UC San Diego

Ocean acidification (OA) and deoxygenation (collectively OAD) increasingly threaten coral reefs, which support marine biodiversity and human economic activity. Seaweed farms have been proposed to locally mitigate OAD conditions by removing dissolved inorganic carbon (DIC) and enhancing pH and dissolved oxygen (DO), but empirical evidence has been limited. Here, we investigated the effects of seaweed farming on net community production (NCPDIC, NCPDO), net community calcification (NCC), and seawater chemistry on a back reef lagoon system in Onna, Okinawa, Japan. Lagrangian drifters were used to measure and calculate rates of NCP, NCC, pH, and aragonite saturation state (Ωarag) in December 2022, April 2023, January 2024, and May 2024. The April 2023 and May 2024 surveys coincided with the annual maximum extents of seaweed cultivation (0.16 and 0.14 km2 respectively). Mean NCP and NCC were positive across sessions, but had high variability. Under saturating solar radiation, we observed elevated NCPDIC and NCPDO in April 2023 compared to January 2024, and stronger increases in pH and Ωarag in April 2023 and in May 2024 than in January 2024. Changes in pH and Ωarag were driven by NCPDIC, which was itself driven by temperature and solar radiation. We noted that NCC had a minor effect on the carbonate chemistry system relative to NCP. While we observed biogeochemical signals consistent with biotically-driven OAD modulation, the lack of spring survey control groups limits our ability to decisively attribute this modulation to the seaweed farm.