²³⁴Th samples were collected along two transects across Cyclone Opal as well as during a time-series within the eddy core during a decaying diatom bloom. Particulate carbon (PC), particulate nitrogen (PN) and biogenic silica (bSiO₂) fluxes at 150 m varied spatially and temporally within the eddy and strongly depended on the ²³⁴Th model formulation used (e.g., steady state versus non-steady state, inclusion of upwelling, etc.). Particle fluxes estimated from a steady state model assuming an upwelling rate of 2 m day⁻¹ yielded the best fit to sediment-trap data. These ²³⁴Th-derived particle fluxes ranged from 332±14 to 1719±53 μmol C m⁻² day⁻¹, 27±3 to 114±12 μmol N m⁻² day⁻¹, and 33±20 to 309±73 μmol Si m⁻² day⁻¹. Although PP rates within Cyclone Opal were elevated by a factor of 2–3, PC and PN fluxes were the same, within error, inside and outside of Cyclone Opal. The ratio of PC export to PP remained surprisingly low at <0.03 and similar to those measured in surrounding waters. In contrast, bSiO₂ fluxes within the eddy core were three times higher. Detailed analyses of ²³⁴Th depth profiles consistently showed excess ²³⁴Th at 100–175 m, associated with the remineralization and possible accumulation of suspended and dissolved organic matter from the surface. We suggest that strong microzooplankton grazing facilitated particulate organic matter recycling and resulted in the export of empty diatom frustules. Thus, while eddies may increase PP, they do not necessarily increase PC and PN export to deep waters. This may be a general characteristic of wind-driven cyclonic eddies of the North Pacific Subtropical Gyre and suggests that eddies may preferentially act as a silica pump, thereby playing an important role in promoting silicic-acid limitation in the region.     

Microbial respiration in the Levantine Sea: evolution of the oxidative processes in relation to the main Mediterranean water masses

First data on microbial respiration in the Levantine Sea are reported with the aim of assessing the distribution of oxidative processes in association with the main Mediterranean water masses and the changing physical structure determined by the Eastern Mediterranean Transient. Respiratory rates, in terms of metabolic carbon dioxide production, were estimated from measured electron transport system activities in the polygonal area of the Levantine Sea (32.5–36.5 N Latitude, 26.0–30.25 E Longitude) and at Station Geo’95, in the Ionian Sea (35°34.88 N; 17°14.99 E). At the Levantine Sea, the mean carbon dioxide production rate decreased from the upper to the deeper layers and varied from 22.0±12.4 μg C h⁻¹ m⁻³ in the euphotic layer to 1.30±0.5 μg C h⁻¹ m⁻³ in the depth range between 1600 and 3000 m. Significant differences were found among upper, intermediate and bottom layers. The euphotic zone supported a daily carbon dioxide production of 96.6 mg C d⁻¹ m⁻² while the aphotic zone (between 200 and 3000 m) sustained a 177.1 mg C d⁻¹ m⁻² carbon dioxide production. In Station Geo’95, the carbon dioxide production rates amounted to 170.4 and 102.2 mg C d⁻¹ m⁻² in the euphotic and aphotic zones, respectively. The rates determined in the identified water masses showed a tight coupling of respiratory processes and Mediterranean circulation patterns. The increasing respiratory rates in the deep layers of the Levantine Sea are explained by the introduction of younger waters recently formed in the Aegean Sea.     

Microbial respiration in the aphotic zone of the Ross Sea (Antarctica)

In January–February 2001, we measured microbial biomass as ATP and community respiration as ETS activity of organisms < 200 μm in the aphotic zone of the Ross Sea. Microbial respiration amounted to 2.14 mmol C m^− 2 day^− 1 in the depth range 200–1000 m. Our daily estimates of carbon export are close to the daily percentage of net community production (NCP), removed as sinking biogenic particles from the upper 100 m in the entire Ross Sea, but lower than those of other oceanic systems. Comparing remineralization determined in this study with that obtained by sediment traps in the Ross Sea, it appeared that about 63% of organic carbon remineralized by respiration derived from POC pool. Such evidence highlighted POC source as the main organic fuel of the biological pump in the Ross Sea.     

Planktonic ciliates in the Mediterranean Sea: longitudinal trends

We analysed samples taken through the euphotic zone from 18 stations between the Ligurian Sea (6°E) and the Levantin Basin (32°E) from 24 May to 25 June 1996. Both ciliate and chlorophyll concentrations ranged over a factor of about 7, but ciliate concentrations (0.4–2.8 mg C m³) varied irregularly compared to a longitudinal decline, west to east, in chlorophyll concentration (0.07–0.48 mg m³). The lower chlorophyll concentrations (0.1 mg m²) of the eastern basin stations corresponded with a relatively high stock of ciliates (0.5 mg C m²). Large mixotrophic ciliates were more abundant, in both absolute and relative terms, in the eastern Mediterranean stations with less chlorophyll. The species diversity of tintinnid ciliates appeared higher in the central and eastern basins compared to the west. Our results suggest a shift from the western to eastern Mediterranean in the planktonic food towards a microbially dominated system.     

Seasonal horizontal and vertical variability in primary production and standing stocks of phytoplankton and zooplankton in the Cretan Sea and the Straits of the Cretan Arc (March 1994–January 1995)

Phytoplankton communities, production rates and chlorophyll levels, together with zooplankton communities and biomass, were studied in relation to the hydrological properties in the euphotic zone (upper 100 m) in the Cretan Sea and the Straits of the Cretan Arc. The data were collected during four seasonal cruises undertaken from March 1994 to January 1995.The area studied is characterised by low nutrient concentrations, low ¹⁴C fixation rates, and impoverished phytoplankton and zooplankton standing stocks. Seasonal fluctuations in phytoplankton densities, chlorophyll standing stock and phytoplankton production are significant; maxima occur in spring and winter and minima in summer and autumn. Zooplankton also shows a clear seasonal pattern, with highest abundances occurring in autumn–winter, and smallest populations in spring–summer. During summer and early autumn, the phytoplankton distribution is determined by the vertical structure of the water column.Concentrations of all nutrients are very low in the surface waters, but increase at the deep chlorophyll maximum (DCM) layer, which ranges in depth from about 75–100 m. Chlorophyll-a concentrations in the DCM vary from 0.22–0.49 mg m⁻³, whilst the surface values range from 0.03–0.06 mg m⁻³. Maxima of phytoplankton, in terms of cell populations, are also encountered at average depths of 50–75 m, and do not always coincide with chlorophyll maxima. Primary production peaks usually occur within the upper layers of the euphotic zone.There is a seasonal succession of phytoplankton and zooplankton species. Diatoms and ‘others’ (comprising mainly cryptophytes and rhodophytes) dominate in winter and spring and are replaced by dinoflagellates in summer and coccolithophores in autumn. Copepods always dominate the mesozooplankton assemblages, contributing approximately 70% of total mesozooplankton abundance, and chaetognaths are the second most abundant group.