OS26A-01
Grazing and Carbon Flux Through Diel Vertical Migration of Copepod Assemblages in the Subarctic Pacific Ocean
Seasonal change of grazing impact and the active transport of carbon by diel vertical migration of copepod community in the Oyashio region, western subarctic Pacific was investigated during 6 cruises from June 2001 to June 2002. In situ grazing rates of copepods community (CGR) at 0-50 m were measured by gut fluorescence method. The CGR was lowest during autumn-winter (8-15 $\mu$ g C m $^{-2}$ d$^{-1}$) and increased during the spring bloom period (195 $\mu$ g C m m $^{-2}$ d$^{-1}$). The highest CGR was observed in June 2002 (397 $\mu$ g C m $^{-2}$ d$^{-1}$) and it contributed 87 % of the primary production. CGRs in other periods were 2-20 % of the primary production. The annual CGR was 37.4 g C m$^{-2}$, accounting for 13 % of annual primary production. These showed that copepods play an important role for phytoplankton dynamics in the study area, especially after the bloom. Potential carbon flux by faecal pellet production was 11.3 g C m $^{-2}$ yr$^{-1}$) (assuming an assimilation efficiency of 70 %). Among six dominant large calanoids, 2 species of {\it Metridia} spp. showed active diel vertical migration but not in {\it Neocalanus} spp. and {\it Eucalanus bungii}. {\it Metridia pacifica} was active migratory species throughout the year and its travel distance was shorter in spring (ca. 100 m) and longer in winter (-300 m). {\it M. okhotensis} migrated only in spring with much longer distance (> 300 m). Annual carbon transport by diel vertical migration of {\it Metridia} spp. was estimated as 1.9 g C m$^{-2}$, corresponding to 17 % of potential carbon flux by the faecal pellet.
OS26A-02
Acoustic Identification and Density Estimates of Mesopelagic Fishes Using the Volume Backscattering Strength (Sv) Difference Method
Mesopelagic fishes are the main component of the sound scattering layer and play a significant role in nutrient cycling between the surface and mesopelagic layer through diel vertical migration. Effective methods for quantitatively monitoring their distribution and behavior are required to better understand the dynamics of pelagic ecosystems. This study describes the acoustic discrimination and density estimation of mesopelagic fishes in the sound scattering layer. As part of the DEEP project, acoustic and biological samplings were conducted between the southern Bering Sea and the northwestern Pacific. Mesopelagic fishes were observed over most of the area studied, and myctophids were the most dominant group in MOCNESS and Midwater trawl samples. We discriminated between swimbladder and non-swimbladder fish based on X-ray observations. Then, the target strength (TS) were estimated at 38 and 120 kHz using existing theoretical models and images of the swimbladder and body shape. The TS values at 38 kHz were 0 to 3 dB higher than at 120 kHz in swimbladder fishes (e.g., Diaphus theta and Lampanyctus jordani). In non-swimbladder fishes (e.g., Stenobrachius leucopsarus and S. nannochir), the TS values at both frequencies were similar in large specimens (over about 8 cm in body length), but the TS at 38 kHz was lower than at 120 kHz in small specimens (not over 8 cm). Based on these TS frequency characteristics, the volume back scattering strength (Sv) difference method was used to extract mesopelagic fishes from the field echo data. Some species were discriminated from the echoes of other sound-scatter components (e.g., euphausids or copepods). Many mesopelagic fish species were distributed in deep water (deeper than 300 m) where they cannot be detected by high frequencies (e.g., 120kHz). Some species (e.g., S. leucopsarus in the Bering Sea, and D. theta off northeast Japan) formed relatively dense schools dominated by a single species and length mode in daytime, and could be discriminated at a single low frequency (38 kHz). The density estimates of myctophid fishes by the acoustic method were much higher than estimates calculated from trawl data.
OS26A-03
Changes in fecal pellet characteristics with depth as indicators of zooplankton repackaging of particles in the mesopelagic zone
The mesopelagic zooplankton community can affect particle flux in the sea by consuming sinking particles and repacking them into fecal pellets, and by fragmenting them into smaller, slower- or non-sinking particles. However, there are very few studies that have examined these processes. As part of the VERtical Transport In the Global Ocean (VERTIGO) project we investigated how fecal pellet characteristics change with depth in order to quantify the extent of particle repackaging by zooplankton. Material from sediment traps deployed at 150m, 300m, and 500m was analyzed from both a mesotrophic (Japanese time-series station K2) and an oligotrophic (Hawaii Ocean Time series-HOT station ALOHA) environment in the Pacific Ocean. We quantified changes in the numbers, size, shape, and color of pellets, which are indicative of major zooplankton groups and their diet, and how these parameters changed with depth. Both regions showed evidence of particle repackaging. At station K2, over 80% of the 150m trap pellets were cylindrical and predominately produced by copepods and euphausiids. However, ellipsoid larvacean fecal pellets, found rarely in 150m traps, made up over 60% of the total number of pellets found in both the 300m and 500m traps, indicating substantial repackaging at depth. At ALOHA, the number of light brown and ovoid/ellipsoid pellets increased with depth, as did the number of broken pellets within the traps, with 20% of the total pellets fragmented at 150m and up to 35% fragmented at 500m. About 2.5% of the carbon in the 150m sediment traps at ALOHA was comprised of intact fecal pellets, increasing to 7% in the 500m traps. Significantly higher numbers of fecal pellets were found in traps at K2, and the type of fecal pellets found in traps was reflective of the disparate zooplankton community structure at the two sites. This comparison of particle repackaging by zooplankton communities at the two sites will help our understanding of how future changes in plankton community structure may affect the biological pump.
