Seasonal structuring of a benthic community exposed to regular hypoxic events

The Western Trough of the Lough Hyne Marine Reserve in southern Ireland features annual episodes of profound hypoxia beneath an oxy-thermocline that develops each summer. Previous work had indicated that the hypoxia caused mass mortality of the sessile benthic fauna, but information about the winter fauna, or about mobile species was lacking. Here we report on a combined remote-operated vehicle, SCUBA and baited trap study, backed by regular oxygen-temperature-depth monitoring of the Trough. Our results show that there is community of resident benthic and mobile species during the oxic winter months that disappears in the summer. Mobile fish and crustaceans avoid the hypoxic areas of the Trough, though the prawn Palaemon serratus will venture into the hypoxic zone to scavenge on baits. At the oxy-thermocline itself, burrows of benthic fauna remain evident in summer, but their inhabitants show much reduced activity or death. Regular hypoxic episodes clearly structure the benthic community of the Trough, but this is not a simple matter of alternating mass mortality and recolonization; mobile species are clearly capable of avoiding the hypoxic zone in summer, but benefit from its productivity in winter and spring.