Groundwater sources of “new” N for benthic microalgal production
in the South Atlantic Bight
James L. Pinckney, Susan Lang, Alicia Wilson, Angela Knapp
National Science Foundation
(grant # OCE 1736557)
2018 – 2022
OVERVIEW: Continental shelves are highly productive, with both ecological and economical importance. Benthic microalgae (BMA) are key primary producers in these locations, as demonstrated by the remarkable fact that BMA biomass in the upper cm of shelf sands can exceed integrated phytoplankton biomass, often by a factor of 4-6 times. Thus as much as 6x the water column biomass of primary producers is compressed into a layer only a few mm thick on the sediment surface. Given the generally low concentrations of organic matter and nutrients in sandy shelf sediments and in the water column, the source(s) of fixed nitrogen (N) supporting such highly concentrated BMA biomass is currently unknown. Recent studies of sub-seafloor groundwater flow at the University of South Carolina have demonstrated that upwelling saline groundwater likely supplies high concentrations of nutrients in the ridge-swale habitats in the South Atlantic Bight (SAB). We suggest that groundwater input of fixed N into surficial sediments is the primary source of N supporting BMA biomass and production in the mid-shelf region of the SAB. The purpose of the proposed research is to determine the primary source of fixed N supporting BMA biomass in the surface sediments of the shallow shelf waters (<30 m), using the SAB as a field area. A secondary objective will be to apply novel and innovative methods to directly quantify groundwater inputs of N into surficial sediments. Research results will fully document the spatio-temporal distributions of BMA and phytoplankton biomass and community structure in the mid-shelf region of the SAB and relate the observed patterns to groundwater inputs of fixed N sources as well as hydrographic and climatic conditions.
INTELLECTUAL MERIT: Understanding the source of fixed N supporting the BMA community is essential for understanding the carbon dynamics and net ecosystem metabolism within the large geographic area of the SAB, which is an excellent model for wide (passive margin) continental shelves in temperate waters around the world. The proposed research will offer new insights into the importance of groundwater nutrient inputs in controlling the balance between net ecosystem autotrophy and heterotrophy, which has implications for CO2 dynamics on a global scale given the large area occupied by continental shelves in the world ocean. Furthermore, the research will challenge the current paradigm that N dynamics on continental shelves is dominated by “regeneration” processes. Our results would be transformative and novel in that we will show that “new” N inputs via groundwater may equal or exceed those supplied by regeneration. This work will also have significant impacts in the area of submarine groundwater discharge (SGD). Hundreds of studies of SGD have demonstrated that SGD delivers significant nutrients, metals, and carbon to the ocean, but the importance of those solutes to coastal ocean ecosystems is very poorly documented. This study will be the first to test the impact of widespread SGD on marine productivity.
BROADER IMPACTS: The proposed research will provide full support and tuition for 2 graduate students, summer support for undergraduate assistants, and involve upper level undergraduates as lab interns. The study team will also work with the Baruch Institute and other partners to develop an “Ocean Schoolyard” program to meet the needs of teachers, students, and community audiences. The project will also provide partial support for Girls Go for I.T., a coding summer camp designed to attract middle-school-aged girls to careers in I.T. and STEM fields. Support from this project will allow us to develop and test a standardized training plan for instructors, which will (1) lay the foundation for future efforts to expand the camp (e.g., to underserved locations like Georgetown, SC, where the Baruch Institute is located) and (2) allow us to begin planning STEM education research around a more standardized camp.