LAUSR

Dissolved oxygen (DO) concentrations of the fully marine Lough Hyne, SW Ireland, were sampled in biologically different habitats between 2014 and 2019 to investigate the declining water quality in the marine reserve and the severity of oxidative stress on benthic communities. DO was measured above and below shallow subtidal rocks, in the Rapids connection to the Celtic Sea, in seagrass meadows (Zostera marina), and at various water depths (1–15 m) in the South Basin. DO values above rocks were normoxic to hyperoxic in daytime (7.8–17.3 mg l−1); below rocks were often hypoxic (0.24–2 mg l−1). South Basin sites experienced hypoxia less often than the North Basin, except for the Goleen, presumably due to differential current flow. DO fluctuations occurred in spring (before ephemeral macroalgae proliferate), summer (when algal mats smother the benthos), and autumn (when macroalgae decay). While the Rapids were normoxic, the seagrass meadows below them exhibited periodic DO stress. Labhra Cliff (9–14.5 m) was normoxic to hyperoxic in autumn but experienced suboxic and hypoxic events in summer. Many organisms in Lough Hyne are experiencing periods of hypoxia below their published sublethal limits, the effects of which could be exacerbated by periodic hyperoxia.