Table of Contents

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

Water samples for radium isotope analysis were collected from both the ship's clean underway intake system ('U' designator in data) and niskin bottles attached to the CTD rosette ('C' designator in data). Salinity data were derived from the ship's clean underway intake system with SBE 21 SEACAT Thermosaliograph. Water samples were filtered through 47 mm GFF filters (unless otherwise noted), then passed slowly through 25 g (dry) acrylic fibers impregnated with MnO2 (Moore, 1976). Fibers were then washed 10x with Ra-free freshwater to remove salts and dried to a mass between 35 and 57 g for optimal humidity levels (Sun and Torgersen, 1998). Fibers were then immediately counted on a Radium Delayed Coincidence Counter (RaDeCC; Moore and Arnold, 1996) for total Ra-224 and Ra-223. Roughly 3 weeks later, fibers were counted again for supported Ra-224. Thus, excess ('XS') Ra-224 is the difference between total and supported Ra-224 measurements. Ra-226 activities were then measured on a radon emanation line as per Peterson et al. (2009).

Minimum detectable activities (dpm/100L) are calculated based on Currie (1968). Any measured values lower than the minimum detectable activity is labeled 'BD' in the dataset.

Data processing was performed in Microsoft Excel (2016).

BCO-DMO Processing:
- modified field names to conform with BCO-DMO naming conventions;
- converted date/time field to ISO8601 format.

NSF Award Abstract:
This is a focused program of field research in waters of the Western Tropical North Atlantic influenced by the Amazon River Plume during the high river flow season. The Amazon Plume region supports diverse plankton communities in a dynamic system driven by nutrients supplied by transport from the river proper as well as nutrients entrained from offshore waters by physical mixing and upwelling. This creates strong interactions among physical, chemical, and biological processes across a range of spatial and temporal scales. The field program will link direct measurements of environmental properties with focused experimental studies of nutrient supply and nutrient limitation of phytoplankton, as well as the transfer of phytoplankton nitrogen to the zooplankton food web. The Amazon Plume exhibits a close juxtaposition of distinct communities during the high-flow season, making it an ideal site for evaluating how nutrient availability, nutrient supply, and habitat longevity interact to drive offshore ecosystem dynamics and function. This project will include German collaborators and will seamlessly integrate education and research efforts. The investigators and their institutions have a strong commitment to undergraduate and graduate education and to increasing the diversity of the ocean science community through active recruiting and training efforts. The team has a strong track record of involving both undergraduate and graduate students in their field and lab research. The two research cruises planned will provide opportunities for students and technicians to interact with an interdisciplinary and international research team.

The ultimate objectives of this project are to understand the processes and interactions that promote distinct communities of nitrogen-fixing organisms (diazotrophs) and other phytoplankton around the Amazon Plume and to explore the impacts of these diazotroph-rich communities on zooplankton biomass and production. The research team includes scientists with expertise in nutrient and stable isotope biogeochemistry, remote sensing as well as specialists in characterizing water mass origin and history using naturally occurring radium isotopes. This combination of approaches will provide a unique opportunity to address fundamental questions related to plankton community structure, primary production, and links to secondary production in pelagic ecosystems. The project will address the following key questions focused on fundamental issues in plankton ecology resulting from previous research in this region:

A. What mechanisms promote the preferential delivery of bioavailable phosphorus and the resulting strong nitrogen limitation associated with the northern reaches of the Amazon Plume during the high flow season?

B. What factors lead to the clear niche separation between diazotrophs within and around the Amazon Plume and how are the distinct diazotroph communities influenced by hydrographic and biogeochemical controls associated with the Amazon River Plume and offshore upwelling processes?

C. How does the nitrogen fixed by the different types of diazotrophs contribute to secondary production, and how efficiently does diazotroph nitrogen move through the food web?