OS26A-04
Vertical distribution of ontogenetically migrating copepods in the Western Subarctic Gyre
In the past decade it has become apparent that diel and seasonal (or ontogenetic) migrating zooplankton contribute to carbon flux through their respiration, excretion, and mortality at depth. Ontogenetically migrating copepods are considered to have a relatively more important role in total carbon flux in the subarctic Pacific and Southern Ocean compared with other oceans because they produce overwintering stock that is resident in the twilight zone. However, there is little knowledge of the downward migration process during late summer that forms the overwintering stock. Here we report temporal changes in vertical distribution of ontogenetically migrating copepods in the Western Subarctic Gyre (Japanese time series site K2) from summertime day/night pairs of zooplankton samples taken at 9 discrete depth intervals between 0-1000 m during the VERTIGO project cruise. Ontogenetically vertical migrants included 2 Calanus, 1 Eucalanus, 2 Metridia and 3 Neocalanus species. Neocalanus spp. and Eucalanus bungii made up the bulk of the carbon-based copepod biomass in the water column (more than 95%). N. plumchrus was abundant near surface but N. cristatus and E. bungii appeared in the subsurface layer between 50 to 100 m. N. flemingeri and a portion of the E. bungii population were observed below the permanent halocline throughout the day. Overwintering stages comprised 98% of Neocalanus spp. and E. bungii. These results indicate August is the beginning of the downward migrating or dormant seasons. Based on the life cycle pattern, ontogenetically exported carbon by N. flemingeri is estimated to be 168.6 mgC m-2. Taking into account for more predominant N. cristatus and N. plumchrus, the estimates are considerably conservative. The impacts of the ontogenetic migration on carbon flux will be discussed.
OS26A-05
An Examination of Sediment Trap Accuracy Issues During VERTIGO
Sediment traps have proven to be a valuable tool in the study of marine particle fluxes. However, a number of issues remain which may impact the quantity and/or quality of material collected in traps, and these issues are particularly important in applications of sediment traps in the upper 500-1000 m of the ocean. During VERTIGO (VERtical Transport In the Global Ocean), we have compared a variety of trapping systems including: surface tethered traps (similar to PITS traps used in many prior studies), a newly developed free vehicle trap, the neutrally buoyant sediment traps (NBSTs), and a tethered trap with an active swimmer avoidance mechanism, the IRS (indented rotating sphere) trap. These were deployed with duplication, at three depths in the mesopelagic "twilight zone" under contrasting flux conditions off Hawaii and in the NW Pacific. Here we present the results of a number of measurements designed to examine the impact of several potential influences on trap accuracy, including: hydrodynamic design of the trap; poison/preservative type and the influence of in-trap solubilization of collected material; and the magnitude and correction for zooplankton "swimmers" in different trap designs. Some findings from our 2004 occupation of sites near station ALOHA and in the NW Pacific gyre include: 1) generally similar fluxes observed for many elements using both tethered traps and NBSTs, however some specific differences for certain elements and during one of two deployments in the NW Pacific; 2) NBSTs were clearly seen to ride internal waves that possessed periods of about 5-7 h; 3) little observable difference in the mass or chemical fluxes of samples poisoned/preserved with either HgCl$_{2}$ or formalin; 4) in-vitro incubations of poisoned trap material indicated that loss of particulate material to the solution phase was not large over a period of time comparable to the trap deployments; 5) zooplankton swimmers removed from trap material by means of a 350 $\mu$m Nitex screen represented "fluxes" comparable to the net sinking particle flux; 6) there was a small, but not insignificant amount of swimmers that passed through the screen as well as for some sinking material caught on screen, both of which if unaccounted for, would bias the measured sinking flux; 7) the IRS trap removed most but not all large swimmers and may underestimate slightly the true flux due to particle loss on the sphere prior to collection in the poisoned trap cup.
OS26A-06
Vertical Flux of Mass, Pigments and Chemical Components in the Mesopelagic "Twilight Zone" of the North Central and Northwest Pacific Ocean during VERTIGO
The mesopelagic "twilight zone" of the ocean (100-500 m) is recognized as a critically important region in determining the export efficiency of the surface biological pump. This region is, however, distinctly understudied. As part of the VERtical Transport in the Global Ocean (VERTIGO) program, we have deployed a number of types of sediment traps in the twilight zones of two contrasting locations in the North Pacific Ocean: at the oligotrophic north central gyre HOT time series site ALOHA, just north of Hawaii and at the western subarctic gyre site K2, which is part of the HILATS time series program. These two locations were occupied for several weeks during the summers of 2004 (ALOHA) and 2005 (K2), during which two deployments of our trap systems were completed. During each effort, multiple systems were deployed at 150, 300 and 500 m. Here we present some of the results from our on-going analyses (mass, C, N, P, bSi, PIC, $^{234}$Th, pigments and selected trace metals) of the sediment trap material and place them in the context of the time series programs underway at each site. Findings include: 1) the fluxes of biogenic material were dramatically different at the two sites, with mass fluxes at K2 about 5-10x greater than at ALOHA; 2) the material quality was also dramatically different, with percent of mass as bSi higher at K2 and percent of mass as PIC higher at ALOHA; 3) material fluxes showed significant attenuation from 150-500 m at ALOHA (500 m mass fluxes approximately 25% of 150 m), while much less attenuation was observed over the same depths at K2 (500 m still 75% of 150 m); 4) C vs. N and P plots from ALOHA indicate that all trapping systems were collecting particles of essentially the same quality, but that individual arrays differed in the quantity collected; 5) export ratios (Flux$_{150m}$/$^{14}$C-Prim. Prod.) were dramatically higher at K2, though primary production did not differ as much between the sites; 6) the material fluxes at 500 m agreed well with deep trap fluxes at ALOHA, indicating that the majority of flux attenuation occurred within the twilight zone; 7) at ALOHA, particle components showed evidence of differential remineralization and attenuation in the general order of {\it Chl a}>P>N>C>bSi>mass; 8) many phytoplankton pigments showed rapid attenuation with depth, with the exception of carotenoids, zeaxanthin and fucoxanthin; 8) application of surface water calibrated algorithms to the sediment trap pigment suite suggests that prymensiophytes were the major contributors of {\it Chl a} in the sinking material; 10) Al and Fe fluxes were consistent with dust inputs, though Fe showed evidence of biological influences as well.
