Abstract
Nutrient enrichment is a major threat to subtidal macroalgal forests. Several studies have shown that nutrient inputs can enhance the ability of opportunistic algal species to acquire space freed by disturbance, at the expense of architecturally complex species that form forests. However, competition between canopy- and turf-forming macroalgae is not limited to the aftermath of disturbance. Canopy-forming macroalgae can provide suitable substratum for diverse epibiont assemblages, including both algae (epiphytes) and sessile invertebrates (epizoans). Despite evidence of enhanced epiphyte loading under eutrophic conditions, few experimental studies have assessed how nutrient enrichment influences the structure of epibiont assemblages on canopy-forming macroalgae at the edge versus inside forests. In oligotrophic waters of the NW Mediterranean, we experimentally tested the hypothesis that nutrient-driven proliferation of opportunistic epiphytic algae would affect the performance of the fucoid, Carpodesmia brachycarpa, and reduce the richness and abundance of the epizoan species they support. We predicted negative effects of nutrient enrichment to be greater at the edge than inside forests and on thalli that had recovered in cleared areas than on those within undisturbed canopy stands. Nutrient enrichment did not affect the photosynthetic efficiency and reproductive output of C. brachycarpa. By contrast, it enhanced herbivore consumption and decreased the cover and diversity of epizoans at forest edges, likely by stimulating the foraging activity of Arbacia lixula, the most abundant sea urchin in adjacent encrusting coralline barrens. Fertilization of areas inside forests had no effect on either C. brachycarpa or epibiont assemblages. Finally, nutrient enrichment effects did not vary between cleared and undisturbed areas. Our results show that moderate nutrient enrichment of oligotrophic waters does not necessarily cause the proliferation of epiphytes and, hence, a strengthening of their competitive effects on canopy-forming macroalgae. Nevertheless, enhanced herbivory damage to fertilized thalli at forest edges suggests that fragmentation could reduce the resilience of macroalgal forests and associated epibiont assemblages to nutrient enrichment.
Highlights
Coastal environments are under siege from human stressors (Halpern et al 2008; Hoegh-Guldberg and Bruno 2010)
Most of the evidence for nutrient-driven decline of macroalgal forests has been generated from eutrophic basins, agricultural or urban catchments (Coleman et al 2008; Mangialajo et al 2008; Gorman et al 2009) and, it is not representative of temperate coasts characterized by lower nutrient status (Menge et al 1999; Bulleri et al 2012; Clausing et al 2020)
To assess how nutrient enrichment influences the physiology of newly established thalli and the structure of the epibiont assemblage they support, canopies were totally removed from marked areas in four FH and four EH, randomly chosen, boulders
Summary
Coastal environments are under siege from human stressors (Halpern et al 2008; Hoegh-Guldberg and Bruno 2010). Brown canopy-forming macroalgae, either Fucoids or Laminariales, are being progressively replaced by less complex, turf-forming species, impairing the functioning of shallow rocky reefs (Kautsky et al 1986; Vogt and Schramm 1991; Bulleri and BenedettiCecchi 2006; Airoldi and Beck 2007; Gorman et al 2009; Crowe et al 2013; Strain et al 2014; Krumhansl et al 2016; Filbee-Dexter and Wernberg 2018). Use of internal N stores, along with efficient external uptake, can provide a competitive advantage to complex macroalgae, such as Laminariales and Fucales, over ephemeral algal species (Pedersen and Borum 1997; Falkenberg et al 2013; Kriegisch et al 2019; Tamburello et al 2019)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.