Particle export within cyclonic Hawaiian lee eddies derived from Pb–Po disequilibrium

Particle export from the upper waters of the oligotrophic ocean may play a crucial role in the global carbon cycle. Mesoscale eddies have been hypothesized to inject new nutrients into oligotrophic surface waters, thereby increasing new production and particle export in otherwise nutrient deficient regimes. The E-Flux Program was a large multidisciplinary project designed to investigate the physical, biological and biogeochemical characteristics of cold-core cyclonic eddies that form in the lee of the Hawaiian Islands. There, we investigated particle dynamics using ²¹⁰Pb–²¹⁰Po disequilibrium. Seawater samples for ²¹⁰Pb and ²¹⁰Po were collected both within (IN) and outside (OUT) of two cyclones, Noah and Opal, at different stages of their evolution as well as from the eddy generation region. Particulate carbon (PC), particulate nitrogen (PN) and biogenic silica (bSiO₂) export fluxes were determined using water-column PC, PN, and bSiO₂ inventories and the residence times of ²¹⁰Po. PC and PN fluxes at 150 m ranged from 1.58±0.10 to 1.71±0.16 mmol C m⁻² d⁻¹ and 0.22±0.02 to 0.30±0.02 mmol N m⁻² d⁻¹ within Cyclones Opal and Noah. PC and PN fluxes at OUT stations sampled during both cruises were of similar magnitudes, 1.69±0.16 to 1.67±0.16 mmol C m⁻² d⁻¹ and 0.30±0.03 to 0.26±0.03 mmol N m⁻² d⁻¹. The bSiO₂ fluxes within Cyclone Opal were 0.157±0.010 mmol Si m⁻² d⁻¹ versus 0.025±0.002 mmol Si m⁻² d⁻¹ at OUT stations. These results of minimal PC and PN export, but significant eddy-induced bSiO₂ fluxes, agree very well with other studies that used a variety of direct and indirect methods. Thus, our results suggest that using elemental inventories and residence times of ²¹⁰Po is another independent and robust method for determining particle export and should be investigated more fully.     

Mesozooplankton biomass and grazing responses to Cyclone Opal, a subtropical mesoscale eddy

As part of E-Flux III cruise studies in March 2005, plankton net collections were made to assess the effects of a cyclonic cold-core eddy (Cyclone Opal) on the biomass and grazing of mesozooplankton. Mesozooplankton biomass in the central region of Cyclone Opal, an area of uplifted nutricline and a subsurface diatom bloom, averaged 0.80±0.24 and 1.51±0.59 g DW m⁻², for day and night tows, respectively. These biomass estimates were about 80% higher than control (OUT) stations, with increases more or less proportionately distributed among size classes from 0.2 to >5 mm. Though elevated relative to surrounding waters south of the Hawaiian Islands (Hawai’i lee), total biomass and size distribution in Cyclone Opal were almost exactly the same as contemporary measurements made at Stn. ALOHA, 100 km north of the islands, by the HOT (Hawaii Ocean Time-series) Program. Mesozooplankton biomass and community composition at the OUT stations were also similar to ALOHA values from 1994 to 1996, preceding a recent decadal increase. These comparisons may therefore provide insight into production characteristics or biomass gradients associated with decadal changes at Stn. ALOHA. Gut fluorescence estimates were higher in Opal than in ambient waters, translating to grazing impacts of 0.11±0.02 d⁻¹ (IN) versus 0.03±0.01 d⁻¹ (OUT). Over the depth-integrated euphotic zone, mesozooplankton accounted for 30% of the combined grazing losses of phytoplankton to micro- and meso-herbivores in Opal, as compared to 13% at control stations. Estimates of active export flux by migrating zooplankton averaged 0.81 mmol C m⁻² d⁻¹ in Cyclone Opal and 0.37 mmol C m⁻² d⁻¹ at OUT stations, 53% and 24%, respectively, of the carbon export measured by passive sediment traps. Migrants also exported 0.18 mmol N m⁻² d⁻¹ (117% of trap N flux) in Cyclone Opal compared to 0.08 mmol N m⁻² d⁻¹ (51% of trap flux) at control stations. Overall, the food-web importance of mesozooplankton increased in Cyclone Opal both in absolute and relative terms. Diel migrants provided evidence for enhanced export flux in the eddy that was missed by sediment trap and ²³⁴Th techniques, and migrant-mediated flux was the major export term in the observed bloom-perturbation response and N mass balance of the eddy.     

