Oxygen allocation dynamics in a seagrass Zostera marina meadow

Photosynthetic activity in seagrasses benefits various marine organisms in the seagrass ecosystem by supplying oxygen (O₂). This study investigated the allocation of photosynthetically produced O₂ between the above- and below-ground tissues of Zostera marina under different irradiance levels and temperature conditions. Annual variations in O₂ production, consumption, and supply to the surrounding ecosystem were also estimated based on tissue O₂ release and respiration rates, along with in situ underwater irradiance, water temperature, and seagrass biomass. The proportion of O₂ allocated to both above- and below-ground tissues varied depending on light conditions. Across all light conditions, the O₂ allocated to above-ground tissues consistently accounted for a larger proportion than that allocated to non-photosynthetic below-ground tissues. As light intensity decreased, the contribution of O₂ allocated to below-ground tissues declined, indicating that O₂ production in above-ground tissues was insufficient to meet the O₂ demands of the below-ground tissues. The estimated O₂ consumption, release, and daily O₂ supply within the Z. marina meadow varied seasonally, driven by changes in temperature and irradiance conditions. In above-ground tissues, O₂ consumption during the night was lowest in winter, while O₂ release during the day was highest in the warmer months (May-July). The mean daily O₂ supply was 46.40 mmol O₂ m^-2 d^-1 (ranging from -69.45 to 231.23 mmol O₂ m^-2 d^-1) from above-ground tissues and -9.23 mmol O₂ m^-2 d^-1 (ranging from -32.43 to 0.25 mmol O₂ m^-2 d^-1) from below-ground tissues. Higher water temperatures and reduced underwater irradiance from late summer to fall resulted in a negative O₂ supply, reducing seagrass shoot density and biomass. These results suggest that environmental changes, particularly those driven by ongoing climate change, significantly influence O₂ dynamics in coastal seagrass ecosystems, with potential ecological consequences for the resistance and resilience of associated marine organisms.