Award: OCE-2205277

Intellectual Merit: Extreme storm events can alter nutrient cycling in highly productive coastal regions. The impacts of hurricanes on the silica cycle, a critical nutrient for the phytoplankton diatom group, in these near-shore areas such as the Mississippi Delta has rarely been investigated. Primary production in coastal zones is dominated by diatoms which have a fixed requirement for dissolved silica that they use to produce shells of biogenic amorphous silica.  After diatoms die and sink to the seabed, biogenic silica can dissolve and be recycled back into the water column where it becomes available again as a nutrient for phytoplankton in the surface water. In the seabed, biogenic silica may also undergo chemical alteration that prevents dissolved silica from being recycled into the water column and bioavailable. This project investigated the impacts of Hurricane Ida, a Category 4 hurricane that moved through the Mississippi Delta in late August 2021, on the recycling efficiency and chemical alteration of biogenic silica from the near-shore seabed which was significantly perturbed by the sustained >130 mph wind speeds of the storm event. Seabed sediment samples were collected from several stations 2 weeks before Hurricane Ida made landfall. Sediment samples were collected 4 more times from the same stations over 1 year with the final sampling conducted in August 2022. Sediment pore fluids were analyzed for major nutrients and trace elements. Solid phases were analyzed for biogenic silica contents. At some stations an increased flux of silica was seen out of the seabed, whereas at other stations a flux of silica was seen into the seabed.  Stable isotope ratios of silicon in sediment pore fluid, which can indicate whether biogenic silica is dissolving or undergoing chemical alteration, were used to determine how efficiently biogenic silica was recycled back into the water column. Stable silicon isotope ratios in the seabed revealed that biogenic silica was chemically altered at all stations, which impacted (lowered) its recycling efficiency. We continued to observe the impacts of the hurricane 12 months after the hurricane perturbed the region. Our study shows that different nutrient cycles may have different response times to extreme events, specifically that the silica cycle may have a longer response time than the carbon cycle in coastal zones. This “legacy” response time may be related to the introduction of oxygen into the seabed by the hurricane which in turn may impact the degree of chemical alteration that biogenic silica undergoes. Results of this project are also one of the few high-resolution sampling campaigns in the region.

Broader Impacts: This project supported three undergraduate researchers, two of whom are from minoritized populations, four field seasons, a poster presentation at an extreme events workshop, an undergraduate oral presentation at regional hydrological sciences symposium, an oral presentation at the biannual 2023 Coastal and Estuarine Research Federation Conference, and a planned poster presentation by an undergraduate at the 2024 Ocean Sciences Meeting. The project provided a firsthand research opportunity for one of the undergraduate researchers whose laboratory courses were suspended due to the Covid pandemic. Further, the project provided data for a  student project through a Research Experiences for Undergraduates program. The grant also provided support for a pre-tenure faculty member (PI Rahman), assisted laboratory infrastructure development and sample archiving for future projects.

Last Modified: 01/05/2024
Modified by: Shaily Rahman