The Importance and Mechanics of Near-Seafloor Oxygenation Events in Shallow Coastal Environments

Abstract

Bottom-water oxygen (O₂) possibly constitutes the strongest regulating factor on ecosystem function of the seafloor environment. The conventional view of bottom-water O₂ regulation in temperate regions is a strong seasonal variability imposed by deposition of fresh, labile organic carbon following the spring and fall plankton bloom generating changing sediment O₂ demand. The supply of O₂ is considered mainly regulated by seasonally-varying thermal stratification or large-scale lateral advection of water masses. However, benthic O₂ also varies over much shorter timescales, but the spatiotemporal regulation of this O₂ variability is not well resolved. We conducted a 10-month-long in situ study (2022–2023) of bottom-water O₂ dynamics at a 40-m-deep coastal site in the Baltic Sea. Following the spring bloom and the onset of thermal stratification, the gradual decline in O₂ was regularly interrupted by rapid (<24 hr) oxygenation events in which O₂ levels spiked, subsided, but remained elevated by between 25 and 180 μmol L⁻¹ relative to conditions before the event. Without these events, extrapolation of the observed O₂ decline implied hypoxic or anoxic bottom-water conditions by mid-May. Similarly, reoxygenation of the lower water column in the late fall was controlled by a few rapid and strong reoxygenation events. These events are consistent with regional-scale ocean circulation and mixing processes, potentially involving a downwelling mixing front and/or coastal trapped/internal waves. However, we lack data to unambiguously distinguish between these mechanisms. We suggest that rapid benthic oxygenation events are likely a common characteristic of coastal seafloors and critical for benthic ecosystem functioning.