Nitrogen dynamics within a wind-driven eddy

Wind-driven cyclonic eddies are hypothesized to relieve nutrient stress and enhance primary production by the upward displacement of nutrient-rich deep waters into the euphotic zone. In this study, we measured nitrate (NO₃⁻), particulate carbon (PC), particulate nitrogen (PN), their stable isotope compositions (δ¹⁵N-NO₃⁻, δ¹³C-PC and δ¹⁵N-PN, respectively), and dissolved organic nitrogen (DON) within Cyclone Opal, a mature wind-driven eddy generated in the lee of the Hawaiian Islands. Sampling occurred in March 2005 as part of the multi-disciplinary E-Flux study, approximately 4–6 weeks after eddy formation. Integrated NO₃⁻ concentrations above 110 m were 4.8 times greater inside the eddy (85.8±6.4 mmol N m⁻²) compared to the surrounding water column (17.8±7.8 mmol N m⁻²). Using N-isotope derived estimates of NO₃⁻ assimilation, we estimated that 213±59 mmol m⁻² of NO₃⁻ was initially injected into the upper 110 m Cyclone Opal formation, implying that NO₃⁻ was assimilated at a rate of 3.75±0.5 mmol N m⁻² d⁻¹. This injected NO₃⁻ supported 68±19% and 66±9% of the phytoplankton N demand and export production, respectively. N isotope data suggest that 32±6% of the initial NO₃⁻ remained unassimilated. Self-shading, inefficiency in the transfer of N from dissolved to particulate export, or depletion of a specific nutrient other than N may have led to a lack of complete NO₃⁻ assimilation. Using a salt budget approach, we estimate that dissolved organic nitrogen (DON) concentrations increased from eddy formation (3.8±0.4 mmol N m⁻²) to the time of sampling (4.0±0.09 mmol N m⁻²), implying that DON accumulated at rate of 0.83±1.3 mmol N m⁻² d⁻¹, and accounted for 22±15% of the injected NO₃⁻. Interestingly, no significant increase in suspended PN and PC, or export production was observed inside Cyclone Opal relative to the surrounding water column. A simple N budget shows that if 22±15% of the injected NO₃⁻ was shunted into the DON pool, and 32±6% is unassimilated, then 46±16% of the injected NO₃⁻ remains undocumented. Alternative loss processes within the eddy include lateral exchange of injected NO₃⁻ along isopycnal surfaces, remineralization of PN at depth, as well as microzooplankton grazing. A 9-day time series within Cyclone Opal revealed a temporal depletion in δ¹⁵N-PN, implying a rapid change in the N source. A change in NO₃⁻ assimilation, or a shift from NO₃⁻ fueled growth to assimilation of a ¹⁵N-deplete N source, may be responsible for such observations.     

A time-series study of particulate matter export in the North Pacific Subtropical Gyre based on Th : U disequilibrium

Depth profiles of total ²³⁴Th (dissolved+particulate) were collected at Station ALOHA (22°45N, 158°00W) in the North Pacific Subtropical Gyre during 9 cruises from April 1999 to March 2000. Samples were collected and processed by a new 2 L technique that enables more detailed depth resolution then previous ²³⁴Th studies. Significant zones of particle export (²³⁴Th deficiency) and particle remineralization (²³⁴Th excess) were measured both temporally and with depth. ²³⁴Th derived particulate carbon (PC) and nitrogen (PN) fluxes were determined with steady-state and non-steady-state models and PC/²³⁴Th and PN/²³⁴Th ratios measured with both in situ pumps and free-drifting particle interceptor traps deployed at 150 m. ²³⁴Th based export estimates of 4.0±2.3 mmol C m⁻² d⁻¹ and 0.53±0.19 mmol N m⁻² d⁻¹, were approximately 60% higher than those measured in PIT style sediment traps from the same time period, 2.4±0.2 mmol C m⁻² d⁻¹ and 0.32±0.08 mmol N m⁻² d⁻¹. Most of this difference is attributable to two large export events that occurred during October and December 1999, when traps undercollected for ²³⁴Th by a factor of 2 to 4. ²³⁴Th export (ThE) ratios based on ²³⁴Th derived PC flux/¹⁴C based primary production ranged from 4% to 22% (average=8.8%). Our results confirm the recent estimates of C export by Emerson et al. (Nature 389 (1997) 951) and Sonnerup et al. (Deep-Sea Research I 46 (1999) 777) and indicate that C export from the oligotrophic ocean must be considered when discussing C sequestration in global climate change.     

