OS12H-01
Diatoms in the Desert
During the third cruise of the E-flux project in the subtropical Pacific (March 2005), we encountered a well-developed, cyclonic cold-core eddy off the big island of Hawaii. The eddy, designated cyclone {\it Opal}, spanned 200 km in diameter and was evident as doming isopycnals and an uplifted deep chlorophyll maximum (DCM). Microscopic examination of transect samples across the eddy revealed a dramatic shift in phytoplankton community structure at depth inside the eddy relative to surrounding waters. Whereas upper mixed-layer populations in the eddy resembled those outside the eddy and were dominated by small phytoplankton, the deep chlorophyll maximum inside the eddy consisted of large chain-forming diatoms, strikingly similar to the iron-induced patch bloom at $66\deg$S, $172\deg$W during the Southern Ocean Iron Experiment (SOFeX). Initial estimates of diatom biomass reached nearly 90 $\mu$g C L$^{-1}$ in the center of the eddy, and protozoan grazer biomass was enhanced 2-3 fold. A distinct and persistent layer of presumably senescent diatom cells overlaid healthy populations, often separated by less than 10 meters, indicating that we sampled a bloom in the state of decline. Visual estimates of fluorescence suggest that 90% of diatom cells in this senescent layer were lacking in chlorophyll and/or cellular protein. The abundance of ``empty`` cells confounds the interpretation of standing stocks, both in the euphotic zone and beneath, affecting such parameters as carbon content per cell, carbon to chlorophyll ratios, and carbon to silica ratios. Time-series sampling over five days also showed a community structure change within the diatom bloom, from the unexpected large chain-forming species to smaller ones more typical of subtropical Pacific DCM. Such rapid and distinct temporal and spatial changes within the eddy indicate a dynamic, yet localized biological response. Shifts in size structure, composition and biomass of phytoplankton in the eddy should result in greater productivity and particle flux inside the eddy than out.
OS12H-02
Microbial Community Dynamics During the Decline Phase of a Diatom Bloom in a Subtropical Cyclonic Eddy
As part of experimental studies of cyclone Opal, a cold-core eddy off Hawaii, in March 2005, we investigated phytoplankton growth and microzooplankton grazing rates during the decline phase of an open-ocean diatom bloom. We used a 2-bottle dilution technique with incubations on a drift array to obtain a time series of depth-profile rate estimates under in situ conditions of light and temperature. During the early period of observation, phytoplankton growth rates in the center of eddy were high, exceeding 1 per day and contemporaneous estimates of grazing. However, growth estimates collapsed precipitously on the 4th day and remained low (less than 0.4 per day) for the final 3 days of study. Grazing rates exceeded growth during this latter period, which was marked by declining cell biomass in the ambient environment. From compositional analyses of these experiments by flow cytometry, HPLC pigments and microscopy, we will reconstruct the taxon-specific dynamics of the microbial assemblage during the bloom decline and consider the varying fates of different phytoplankton taxa as they relate to cycling and export processes.
OS12H-03
Gross Primary and Net Community Production Rates Inside and Outside Eddies off Hawaii Based on Isotopic Measurements of Dissolved Oxygen and Inorganic Carbon
We measured the triple oxygen isotopic composition of dissolved O$_{2}$, the dissolved O$_{2}$/Ar gas ratio and the $^{13}$C/$^{12}$C of dissolved inorganic carbon (DIC) on three cruises during the E-Flux project between November 2004 and March 2005. The objective of these measurements was to determine rates of gross primary productivity (GPP) and net community productivity (NCP) and compare the rates inside and outside eddies formed near Hawaii in the subtropical N. Pacific. GPP determined using a mixed layer oxygen isotope budget resulted in rates of 1.4 - 1.8 gms C/m$^{2}$/d inside the eddies that, on average, was almost 2x higher than GPP rates outside the eddies (0.9 gms C/m$^{2}$/d). There was, however, a large range (4x) in the oxygen isotope based GPP rates inside the eddies during the three cruises that was twice the GPP range outside the eddies. There was up to a 3x range in GPP rates determined for different stations within the same eddy. On average, the oxygen isotope derived GPP rates outside the eddy agreed with our previous determinations at the subtropical time series Station ALOHA. Rates of NCP within the eddies determined from the oxygen isotope and O$_{2}$/Ar measurements averaged 50 mg C/m$^{2}$/d but with large variability. We will examine the effect that the doming of isopycnals has on the oxygen isotope based estimate of GPP and NCP. Typically, the mixed layer δ$^{13}$C of DIC values measured inside and outside the eddies were similar. The rate of NCP determined from a mixed layer DIC$^{13}$ budget averaged 60 mg C/m$^{2}$/d (1.8 moles C/m$^{2}$/yr) both inside and outside the eddies which was slightly lower than our previous estimates at Station ALOHA.
