The benthos of Lough Hyne (Europe’s first marine reserve) in SW Ireland has changed dramatically in the past decade with declining invertebrate communities and proliferating ephemeral macroalgae. In summer 2011 and 2012, we measured dissolved oxygen (DO) levels on shallow subtidal rocky shores of the lough and recorded both benthic hypoxia (<2 mg O2 L−1) and hyperoxia (∼10–16 mg O2 L−1). In late summer 2013, we systematically characterized the spatial and temporal patterns of DO in the lough seawater. Daytime DO levels were hyperoxic either at or near the rocky shore, declined with increased distance for the first 5 m offshore, then stabilized at normoxic levels (∼8 mg O2 L−1). DO levels above and below rocks were variable and often showed large differences (max. difference of 15.6 mg O2 L−1), particularly at rocky shore sites with low current flow. At night, the DO in shallow water declined, becoming hypoxic to anoxic during calm weather. These extreme oxygen fluctuations (i) are amongst the first records to report severe conditions within Irish coastal waters and (ii) could contribute to the known recent decline in benthic invertebrate diversity within the marine reserve
Although red tides are typically formed by dinoflagellate blooms, aloricate ciliates in the genus Mesodinium also produce striking red blooms throughout the world. In mid-March 2015, we confirmed the occurrence of dense blooms of Mesodinium major in Lough Hyne Marine Nature Reserve, a fully marine sea lough in County Cork, SW Ireland. Daytime aggregations of these highly mobile ciliates occurred in the surface and subsurface waters of the lough, particularly along the western shore and in the Goleen. Fine-scale comparisons of dissolved oxygen (DO) and water temperature values indicated that the surface water supporting dense blooms was slightly but significantly more oxygenated and warmer than closely juxtaposed water without visible blooms. Coarse-scale comparisons, however, showed no significant difference in DO or temperature between bloom and non-bloom areas, although there were highly significant differences in the two parameters by water depth and location within the lough. Although this is the first formal report of redwater ciliate blooms for Lough Hyne and County Cork, it is not the first known occurrence; earlier spring red tides at the lough were assumed (probably in error) to be caused solely or primarily by dinoflagellates and/or loricate ciliates (tintinnids).
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