Table of Contents

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

Protist rates based on epifluorescence counts; cyanobacteria rates based on flow cytometry data (Mike Lomas' group); bulk rates based on extracted chlorophyll-a. Samples were collected during four cruises in the Sargasso Sea during spring and summer 2011-2012.

Water Column Sampling:
Water column sampling was performed on four cruises during the spring and the summer of 2011 and 2012 at the Bermuda Atlantic Time-series Study station (31’40°N 64’10°W, BATS) and in the mesoscale eddies found in the surrounding area of the Sargasso Sea. For each cruise, two stations were sampled, usually in the center of a mesoscale eddy and at BATS. The edge of the eddy was sampled two times, as well. To be able to get a better reproducibility of data, each experiment was replicated.

For each experiment, seawater samples were collected pre-dawn (on deck 2:30-4:00, local time) at four different depths within the euphotic zone (20m, 50m, 80m and the Deep Chlorophyll Maximum, DCM). Twenty-one 10L Niskin bottles were attached to a rosette with conductivity, temperature, depth sensors (CTD), and an in vivo fluorometer. This sensor allowed for recording in real time of chlorophyll fluorescence and the DCM for each station. The water that was collected from the 10L Niskin bottles was sampled for abundance and biomass of the plankton community.

Dilution experiments:
The growth and grazing rates of the phytoplankton community were determined using the dilution method with a two-point modification (Landry et al. 1984, Landry et al. 2008a, Selph et al. 2011). To check the applicability and resolution of the two point modification, two experiments at two different stations (one replicate of C3 and one replicate of B3) were carried out with a complete set of dilutions (25, 50, 75 and 100%) on-deck; the results of these experiments confirmed the validity of the 2 point dilution experiment (data not shown). The Landry-Hassett dilution method was used because it can separate autotrophic and heterotrophic processes with relatively little manipulation (Landry & Hassett, 1982, Landry et al 1984). The method is based on the concept that the dilution lowers the encounter probability of grazer and prey and enables one to calculate from the net phytoplankton population growth (k) in the diluted and undiluted incubation bottles the instantaneous growth rate (u) and grazing mortality (g), where k= u - g (Landry & Hassett, 1982). This method is based on three assumptions.  The first one is that the instantaneous growth rate of any phytoplankton group is not influenced by the dilution of the “seawater/sample” with particle free water. The second assumption is that the probability of a phytoplankton to be grazed by the consumer is directly related to the abundance of the consumer itself, which means that with a higher density of grazers the probability of the prey to be grazed is higher as well, making the grazing a function of the dilution. The third assumption is the sum of the first two assumptions, that the change in the abundance of the phytoplankton community over time can be expressed by the exponential growth equation:

Pt=P0 e ^ [(u -xg)t]

where P0 and Pt  are the initial and the final abundances or biomass of bulk or specific phytoplankton groups, t is time in day (d), u is the instantaneous phytoplankton growth rate (d-1), g is the grazing mortality (d-1), and x represents the dilution factor (Landry and Hassett 1982; Landry et al. 1984; Neuer and Cowles 1994). M and g are calculated as the y-intercept and the slope of the linear regression plotted against the dilution factors where k=1/t ln (Pt/P0) (Landry & Hassett, 1982; Landry et al. 1984). For the two-point dilution, this converts mathematically to solving the equations for u and g (Landry et al., 2008a).

Bulk measurements:
Chlorophyll-a was extracted from seawater (250 ml and 400 ml depending on the dilution), with 90% acetone and measured after 24hrs at 4 degrees C in the dark onboard the ship using a TD 700 Laboratory Fluorometer using the non-acidification technique (Welschmeyer 1994). These data were used as a proxy for the phytoplankton biomass in the water column and to calculate the bulk growth and grazing rates of the phytoplankton community.

BCO-DMO Processing Notes:
- Added lat and lon for each station & cast from the metadata form.
- Replaced spaces with underscores.
- Replaced blanks with 'nd' to indicate 'no data'.
- Dates/times assumed to be in UTC/GMT, based on comparison with other Trophic BATS datasets.

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.