Sediment-water interface nitrogen removal and cycling in native and invasive seagrass meadows in Jobos Bay, Puerto Rico

Jobos Bay is a National Estuarine Research Reserve in southern Puerto Rico. The role of seagrasses on sediment-water interface nitrogen (N) transformations is unknown in Jobos Bay. Intact sediment cores from invasive (Halophila stipulacea) and native (Thalassia testudinum) seagrass meadows, and unvegetated sediments, were incubated in a continuous-flow system with and without 15N-ammonium or 15N-nitrate amendments to measure net nutrient (N and P) fluxes and denitrification/anammox, N₂ fixation, and dissimilatory nitrate reduction to ammonium (DNRA) rates. Dinitrogen gas fluxes varied between net N₂ fixation along an exposed mangrove shoreline (in mixed seagrasses and an adjacent bare sediment area) to net denitrification (no anammox was detected) in a semi-enclosed, mangrove-lined embayment and on the inland side of a barrier island. Net N₂ fixation was most pronounced in bare sediments along the exposed mangrove shoreline during the dry season (March). Vegetated sediments along this shoreline switched to net denitrification in the wet season (July). Net denitrification was highest in the semi-enclosed bay in the dry season and at the barrier island in the wet season (July). Denitrification was mostly coupled to nitrification, with typically <15% of total N₂ production comprised of ^29/30N₂ in 15-nitrate amended sediments. Evidence for DNRA was detected in preliminary sample analyses, but its relative importance is pending additional results. This study will help resource managers advocate for watershed conservation activities to reduce nutrient loading to the estuary.

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

Authors:

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

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

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

Angel Dieppa-Ayala, Jobos Bay Estuarine Research Reserve  (adieppa@dnra.pr.gov)

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

Thomas McElroy, Kennesaw State University  (tmcelro2@kennesaw.edu)

Milton Munoz-Hincapie, Jobos Bay Estuarine Research Reserve  (mmunoz@dnra.pr.gov)