OS14H-01
Vertical Nutrient Transport in Cyclonic and Anticyclonic Eddies in the Sargasso Sea as Evidenced from $^{3}$He Distributions
The physical transport of thermocline tritiugenic $^{3}$He into the surface layers of the Sargasso Sea must be accompanied by a proportionate flux of regenerated nutrients. This has been used to make estimates of the long term average new production in the oligotrophic waters of the Sargasso Sea, and implies the existence of a subtropical nutrient spiral whereby thermocline nutrients are ultimately brought into the seasonally accessible layers of the subtropical gyre. From there, deep winter convection and eddy pumping processes are responsible for the final leg of the journey, where nutrients are lifted and/or mixed into the euphotic zone to fuel new production. As part of the Eddies Program, we surveyed a cyclonic and anticyclonic eddy for 3He and nutrient distributions. While the two tracers are transported in proportion by the physical processes of advection and mixing, they differ in an important way in their behavior near the sea surface: nutrients can and will be taken up by community production when light is available, but 3He can only be lost from the mixed layer by gas exchange with the atmosphere. That is, fluid parcels rich in both nutrients and $^{3}$He that have been advected or mixed into the euphotic zone below the mixed layer will tend to lose nutrients more rapidly than $^{3}$He. Thus below the mixed layer, we can combine the two tracer fields to compute a "missing nitrate" in the form MN = 2.4 (Δ^{3}$He + 1.66) - NO3 where Δ^{3}$He is the helium isotope ratio anomaly in percent relative to air. Thus missing nitrate is a measure of the amount of nitrate that would be expected in the absence of biological productivity, and may represent a lower bound on the amount of new production that has occurred. We compare the patterns of missing nitrate for the cyclone and anticyclone, and discuss the similarities and apparent differences in implied nutrient transport and biological production of both features.
OS14H-02
Biogeochemical Impacts of a Cyclonic Eddy in the Sargasso Sea
Mesoscale eddies have been hypothesized as a mechanism for vertical nutrient pumping into the surface euphotic zone in the oligotrophic Sargasso Sea, but the full range and magnitude of biogeochemical impacts by eddies remain uncertain. Here, we report findings from a cyclonic eddy located near Bermuda that we evaluated for its impact on water column biogeochemistry. We find evidence that the eddy may not be formed with water local to the western Sargasso Sea (the waters surrounding the eddy). In the density range 26.1 to 26.7, an eddy core with anomalous salinity, temperature and biogeochemical properties was clearly observed. Here we combined quasi-conservative hydrographic and biogeochemical tracers (including tritium and 3He) to address whether the origin of the eddy core water may be an area several hundred km to the southeast of the eddy location at sampling. By comparing the observed eddy core's biogeochemical properties with those of waters located near the proposed origin, we estimate the net changes that would have occurred, and thence the implied biogeochemical impacts by these changes. The observed biogeochemical variations in the subsurface eddy core are the results of remineralization of organic materials, requiring new production of 0.5 $\pm$ 0.3 mol$\cdot$N$\cdot$m$^{-2}$ since eddy formation. Our results suggest that transport of water in mesoscale eddies may be an important mechanism for transferring biogeochemical signals over considerable distances, in this case from the SW Sargasso Sea to the western Sargasso Sea. Given that the proposed location of eddy water formation covers a very large area, we cannot be certain as to the true initial biogeochemical conditions in the source water, thus making a precise estimate of the biogeochemical impact of the eddy difficult. We conclude that uncertainty in the initial conditions of eddies, and uncertainties in processes such as mixing occurring between eddy formation and distant sampling, imposes uncertainty in ascertaining the impact of eddies on ocean biogeochemistry.
OS14H-03
Dissolved iron distribution in a mode-water eddy in the Sargasso Sea
Dissolved Fe distribution is studied in a mode-water eddy in the Sargasso Sea during August 29 to September 14, 2005. Seawater samples were collected using UAF ATE/vane automatic Fe sampler. Dissolved Fe in the samples was determined by isotope dilution ICPMS. The results show that iron concentration is high at the surface, decreases sharply to a minima at about100 m and then increases with increasing depth to a small maximum at about 1000 m. The dissolved Fe minimum coincides with chlorophyll maximum, implying Fe removal by active algal growth. The Fe:P ratio decreases with increasing depth, reflecting a balance between upward nutrient supply and downward mixing of eolian Fe. Temporal variation of this balance may control the dissolved Fe concentration and the Fe status of phytoplankton at the chlorophyll maxima.