OS26A-07
A High Resolution Study of Particle Export Using Thorium-234 in the N. Central Pacific and NW Pacific as Part of the VERTIGO Project
As part of the VERTIGO project (VERtical Transport In the Global Ocean) we used thorium-234 as a natural proxy for particle export. As developed in recent years, application of a small volume sampling method allowed for relatively high resolution total 234Th sampling in both space and time, during 3 week occupations of two contrasting flux sites off Hawaii (ALOHA) and in the NW Pacific (K2). The higher vertical resolution allows us to constrain not only particle export out of the euphotic zone, but export and remineralization processes below. We have made extensive use in the VERTIGO project of different sediment traps as well as large volume pumping systems for size fractionated filtration, and thus can compare for export calculations of C, N, bSi and PIC, the ratios of these elements to particulate 234Th for a wide variety of sinking and suspended materials. Results to date indicate a rather large difference in 234Th distributions at ALOHA and K2. At ALOHA, the 234Th:238U disequilibrium is small, and hence 234Th fluxes on particles are low, on the order of 300-500 dpm/m2/d. Variability in space and time within a 200 km2 area is also small and hence a 1-D steady-state model is a good approximation of the flux conditions. The predicted fluxes are consistent within errors with simultaneously measured 234Th trap fluxes. As developed in prior studies, we can apply the ratio of E/Th on particles to convert from Th to other Elemental fluxes, such as C, N, bSi and PIC. At ALOHA C/Th on size fractionated particles decreased with depth, and increased with size for 1-10, 10-53 and >53 micron pore sized filters. Interestingly, the 10-53um fraction was closest in C/Th ratio to the sinking material caught in traps. At K2, the 234Th:238U disequilibrium was much larger, with total activities as low as 1 dpm/L within the mixed layer. By sampling with up to 20-24 point vertical resolution in the upper 300m, we can see a regular "excess" 234Th feature at about 100-120m at the base of the subsurface Chlorophyll maximum, and in many cases a small disequilibrium between 100-250m. This deeper feature may be an indication of repackaging of suspended material into sinking particles. Fluxes calculated from a 1-D SS model thus increase from about 1700 to 2200 dpm/m2/d between 100-300m, consistent with sediment traps during our first deployment. Trap fluxes drop off during the cruise to values closer to 500-700 dpm/m2/d, and the average 234Th disequilibrium drops slightly, but there is significant spatial and temporal variability at K2. Final chemical yields from K2 will be needed to finalize this 234Th data set, but early indications are that the increase in 234Th over time will lead to a lower predicted 234Th export using a non steady-state approach to modeling the changing 234Th activities over the 3 week observation period.
http://cafethorium.whoi.edu
OS26A-08
Nitrogen and oxygen isotopes in nitrate from contrasting sites in the Pacific
As part of the VERTIGO research program, samples for nitrogen (d15N) and oxygen (d18O) isotopic signatures in nitrate were analyzed from depth profiles and surface transects from the northwest pacific, near station K2 and the subtropical gyre, near station ALOHA. Contrasting productivity, ecology, nutrient cycling, and particle flux dynamics of these two locations are reflected in the nitrate isotopic signatures. At ALOHA, low nitrate concentrations in near-surface waters were coupled to some of the lowest d15N signatures yet reported for nitrate in seawater. Oxygen isotopic signatures in nitrate also decreased into the shallow layers, most likely due to active cycling of nitrate and production through nitrification. By accessing the isotopic signatures of nitrate at the low concentrations typical of near surface waters at station ALOHA, we are able to observe the "resetting" of nitrogen and oxygen isotopic signatures in nitrate resulting from active cycling and local sources of nitrate within this near-surface layer. In regions where surface nitrate concentrations are higher,and the turnover time of the nitrate pool is longer, nitrate consumption by phytoplankton is the dominant process impacting near-surface nitrate isotopic signatures. Through the datasets compiled simultaneously from the VERTIGO program, we are able to combine isotopic distributions in nitrate with suspended and sinking PN-d15N as well as rate measurements for new production via nitrate uptake and N2 fixation to evaluate different processes involved in nitrogen cycle dynamics.
