Stressed from above and stressed from below: dissolved oxygen fluctuations in Lough Hyne, a semi-enclosed marine lake

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.

Long-term variation of trochid populations in an Irish sea lough

Snapshot assessments of species, which live for a number of years, are often misleading if they are conducted at an inappropriately short temporal scale. Densities of the trochid gastropods Steromphala umbilicalis (da Costa, 1778) and S. cineraria (Linnaeus, 1758) fluctuated widely between 1996 and 2014 at ten rocky-shore sites within Lough Hyne, southwest Ireland. Between 2014 and 2019, the abundance of S. cineraria decreased to levels not seen in the previous two decades. This significant reduction was possibly in response to extreme dissolved oxygen fluctuations related to eutrophication of the semi-enclosed lough. Steromphala umbilicalis showed low density in 2018 and 2019, but this was within the limits of interannual fluctuations. The Lusitanian trochid Phorcus lineatus (da Costa, 1778) was first recorded by us in the lough in the early 2000s, just as it was establishing a flourishing population with densities of c. 6 snails/0.25 m2. Although this native trochid survived the cold winters of 2009–2012, the population suffered high adult mortality and little recruitment. Between 2013 and 2019, the population increased and the population density stabilized. The increase in P. lineatus reflects its position high on the shore, where air temperatures have risen over the last two decades. It may, therefore, be a sentinel species to detect the effects of climate change. By contrast, the decrease in S. cineraria was related to its shallow subtidal habitat where hypoxia and superoxia have recently become marked in the lough. This trochid, therefore, offers potential as an indicator of eutrophication-associated stress. These temporal variations in trochid species emphasize the need for longer term assessments when estimating the importance of any one species to the ecosystem.