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 – 2021
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.
Development of models for phytoplankton-nutrient responses in support of numeric nutrient criteria for estuarine water quality
James L. Pinckney and Erik Smith
South Carolina Sea Grant
The primary objective of the proposed research is to develop and test a family of empirical mathematical models to quantify the responses of the total phytoplankton community as well as phytoplankton groups to increases or reductions in total N loading. Models will be constructed for a range of N loading scenarios under both high and low light exposure conditions. These models will be invaluable for developing and validating numeric nutrient criteria for Winyah Bay and provide a “proof of concept” for determining criteria in other estuarine systems.
Phytoplankton-nutrient response curves will be constructed using natural phytoplankton communities collected along a salinity gradient in Winyah Bay, SC. Surface water (0.5 m depth) will be obtained seasonally over 2 years from 2 locations in Winyah Bay. The N treatment for the bioassays will consist of a range of concentrations (1 – 100 µmol N l-1 in increments of 10 µmol N l-1) and will be composed of an equimolar mixture of NO3–, NH4+, and urea (CO(NH2)2) to simulate the types of N compounds likely available in the estuary. Data from the bioassays will be used to derive an empirical numerical relationship between N loading and phytoplankton community biomass (as chl a) responses.
Regionally and globally, fixed nitrogen (N) is usually the primary nutrient controlling or “limiting” estuarine and coastal primary production. Rapidly growing and diversifying anthropogenically-generated N compounds associated with agricultural, urban and industrial expansion, have been identified as key “drivers” of accelerating planktonic primary production, or eutrophication, in N-sensitive waters. Fast-paced and widespread development, agricultural practices in the watershed, and an increasing human population in the coastal zone of SC have resulted in a general decline in water quality in estuaries. Concerns about this decline have led to the need to determine minimum nutrient criteria to insure acceptable water quality conditions for both biota and recreational uses. Numeric nutrient criteria are established by state and federal management agencies to provide targets for pollutant reduction and maintenance of acceptable water quality in aquatic systems. A major component of numeric nutrient criteria modeling is the understanding of the quantitative relationship between nutrient loading and phytoplankton productivity responses, in terms of both total biomass and community composition. Phytoplankton responses to excessive N loading result in a variety of negative impacts such as hypoxia, anoxia, fish kills, and harmful algal blooms. The research proposed here will use a quantitative empirical approach for predicting the magnitude of phytoplankton group-specific (i.e., diatoms, cyanobacteria, dinoflagellates, chlorophytes, etc.) responses to a range of nutrient loading conditions. The empirical models we will develop can be directly used by SC DHEC to evaluate numeric nutrient criteria for this system under a variety of N addition/reduction scenarios. Thus the proposed research will provide an independent tool for developing numeric nutrient criteria models with potential methodological applications to other river-dominated estuarine systems. The proposed research will address Goal 1, Objectives 1.1, 1.2, and 1.3 of SC Sea Grant Program priorities.
Statewide and nationally, estuarine waters are exposed to unprecedented and alarming rates of human occupation and development. Growing inputs of anthropogenic nutrients will likely have cascading impacts on the biota and environmental quality of coastal waters. The proposed work will provide functional insights and mechanistic explanations of the potential negative impacts of excessive nutrient inputs on planktonic communities in estuarine ecosystems. For managers, the most useful products of the proposed research will be the determination of phytoplankton responses over a range of realistic nutrient concentrations, and an ecological basis for establishing water quality guidelines that insure the long-term health and sustainability of the State’s water resources. The results and experimental approaches used in this project will be widely applicable to similar temperate estuaries.