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.     

Depth-stratified phytoplankton dynamics in Cyclone Opal, a subtropical mesoscale eddy

As part of E-Flux III cruise studies in March 2005, we investigated phytoplankton community dynamics in a cyclonic cold-core eddy (Cyclone Opal) in the lee of the Hawaiian Islands. Experimental incubations were conducted under in situ temperature and light conditions on a drift array using a two-treatment dilution technique. Taxon-specific estimates of growth, grazing and production rates were obtained from analyses of incubation results based on phytoplankton pigments, flow cytometry and microscopy. Cyclone Opal was sampled at a biologically and physically mature state, with an 80–100 m doming of isopycnal surfaces in its central region and a deep biomass maximum of large diatoms. Depth-profile experimentation defined three main zones. The upper (mixed) zone (0–40 m), showed little compositional or biomass response to eddy nutrient enrichment, but growth, grazing and production rates were significantly enhanced in this layer relative to the ambient community outside of the eddy. Prochlorococcus spp. dominated the upper mixed layer, accounting for 50–60% of its estimated primary production both inside and outside of Opal. In contrast, the deep zone of 70–90 m showed little evidence of growth rate enhancement and was principally defined by a 100-fold increase of large (>20-μm) diatoms and a shift from Prochlorococcus to diatom dominance (80%) of production. The intermediate layer of 50–60 m marked the transition between the upper and lower extremes but also contained an elevated biomass of physiologically unhealthy diatoms with significantly depressed growth rates and proportionately greater grazing losses relative to diatoms above or below. Microzooplankton grazers consumed 58%, 65% and 55%, respectively, of the production of diatoms, Prochlorococcus and the total phytoplankton community in Cyclone Opal. The substantial grazing impact on diatoms suggests that efficient recycling was the major primary fate of diatom organic production, consistent with the low export fluxes and selective export of biogenic silica, as empty diatom frustules, in Cyclone Opal.     

Diatoms in the desert: Plankton community response to a mesoscale eddy in the subtropical North Pacific

As part of the E-Flux project, we documented spatial variability and temporal changes in plankton community structure in a cold-core cyclonic eddy in the lee of the Hawaiian Islands. Cyclone Opal spanned 200 km in diameter, with sharply uplifted isopycnals (80–100 m relative to surrounding waters) and a strongly expressed deep chlorophyll a maximum (DCM) in its central core region of 40 km diameter. Microscopic and flow cytometric analyses of samples from across the eddy revealed dramatic transitions in phytoplankton community structure, reflecting Opal's well-developed physical structure. Upper mixed-layer populations in the eddy resembled those outside the eddy and were dominated by picophytoplankton. In contrast, the DCM was composed of large chain-forming diatoms dominated by Chaetoceros and Rhizosolenia spp. Diatoms attained unprecedented levels of biomass (nearly 90 μg C l⁻¹) in the center of the eddy, accounting for 85% of photosynthetic biomass. Protozoan grazers displayed two- to three-fold higher biomass levels in the eddy center as well. We also found a distinct and persistent layer of senescent diatom cells overlying healthy populations, often separated by less than 10 m, indicating that we were sampling a bloom in a state of decline. Time-series sampling over 8 days showed a successional shift in community structure within the central diatom bloom, from the unexpected large chain-forming species to smaller forms more typical of the subtropical North Pacific. The diatom bloom of Cyclone Opal was a unique, and possibly extreme, example of biological response to physical forcing in the North Pacific subtropical gyre, and its detailed study may therefore help to improve our predictive understanding of environmental controls on plankton community structure.     

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.     

