Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
• Data Files
• Parameters
• Instruments
• Deployments
• Project Information
• Program Information
• Funding

Pigment concentrations are reported from four cruises in the Sargasso Sea during 2011 and 2012.

Study Site and CTD Casts
Data were collected on four cruises in the Sargasso Sea on board the R/V Atlantic Explorer. On each cruise, sampling was conducted at three stations: the center and edge of a mesoscale eddy and at one station outside of the eddy. Eddies were identified using satellite-derived sea level anomaly (SLA) data provided by Dr. Dennis McGillicuddy and Dr. Valery Kosnyrev of the Woods Hole Oceanographic Institution. Target eddies (one per cruise) were initially identified on the day of departure; the ship's position within the eddy (at the center or the edge, as appropriate) was confirmed by daily checks of SLA data.

At each station, high resolution CTD casts to ~2000 m were performed at noon to measure core physical, chemical and biological parameters of the water column. In addition to the core CTD casts, pre-dawn "Productivity" CTD casts were performed to collect water for measurements of size-fractionated biomass (as chl a) and size-fractionated primary productivity. Samples were obtained using the 24 bottle Niskin rosette from 3-4 depths (20 m, 40-50 m, deep fluorescence maximum (~80 m), and 100 m). Ten-liter polycarbonate collection bottles (covered with black tape) were pre-rinsed with sample water and were filled by draining the Niskin bottles through opaque tubing. All samples were pre-filtered through a 200 um Nitex screen. Further handling of the samples was done in the dark or under red light.

The 200 um pre-screened water from pre-dawn productivity casts was used for measurements of size-fractionated biomass (as chl a) and biomarker photopigments by HPLC and for measurements of size-fractionated primary productivity.  HPLC pigments were also used for taxonomic identification of total and size-fractionated phytoplankton groups using ChemTax analyses. Samples for microscopy were also taken from productivity casts as verification of the ChemTax results using methods described above for core CTD casts. 

Total phytoplankton biomass was measured directly by filtering triplicate aliquots of 1 to 2 liters of pre-screened water onto GF/F filters. This gave total chl a in the size fraction 0.7 to 200 um. The biomass of three size classes of phytoplankton was quantified by differential filtration: the picophytoplankton (0.7-2 um), the nanophytoplankton (2-20 um) and the microphytoplankton (20-200 um) as follows. Triplicate aliquots of 1 to 2 liters of pre-screened water were filtered through a 2 um Nuclepore filter then onto a GF/F filter (= picophytoplankton, 0.7-2 um). Triplicate aliquots of pre-screened water were also filtered through a 20 um Nitex mesh then onto a GF/F filter (= 0.7-20 um). Biomass of the nanophytoplankton size class was determined by subtracting the picophytoplankton biomass from the 0.7-20 um biomass. Microphytoplankton biomass was determined by subtracting the 0.7-20 um biomass from the total chl a value. Filters were folded and placed in 1.5 ml cryotubes and frozen at -80° C until later analysis at the University of South Carolina (USC) using the methods below. 

Primary Prodcutivity Measurements
For size-fractionated primary productivity measurements, 200 um pre-screened water collected from discrete depths were dispensed into Nalgene polycarbonate incubation bottles (7-8 clear bottles plus 1-2 dark bottles per depth; 800-1200 ml each). Bottles were spiked with 14C-labeled sodium bicarbonate (PerkinElmer Health Sciences Inc.) to a final activity 0.04-0.08 uCi ml-1 per bottle. An additional bottle per depth was used as a particulate blank (T0) (Barber et al., 1996). The T0 bottles were immediately filtered onto a GF/F, acidified with 500 ul 0.5 N HCl and left open to fume for 24 hours (Barber et al., 1996). Samples for total counts (Tc; 100 ul) were collected from one bottle per depth and combined with 200 ul of phenylethylamine (PEA) and 5 ml of scintillation cocktail (EcoLume, MPBiomedicals, Solon, Ohio). All bottles were incubated in situ at the depth of collection. Incubations were started before sunrise (usually between 05:00 and 06:00 h) and were terminated 24 h later. The productivity array was tracked using a Telonics, Inc. transponder platform subscribed to the ARGOS satellite tracking system.

Total phytoplankton primary productivity was measured directly by filtering triplicate incubation bottles onto GF/F filters. This gave total primary productivity in the size fraction 0.7 to 200 um. Dark bottle productivity was also measured directly by filtering dark bottles directly onto GF/F filters (= dark productivity; 0.7-200 um). Size-fractionated rates of primary productivity of the picophytoplankton, nanophytoplankton and microphytoplankton were made by differential filtration. Two 1 liter bottles were filtered through a 20 um Nitex mesh then onto a 2 um Nuclepore filter (= nanophytoplankton, 2-20 um). Two or three 1 liter bottles were filtered through a 20 um Nitex mesh then onto a GF/F filter (= 0.7-20 um).  Filters were treated with 500 ul of 0.5 N HCl and left under a fumehood for 24 hours, then combined with 10 ml scintillation cocktail.  Radioactivity was determined in disintegrations per minute (DPM) by the shipboard liquid scintillation analyzer (Packard Tri-Carb 2000CA). 

Rates of primary productivity (PP) were calculated in units of mg C m-3 d-1 using the methods of Barber et al. (1996) with the addition of dark bottles:

    PP = (DPM24 – DPM0 – DPMd)/(1.05)(25200 mg C m-2)(DPMtot * time)-1       

where DPM24 = activity on filter after 24 hour incubation; DPM0 = activity of (depth-specific) T0 particulate blank; DPMd = average of (depth-specific) dark bottles; DPMtot = total activity DPM of isotope added multiplied by volume of water filtered (DPM ml-1); 1.05 = constant that accounts for preferential uptake of the lighter isotope 12C over 14C; 25,200 = concentration (in mg m-2) of inorganic carbon in seawater.

