The role of local environmental changes on maerl and its associated non-calcareous epiphytic flora in the Bay of Brest

Large stands of free living (calcareous) coralline algae - called maerl beds - play a major role as ecosystem engineers in coastal areas throughout the world. They are also subject to strong anthropogenic pressures at global and local scales, which threaten their survival. However, the macroalgal epiphytes growing on maerl may benefit from these pressures, developing to the detriment of maerl algae. Here, we sought to gain insight into how maerl beds and their epiphytic algae are disturbed by variations in the local environment, and how these variations affect their capacity to respond to global change. In 2015, we monitored three maerl beds located in the Bay of Brest (Brittany, France). Sites with contrasting conditions were selected, with one station lying in a zone close to the harbor (northern basin S1) and two stations (S2 and S3) located in areas away from the main urban effluents but subject to other sources of local change: higher currents at S3 (PREVIMER Ocean Forecast) and higher sedimentation rates at S2 (Ehrhold et al., 2016). We observed significant temporal variations of physico-chemical parameters, on an annual but also on a daily basis. Results showed that S2 differentiated itself from the other stations, this station experienced higher fluctuations of salinity, nutrient concentrations and carbonate system parameters and hosted the lowest (living) maerl biomass (4.38 ± 1.54 kg DW m−2). S3 observed the highest living maerl biomass (14.56 ± 1.61 kg DW m−2) and the lowest non-calcareous epiphytic macroalgal abundance (0.1-7.9 g DW m−2). S1 displayed the highest heterogeneity in terms of living maerl biomass (it varied from 0.8 to 8.6 kg DW m−2), and the highest Chl a content. However, we did not record differences in terms of physico-chemical parameters between S1 and S3. No positive relationship was observed between nutrient enrichment and macroalgal epiphyte abundance, but epiphyte abundance was higher at stations with lower maerl biomass (S1 and S2) (mean value ranged from 4.6 to 49.0 g DW m−2 at S1 and from 7.4 to 23.7 g DW m−2 at S2). Our results highlight the importance of local changes on the development, survival and capacity to adapt to global change of maerl beds.