The transient oasis: Nutrient-phytoplankton dynamics and particle export in Hawaiian lee cyclones

Macronutrients, photosynthetic pigments, and particle export were assessed in two eddies during the E-Flux I and III cruises to investigate linkages between biogeochemical properties and export flux in Hawaiian lee cyclonic eddies. Cyclone Noah (E-Flux I), speculated to be in the ‘decay’ stage, exhibited modest increases in macronutrients and photosynthetic pigments at the eddy center compared to ambient waters. Cyclone Opal (E-Flux III) also exhibited modest increases in macronutrient concentrations, but a 2-fold enhancement in total chlorophyll a (TChl a) concentration within the eddy center. As indicated by fucoxanthin concentrations, the phytoplankton community in the deep chlorophyll maximum (DCM) of Opal was comprised mainly of diatoms. During an 8-day time series in the center of Opal, TChl a concentration and fucoxanthin in the DCM decreased by 50%, which was potentially triggered by silicic acid limitation. Despite the presence of a substantial diatom bloom, Opal did not deliver the expected export of particulate carbon and nitrogen, but rather a large biogenic silica export (4-fold increase relative to export in surrounding waters). Results suggest that controls on the life cycle of a Hawaiian lee cyclone are likely a combination of physical (eddy dynamics), chemical (nutrient limitation), and biological (growth and grazing imbalance) processes. Comparisons between Noah and Opal and previously studied cyclones in the region point to a relationship between the spin-up duration of a cyclone and the resulting biological response. Nonetheless, Hawaiian lee cyclones, which strongly influence the biogeochemistry of areas 100's of km in scale in the subtropical North Pacific Ocean, still remain an enigma.     

Factors influencing fatty acid and hydrocarbon composition of sedimenting particles in the northeastern Adriatic Sea

Fatty acids and hydrocarbons of sedimenting particles were investigated in the northeastern Adriatic Sea from November 1988 to December 1989. Particles were collected at approximately monthly intervals, using sediment traps deployed at 30 m depth (2 m above bottom). Seasonal changes in sedimentation of particulate matter were very pronounced. Hydrocarbon fluxes and concentrations were found to vary significantly depending on the season. They averaged 2.69 ± 1.44 mg m⁻² day⁻¹ and 232.4 ± 90.93 μg g⁻¹ in winter, respectively. In late spring-early summer the corresponding values amounted to 0.045 ± 0.015 mg m⁻² day⁻¹ and 13.72 ± 5.56 μg g⁻¹, and they increased towards autumn, when mean values of 0.517 ± 0.228 mg m⁻² day⁻¹ and 98.86 ± 48.72 μg g⁻¹ were obtained. In contrast, fatty acid fluxes and concentrations were low during winter (0.26 ± 0.08 mg m⁻² day⁻¹ and 21.95 ± 3.35 μg g⁻¹), increased slightly towards the summer (0.48 ± 0.12 mg m⁻² day⁻¹ and 139.9 ± 44.6 μ g⁻¹) and reached maximum rate and concentration in autumn, when average values were 1.98 ± 1.30 mg m² day⁻¹ and 489.1 ± 186.7 μg g⁻¹, respectively. The differences in composition, concentrations and fluxes of the fatty acids and hydrocarbons were related to the sources of sedimenting material, reflecting the influence of resuspension of bottom sediments during winter and the appearance of mucus aggregates during summer and their subsequent deposition in autumn.     

Was a carbon balance measured in the equatorial Pacific during JGOFS?

