By Megan Sam
Why does one branch of the St. Lucie Estuary consistently struggle with low oxygen levels while the other remains relatively healthy? That’s the puzzle a team of researchers, led by Scott Lee Young, a graduate student in the Department of Civil and Coastal Engineering, is working to solve. Each year, the North Fork of the estuary experiences hypoxic events — periods when oxygen levels drop too low to support life — while the South Fork rarely does. The team suspects that hydrodynamics, or how water moves through the system, plays a role.
“There clearly seems to be a major difference between both regions of the same estuary,” said Lee Young, a student affiliate of the Center for Coastal Solutions (CCS). “We suspect those differences are significantly impacted by hydrodynamic processes such as differences in how water mixes across the system and transport processes, and we’re aiming to identify them and quantify how strongly they influence the development of hypoxia.”
To explore these questions, the team conducted a five-week field mission from May 15 to June 19. Acoustic Doppler Current Profilers (ADCPs), which measure the speed and direction that water is moving, dissolved oxygen concentration sensors and pressure gauges were deployed at key locations throughout the estuary at the start of the mission. In addition, vessel mounted surveys — which collect measurements using instruments attached to a moving boat — were conducted along the length of the estuary and at the junction of the North and South Forks across a tidal cycle, on separate days. These surveys collected data on dissolved oxygen, salinity, water temperature and water levels.
By combining dynamic sampling — collecting data while moving through the water — with static sampling using anchored instruments, the team is building a detailed picture of how water flows through the estuary and how those movements affect dissolved oxygen levels.
In mid-June, the team returned to retrieve the deployed instruments and to conduct another round of dynamic sampling. This follow-up helps them track changes over time and deepen their understanding of the physical conditions affecting the estuary’s health.
The success of the fieldwork depended on a skilled and well-coordinated team. Civil and coastal engineering graduate students Juan Torres, Oscar Caballero Martinez and Ella Bear assisted Lee Young during the field experiment. Maitane Olabarrieta, Ph.D., and Arnoldo Valle-Levinson, Ph.D., professors in the Department of Civil and Coastal Engineering and members of Lee Young’s Ph.D. committee, provided integral support and guidance. CCS field crew members Todd Van Natta and Patrick Norby captained the research vessel and offered valuable insights for adjusting the deployment plan and transect routes in response to field conditions.
While the current focus is the St. Lucie Estuary, the implications are much broader.
“The goal isn’t just to create a handbook for the St. Lucie,” said Lee Young. “We want to use this system as a model for understanding hypoxia in other estuaries — anywhere these low-oxygen events are happening.”