OS12H-04
Eddy Induced Biological Export in the lee of Hawaii
Mesoscale eddies are dynamic and ubiquitous features of ocean circulation. Recent studies suggest that they play an important role in supplying new nutrients to the euphotic zone, thereby enhancing primary production and carbon export in otherwise nutrient-deficient systems. The goal of this study was to examine particle export using $^{234}$Th-$^{238}$U disequilibria associated with cyclonic eddies that form in the lee of the Hawaiian Islands. High resolution sampling was conducted within several eddies that appeared to represent various stages of an eddy's biogeochemical life cycle: spin-down and decay (Cyclone {\it Noah}, November 2005, E-Flux I), spin-up (Cyclones {\it Akiko} and {\it Victor}, January, 2006 E-Flux II) and maturity (Cyclone {\it Opal}, March 2006, E Flux III). Each eddy was bisected by several transects and at multiple time points within the eddy center (IN stations). These data were then compared with data generated from samples collected at control stations unaffected by eddy activity (OUT stations). Transects through mesoscale Cyclones {\it Noah} (spin down) and {\it Opal} (mature) showed increasing particle flux towards the eddy center, whereas the submesoscale Cyclones {\it Akiko} and {\it Victor} (spin-up) had little to no $^{234}$Th-$^{238}$U disequilibria. The magnitude of $^{234}$Th-derived export within Cyclone {\it Noah} was also consistently higher than that measured within Cyclone {\it Opal}. These patterns in particle flux are likely due to the time lag that typically occurs between initial production and particle export and confirm the hypothesized eddy life cycle stage. $^{234}$Th derived particulate organic carbon export rates will be determined using C/$^{234}$Th ratios collected using {\it in situ} pumps and free-floating sediment traps. Results will further be compared with those determined from other estimates of particle flux made during each cruise, e.g. sediment traps, $^{210}$Pb/$^{210}$Po, δ$^{18}$O, δ$^{13}$C and δ$^{15}$N. Hence, E-flux will be one of the few comprehensive studies that not only examines the role of eddies in organic carbon export, but also details the temporal and spatial variability in organic carbon export associated with the entire biogeochemical life cycle of cold-core cyclonic eddies.
OS12H-05
Mesoscale Jets and Eddies in the Southern Drake Passage
Strong biological gradients exist in the southern Drake Passage near the South Shetland Islands and Elephant Island, forming highly productive regions of all trophic levels from primary production, zooplankton aggregations to birds and mammals. It has been hypothesized that the primary production in the Southern Ocean is limited by available iron, and the offshore transport of iron-rich shelf waters determines the biological gradients and enhanced primary productivity. A field survey was conducted between February 12 and March 24, 2004 for studying the Antarctic Circumpolar Current (ACC) and its interactions with topography and shelf waters using Acoustic Doppler Current Profiler (ADCP) and high resolution CTD surveys. The results visualize the large scale topographic steering of the ACC, small-mesoscale jets and eddies associated with fronts and baroclinic instabilities, and their impacts on transport and retention of biota in the area. The ACC intrudes onto the shelf by topographic steering, forming jets of greater than 50 cm/s, fronts of approximately 2-6 km wide and 1000 m deep, eddies of 10s km translating off shelf containing shelf iron rich waters, and high productivities associated with such small-mesoscales. The findings indicate the importance of resolving small-mesoscales for better understanding the coupled physical and biogeochemical processes in the Southern Ocean.
OS12H-06
Intense submesoscale vertical motion in the upper ocean and its effect on surface biogeochemical distributions
A profusion of submesoscale, secondary circulation features are formed in the upper ocean in the vicinity of fronts. The associated vertical motions can be intense; narrow, elongated cells of intense up- and down-welling form close to the surface, overlying broader cells associated with the mesoscale meanders of the front. The submesoscale down-welling is considerably stronger than up-welling and is concentrated in 1-2 km width filaments within which vertical velocities can attain magnitudes as high as 200 m/day. A fully nonhydrostatic, three-dimensional, free-surface model with horizontal grid resolutions of 0.5 to 1 km is used to explore the dynamics of these phenomenon and assess the role of nonhydrostatic effects, wind forcing and loss of balance. The submesoscale motions investigated play a significant role in the exchange of properties between the surface ocean and thermocline and in the transfer of energy between the energetic, non-dissipative, mesoscale flow field and small-scale, three-dimensional regions of mixing in the surface ocean. We examine their effects on the transport and distribution of biogeochemical properties in the upper ocean.
http://varun.bu.edu/amala