Table of Contents

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

A custom instrument, called CLASS, was used to measure CDOM, chlorophyll, and phycoerythrin fluorescence (among others) on R/V Endeavor cruise EN416 in May 2018.  

** Need additional details from the PI **

** Need additional details from the PI **

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?

NSF Award Abstract

This research will study the processes that cause the relative biological availability of nitrogen (N) and phosphorus (P) in the upper ocean to change as water moves from the coast to offshore in the northern Gulf of Mexico. Both N and P are required nutrients. Consequently, the ratio of N to P in the upper ocean has important consequences for plant growth and the marine food web structure. Typically, as water moves offshore bioavailable N declines faster than bioavailable P. While processes that alter either element will affect the N to P ratio, previous research has focused mainly on the N side of the relationship, examining cycling and the selective removal of different forms of N from the water by marine microorganisms. This project will focus instead on the less-studied P side of the N to P ratio in the upper ocean. It will use shipboard experiments to quantify microbiological processes that maintain P availability in the upper ocean, even as N availability declines. Given that low N availability relative to P limits plant growth in most of the ocean's sunlit surface waters, understanding how this chemical ratio develops as water moves offshore is of fundamental importance for the study of marine ecosystems worldwide. Educational impact will include at least seven students' direct participation in the research, providing hands-on and cross-disciplinary training, as well as practical experience at sea. Two middle school teachers will also participate in the oceanographic cruises. They will incorporate field results and personal experiences into lesson plans and teachers' workshops. The project will also develop public outreach activities that focus on the unique value of marine ecosystems of the Gulf of Mexico.

This project will test the hypothesis that the decline in the ratio of bioavailable N to P in surface water as it moves offshore develops from preferential phosphorus retention as opposed to removal of biologically-available forms of nitrogen. As part of the research associated with this central hypothesis, the project will quantitatively compare the relative importance of different phosphorus-retention mechanisms during two oceanographic cruises in the northern Gulf of Mexico. Previous observations of spatial changes in N and P availability are common. The researchers will track discrete water masses with Lagrangian drifters for time course sampling, and use physical oceanographic measurements to quantify potential N to P ratio changes contributed by vertical and horizontal mixing. Shipboard incubation experiments will quantify and compare rates for the key microbiological processes thought to affect phosphorus retention in the upper ocean. This focus on potential P-retention processes rather than N loss as an explanation of commonly observed declines in surface ocean N to P ratio represents a unique contribution to the complete understanding of the complex feedback mechanisms between nutrient cycles and marine ecosystem function.