Health status and characterisation of Gibraltar’s Maerl beds

Abstract

Maerl beds form heterogeneous, three-dimensional habitats for numerous species, housing high levels of biodiversity and species richness across the globe. Despite the importance of maerl’s ecological and carbon-regulating role; there is still a lack of information on methods to characterise its health. Therefore, this research set out to develop a standardised health assessment framework to assess the overall health of maerl a) structural function and b) habitat function. The framework was tested in an area of Gibraltar that has historically been severely impacted from various pressures including water quality. The maerl health is in a critical status, as over 92% of it is deemed dead, with high levels of surface erosion and unusual colourations observed, and rhodoliths were generally small in size (mean: 1.35cm ± 0.19cm). The habitat health was labelled as intermediate, based on the species richness values, signs of nursery function, high number of Terebellidae (n = 233), distribution and coverage, and type of sediment composition. The maerl-forming species morphologically identified as Lithophyllum cf. racemus and Lithothamnion cf. corallioides were found to be present in the study site, which are first records within Gibraltar’s waters.