Seminar: Nathan Hall – UNC IMS

UNC-CH’s Department of Marine Sciences is proud to host a seminar by Nathan Hall Ph.D. This event is scheduled for Wednesday, March 6th, at 3:35pm in room G201 on the ground floor of Murray Hall at the University of North Carolina at Chapel Hill. This seminar will also be broadcast live to both UNC’s Institute of Marine Sciences room 222 and online via Zoom.

Presenter Affiliation: Research Assistant Professor; Institute of Marine Sciences, The University of North Carolina at Chapel Hill

Title:  Multifaceted impacts of flow as the master variable for bloom dynamics in rivers

Abstract: Phytoplankton blooms in large rivers are rare, and in most cases are linked to drought and/or human hydrological modifications (e.g. low head dams). Over the past decade, the Cape Fear River, NC has experienced summertime blooms of the toxigenic cyanobacteria, Microcystis that threatens its use as the water supply for 0.5 million people. Point source nutrient loads to the Cape Fear River have increased significantly over the past two decades, and river flows are controlled by two major reservoirs and further modified by three lock and dam structures. Using a combination of experiments, long term trend analyses, field observations, and modeling efforts we explored the roles of nutrients and hydrology on the blooms to help guide strategies for bloom prevention. A nutrient/light bioassay experiment indicated that nutrients were replete and phytoplankton growth was light limited. Long term analysis of river flow indicated an increasing frequency of summertime low flow events. Low flow enhanced light availability due to increased water transparency and shallower depths, and nutrient levels also significantly increase due to low dilution of point source inputs. The lowest flows lengthened transit times to about two weeks. Reduced vertical mixing during low flows also allowed weak thermal stratification, and chlorophyll a vertical profiles suggested that the stratification had likely persisted for a few days. A 1-D Lagrangian competition model of positively buoyant Microcystis and a negatively buoyant diatom was developed and reproduced the timing of observed Microcystis blooms based solely on river flow, temperature, and incident irradiance. Model simulations with stratification that persisted overnight versus diel stratification showed that persistent stratification accelerated bloom development by maintaining a larger number of cells closer to the surface throughout the daytime period. To determine whether the stratification regime was diel or persistent, two thermistor strings and a remote meteorological station were deployed during summer 2018 and a heat budget model was constructed to hind cast stratification conditions over the past decade. Thermistor string data show a few days during summer 2018 when stratification almost persisted overnight with complete mixing not occurring until just before sunrise. The heat budget model reasonably predicted stratification during summer 2018, but hind casts for prior summers indicate that persistent stratification is not likely to occur, even under the lowest observed flows. The currently unresolved question of whether persistent stratification occurs is significant because modest changes in flow management may be capable of preventing weak but biologically important persistent stratification.