OS26A-09
Nitrogen uptake regime at station ALOHA during VERTIGO I
Uptake of nitrate, ammonium, nitrite and nitrogen fixation was surveyed in the photic zone (0-125m) at station ALOHA (Hawaii) during June-July 2004. Combined 15N, 13C tracer experiments were conducted over periods of 12hours under constant illumination (use of algal cabinet) and neutral density screens to mimic in-situ light attenuation. For nitrogen, tracer additions were systematically higher than 10% of the ambient substrate concentration, particularly when determinations were below the level of analytical detection. Consequently, flux rates were adjusted for estimating ambient uptake rates based on the assumption that increases in uptake rates following isotope addition were proportional to the increase in substrate concentrations, following Eppley et al. A total of 7 uptake experiments were conducted over a 3 week period at station ALOHA. Total N-uptake shows a light dependent uptake relationships with convex type curves. Day to day profiles looked very similar showing little temporal variability. Ammonium uptake generally exceeded nitrate uptake rate particularly under 50m depth. Nitrite uptake and N2 fixation represented generally less than 15% of total N-uptake. Average f-ratio (i.e. (NO3 uptake + N2 fixation) / ( A total N-uptake) ranged between < 0.1 to 0.2, suggesting predominance of regenerated production. Daily primary production integrated over the upper 125m amounted to 10.2 -O 1.0 mmol C /m2/d. This C-fixation flux is ten-fold higher than the POC flux samples by neutrally buoyant traps 150m (Buesseler et al.) suggesting export ratio was about 0.1. Moreover, this indicates satisfactory agreements between estimates of new production and export production with the former exceeding the latter maximally by a factor 2. N-uptake Results for the VERTIGO II (July - August 2005) at station K2 in the North West Pacific are pending. They will be compared with results for station ALOHA.
OS26A-10
The Neutrally Buoyant Sediment Trap (NBST); a new tool for "Twilight Zone" Particle Exploration.
NBST's were deployed for the first time in support of a major experiment during VERTIGO (VERTical Transport In the Global Ocean). Estimating particle flux in the upper ocean can be challenging due to hydrodynamic effects with surface tethered traps. The NBST was developed to mitigate these effects on the PITS style of sampling tubes. The NBST once deployed, operates autonomously - controlling it's displacement to maintain a programmed depth over the course of its mission, which in this study was 3-5 days. Over the term of its deployment the instrument status is monitored by the on-board CPU; any unusual conditions will result in a `bailout' returning the instrument to the surface. Depth records indicate that the sampling depths of 150, 300 and 500m were reached within 4-8 hours and maintained a fixed depth within +/- 10 m of target. At the conclusion of the mission the sample tubes are closed utilizing a "Clap Trap" mechanism, similar to that on the surface tethered PITS array that was deployed during VERTIGO. Neutrally buoyant instruments by virtue of design generally exhibit a small surface profile which could make relocation and recovery, difficult and time consuming. However, the NBST has incorporated a suite of recovery aids, these include an Argos / GPS beacon transmitter and a low power LED flasher. Generally speaking most instruments were relocated within an hour of their surfacing.
OS26A-11
Estimating Export Production Using a Novel Drifting Sediment Trap: Results From the Crozet Plateau
The measurement of export production, and downward particle flux is of great biogeochemical interest as it sequesters carbon from the atmosphere and has the potential to buffer against atmospheric CO2 increases. Numerous models exist that describe the remineralisation of organic carbon with depth, but these have large errors associated with them in the upper part of the water-column where it is most difficult to make reliable estimates of export. PELAGRA provides reliable measurements of flux at prescribed depths in the top 600m. The CROZEX programme is a natural iron fertilisation experiment that was conducted in the Indian Sector of the Southern Ocean in an attempt to ascertain the effect of natural iron fertilisation on the biogeochemistry of the oceans. PELAGRA was deployed in order to constrain export measurements from the study area. The biogeochemical composition of export has been studied and results on organic carbon, nitrogen, calcium carbonate and biogenic silica will be presented. The composition of flux is compared with surface productivity data to estimate export production, and the differences in elemental ratios North and South of the Island are discussed. Small scale variability results in huge variations in the magnitude of export, at a high flux station M8W, the total particle flux recorded was 2716 mg/m2/day compared with 526 mg/m2/day at the low flux station M8E. Although both stations have different phytoplankton community structures, SEM imaging suggests that the exported material is almost exclusively comprised of diatoms. Chemical analysis shows that opal accounts for about 60% of the sinking material at both sites, however there are significant differences with respect to the elemental ratios. The low flux site, M8E, has significantly higher Si:C and Si:N ratios suggesting potential light limitation caused by small scale hydrographical variability. At M8W the export of organic carbon is 197 mg/m2/day, a factor of around 8 greater than the 24 mg/m2/day observed at M8E. The data highlights the complexity surrounding the determination of export in the upper part of the water column, and demonstrates the need to develop novel technologies to try and combat these difficulties.
OS26A-12
Determination of Pu Isotopes in Settling Particles by SF-ICP-MS: Implications for the Lateral Transport of Particles in the Continental Margin of the East China Sea
In recent years, a series of studies on Pu inventories and 240Pu/239Pu atom ratios in the water column and sediments in the Pacific have revealed the wide presence of Pacific Proving Grounds (PPG) close-in fallout Pu in the western North Pacific and its marginal seas. Although the oceanic current transportation of Pu from the PPG and the early direct tropospheric fallout have been proposed to be responsible for the wide presence of PPG close-in fallout Pu signature, details for the Pu transportation are not well understood. Studying the Pu scavenging process by biogenic and inorganic particles using a sediment trap sampling technique is essential to elucidate the details of the oceanic Pu transportation process, because sinking particulate material has been regarded as the primary means by which surface-introduced transuranics, for instance, Pu are transported to the sediments. In this work, we present an analytical method for the determination of Pu concentration and its isotope ratio (240Pu/239Pu) for settling particle samples by inductively coupled plasma mass spectrometry (ICP-MS). The generally used approach for Pu preconcentration by increasing the amount of samples is not applicable because of the small size of settling particle samples available for the analysis for Pu isotopes. Efforts were made to improve the sensitivity of a sector-field ICP-MS (SF-ICP-MS) and reduce the 238UH+ interference for Pu analysis by combining a high-efficiency sample introduction system (APEX-Q). An extremely low detection limit of 0.07 fg Pu was achieved, which allowed the determination of Pu isotope ratio at femtogram levels. The precision and accuracy of 240Pu/239Pu isotope ratio analysis were carefully examined with a certified Pu isotope standard and an ocean sediment reference material. Simple anion-exchange chromatography for the separation and purification of Pu was combined with the APEX-Q/SF-ICP-MS system to determine Pu isotopes in settling particles collected in the East China Sea continental margin. The obtained results supported a previous observation on the lateral transport of Pu containing particles in this continental margin.