The rate of primary productivity for the picophytoplankton size class was determined by subtracting the nanophytoplankton productivity from the 0.7-20 um productivity. Primary productivity for the microphytoplankton size class was determined by subtracting the 0.7-20 um productivity from the total primary productivity, 0.7-200 um. Total and size-fractionated rates of primary productivity were integrated to 100 meters using trapezoidal integration (mg C m-2 d-1).

HPLC and ChemTax
Samples for HPLC analysis were lyophilized for 24 h at -50° C, placed in 90% acetone (0.45-0.55 ml), and extracted at -20° C for 24 h.  Filtered extracts (350 µl) were injected into a Shimadzu HPLC equipped with a monomeric (Rainin Microsorb-MV, 0.46 x 10 cm, 3 µm) and a polymeric (Vydac 201TP54, 0.46 x 25 cm, 5 um) reverse-phase C18 column in series. A nonlinear binary gradient consisting of the solvents 80% methanol: 20% 0.50 M ammonium acetate and 80% methanol: 20% acetone was used for pigment separations (Pinckney et al. 1996). Absorption spectra and chromatograms (440 ± 4 nm) were acquired using a Shimadzu SPD-M10av photodiode array detector. Pigment peaks were identified by comparison of retention times and absorption spectra with pure standards (DHI, Denmark). The synthetic carotenoid ß-apo-8'-carotenal (Sigma) was used as an internal standard.

BCO-DMO Processing:
- Separated original Date/Time field into the separate date_gmt and time_gmt columns.
- Added date/time formatted to ISO 8601 standard.
- Replaced blanks with 'nd' to indicate 'not determined' (or 'no data').
- Modified paramater names to conform with BCO-DMO naming conventions.

Fluxes of particulate carbon from the surface ocean are greatly influenced by the size, taxonomic composition and trophic interactions of the resident planktonic community. Large and/or heavily-ballasted phytoplankton such as diatoms and coccolithophores are key contributors to carbon export due to their high sinking rates and direct routes of export through large zooplankton. The potential contributions of small, unballasted phytoplankton, through aggregation and/or trophic re-packaging, have been recognized more recently. This recognition comes as direct observations in the field show unexpected trends. In the Sargasso Sea, for example, shallow carbon export has increased in the last decade but the corresponding shift in phytoplankton community composition during this time has not been towards larger cells like diatoms. Instead, the abundance of the picoplanktonic cyanobacterium, Synechococccus, has increased significantly. The trophic pathways that link the increased abundance of Synechococcus to carbon export have not been characterized. These observations helped to frame the overarching research question, "How do plankton size, community composition and trophic interactions modify carbon export from the euphotic zone". Since small phytoplankton are responsible for the majority of primary production in oligotrophic subtropical gyres, the trophic interactions that include them must be characterized in order to achieve a mechanistic understanding of the function of the biological pump in the oligotrophic regions of the ocean.

This requires a complete characterization of the major organisms and their rates of production and consumption. Accordingly, the research objectives are: 1) to characterize (qualitatively and quantitatively) trophic interactions between major plankton groups in the euphotic zone and rates of, and contributors to, carbon export and 2) to develop a constrained food web model, based on these data, that will allow us to better understand current and predict near-future patterns in export production in the Sargasso Sea.

The investigators will use a combination of field-based process studies and food web modeling to quantify rates of carbon exchange between key components of the ecosystem at the Bermuda Atlantic Time-series Study (BATS) site. Measurements will include a novel DNA-based approach to characterizing and quantifying planktonic contributors to carbon export. The well-documented seasonal variability at BATS and the occurrence of mesoscale eddies will be used as a natural laboratory in which to study ecosystems of different structure. This study is unique in that it aims to characterize multiple food web interactions and carbon export simultaneously and over similar time and space scales. A key strength of the proposed research is also the tight connection and feedback between the data collection and modeling components.

Characterizing the complex interactions between the biological community and export production is critical for predicting changes in phytoplankton species dominance, trophic relationships and export production that might occur under scenarios of climate-related changes in ocean circulation and mixing. The results from this research may also contribute to understanding of the biological mechanisms that drive current regional to basin scale variability in carbon export in oligotrophic gyres.

The Ocean Carbon and Biogeochemistry (OCB) program focuses on the ocean's role as a component of the global Earth system, bringing together research in geochemistry, ocean physics, and ecology that inform on and advance our understanding of ocean biogeochemistry. The overall program goals are to promote, plan, and coordinate collaborative, multidisciplinary research opportunities within the U.S. research community and with international partners. Important OCB-related activities currently include: the Ocean Carbon and Climate Change (OCCC) and the North American Carbon Program (NACP); U.S. contributions to IMBER, SOLAS, CARBOOCEAN; and numerous U.S. single-investigator and medium-size research projects funded by U.S. federal agencies including NASA, NOAA, and NSF.

The scientific mission of OCB is to study the evolving role of the ocean in the global carbon cycle, in the face of environmental variability and change through studies of marine biogeochemical cycles and associated ecosystems.

The overarching OCB science themes include improved understanding and prediction of: 1) oceanic uptake and release of atmospheric CO2 and other greenhouse gases and 2) environmental sensitivities of biogeochemical cycles, marine ecosystems, and interactions between the two.

The OCB Research Priorities (updated January 2012) include: ocean acidification; terrestrial/coastal carbon fluxes and exchanges; climate sensitivities of and change in ecosystem structure and associated impacts on biogeochemical cycles; mesopelagic ecological and biogeochemical interactions; benthic-pelagic feedbacks on biogeochemical cycles; ocean carbon uptake and storage; and expanding low-oxygen conditions in the coastal and open oceans.