Collaborative Research: Impact of the Amazon River Plume on Nitrogen Availability and Planktonic Food Web Dynamics in the Western Tropical North Atlantic

DESCRIPTION: 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. 

OUTCOMES: We conducted 11 nutrient amendment experiments in the plume waters to measure phytoplankton response to additions of N, P, Si and combinations of these nutrients at salinities ranging from 19.1 to 35.9 PSU.  Throughout the Amazon plume, the phytoplankton communities are serially limited, first by nitrogen and then by phosphorous. Though the amount of growth varied, the communities showed significant growth in all of the treatments with addition of nitrogen with even greater growth when nitrogen and phosphorus were added in conjunction. The composition of the community changed, with cyanobacteria comprising a smaller percentage at the end of the 48 hour experiment. However, this change is not directly linked to the addition of any specific nutrient(s).