The likelihood that the carbon fluxes measured as part of the US-JGOFS field program in the equatorial Pacific ocean (EgPac) during 1992 yielded a balanced carbon budget for the surface ocean was determined. The major carbon fluxes incorporated into a surface carbon budget were: new production, particulate organic carbon (POC) and dissolved organic carbon (DOC) export, CaC0₃ export, C0₂ gas evasion, dissolved inorganic carbon (DIC) supply, and the time rate of charge. The ratio of the measured concentration gradients of DOC and DIC provided a constraint on the ratio of POC/DOC export. Uncertainties of ±30–50% for individual carbon flux measurements reduce the likelihood that a carbon balance can be measured during a JGOFS process-type study. As a benchmark, carbon fluxes were prescribed to yield a hypothetical surface carbon budget that was, on average, balanced. Given the typical errors in the individual carbon fluxes, however, there was only about a 30% chance that this hypothetical budget could be measured to be balanced to ±50%. Using this benchmark, it was determined that there was a 95 % chance that the carbon flux measurements yielded a surface DIC budget balanced (to ±50%) during El Nino conditions in boreal spring 1992, when the total organic carbon export rate was - 5 mmol C m-2 day- 1 and the POC export was 3 mmol C m⁻² day⁻¹. In boreal fall 1992, during cold period conditions, there was a 70% chance that the surface carbon DIC budget was balanced when the total organic carbon export rate was 20 mmol C m⁻² day⁻¹ and export was -13 mmol C m-2 day-'. The DOC to DIC concentration gradient ratio of - -0.15, measured in depth profiles down to 100m and in surface waters, was used as an important constraint that most (> 70%) of the organic carbon exported from the euphotic zone was POC rather than DOC. If a balanced surface DIC budget was used to test the compatibility of individual carbon fluxes measured during EgPac, then a three- to four-fold increase in total and particulate organic carbon export between spring and fall is indicated. This increase was not reflected in the POC loss rates measured by drifting sediment trap collections or estimated by²³⁴Th deficiencies coupled with the C/Th measured on suspended particles.     

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.     

Dimethylsulfide production in Sargasso Sea eddies

Lagrangian time series of dimethylsulfide (DMS) concentrations from a cyclonic and an anticyclonic eddy in the Sargasso Sea were used in conjunction with measured DMS loss rates and a model of vertical mixing to estimate gross DMS production in the upper 60 m during summer 2004. Loss terms included biological consumption, photolysis, and ventilation to the atmosphere. The time- and depth (0–60 m)-averaged gross DMS production was estimated to be 0.73±0.09 nM d⁻¹ in the cyclonic eddy and 0.90±0.15 nM d⁻¹ in the anticyclonic eddy, with respective DMS replacement times of 5±1 and 6±1 d. The higher estimated rate of gross production and lower measured loss rate constants in the anticyclonic eddy were equally responsible for this eddy's 50% higher DMS inventory (0–60 m). When normalized to chlorophyll and total dimethylsulfoniopropionate (DMSP), estimated gross production in the anticyclonic eddy was about twice that in the cyclonic eddy, consistent with the greater fraction of phytoplankton that were DMSP producers in the anticyclonic eddy. Higher rates of gross production were estimated below the mixed layer, contributing to the subsurface DMS maximum found in both eddies. In both eddies, gas exchange, microbial consumption, and photolysis were roughly equal DMS loss terms in the surface mixed layer (0.2–0.4 nM d⁻¹). Vertical mixing was a substantial source of DMS to the surface mixed layer in both eddies (0.2–0.3 nM d⁻¹) owing to the relatively high DMS concentrations below the mixed layer. Estimated net biological DMS production rates (gross production minus microbial consumption) in the mixed layer were substantially lower (by almost a factor of 3) than those estimated in a previous study of the Sargasso Sea, which may explain the relatively low mixed-layer DMS concentrations found here during July 2004 (3 nM) compared to previous summers (4–6 nM).     

Mesoscale physical processes and zooplankton transport-retention in the northern Norwegian shelf region

Northern Norwegian shelf regions are highly productive, supporting fisheries rich in commercially important species such as cod, herring and capelin. It has been long recognized that the mesoscale jets, meanders and eddies associated with interactions between the North Atlantic Current, Norwegian Coastal Current and regional bottom topographic features such as troughs, banks and shelfbreaks play important roles in transporting and retaining zooplankton. To investigate zooplankton distributions and their correspondence with the physical fields, three large-scale surveys with mesoscale resolutions on physical and biological fields were conducted in northern Norwegian shelf regions between latitudes 68°15′N and 70°15′N in springs of 2000–2002. Survey results provide insights into the relationships between zooplankton distributions and the physical features such as fronts, the Norwegian Coastal Current and eddies related to topographic features. The physical and biological data are integrated and analyzed focusing on water types, estimation of geostrophic currents from direct current measurements, along-shelf transport of zooplankton, and retention of zooplankton by the mesoscale meander–eddy over a typical bank area on the shelf. The estimated mean transport in the upper 100 m on the shelf in the survey region is approximately 6.4×10³ tonnes wet weight day⁻¹ northward. High zooplankton abundances were found over both Malangsgrunnen and Sveinsgrunnen banks. The specific accumulation rate from northward–southward transport in the upper 100 m over Malangsgrunnen was approximately 0.08 day⁻¹, while variable currents with an offshore gradient of zooplankton abundance over Sveinsgrunnen implies an offshore dispersion of coastal-originated zooplankton cohort.     