OS26A-13
Time-series observation of POC fluxes estimated from 234Th in the northwestern North Pacific
Time-series measurements of 234Th activities and particulate organic carbon (POC) concentrations were conducted in the northwestern North Pacific from October 2002 to August 2004. Seasonal changes in POC fluxes from surface layer (upper 100 m) were estimated using 234Th as a tracer. POC fluxes varied seasonally from 0-310 mg-C m-2 d-1; values were higher from spring to summer than from autumn to winter. The export ratio (e-ratio) ranged from 9-100% and was also higher between spring and summer. The annual POC fluxes were estimated to be 59 g-C m-2 y-1 (or 160 mg-C m-2 d-1) in the subarctic region and 38 g-C m-2 y-1 (or 100 mg-C m-2 d-1) in the boundary region between the subarctic and the subtropics. POC fluxes and e-ratios in the northwestern North Pacific were considered much larger than those in much of the world's oceans. The seasonal (from April to October) POC flux based on 234Th was 39 g-C m-2 and was 77-87% of seasonal new production based on the seasonal difference of the nutrients in the Western Subarctic Gyre (45-51 g-C m-2). Much of the organic carbon produced in the surface layer in the northwestern North Pacific was considered to transfer via particulate materials to deeper layers.
OS26A-14
MedFlux: Particulate Organic Carbon-$^{234}$Th Relationships in Particles Separated by Settling Velocity in the Northwest Mediterranean
The ratio of particulate organic carbon (POC) to the natural U-series radionuclide $^{234}$Th is an important parameter in the determination of POC export from $^{234}$Th deficits. As part of the MedFlux program, we measured the POC/$^{234}$Th ratio of sinking material at the DYFAMED site in northwest Mediterranean. During early March to early May in 2003 (61 d deployment) and early March to late April in 2005 (55 d deployment), material was collected from depths of 200-1800 m and separated by settling velocity using IRS sediment traps operated in a settling velocity mode. The POC/$^{234}$Th ratio of exported material was determined for 11 settling velocity classes ranging from <1 to >980 m/d. In 2003, the POC/$^{234}$Th ratio of the settling velocity classes at 200 m ranged from 1.9 $\pm$ 0.1 to 4.9 $\pm$ 0.3 $\mu$mol C/dpm Th. The POC/$^{234}$Th ratios increased slightly with decreasing settling velocity. This is the reverse of the trend expected if particle size (or surface area: volume ratio) were the dominant control on particulate $^{234}$Th. In 2005, the POC/$^{234}$Th ratios for the settling velocity classes (<1 to >980 m/d) ranged from 1.0 $\pm$ 0.1 to 2.5 $\pm$ 0.3 $\mu$mol C/dpm Th at 200 m, 0.8 $\pm$ 0.03 to 2.0 $\pm$ 0.1 $\mu$mol C/dpm Th at 400 m, and 0.4 $\pm$ 0.01 to 1.3 $\pm$ 0.1 $\mu$mol C/dpm Th at 1800 m. In contrast with 2003, there is a bimodal relationship between POC/$^{234}$Th and settling velocity in all 2005 settling velocity traps. Elevated POC/$^{234}$Th occurred at high and low settling velocities (peaks centered at $\sim$490 m/d and $\sim$11 m/d). The differences observed between 2003 and 2005 in terms of overall values of POC/$^{234}$Th and its relationship to settling velocity most likely reflects differences in the dominant particle types exported. Field observations and analyses of biomarkers such as pigments, fatty acids, and amino acids support these conclusions. In 2003, material exported from 200 m consisted of fresh and degraded phytoplankton-derived (mostly diatom) organic carbon. Biomarker analyses have not yet been completed for the 2005 trap material; however, visual observation of plankton tows and trap material collected during this time indicates that there were large numbers of zooplankton and zooplankton fecal pellets, and relatively few phytoplankton cells present. The zooplankton were predominantly crustaceans (mostly copepods) and Pteropods mollusks and there was little gelatinous zooplankton. This is in striking contrast to 2003 when salps were the dominant zooplankton. The gut passage time for salps is short and their fecal pellet material can be chemically associated with phytoplankton-derived carbon since in 2003 phytoplankton biomass was important. Thus, the comparatively higher POC/$^{234}$Th in 2003 are consistent with algal material, while the lower values in 2005 are consistent with expectations for fecal pellet material, enriched in $^{234}$Th relative to POC. Overall, it is clear that particle size alone does not predict the relationships between POC/$^{234}$Th and settling velocity, but rather it is likely that additional factors such as particle composition, aggregation, disaggregation, and remineralization are responsible for the observed patterns.
