Relationships between organic matter content, sediment nutrient pools, and oxygen demand in mixed seagrass beds in Jobos Bay, Puerto Rico

The invasive seagrass, Halophila stipulacea (Forsskål and Niebuhr) Ascherson, spread widely throughout Puerto Rico in the last decade, particularly after Hurricane Maria in 2017. The seagrass now forms large monospecific beds in Jobos Bay and also co-occurs with the native seagrass, Thalassia testudinum Banks ex König. Because of its different morphology and life cycle, contributions of the invasive seagrass to detrital pools in sediments may differ in quantity and quality, potentially altering carbon (C) and nitrogen (N) cycling within the estuary. We measured sediment oxygen demand (SOD), organic matter and its elemental content in surface sediments, and sediment ammonium pools to explore potential relationships between microbial activity and C and N pools within the sediments of four sites with differing seagrass composition. Initial results from March 2023 indicate average organic matter content (±s.e.) of the upper three cm of sediments was 5±1 kg m^-2 in beds where T. testudinum was abundant and 4±1 kg m^-2 in beds with abundant H. stipulacea; whereas, average porewater ammonium concentrations varied across sites (85±28 to 213±202 µM). SOD ranged from 749±22 to 1239±142 at sites containing seagrass. Data were also collected in July and October 2023 and March 2024. Potential changes in sediment conditions associated with the spread of H. stipulacea will generate important insights into C and N budgets within the bay.

Presentation Preference: Oral

Primary Presenter: Troy Mutchler, Kennesaw State University (tmutchle@kennesaw.edu)

Authors:

Troy Mutchler, Kennesaw State University  (tmutchle@kennesaw.edu)

Kayla Gonzalez-Boy, Kennesaw State University  (kgonza36@students.kennesaw.edu)

Ashlyn Stanalonis, Kennesaw State University  (astanalo@students.kennesaw.edu)

Jonathan Herald, Kennesaw State University  (jherald3@students.kennesaw.edu)

Ángel Dieppa-Ayala, Jobos Bay National Estuarine Research Reserve  (adieppa@drna.pr.gov)

Milton Muñoz-Hincapié, Jobos Bay National Estuarine Research Reserve  (mmunoz@drna.pr.gov)

Mark McCarthy, Estonian University of Life Sciences  (mmccar49@kennesaw.edu)