Biogeochemical responses to late-winter storms in the Sargasso Sea,III—Estimates of export production using 234Th:238U disequilibria and sediment traps

Direct measurements of new production and carbon export in the subtropical North Atlantic Ocean appear to be too low when compared to geochemical-based estimates. It has been hypothesized that episodic inputs of new nutrients into surface water via the passage of mesoscale eddies or winter storms may resolve at least some of this discrepancy. Here, we investigated particulate organic carbon (POC), particulate organic nitrogen (PON), and biogenic silica (BSiO₂) export using a combination of water column ²³⁴Th:²³⁸U disequilibria and free-floating sediment traps during and immediately following two weather systems encountered in February and March 2004. While these storms resulted in a 2–4-fold increase in mixed layer NO₃ inventories, total chlorophyll a and an increase in diatom biomass, the systems were dominated by generally low ²³⁴Th:²³⁸U disequilibria, suggesting limited particle export. Several ²³⁴Th models were tested, with only those including non-steady state and vertical upwelling processes able to describe the observed ²³⁴Th activities. Although upwelling velocities were not measured directly in this study, the ²³⁴Th model suggests reasonable rates of 2.2–3.7 m d^?1.Given the uncertainties associated with ²³⁴Th derived particle export rates and sediment traps, both were used to provide a range in sinking particle fluxes from the upper ocean during the study. ²³⁴Th particle fluxes were determined applying the more commonly used steady state, one-dimensional model with element/²³⁴Th ratios measured in sediment traps. Export fluxes at 200 m ranged from 1.91±0.20 to 4.92±1.22 mmol C m^?2 d^?1, 0.25±0.08 to 0.54±0.09 mmol N m^?2 d^?1, and 0.22±0.04 to 0.50±0.06 mmol Si m^?2 d^?1. POC export efficiencies (Primary Production/Export) were not significantly different from the annual average or from time periods without storms, although absolute POC fluxes were elevated by 1–11%. This increase was not sufficient, however, to resolve the discrepancy between our observations and geochemical-based estimates of particle export. Comparison of PON export rates with simultaneous measurements of NO₃^? uptake derived new production rates suggest that only a fraction, <35%, of new production was exported as particles to deep waters during these events. Measured bSiO₂ export rates were more than a factor of two higher (p<0.01) than the annual average, with storm events contributing as much as 50% of annual bSiO₂ export in the Sargasso Sea. Furthermore it appears that 65–95% (average 86±14%) of the total POC export measured in this study was due to diatoms.Combined these results suggest that winter storms do not significantly increase POC and PON export to depth. Rather, these storms may play a role in the export of bSiO₂ to deep waters. Given the slower remineralization rates of bSiO₂ relative to POC and PON, this transport may, over time, slowly decrease water column silicate inventories, and further drive the Sargasso Sea towards increasing silica limitation. These storm events may further affect the quality of the POC and PON exported, given the large association of this material with diatoms during these periods.     

Remotely sensed sea-surface chlorophyll and POC flux at Deep Gulf of Mexico Benthos sampling stations

Biweekly composite averages of the standing stock of sea-surface chlorophyll (SSC) were derived from SeaWiFS satellite ocean-color data at 44 benthic sampling stations occupied along the continental slope and rise by the Deep Gulf of Mexico Benthos (DGoMB) program. At the 22 DGoMB sites north of 26°N and west of 91°W in the NW Gulf of Mexico, annual average SSC was 0.19 mg m⁻³, ranging at most locations from annual highs of about 0.3 mg m⁻³ in November–February to lows of about 0.1 mg m⁻³ in May–August. Comparison of three years of SeaWiFS data (January 1998–December 2000) showed little inter-annual variation at these NW Gulf stations. In contrast, at the 22 NE Gulf sites north of 26°N and east of 91°W, SSC averaged 2.8 times higher than in the NW Gulf, showing also strong inter-annual variation. Maxima in the NE region occurred in November–February and also during summers. The summer maxima were associated with Mississippi River water transported offshore to the east and southward by anticyclonic eddies in the NE Gulf. The apparent increases in SSC in June–August at NE Gulf stations reached average monthly concentrations >50% greater than in November–February. Based on a primary productivity model and a vertical flux model, the calculated export of particulate organic carbon (POC flux reaching the seafloor) was estimated as 18 mg C m⁻² day⁻¹ at the 22 NE Gulf stations, and 9 mg C m⁻² day⁻¹ at the 22 NW Gulf stations. These estimates are comparable to fluxes measured by benthic lander by others in the DGoMB program, which may drive the differences in west versus east bathymetric zonation and community structure of macrobenthos that were sampled with large box corers by others in the DGoMB program.     

