Seasonal oxygen-driven migration of mobile benthic fauna affected by natural water column stratification

Abstract

Changes in mobile benthic fauna affected by a seasonal oxycline were studied at three-monthly intervals for one year at Lough Hyne, a marine reserve in county Cork, Ireland. This marine lake features regular seasonal anoxic conditions in the deepest part (the Western Trough). Building on previous studies of the effects of the oxycline on demersal fauna, a ROV was used to film the benthic assemblage. Transects above, within and below oxycline depth were filmed on each sampling occasion. Animals were identified and their sizes measured. Data on oxygen concentration of the water column were taken monthly during this period to correlate with the presence and absence of animals in the Trough. Most noteworthy was the establishment of a demersal fish assemblage during normoxic conditions in the deeper areas of the Trough. The goby Lesueurigobius friesii was the most abundant species and their population contained all size classes. They are territorial and build burrows into which they retreated when the ROV approached. The main factor influencing benthic fauna below oxycline depth was oxygen concentration, while the assemblage above the oxycline was never affected by hypoxia. This study shows that mobile animals make use of areas affected by hypoxia as soon as conditions improve, and can establish resident communities. •Changes in a macrofaunal community affected by a seasonal thermocline were studied.•Transects above, within and below thermocline depth were filmed at 3-month intervals for 1 year.•In March, a macrofaunal community was established below the oxycline depth.•Fries' gobies were the most abundant established macrofauna during periods of normoxia.•Crabs dipped into the area beneath the breaking down-oxycline to feed in autumn. © 2013 Elsevier Ltd.