Algal mats reduce eelgrass (Zostera marina L.) growth in mixed and monospecific meadows

• Published in: Journal of experimental marine biology and ecology 2014-12, Vol.461, p.85-92
• Main Authors: Gustafsson, Camilla, Boström, Christoffer
• Format: Article
• Language: English
• Subjects: Animal and plant ecology; Animal, plant and microbial ecology; Baltic Sea; Biological and medical sciences; Drifting algae; Eutrophication; Fundamental and applied biological sciences. Psychology; Marine; Plant diversity; Potamogeton perfoliatus; Ruppia cirrhosa; Sea water ecosystems; Synecology; Terrestrial ecosystems; Zostera marina
• ISSN: 0022-0981; 1879-1697
• DOI: 10.1016/j.jembe.2014.07.020

Harmful algal blooms are considered to be one of the biggest threats to benthic vegetation. The amount of annual algal mats has increased due to eutrophication but their effects on perennial vegetation have rarely been tested in field experiments. In regions where eelgrass (Zostera marina) grows in mixed meadows, plant species richness may ameliorate algal-induced stress through facilitative mechanisms and improve eelgrass growth and survival. To test this, we conducted a 3mo. field experiment at 3.5m depth in the Baltic Sea, where we applied filamentous brown algae (300g ww) to eelgrass monocultures and polycultures (Z. marina, Ruppia cirrhosa, Stuckenia pectinata and Potamogeton perfoliatus). The algal stress period lasted for 13d and plots were sampled after a 5wk recovery period. In both algal-stressed mono- and polycultures, the presence of algal mats significantly reduced eelgrass shoot and root biomass production and affected the leaf growth negatively. Plant richness did not have strong effects on eelgrass growth and eelgrass in both algal-stressed mono- and polycultures showed significantly reduced productivity. Our results clearly demonstrate that loose-lying algal mats pose a serious threat to seagrass meadows and that plant diversity cannot alone ameliorate negative effects by macroalgal mats. •Eelgrass productivity and growth were suppressed by the presence of macroalgal mats.•Plant richness did not ameliorate algal mat-induced stress in eelgrass.•Algal mats pose a serious threat to seagrass meadows.