Seasonal deposition and reworking at the sediment-water interface on the northwestern Iberian margin

Seabed distributions of ²³⁴Th excess (Th_xs) were determined in the upper centimetres of 38 sediment cores from the north-western Iberian Margin, sampled from 41–44°N and from 9–12°E during five OMEX II cruises. Three main areas, a northern, and at 42°38 and 42°N, were investigated during representative seasons (winter, spring and summer). Low ²³⁴Th_xs activities in summer 1998 (18–252 Bq per kg) were similar to those measured in summer 1997. In winter ²³⁴Th also showed moderate excess. The highest values were observed in spring with surface ²³⁴Th_xs values up to 402 Bq kg⁻¹. Maximum penetration depths of ²³⁴Th_xs ranged from a few mm to 3 cm. ²³⁴Th_xs activities always showed a smooth decrease with depth, without any evidence of non-local mixing. Thus particle mixing on a short time scale can be described as an eddy diffusive process, and bioturbation rates, calculated on this basis, range from 0.02 to 3.07 cm² per year. Data (activities, inventories, bioturbation rates) are discussed in order to relate the observed surface and down-core variations to spatial and seasonal trends. Using ²³⁴Th_xs data in sediment as a substitute for sediment trap estimates, particle fluxes were calculated from ²³⁴Th_xs inventories. The range of ²³⁴Th-derived particle fluxes for the north-western Iberian Margin is 16–1418 mg.m⁻².d⁻¹. Mean values indicate a gradual decrease of mass fluxes from the shelf to the open ocean. On a 100-day scale, the northern area (43–44°N) represents a low sedimentation regime. Further south, around 42°–43°N, particle inputs are more important. On the middle slope, around 1000 to 2000 m depth, high inventories and bioturbation rates indicate enhanced, and probably organic-rich, particle fluxes to the seafloor, particularly in spring.     

Biological components of Ross Sea short-term particle fluxes in the austral summer of 1995–1996

The Ross Sea, a region of high seasonal production in the Southern Ocean, is characterized by blooms of the haptophyte Phaeocystis antarctica and of diatoms. The different morphology, structural composition and consumption of these two phytoplankton by grazing zooplankton may result in different carbon cycling dynamics and carbon flux from the euphotic zone. We sampled short-term (2 days) particle flux at 5 sites from 177.6°W to 165°E along a transect at 76.5°S with traps placed below the euphotic zone at 200 m during December 1995–January 1996. We estimated carbon flux of as many eucaryotic organisms and fecal pellets as possible using microscopy for counts and measurements and applying volume:carbon conversions from the literature. Eucaryotic organisms contributed about 20–40% of the total organic carbon flux in both the central Ross Sea polynya and in the western polynya, and groups of organisms differed in contribution to the carbon flux at the different sites. Algal carbon flux ranged from 4.5 to 21.1 mg C m⁻² day⁻¹ and consisted primarily of P. antarctica (cell plus mucus) and diatom carbon at all sites. Different diatom species dominated the diatom flux at different sites. Carbon fluxes of small pennate diatoms may have been enhanced by scavenging, by sinking senescent P. antarctica colonies. Heterotrophic carbon flux ranged from 9.2 to 37.6 mg C m⁻² day⁻¹ and was dominated by athecate heterotrophic dinoflagellate carbon in general and by carbon flux of a particular large athecate dinoflagellate at two sites. Fecal pellet carbon flux ranged from 4.6 to 54.5 mg C m⁻² day⁻¹ and was dominated by carbon from ovoid/angular pellets at most sites. Analysis of fecal pellet contents suggested that large protozoans identified by light microscopy contributed to ovoid/angular fecal pellet fluxes. Carbon flux as a percentage of daily primary production was lowest at sites where P. antarctica predominated in the water column and was highest at sites where fecal pellet flux was highest. This indicates the importance of grazers in carbon export.     