OS26A-15
Seasonal Variations of Coccolithophore Export Production in the Northwest Pacific Ocean
Coccolithophore fluxes and assemblages were investigated by three sediment traps, collected from November 1997 to August 1999, along the Kuroshio Extension in the NW Pacific (station WCT-3), in the temperate NW Pacific (station WCT-2) and in the subarctic NW Pacific_istation WCT-5_j. The coccolith fluxes of station WCT-5, WCT-2, and WCT-3 were occurred 307.9, 674.8, and 104.6 \times 10$^{6}$ coccoliths m$^{-2}$ year$^{-1}$, respectively. The mixed water region showed that the coccolithophorid production was the highest. Both WCT-2 and WCT-3 stations were characterized by a strong seasonality in the coccolith fluxes. High coccolith fluxes were observed in late spring, while very low fluxes occurred in autumn and early spring. At station WCT-5, maximum coccolith fluxes were reached in late spring and early summer. At station WCT-2, the seasonal change of coccolith assemblages were complexly influenced of the warm current and the cold current because this trap site was located in the mixture waters of the Kuroshio Current and Oyashio Current. Emiliania huxleyi, Gephyrocapsa oceanica and Florisphaera profunda and Coccolithus pelagicus were the most abundant species of coccolith flora. E. huxleyi dominated the community in the three stations. Recently, it was reported that two different genotypes (types A and B) existed in E. huxleyi. Then, the seasonality between two types of E. huxleyi was examined at three stations. The frequency of type A of E. huxleyi was corresponding at time when the influence of the cold eddy of Oyashio Current strengthened at stations WCT-2 and WCT-5. C. pelagicus s.l. flux pattern was closely correlated with that of opal flux. Moreover, the abundance of C. pelagicus s.l. is considered to be associated the supply of the nutrient.
OS26A-16
An Uneven Pair: Organic Carbon Export in the Eastern and Western Compartment of the North Atlantic Subtropical Gyre
The North Atlantic subtropical gyre can be subdivided into eastern and western provinces. Both compartments are characterized by oligotrophic conditions and low production, at times enhanced by the influence of wind-induced surface mixing, eddy activity or lateral nutrient input. Surface chlorophyll seasonality and production are apparently similar, but zonal differences in new production and export of particulate organic carbon (POC) have been observed. Here we present satellite derived surface chlorophyll, primary production and export fluxes in these two regions, represented by results from the European Station for Time-series in the Ocean, Canary Islands (ESTOC) in the east and the Oceanic Flux Program (OFP) time-series sediment traps in the west. To quantify region-wide POC export we used a correlation-based approximation to the source region of sample material and fitted a regression to satellite deduced surface primary production in the source region and trap-derived POC flux. In contrast to ESTOC, the strongly seasonal signal at the OFP station is in good accordance with the corresponding surface chlorophyll signal, whereas at ESTOC, the temporal correlation is generally less pronounced, and POC export is lower. Recent studies suggest that eddy-driven vertical advection may be the clue to explain higher nutrient input and hence POC export in the western province. The regional adaptation of the regression model enables us to calculate region-wide POC export with a higher precision than would be possible with basin wide approaches.
OS26A-17
Using Settling Velocity to Model Particle Stickiness in a Mesocosm
Mesocosm experiments are increasingly being used to study marine ecosystem dynamics and the mechanisms of marine particle interactions. The probability of adhesion of two particles upon collision, particle stickiness, is one of the principle factors affecting the interaction and subsequent flux of particles in the ocean. However, parameterization and accurate estimation of particle stickiness is difficult. Conventional determination of particle stickiness relies on changes in the Particle Size Distribution (PSD) through the use of a horizontal couette device having well-characterized shear. This technique requires that the velocity of the fluid within the couette device be much larger than the range of possible settling velocities of the interacting particles. Under low energy levels in a mesocosm experiment, the effects of the loss of particles due to settling will affect the PSD such that such that current models for directly estimating particle stickiness are inadequate. We conducted mesocosm experiments, with phytoplankton monocultures, in a 1500 liter rotating annular flume. The annular flume has near laminar flow and our mesocosm experiments were conducted with a mean shear of 0.0946/s. Variability of compared estimates of mesocosm particle stickiness as determined in both a couette device and directly from the flume could not be explained using traditional models (r^2 < 0.12). This variability can be explained by loses from the PSD due to the settling of particles. Our experiments show that the vertical loss of particles in the low shear mesocosm is an important factor that contributed to the variability of our direct estimates of particle stickiness. To improve direct estimates of particle stickiness in mesocosm experiments, we have developed a model that utilizes measurements of the PSD and the settling velocity of particles within the mesocosm. Our model is derived from the basic equations for algal cell aggregation as presented in Jackson [1990]. Initialization of the model with the PSD and average particle settling velocity produces reasonable estimates of particle stickiness, compared to previously published values. The use of our model, when combined with the low shear of our mesocosm allows for estimates of particle stickiness with minimal exposure of fragile marine particles to the high energy levels inherent in conventional techniques.
OS26A-18
Importance of Calcareous Shells in Aggregate Formation and Particle Decomposition for the Coccolithophorid Emiliania Huxleyi
Reports showing a tight relationship between the deep export of organic carbon and minerals indicate that organisms with mineral shells such as coccolithophorids are of special importance for carbon export in the ocean. Several hypotheses about the mechanism behind this coupling of mineral and organic fluxes have been raised, such as the ballast hypothesis, or the mineral protection hypothesis.We examined aggregate formation and sinking as well as organic matter decomposition of calcareous and non-calcareous Emiliania huxleyi cells in an experiment that allowed aggregation and settling of the cells similar to what might occur in the water-column. Marked differences between the two cultures were observed regarding the time of aggregate formation, aggregate properties and the decomposition rates of organic matter. Our results indicate that calcareous cells are exported much faster and seem to be better protected against decomposition. The concurrence of both ballast and mineral protection observed in our experiment may explain the robustness of the relationship between organic and mineral matter fluxes in the deep ocean.