OS14H-04
The Impact of Mesoscale Eddies on Phytoplankton Communities as Determined by Fluorescent Induction and Relaxation Kinetics
Here we present data describing the effect of mesoscale eddies on the photosynthetic capacity of the phytoplankton community from both the Atlantic `EDDIES' and Pacific `E-Flux' research projects. The fluorescence induction and relaxation emission (FIRe) fluorometer can measure, non-invasively, in real time and at high sensitivity, several photosynthetic parameters of a phytoplankton community. These include the ratio of active to maximal fluorescence (Fv/Fm), which, together with other measurements, can provide estimates of phytoplankton productivity and determine community photosynthetic health. The physical perturbation generated by eddy systems can uplift phytoplankton communities at the nutracline higher into the euphotic zone. Such features are often associated with a phytoplankton bloom and a shift in phytoplankton community structure to larger cells. Discrete samples taken from both inside and outside the eddies have provided spatial and temporal insight into the changing photosynthetic parameters and community structure associated with bloom formation and development. A strong correlation of photosynthetic efficiency (Fv/Fm) to macro-nutrient availability was observed throughout the features. However, despite the presence of available macro-nutrients, Fv/Fm at the bloom site was often lower than expected, suggesting another limiting factor and/or a mature eddy feature. Intriguingly, whilst the cyclonic eddies studied in the Pacific were associated with diatom blooms cyclonic eddies from the Atlantic induced blooms of picoplankton. Rather, it was Atlantic mode-water eddies that were associated with diatom bloom events. This may possibly indicate a fundamental difference in the physical dynamics and nutrient availability within an individual eddy feature. We investigated potential nutrient differences using bioassay studies in an attempt to correlate Atlantic and Pacific eddy events. In addition, during the Atlantic EDDIES project, a submersible fluorosensor was deployed as one of the instruments associated with the video plankton recorder (VPR). This generated high-resolution data sets of the photosynthetic parameters of the phytoplankton community within a complex and dynamic eddy feature. In combination with species abundance and composition, data generated from the VPR provide insight into the linkages between community structure and phytoplankton photosynthetic capacity.
http://science.whoi.edu/users/mcgillic/eddies/EDDIES_Project.html
OS14H-05
Elevated Biomass of Mesozooplankton and Enhanced Fecal Pellet Flux in Cold-Core and Mode-Water Eddies in the Sargasso Sea
Accumulating evidence points to the importance of mesoscale eddies in supplying nutrients to surface waters in oligotrophic gyres. However, the nature of the biological response and its impact on export has yet to be elucidated. As mesozooplankton play a fundamental role in food web interactions and the flux of material out of the surface waters, changes in mesozooplankton community composition due to physical perturbation by eddies could profoundly affect biogeochemical cycling. As part of the EDDIES project we followed the evolution of 2 distinct features in the Sargasso Sea, a cold-core eddy and a mode-water eddy. We conducted zooplankton tows across both eddies using a Multiple Opening and Closing Net Environmental Sampling System (MOCNESS) and coupled these data with bioacoustical backscatter from a hull mounted ADCP. Size-fractionated (>150 $\mu$m) biomass was quantified from the surface to 700 m over 9 discrete depth intervals. Zooplankton biomass (dry weight) in the upper 150 m was similarly enhanced for both the cold-core cyclone and mode-water anticyclone at 752 mg m$^{-2}$ and 674 mg m$^{-2}$, respectively, compared with an average of 462 mg m$^{-2}$ for summer samples collected over the last decade at the Bermuda Atlantic Time-series (BATS) site. In the cyclone, maximum zooplankton biomass occurred on the eddy periphery associated with chlorophyll hotspots. Biomass increased for the largest size class (>5 mm) from 0-500 m and for all size classes in the upper 150 m, where an average of 67% of the total biomass occurred. In the anti-cyclone, peak biomass occurred at eddy center but was highly variable (by >3 fold) over time, perhaps resulting from the high numbers, but patchy distribution of diatom chains in this region. Night:day biomass ratios in both eddies decreased compared to the BATS site, potentially indicative of reduced vertical migration in eddies. Shifts in both zooplankton community composition and abundance were reflected in sedimenting fecal pellets which were an important component of particulate organic carbon flux. Inside the cold-core cyclone the number of zooplankton fecal pellets in sediment traps at 150 m was 2 fold higher than outside the eddy, accounting for 25% of total carbon flux. Eddies thus provide us with natural perturbation experiments in which the linkages between the physical environment, the biota and biogeochemistry can be elucidated.
OS14H-06
Particle Export During EDDIES
Prior studies suggest that one of the impacts of mesoscale eddies on upper ocean biogeochemistry might include increased particle export due to enhanced biological activity. As part of the EDDIES project (EDdy Dynamics, mIxing, Export, and Species composition), we obtained detailed sections of the naturally occurring radionuclide thorium-234 to use as a proxy of particle flux. New small volume 234Th methods allow us to obtain on the order of 150 samples per 10 day cruise at roughly 15 stations allowing for resolution of submesoscale patterns of export. In 2004 and 2005, we were able to sample the same eddy twice with a 3-4 week gap in between. This allows us to look for temporal changes in 234Th distributions on time scales similar to 234Th`s half life (24.1 days), which enables us to consider changes over time in upper ocean food webs and export, and the application of more precise non-steady state models of export fluxes. In 2004, we saw considerable small scale variability in total 234Th within a cyclonic eddy, with enhanced export of 234Th being observed along the perimeter of the eddy and in vertical layers across the feature. There is a remineralization feature as evidenced by ``excess`` 234Th at 200m, as well as scavenging at 300m, perhaps due to enhanced packaging of suspended particles into sinking ones. While features within the eddy thus appear to be influenced by the eddy structure, there was no significant difference between fluxes in the eddy and a single control station outside the main feature. Drifting sediment trap data confirm this lack of significant difference in particle flux within the eddy in 2004 and outside at control stations. During 2005, a mode water eddy was sampled with similar resolution. These data will be presented and compared to results from 2004 and the flux of particles seen at the BATS site, where prior 234Th data are available. Using in situ pumps, ratios of C/Th on filtered particles allow us to convert from 234Th flux to C export for comparison to rates of primary production, new production and the nutrient, carbon and oxygen budgets.
http://cafethorium.whoi.edu