Impact of the Mediterranean Outflow Water on particle dynamics in intermediate waters of the Northeast Atlantic, as revealed by Th and Th

Over the last decade ²³⁴Th has become increasingly used to study particle transport in the ocean on a timescale of weeks. The application of ²³⁴Th is mainly focused on the determination of particle and associated carbon fluxes from oceanic surface water. However, ²³⁴Th is also suitable for investigating particle dynamic from the upper ocean down to interface sediments, as illustrated by the present work which reports unexpected behavior of ²³⁴Th in intermediate waters associated with the Mediterranean Outflow Water (MOW). Concentration profiles of dissolved ²³⁸U and ²²⁸Ra, and dissolved and particulate ²³⁴Th and ²²⁸Th were measured in the Mediterranean Outflow Water (MOW) near the Gibraltar Straits and at two sites (36°30′N–15°35′W, Nicole; 36°27′N–10°35′W, Yseult) which had hydrographic characteristics of Meddies, i.e. MOW that propagates as eddies in the Northeastern Atlantic at intermediate depths.There are marked differences in the distribution of thorium between MOW and the surrounding Atlantic waters. At the youngest Meddy Nicole salinity maximum at 1000 m depth, ²³⁴Th(total) : ²³⁸U and ²²⁸Th(total) : ²²⁸Ra activity ratios are significantly lower than radioactive equilibrium, indicating an unusual deficit of short half-life thorium nuclides. This implies an export of thorium, presumably on particles, from intermediate Meddy Nicole waters. This process is supported by an increase of particulate thorium fluxes measured in sediment traps deployed for two weeks above and within Meddy Nicole. In contrast, offshore Meddy Yseult has more typical profiles of both thorium nuclides that are nearly in equilibrium with their parents. These results indicate that at intermediate depths, the presence of MOW affects the exchange of reactive elements between particles and dissolved forms and enhances the downward flux of particles from intermediate waters in the Northeast Atlantic.     

Effects of irradiance on benthic and water column processes in a Gulf of Mexico estuary: Pensacola Bay, Florida, USA

We examined the effect of light on water column and benthic fluxes in the Pensacola Bay estuary, a river-dominated system in the northeastern Gulf of Mexico. Measurements were made during the summers of 2003 and 2004 on 16 dates distributed along depth and salinity gradients. Dissolved oxygen fluxes were measured on replicate sediment and water column samples exposed to a gradient of photosynthetically active radiation. Sediment inorganic nutrient (NH₄⁺, NO₃⁻, PO₄³⁻) fluxes were measured. The response of dissolved oxygen fluxes to variation in light was fit to a photosynthesis–irradiance model and the parameter estimates were used to calculate daily integrated production in the water column and the benthos. The results suggest that shoal environments supported substantial benthic productivity, averaging 13.6 ± 4.7 mmol O₂ m⁻² d⁻¹, whereas channel environments supported low benthic productivity, averaging 0.5 ± 0.3 mmol O₂ m⁻² d⁻¹ (±SE). Estimates of baywide microphytobenthic productivity ranged from 8.1 to 16.5 mmol O₂ m⁻² d⁻¹, comprising about 16–32% of total system productivity. Benthic and water column dark respiration averaged 15.2 ± 3.2 and 33.6 ± 3.7 mmol O₂ m⁻² d⁻¹, respectively Inorganic nutrient fluxes were generally low compared to relevant estuarine literature values, and responded minimally to light exposure. Across all stations, nutrient fluxes from sediments to the water column averaged 1.11 ± 0.98 mmol m⁻² d⁻¹ for NH₄⁺, 0.58 ± 1.08 mmol m⁻² d⁻¹ for NO₃⁻, 0.01 ± 0.09 mmol m⁻² d⁻¹ for PO₄³⁻. The results of this study illustrate how light reaching the sediments is an important modulator of benthic nutrient and oxygen dynamics in shallow estuarine systems.