OS26A-19
The Protection of Particulate Organic Carbon Within Aggregates in the Water Column
The occlusion of organic matter within aggregates of mineral particles has been proposed as one mechanism for its preservation in terrestrial soils and marine sediments. This mechanism of protection may also be important in the water column, where it may contribute to the export of organic matter from the surface mixed layer.\\\\To determine if the compartmentalization of organic matter within aggregates has any effect on decomposition, we approached this question in reverse, by testing the effect of disaggregation on decomposition. For comparison of different particle types, we performed experiments using natural aggregates from Long Island Sound, a coastal estuary, and from the DYFAMED site in the northwest Mediterranean Sea (as part of MedFlux), an area relatively free from coastal influence. Long Island Sound samples were collected in October, 2004 from a depth of 2 m using a 125-$\mu$m mesh plankton net. Mediterranean samples were collected in March, 2005 from a depth of 200 m using a 50-$\mu$m mesh NetTrap, a surface-tethered particle collection apparatus designed to collect sinking particles. Half the particles were sonicated to disrupt aggregates, and half were not. Decomposition of particles within the two treatments was monitored through analysis of particulate organic carbon and nitrogen concentrations and amino acid composition.\\\\In the case of the Long Island Sound experiment, the rate and final extent of POC and PN decomposition was slightly greater (by a factor of 2) for the disaggregated samples compared with the natural aggregates. Principal components analysis of the amino acid compositions revealed that the disaggregated particles were also more extensively degraded than the natural aggregates. These results support the hypothesis that organic matter is protected by its occlusion within aggregates. Comparison with Mediterranean particles, as well as the possible role of aggregate porosity in determining protective capacity, will also be discussed.
OS26A-20
Composition and Degradation of Marine Particles with Different Settling Velocities from MedFlux Experiments
In recent MedFlux experiments in the western Mediterranean, we fractionated unpoisoned sinking particles by their settling velocity. Our goal was to characterize the in-situ sinking nature of particles, followed by analysis of their chemical composition to evaluate relationships between organic matter, mineral ballast, sinking, and decomposition. Analyses of lipids and amino acids in these trap samples and application of multivariate analysis separates trap samples dominated by fresh diatom-derived organic matter from those that contain substantial amounts of organic matter that has been reworked by zooplankton and bacteria. Most particles (40 percent of total mass) sank at greater than 200 m/d and were dominated by large diatom-derived aggregates produced during the spring bloom period. More slowly-settling particles carried with them a greater zooplankton and bacterial signature. Particles of the different settling velocity classes were also obtained by elutriation and were incubated for 5 days at in-situ temperature in the dark in seawater from the depth of collection and with its natural bacterial community. The organic composition of particulate material was determined over the time-course of the incubation to assess biological lability of the settling-velocity classes. As the classes of particles decomposed, their compositions changed and became more similar with time, with a dominance of compounds that suggests a more degraded state. Biogenic opal in the particles dissolved faster in more-degraded particles than in fresher particles, suggesting that dissolution of opal might release labile organic matter that is then degraded. Coupling measurements of in-situ settling velocity of particles with turnover rates of particulate lipid biotracers showed that most small, slowly sinking particles would decompose in the twilight zone.
OS26A-21
MEDFLUX: Effects of Mercuric Chloride and Protease Inhibitor on Degradation of Particulate Organic Matter
Mercuric chloride (Hg) and protease inhibitor (PI) are used to inhibit bacterial or enzymatic activities in laboratory and field studies of marine particulate matter. We investigated effects of Hg and PI on degradation of particulate organic matter (POM) with emphasis on compositional changes of individual organic compounds over a one-month time scale (33 d). For these lab studies, we used a diatom culture, Thalassiosira pseudonana 3H, to simulate POM. In control samples, concentrations of POC, PN and particulate amino acids (PAA) decreased by 50%. Most fatty acids were degraded, but neutral lipids did not change much. Chl-a increased by 30% after 33 d, suggesting that some free Chl-a was released from Chl-protein complexes due to decomplexation or degradation of proteins. Samples treated with PI showed similar quantitative and compositional changes as in control samples, although degradation of fatty acids was even more intense, indicating that PI did not stop bacterial activity. For samples treated with Hg, 10-20% of POC, PN, PAA and lipids were lost after a month. In addition, certain polyunsaturated fatty acids including 16:3, 18:3 and 18:4 were lost preferably in Hg-preserved samples, possibly due to oxidation. Dissolved amino acids in these samples increased as much as particulate amino acids decreased, suggesting that dissolution might explain the loss of POM. In Hg-treated samples, Chl-a decreased by a factor of 4 compared to control samples. Simultaneously, a significant amount of a Chl-a allomer (identified by HPLC-MS) was produced. In contrast, allomerization in sediment-trap samples preserved by Hg was not as intense as in the diatom cultures. We recommend summing the Chl-a and the allomer peaks together to avoid underestimating Chl-a concentration.
