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The nursery function of coastal habitats is one of the most frequently mentioned and recognized ecosystem services in the valuation of coastal ecosystems. Despite its importance our understanding of the precise habitat parameters and mechanisms that make a habitat important as a nursery area is still limited for many species. The study aimed to establish the importance of different algae morphotypes in providing shelter and food for juvenile coastal fish during the main settlement peaks in early spring and late summer in littoral rocky reef systems in the Northwestern Mediterranean. The results of our study showed strong seasonal differences in algae cover, composition and height between the two sampling periods. Overall, during spring the algae were well developed, while in late summer, both density and height, of most algae decreased considerably. Equally, prey biomass, in form of suitable sized invertebrate fauna associated to the algae, decreased. Accordingly, the shelter and food for the fish settling in this habitat during late summer were less abundant, indicating a mismatch between the observed presence of juvenile fish and optimal habitat conditions. Differences in prey densities were detected between algae morphotypes, with structurally more complex algae, such as Cystoseira spp. and Halopteris spp consistently contain more prey independent of season compared to simpler structured morphotypes such as many Dictoytales. The study furthermore related juvenile fish density to habitats dominated by different algae morphotypes. Out of the three-study species (Diplodus vulgaris, Symphodus ocellatus,Coris julis) only S. ocellatus showed a significant association with an algae habitat. S. ocellatus related positively tohabitats dominated by Dictoytales which provided the highest cover during late summer but had the lowest prey densities. A strong association of this species with Cystoseira as reported by other studies could not be confirmed.Cystoseira was abundant within the study area but in a state of dieback showing loss and reduced height of foliage, typical for the time of year within the study area. It is therefore likely that algae-fish associations are context dependent and that several algae species may fulfil similar functions. We also discovered that prey biomass did not appear to have an important effect on juvenile abundances. Nevertheless, the availability of prey may influence juvenile fish condition, growth performance and ultimately long-term survival. We therefore suggest that future studies on habitat quality should also include, besides abundance, indicators related to the condition and growth of juveniles.
Length mass conversion of invertebrate fauna of algae
Length mass conversions used for the invertebrate fauna within algae morphotypes. The final equations can also be found summarized in the supplementary material. Relationship for a taxonomic group were sometimes derived from equations or data from various species available in the published litrature. In this case a mean relationship was used to derive the taxonomic group equation.
Average squared distance between transects belonging to tested seasons and Islands sampling events. Shaded boxes show average squared distance between transects within season and island grouping. The lager the average squared distance the more dissimilar are the dive transect groupings with respect to their algae cover composition. Bold numbers are representing the highest values measured indicating that transects sampled in summer in Menorca were distinct from the other sampling seasons.
Results of the post-hoc Tukey test related to the analyses performed in table S11. Differences in the same morphotypes between seasons in the tow islands and differences between morphotypes within seasons.
Results of the pairwise comparisons between prey availability (abundance and biomass) in different algae morphotypes for the three study fish species from the post-hoc turkey test. For the original one-way ANOVA analysis results see S14.
Abundance and Biomass of invertebrate community per algae morphotype
Abundance (0.062 m2) and estimated biomass (AFDW mg per 0.062 m2) of invertebrate fauna associated to different algae morphotypes collected in spring (May 2014) and summer (August 2014) in Mallorca and Menorca. Error bar are S.E.