Long-term fluctuations in epibiotic bryozoan and hydroid abundances in an Irish sea lough

Recent declines in coastal water quality in SW Ireland have led to changes in the abundance of algal-associated bryozoans and a hydroid. These common North Atlantic species offer considerable potential as indicator species for reduced oxygen conditions due to habitat degradation. Annual monitoring for more than two decades (1994–2016) at 10 rocky shore sites within Lough Hyne Marine Reserve, SW Ireland revealed a significant reduction in abundance of the ctenostome bryozoan, Flustrellidra hispida, through time, coincident with the development of daily, extreme, shallow water oxygen fluctuations (hyperoxia and hypoxia) due to eutrophication. In contrast, the ctenostome, Alcyonidium species-complex, increased significantly during the two-decade period. The thecate hydroid, Dynamena pumila, did not show any significant decadal-scale pattern but temporal variability in abundance increased since 2010. Abundances of F. hispida and D. pumila were positively correlated with estimates of water flow. F. hispida inhabited a variety of algal hosts but appeared most sensitive (of the taxa surveyed) to variation in habitat quality, especially experimentally reduced levels of dissolved oxygen ( −1 ). Such sensitivity indicates F. hispida could be useful as an indicator of environmental change, whereas D. pumila was more robust in response to reduced oxygen with 38.6% of the hydroid zooids being able to survive under hypoxic conditions as compared to only 0.2% of the bryozoan zooids. Given the widespread European distribution of these species, they offer the opportunity to act as important bioindicators of water quality and hence environmental degradation in north temperate intertidal systems.