OS26A-22
Discovery of a Serious Problem in the Methods for the Determination of Organic Carbon Contents in Marine Sediments or Particles: organic carbon losses caused by drying at 60°C prior to instrumental measurements
Current methods for the determination of organic carbon contents in marine sediments or settling particles usually included one or both of the following two sample preparation steps before the solid samples were loaded onto an elemental (CHN) analyzer for organic carbon measurement: drying at temperature 60°C of (1) the wet sample and (2) the spent acid solution, after the addition of an acid solution to the sample to eliminate inorganic carbon. We conducted parallel tests to determine the effects of these two drying steps on organic carbon measurements: the traditional heat-drying at 60°C in an oven versus the new freeze-drying at -44°C and under vacuum in a freeze-drier. Measurements of organic carbon contents of five fresh marine sediment samples (three hemipelagic and two coastal, organic carbon from 0.4% to 3.0%) showed that the results obtained by the heat-drying test were 16.4% to 24.7% lower than those of the freeze-drying test. Gas chromatographic analysis of the vapor collected above the wet sample during heat-drying at 60°C revealed the existence of organic carbon in this vapor, indicating organic carbon losses or evaporation during heat-drying. Freeze-drying of a sediment sample with different durations, on the other hand, yielded identical organic carbon content results, suggesting that no organic carbon losses occurred during freeze-drying. Based on these observations, we believe that (1) heat-drying in any current methods for the determination of organic carbon contents needs to be avoided or replaced by freeze-drying and (2) many of the published organic carbon data of marine sediments or particles, unfortunately, are erroneous or may have to be reevaluated.
OS26A-23
A One Dimensional Model of Particle Flux in the Water Column
The mechanisms by which particulate organic carbon (POC) makes it to the sea floor are still somewhat unclear. A major issue of disagreement is the dominant process by which POC reaches the sea floor. Is it the amount of ballast available (Armstrong, 2002; Klaas and Archer, 2002) or is it the amount of POC available to bind the ballast to it (Passow, in press) that is the dominating factor? In an attempt to answer this question and learn more about fluxes in the ocean we have designed a 1 dimensional model. It looks at individual particles of different sizes, composed of organic carbon, calcite, opal and lithogenic material. The relationships between porosity, diameter and volume are from Alldredge and Gotschalk (1988). The mechanisms included in the model are a modified Stoke's law (Alldredge and Gotschalk, 1988), bacterial consumption of organic carbon (Hirose and Kamiya, 2003) and dissolution of calcite and opal. The model is forced from the surface with different export ratios and the effect of them on the rain ratio and flux of POC to the sea floor examined. Using a particle spectrum from Stemmann at al. (2002) we constrain our spectrum and observe the total flux of POC and ballast towards the sea floor. In agreement to Contes' (2001) observations we find a constant mass ratio of organic carbon to the seafloor and also a rain ratio of approximately 0.7. Alldredge, A.L, and C. Gotschalk, In situ settling behavior of marine snow, Limnol. Oceanogr. 33(3), 339-351, 1988.\\ Archer, D.E., and E. Maier-Reimer, Effect of deep-sea sedimentary calcite preservation on atmospheric CO2 concentrations, Nature. 367, 260-264, 1994.\\ Armstrong, R., C. Lee, J. Hedhes, S. Honjo, S. Wakeham, A new, mechanistic model for organic carbon fluxes in the ocean based on the quantatative association of POC with ballast minerals, Deep Sea Research II, 49, 219-236, 2002.\\ Conte, M.,, N. Ralph, E. Ross, Seasonal and interannual variability in deep ocean particle fluxes at the Oceanic Flux Program (OFP)/Bermuda Atlantic Time Series (BATS) site in the western Sargasso Sea near Bermuda, Deep-Sea Research II 48 1471-1505, 2001\\ Hirose, K. and H. Kamiya, Vertical Nutrient Distributions in the Western North Pacific Ocean: Simple Model for Estimating Nutrient Upwelling, Journal of Oceanography, Vol. 59, pp. 149 to 161, 2003 Klaas, C., and D.E. Archer, Association of sinking organic matter with various types of mineral ballast in the deep sea: Implications for the rain ratio, Global Biogeochemical Cycles, 16, 2002.\\ Passow, U., Switching perspectives: do mineral fluxes determine particulate organic carbon fluxes or vice versa?, Geochem. Geophys. Geosyst., 5, 2004. U. Passow, Christina L. De La Rocha, The Accumulation of Mineral Ballast on Organic Aggregates, in press\\ Stemmann, L., Gorsky, G., Marty, J., Picheral, M., and J. Miquel, Four-year study of large-particle vertical distribution (0-1000m) in the NW Mediterranean in relation to hydrology, phytoplankton, and vertical flux, Deep-Sea Research II. 49, 2143-2162, 2002.\\
OS26A-24
On the variability of the exponent in the powerlaw depth dependence of POC flux estimated from sediment traps
Using the powerlaw relationship $J(z) = J(z_o)(z/z_o)^{-b}$ relating the sediment flux at depth $z_o$ in the water column to the flux,$J(z)$ at depth $z$ we have reanalyzed the sediment-trap data set compiled by Berelson (2001) to better understand the variability in the exponent $b$. We show that the observed correlation between the estimates of the exponent $b$ and the export flux at $z_o=100$ m noted by Berelson (2001) can be accounted for by the non-independence of errors in the parameter estimates and that the data is consistent with the hypothesis that the true parameters are independent. Furthermore we found that the sediment-trap dataset is consistent with a universal exponent $b=0.68 \pm 0.04$ (1 s.e. $\nu= 88$ d.f.). At the NABE site however the above conclusion was dependent on which POC flux data were used at depths 150 m and 300 m. POC flux data based on floating-traps did not result in a significantly different exponent, but the thorium-based flux measurement resulted in an exponent that was significantly different $b=1.49 \pm 0.18$ (1 s.e. $\nu = 8$ d.f.). The thorium-based exponent is probably biased high because of the different measurement technique in the intermediate depth range of (300-2000 m).